Scientific evidence against CMB originating from “Oort Soup”? In this article (and references therein) the author claims that the Cosmic Background Radiation can be explained as black-body radiation originating from the cold bodies filling the interstellar space (which the author refers to as “Oort soup”).

As far as I know, the mainstream point of view is that CMB is an echo of the Big Bang which dates back to the electroweak symmetry breaking,and has cooled down since then due to the expansion of Universe.

This is the first time I learned about the alternative “Oort soup” hypothesis, which, according to the Author, also favors the Dark Matter model made of cosmic dust. I don’t have enough evidence to judge for myself whether it is pseudoscience or a valid alternative explanation of the CMB.

I would like to know what the position of the scientific community is — is the hypothesis considered pseudoscientific, or a valid (though maybe unlikely) alternative? What scientific evidence against the author’s hypothesis do we have?

The idea that belts or spheres of dust might be responsible for (some) microwave emission is not crazy. Indeed we know that dust does emit microwaves and indeed the contribution of such dust has to be removed from the CMB signal before it can be interpreted cosmologically.
There is some debate about some of the larger scale anisotropies (particularly the "quadrupole anisotropy" (of about 1 part in $10^5$) might be the result of badly-subtracted emission from within the solar system because the anisotropy is roughly aligned with the ecliptic plane.
However, the bulk of the article you refer to is nonsense and betrays a basic lack of understanding of physics and a lack of knowledge about the microwave background.
Just to address a couple of things.

The CMB is an almost perfect black body spectrum. A blackbody spectrum is emitted by perfect blackbodies. The thermal emission from dust is not blackbody radiation. Thermal radiation $\neq$ blackbody radiation. To become blackbody radiation the emission has to come from something that is "optically thick" to that radiation - i.e. it should have a surface that we can see. In other words if we had an opaque sphere of dust around us that had somehow come into perfect thermal equilibrium at 2.7K then this would produce something that looked like the CMB. Unfortunately we would then be unable to see anything else beyond that! For a cosmological interpretation that isn't a problem. We cannot see (at any wavelength) "beyond" the CMB at a redshift of $\sim 1100$. It is a fatal problem for any model where the CMB is optically thick emission from a sphere surrounding the solar system, since we wouldn't be able to see anything beyond the solar system!

The CMB is anisotropic. The most obvious anisotropy is the large (0.1%) dipole anisotropy caused by the motion of the solar system with respect to the rest frame of the CMB. Why should the Sun be moving at about 370 km/s with respect to a spherically symmetric Oort cloud (in the direction with Galactic coordinates $l=264$, $b=48$? Even if the Oort cloud were at a distance of 10,000 au, the Sun would travel beyond it in only $\sim 100$ years! If the sphere is supposed to be further away, what is preventing gravity disturbing its almost perfect uniformity and how does it maintain a perfect thermal equilibrium?


How frequently are other planets in the Solar system hit by meteorites? How frequently are other planets in the Solar system hit by meteorites? For example, is the rate per unit area about the same?

Moon
Even though the Moon obviously isn't a planet, it's a good place to start as most of the observations of meteorites hitting other bodies has been focussed on the Moon. NASA runs the Meteoroid Environment Office which in part monitors lunar impacts. They state;

  The lunar impact rate is very uncertain because observations for objects in this mass range are embarrassingly few. Source

This statement from what I can tell hold true for most objects in the solar system aside from the Earth. However according to this source reckons the moon receives around 2800kg of meteor material each day, which seems about right. The LRO and increased amounts of astronomical observations have led to this increased knowledge on the issue.

Jupiter
Aside from the Moon, Jupiter is probably the second most studied object in regards to meteorites. This is because it regularly has some fairly energetic collisions due to its large gravitational pull. According to this paper Jupiter can receive around 12-60 meteorites with comparable strength to the Chelyabinsk airburst each year.

Also noteworthy was the Shoemaker-Levy impact in 1994, which gave a lot of importance to the study of collisions and NEO studies.

Comet Shoemaker-Levy 9 Impact

Source

Mars
Noteworthy planet as its only planet where we've found a meteorite on the surface and studied it up close. Opportunity, Spirit and Curiosity all found candidates for meteorites. Curiosity studied up close the iron meteorites Lebanon A and B.

Source

Earth

  On average, 33 metric tons (73,000 lbs) of meteoroids hit Earth every day. Source

Now while meteoroids don't count as meteorites, but this is useful in gaining a sense of the relative scale. The atmosphere of the Earth means that the meteoroids don't reach the surface, which separates Earth from objects without an atmosphere. Most atmosphere-less objects in the solar system have a fossil record of the amount of collisions they have received, in the form of all the craters on the surface.

Summary;
There's still a lot of unknown information in the area, but observations are beginning to ramp up and with that we can began to gain an idea of just how frequently objects in the solar system are hit by meteorites, so in the future a number could be put forth for each planet in the solar system.


Do objects lose momentum as space expands As far as I know, photons' wavelengths can be considered increasing as space expands, making them lose energy and momentum. Does the same apply to physical objects? I understand a photon's speed is the same regardless of the system of reference, and the same does not hold for physical objects, but is it possible to draw any kind of analogy, for instance, would the momentum of an object launched from Earth (neglect non-cosmological effects) be steadily decreasing when looked at from Earth?

GR doesn't have global frames of reference, so we can't say whether a projectile launched from galaxy A slows down relative to galaxy A due to cosmological expansion when it's at a cosmological distance. However, suppose that galaxy A and galaxy B are both at rest relative to the Hubble flow. We can ask whether the velocity of the projectile relative to B, when it gets to B, is lower than its velocity had been relative to A, when launched from A.
There are a couple of easy ways to see that the answer is yes.
One is to consider the fact that ultrarelativistic massive particles have to have same behavior as massless particles. For example, people didn't even used to know that neutrinos had mass. So an ultrarelativistic neutrino, just like a photon, has to lose momentum and energy by the time it gets to B. If this holds for ultrarelativistic particles that have mass, then we expect it to hold as well for lower-energy particles that have mass, because we expect the behavior to vary smoothly with energy.
Another way to see this is that we know the universe cooled down as it expanded. This means that massive particles must have lost energy. We can't blame this on interactions, because actually the matter in standard cosmological models is an ideal gas. So the result must be the same, on the average, for a particle that just travels freely. If there was no such tendency for motion to settle down to the Hubble flow, then we wouldn't have a Hubble flow now.
It's not true, however, that redshift factors are the same for ultrarelativistic particles as for nonrelativistic ones. The effect is bigger for ultrarelativistic particles, which is why the universe is no longer dominated by radiation, even though it was at one time.


Is it possible to calculate the time of day given latitude, longitude and solar elevation angles (zenith, azimuth, elevation)? I am trying to determine the time of day of some images I have.  I know the lat/lon, the day and year of image.  I also know the 3 solar angles (zenith, azimuth, and elevation).  Is there MATLAB or any other code that can do this calculation?

Thanks!

Of course it is! How do you think sundials work? That said, there are better, more precise ways to keep time. I don't know how to do it for an image; that is really a computational problem rather than an astronomy question. You might be better off asking it at Stack Overflow.


Can Newton's gravity equation explain why black holes are so strong? I was just wondering why black hole's gravitational forces are so powerful. I know it's usually explained by Einstein's relativity which states that when an object becomes infinitely dense (a compact mass) it can exert such a force of gravity and warp spacetime. But I also learned about Newton's Law of Gravity equation F = $GM/r^2$. Considering this equation, if the radius of an object becomes super small, then it can technically have immense gravity. So, can the gravitational pull of a black hole be explained by Newton's Law of Gravity or am I missing something? Thanks.

No you can't and the behaviour of bodies with mass and of light is completely different near a compact, massive object if you use Newtonian physics rather than General Relativity.
In no particular order; features that GR predicts (and which in some cases have now been observationally confirmed) but which Newtonian physics cannot:

An event horizon. In Newtonian physics there is a misleading numerical coincidence that the escape velocity reaches the speed of light at the Schwarzschild radius. But in Newtonian physics you could still escape by applying a constant thrust. GR predicts that no escape is possible in any circumstances.

Further; this numerical coincidence only applies to light travelling radially. In Newtonian physics the escape "speed" is independent of which direction you fire a body, but in GR light cannot escape from (just above) the Schwarzschild unless it if fired radially outwards. For other directions, the radius at which light can escape is larger.

GR predicts an innermost stable circular orbit. A stable circular orbit is possible at any radius in Newtonian physics.

In GR a particle with some angular momentum and lots of kinetic energy will end up falling into the black hole. In Newtonian physics it will scatter to infinity.

Newtonian physics predicts no precession of a two-body elliptical orbit. GR predicts orbital precession.

Newtonian physics predicts that light travelling close to a massive body has a trajectory that is curved by about half the amount predicted by GR. Even stranger effects are predicted close to the black hole including that light can orbit at 1.5 times the Schwarzschild radius.

The GR approach to gravity is fundamentally and philosophically different to Newtonian gravity. For Newton, gravity is a universal force. In GR, gravity is not a force at all. Freefalling bodies are said to be "inertial". They accelerate, not because a force acts upon them, but because spacetime is curved by the presence of mass (and energy).
In most cases, where Newtonian gravitational fields are weak, the consequences of this difference are small (but measureable - e.g. the orbit precession of Mercury or gravitational time dilation in GPS clocks), but near large, compact masses, like black holes and neutron stars, the differences become stark and unavoidable.


How much of an effect does the moon have on Earth's liquid mantle? The gravitational pull of the moon is enough to create tidal forces of large bodies of liquid, i.e the sea.

I was having a conversation the other day about how to terraform Mars, and someone suggested that if Mars was to have a moon artificially placed there then its gravitational effects would help keep a liquid core flowing.

Does the moon therefore also have an effect on other large bodies of liquid on Earth? Does the moons gravitational pull affect in any way the flow of Earths liquid mantle?

Interesting question. I would say from an energy standpoint, it almost certainly it has no effect.

Of course, the extreme case is Io, one of the Galilean moons whose heat source comes from the gravitational tidal stretching as it orbits very closely to the planet Jupiter. The heat that sustains the core of the Earth, however, is left over from its formation and also comes from radioactive decay of heavy elements.

The differential potential energy (and hence the tidal force) over the planet Io due to Jupiter, which is approximately 1300 times more massive than the Earth, is much larger than that of the Earth due to the moon. The relationship between force and differential potential energy is:
$$ F = - \nabla U $$
At a given location on the potential energy curve, the strength of the force is determined by its steepness (derivative) at that same location. Below is a quick plot I've generated for the Earth-Moon system, where the vertical red line represents the average Earth-Moon distance over a one-year period. As you can see it doesn't appear to be very 'steep', though keep the scales of the x- and y-axes in mind.

Admittedly this isn't that exciting of a plot. But, for comparison one could be made for the Jupiter-Io system, and numerical derivatives could be taken for both to calculate the magnitude of the tidal force in each situation.

To answer the question:

If the difference in the gravitational potential energy of object A on B over the scale of B is comparable to the self-gravitational energy of object B, then tidal forces will become important. This self-gravitational energy is the amount required to completely pull apart all massive particles infinitely far away. Formally, this limit is called the Roche Limit.


What is the name of that which exists beyond the Universe? "The Universe is commonly defined as the totality of existence" but that is more of a philosophical position. Empirically we know (or believe) that it is of finite size some believe it is expanding others that is (or will be) contracting.

As we gain knowledge about distant places, our scope of area has expanded from Planet, to Solar system, to Galaxy, to Universe. In the same way we continue to find smaller and smaller building blocks, from Elements, to Atoms, to Quarks.

It is reasonable to assume, that the same gains in knowledge the removed the burden from Atlas, will find the edge of the Universe, and something that is beyond that edge.

What is the name of that which exists beyond the Universe?

Naysayers, who may claim that there is "nothing" beyond universe, are reminded that their beliefs are no less strong than, those who claimed that "nothing" was supporting the Turtle or the Elephant who like Atlas also supported the world.  Lack of knowledge does not mean lack of existence.

By definition, the capitalized word Universe denotes everything there is, so even if we one day discovered we're just a part of a Multiverse, all the parallel universes of it would still be parts of the Universe as a whole, where Multiverse would just describe its nature. Or, if some yet undiscovered regions of it would defy our current understanding of its physical laws and constants as we know them, all of it would still be a part of the whole Universe. So it really doesn't matter, if beyond the known universe, there are regions of honey, milk and chocolate biscuits and all of it is carried by a giant tortoise. All of it would be the Universe, the physical universe as we can observe, the honey, milk, chocolate biscuits and the tortoise. The lot. All of it. The whole shebang.

Notice that I use capitalisation here, i.e. you can have more than one universe, but they're all a part of the Universe. Without capitalisation, it's just any domain, a particular sphere in physical or metaphysical sense, and only a part of the whole Universe. Sadly, this capitalisation is often neglected or used inconsistently, as is often the case with earth vs the Earth (the top soil vs the planet), sun vs the Sun (any star with planets vs our Sol), moon vs the Moon (any natural satellite vs our Luna), even galaxy vs the Galaxy (any galaxy vs our Milky Way). For example, observable universe is a sphere, a region of space, within the Universe.

The beauty of this naming convention is, that we already know the name for (but not necessarily of) everything there is, even if we don't know or can agree on what all that encompasses, or what laws govern some regions of space, time, or some other, yet unknown existence of it. It is universally true regardless of anyone's beliefs, even if they choose to call all of it by other names or attribute this existence to a sentient being, super being, or God. We're all a part of everything there is - the Universe.


Does red shift need to compensate for space expansion Distant galaxies are moving as space expands, not moving through space.  So a photon coming to us from them has to come back through distorted space.  So is not spacetime distorted, so the photon has to go through an area where time slows down.  So would this cause red shift.  So maybe we have a simplistic view of red shift and maybe those galaxies are not going as fast as we think

Redshift is the "stretching" of the wavelength of light as it travels, over finite time, towards us, due to the ongoing expansion of space.

The effects of this redshift are indistinguishable from that caused by the doppler effect, so that is why some elementary treatments choose to discuss it in that way.

Most galaxies only have peculiar velocities with respect to thei comoving volume of a few hundred km/s, and this does perturb the cosmological redshift by that (small) amount. That is why Hubble's law only applies at distances such that cosmological expansion dominates.


Trying to categorize moon phase using fraction illuminated I have a dataset with events that are occurring over a period of 5 years. I am trying to determine whether these events are related to moon phase. I downloaded the "fraction of moon illuminated" for the time period, but am struggling to define the cutpoints for crescent, quarter, and gibbous. These are defined by the USNO as 0.25, 0.5, and 0.75 (http://aa.usno.navy.mil/data/docs/MoonFraction.php) but I am seeking to divide the period into 8 bins of equal time. I think it is a sine function, where the period is 29.53059 days. If I divide this by 8, I get: 3.69132375 days.

Can anyone help me determine the fraction illuminated (y) that corresponds to these outpoints?

0 days

3.69132375 days

7.3826475 days

11.07397125 days

14.765295  days

18.45661875  days

22.1479425  days

25.83926625  days

???

Thanks

I used pyephem to calculate the fraction illuminated at each time

Program

import ephem
m = ephem.Moon
d1 = ephem.Moon('2017') # d1 is the date of the first new moon in 2017
days = [3.69132375*i for i in range(1,8)]
for day in days:
    m.compute(d1+day)
    print(day, m.moon_phase)

Output

3.69132375   0.1482372306829685
7.3826475    0.5259582805101828
11.07397125  0.8929835990614957
14.765295    0.9932503040700423
18.45661875  0.8004766309895268
22.1479425   0.4698580677294108
25.83926625  0.15043495343265506

Note that this is dependent on the particular month. There is an asymmetry as the motion of the moon is not perfectly even, it moves in an elliptical orbit.


How do black holes form from stars that go supernova I know that the cores of stars with more than 20 times the mass of the Sun collapse into black holes at the end of the star’s life. However, as far as my understanding goes, stars with cores that become black holes can explode in a supernova. This doesn’t make sense as in order for that event to happen, a star needs to be torn apart by a shockwave created from core bounce. How can this happen for stars more than 20 time the mass of the Sun?

Stars are not torn apart by supernovae, or at least their cores are not$^*$. At the centre of the explosion is a proto-neutron star. Indeed it is the formation of this proto neutron star that drives the explosion.
If the core is massive enough, then the neutron star will be too massive to be supported by any source of pressure and would quickly collapse to a black hole. This might be very likely if the supernova is insufficiently energetic to expel the entire envelope and some of it falls back onto the proto neutron star, causing it to implode.
It also seems likely that many black holes could be formed without a supernova explosion at all. A direct collapse black hole is probably required to produce $>10M_\odot$ black holes.
These possibilities and the observational evidence for them are discussed by Mirabel (2016).
$*$ The exceptions being pair instability supernovae at very high masses, which may set an upper limit to the mass of black hole that can be produced in this way.


Other than relative temperatures, is there any difference between a red sun and a yellow sun? As time goes by, explanations for unusual things that exist in fiction get disproven. We know radiation probably isn't going to create a Giant ant that destroys everything, we're pretty sure we can't use ray guns, and the idea that falling into a black hole would put you face to face with Ming the Merciless is laughable.

Comic nerds might have guessed from the title why I'm asking, but I'm curious if there could still be any reason to latch on to to explain why superman is powered by a yellow sun, but not a red one? Does the higher temperature of a yellow sun create a higher level of some concentration of exotic particles, or once you get to the point of [solar fusion] is the output pretty much exactly the same?

Note: This question is not about superman, or his powers, just if there is any (temperature aside) meaningful difference between a yellow and red sun (that could be conveniently used by DC to still justify superman's powers).

No.

Small red dwarfs may be fully convective. Larger stars like the sun have a region in which the energy is transferred by radiation. So in small red dwarves, the star becomes fully mixed.

Red Giants are quite different and are large and old stars, swollen by their powerful cores.

Both red and yellow stars produce large amounts of neutrinos. Yellow stars produce more than red dwarfs because they are more powerful. Red Giants would produce more than yellow dwarfs, as they more powerful.

Nothing that could explain superman.


What is beyond the observable universe? What is beyond the edge of the observable universe? Scientists say that the Big Bang is the cause of all creation in the known universe, but it would seem the bang must have happened in the center of the universe ?

What is beyond the observable universe?

There's no way to know because we can't see it.  No information from the unobservable part of the universe can ever get to us.  If we can't see it, we can't know what's there.

Over time, the observable universe grows and we see more of the universe, though this is actually a pretty complicated due to the rate of expansion and I'd rather not open that can of worms, but for further reading, see this question or this one.

But avoiding the complexities of dark energy expansion, we have a pretty good idea that the unobservable universe is very much like what we can see.   This makes logical sense because it likely formed the same way at roughly the same time and out of the same stuff.   So, we expect it's just more galaxies and more space.

In addition to this, if there was a point where the galaxies stopped being there at a theoretical 'edge', not far from the edge of the observable universe, we'd expect to see that lack of gravitation resulting in the galaxies moving away less fast and that's not observed.   The galaxies appear to be moving away from us equally fast in all directions, so we have good reason to believe that the universe is uniform out considerably beyond what we can observe.   It's at least several times larger than what we can see.

If we think of the big bang at happening at a point, which is often depicted in sciency-tv shows, then we shouldn't expect uniformity due to gravity and faster moving parts spreading out more, but what we see when we observe the universe is uniformity.   That means that if it did explode from a singular point, it must be so large that we can't see any variation.

The more popular approach is that the big bang didn't happen at a point but it happened everywhere.   Google Everywhere Stretch for some explanations of this.

It's worth noting that not all shapes (or manifolds) have centers.   The surface of the Earth has no center on the surface.   So even if we could see the entire universe, finding it's center might still not be possible.

Determining the center of the observable universe, however, is easy.   We can see in all directions just about equally so we're the center.  But we can't get a good grasp on the size and shape of the entire universe, or just universe is more correct.   Since we can't see all of it, finding the center is impossible.

  The bang must have happened in the center of the universe.

Keep in mind that the big bang isn't well understood.  It's what physicists call a singularity which means, the math breaks down.     The point exploding outwards is often represented, but shouldn't be considered accurate.

Mathematical approaches can come up with a variety of possibilities like hyperplanes intersecting or 4 or 5 dimensional spheres.   (Stephen Hawking had a 5 dimensional sphere argument where no big bang happened, it's just all a circle - that's in one of his books, I think blackholes and baby universes).   He was able to create a universe without a big bang.   That the universe is infinite and the big bang happened across infinite space is also a popular idea today.    The singular point that blew up into the universe isn't big right now, though that viewpoint still applies to the observable universe, but it's fallen out of favor for the entire universe.

My two cents anyway, as a layman.


How can I estimate how long will a main sequence star stay on the main sequence given its temperature? I have a generic zero-age main sequence star. The only thing I know about it is its temperature. How can I estimate time time it spends on the main sequence (in millions of years)? I've seen equations that estimate it based on mass, like $L=M^{-2.5}\cdot10^{10}$, but I do not have the mass, only the temperature. Is there any definite formula (accurate to a reasonable degree) for this?

The website on Main sequence stars fromr the Austalian national telescope facility lists star mass, temperature and life span:

Mass/MSun
Luminosity/LSun
T=Effective Temperature/K
Radius/RSun
t=Main sequence lifespan/yrs

0.10
$3×10^{-3}$
2,900
0.16
$2×10^{12}$

0.50
0.03
3,800
0.6
$2×10^{11}$

0.75
0.3
5,000
0.8
$3×10^{10}$

1.0
1
6,000
1.0
$1×10^{10}$

1.5
5
7,000
1.4
$2×10^9$

3
60
11,000
2.5
$2×10^8$

5
600
17,000
3.8
$7×10^7$

10
10,000
22,000
5.6
$2×10^7$

15
17,000
28,000
6.8
$1×10^7$

25
80,000
35,000
8.7
$7×10^6$

60
790,000
44,500
15
$3.4×10^6$

If we suspect a lifespan, $t$ is related to the temperature $T$ by a power relationship, $t=aT^k$, we can plot log(t) against log(T) and look for a linear relationship

That looks plausible, and gives a relationship of $t = 10^{28}\times T^{-4.7}$ (rounding to 2sf)
looking more closely, this tends to underestimate the life span of mid-range stars, but overestimate large and small stars. You can adapt for this by considering smaller and larger stars separately: For stars less than 10000K, a model $t = 10^{37}\times T^{-7.6}$ and for hotter stars $t = 10^{20}\times T^{-3.0}$ gives reasonable fit to the data.
This is, of course, a purely empirical fitting of a curve, and isn't based on an astrophysical model. (though I speculate that the change in gradient is related to the change from the proton-proton to the CNO cycle fusion of hydrogen.)


Why Jupiter's Great Red Spot is Reddish? The Great Red Spot is a persistent anticyclonic storm, 22° south of Jupiter's equator, but why is it reddish?

From Wikipedia:

  It is not known exactly what causes the Great Red Spot's reddish color.

Are there updated data?

It's red, because it's a 'sunburn'. The clouds in the red spot reach to higher altitudes than the surrounding and are more exposed to Solar UV radiation, which in turn changes the structure of some of the organic molecules etc. This is at least the explanation suggested by recent data from NASA's Cassini mission, see this 5 day old press release.


I too saw a star like object moving across the sky it appeared to be a star moving! I too saw a star like object moving across the sky from Bournemouth towards say Parkstone, it appeared to be a star moving for quite a long time until it disappeared from my view!
I have seen this the last two nights and so have my family, it's not the Space Station as I see this regularly as my Star Walk app, you can add in when you know what the object is! But unless I know the name of this object I can't track it on my app, does anyone know what it could Be?
It is not a plane as far too high like a star moving but fast across the sky? This was timed 22:10 on Sunday August 28th 2017

This is very probably another artificial satellite, there are hundreds of them

references :
http://www.pixalytics.com/sats-orbiting-earth-2016/
https://www.universetoday.com/42198/how-many-satellites-in-space/


Telescope focal point and inversion When looking at a diagram of the optical path through a newtonian reflector, the lines always cross before the eyepiece. I understand that the eyepiece is used to focus this diverging light cone so that your eye can observe an image.

If you use a camera in "prime focus" i.e. Without an eyepiece to rectify this diverging light cone, it can discern an image anyway? Would this mean that the focus adjustment (distance from the focal point) merely determines the size of the image on the sensor? Or is there still a point where the image is in focus?

How do we ensure that this occurs while the light cone is still smaller than the sensor? Or is the whole image always varied in focus across the cone, just that the depth of field allows all of it to be in focus enough for a good image?

The main mirror of a telescope forms an image at the prime focus.

You see the lines being brought to a single point, but that doesn't mean all the light entering the telescope is focused to a single point. It means that all the light from a single point in space is focussed at a single point. Light from different sources is focussed at different points, these points form a plane in the body of the telescope, the "focal plane".

This image can be seen. You can project an image using a single lens, or if you hold up a lens at arm's length and look through it, you will see a small inverted image. The purpose of the eyepiece is to act as a magnifying glass and make this small inverted image appear bigger.

The image is in focus only at the focal plane. If a sensor (without any further lens) is placed at any other point, no image will be seen, or at best a blurry image. We can see the image when we look through the eyepiece only because we have another lens (the cornea of our eye) bringing the light to a second focus on our retina.


In which direction does the Milky Way rotate? We don't have a photo of our Milky Way the whole, all the pictures are artists' interpretations showing a bulge, a central bar, and several spiral arms. The milky way is spinning. I would like to know:

a) Does it spin with the arms trailing or leading?

b) I read if the direction of spin clockwise or counterclockwise depends on whether the Milky Way is observed from above or below. I don't understand how the direction of spin (clockwise & counterclockwise) depends on the point of view of the observer.

The Milky Way has arms that form due to density waves. Like the majority of spiral galaxies, the arms are trailing. Individual stars orbit in circles (roughly), neither towards or away from the centre.

If you consider a common map of the Milky way (imagined from a point North of the Earth, Celestial North is not the same as Ecliptic North, which are both about 60 degrees off from Galactic North)

The stars in the galaxy would be moving in a clockwise fashion.

If you were to view from the other side, it would be as if you had made a mirror image of the galaxy, so the motion would be counterclockwise. If you look at the back of a clock, then from the wrong side, the hand would move counterclockwise. However the rotation of the galaxy is still with its arms trailing.

There are some galaxies that rotate with arms leading: NGC 4622 is one example.


Is Jupiter really shepherding the main belt, and Neptune the Kuiper belt? By "dominating another object's orbit" my understanding is that the most massive body's gravity has so much influence that, when they come close, it makes the other body/bodies' orbits shift or change, either certain parameters (such as apoapsis, periapsis, inclination etc.) or up to pulling it completely out of its current orbit (thereby perhaps ejecting the body from it or making the body collide with the dominating body or a satellite of the dominating body). However, as far as I can determine, Jupiter is unable to alter the orbit of Ceres; both of them are in stable orbits, nor can any other of the main belt's equilibrium-shaped object's orbits (Interamnia, Hygiea, Pallas and Vesta) ever influence themselves to the point that one's orbital parameters are altered nor does Jupiter exert enough gravitational influence on any equilibrium-shaped main belt object to alter its orbit.
The same seems to be true for the Kuiper belt: For every two complete orbits made by Pluto, Neptune makes exactly three orbits. So both object's orbits are stable and Neptune doesn't influence Pluto's orbit in a way that it would be altered, nor is any other spherical TNO's orbit manipulated either by Neptune or by themselves between each other.
Am I right and is there even any equilibrium-shaped object directly orbiting the Sun that ever altered another equilibrium-shaped object's orbit in recorded history, or would be able to do so in a close approach? As I see it, all ellipsoidal objects orbiting the Sun in our system are on stable orbits that won't change unless some interstellar 'visitor' might meddle any up.

Orbits aren't knife-point balances of speed, distance and direction. For two objects separated by distances much larger than their diameters, almost any combination of the three where the speed is below the local escape velocity results in a stable elliptical orbit whose specific parameters are determined by the combination.
Ceres (and all the other objects in the solar system) are affected in their orbits by the gravitational influence of Jupiter (and everything else, for that matter). However, because the orbital periods of Jupiter and Ceres aren't low-denomination multiples of each other, the influence of Jupiter, while present, remains small, and the orbit remains pretty much the same, year to year.
Where the periods are a low-denomination multiple of each other, however,  the gravitational interactions reinforce repeatedly over billions of years, and can serve to both alter orbits such that the resonance is no longer maintained (see the Kirkwood gaps in the asteroid Belt), or in specific situations, lock an object into maintaining that resonance.
With Pluto and the Plutinos, the fact that their periods are clustered around 3/2 Neptune's orbital period is a direct result of Neptune's gravitational influence over time, rather than in spite of it; The regularity in the amount and the direction of Neptune's perturbations to their solar orbits keeps them near the 3:2 resonance, similar to the way Jupiter keeps the Jovian Trojans near the 1:1 resonance and the Hildas near the 2:3 resonance.
Note that all the orbital parameters of all the bodies in the solar system are changing over time (For example, the perihelion of Jupiter precesses approximately 6.55 arcseconds per year). The Keplerian Orbital Elements of the planets that you may find online are based on measurements taken at a specific time, and define orbits that assume that the planet and the Sun are the only bodies in the universe.


Did the earth form outside the solar system? Why isn't it possible that the earth formed outside the solar system and got attracted later by the Sun?

I just need arguments to defeat this hypothesis.

The chance happenstance that Earth happened to be floating along and got captured is minuscule. How did Earth wind up floating through space? There's no established mechanism for terrestrial planets to form on their own. As far as we know, they need a host star to form around. So if our Sun captured Earth, it must've formed around another Star, got ejected somehow, and then came under the influence of our Sun where it was captured and nicely placed at its current position. This is all highly, highly unlikely. I'm not going to run through the math, but you probably have a better chance of getting struck by lighting at the exact moment you buy a winning lottery ticket (twice).
Aside from a statistical argument, you can look at it from the vantage point that the composition of the Earth properly matches everything else in our solar system. People have made a very close study as to the physical composition of Earth, the Moon, and the other planets and bodies in our solar system. We would have noticed if there was a large disparity in composition or something just didn't match up. As it is, Earth matches up pretty darn well, especially with our Moon. The close resemblance in composition between our Moon and Earth gives strong evidence that the Moon was formed from the Earth. If Earth was captured, so too was the Moon which makes point 1 even less likely.
Many people have spent a lot of time simulating the formation of our solar system, from the start up until now. There are a few open questions (such as why Jupiter resides where it does), but by and large, people agree that a planet should form about where Earth is. So if Earth wasn't here during formation and was later captured, why isn't there an extra planet that did form where Earth is? All simulations suggest such a planet should exist. The most likely scenario is that Earth is that planet!
It is believed that basic life (i.e., single celled organisms) started on Earth around 3.8 to 4.1 billion years ago. Earth itself has been reliable dated to about 4.53 billion years old. This means life was forming a mere 400-700 million years after Earth first formed. Life requires very very specific conditions (as we know it) to start forming and that can't happen in the dead of space. Earth had to be around a star from the beginning. Any time spent drifting through space until it was captured would have delayed the onset of the formation of life.

I'm sure there are plenty of arguments that could be added to the list, but that's a good start I think.

Edit: I didn't expect such a spirited debate in response to this answer. First off, I want to say that none of my arguments above are bullet-proof. I'm going to incorporate some of the comments below to help round out this answer. This will be easier than creating a long conversation of comments.

Matthew Whited - [It] is possible that other rouge planets could have formed outside of their own solar systems. It is also possible that they could form within solar systems and be ejected.

This is certainly true. One cannot soundly reject the hypothesis that Earth formed elsewhere (be it in free space or around another star) and was later captured by our Sun. My arguments are mostly ways to point out that the chance of such an occurrence is diminishingly small due to the unlikely circumstances that must occur for that hypothesis to be true. The betting man would not side with the hypothesis that the Earth formed outside our solar system and was later captured.

Rob Jeffries - Almost the only way Earth could be captured is during the clustered star formation stage (well established theoretical possibility). In which case it would have the same age and metallicity as if it had formed around the Sun. So age and metallicity are weak arguments.

I don't know if they're as weak as that. If the Earth did come from another stellar system, it would of course come from a nearby one, which would of course have a similar metallicity as our Sun. But I think a point which makes the above arguments stronger is the specific composition of the Earth. If it formed around a star which was slightly different in nature (say an M Dwarf) or at a different distance from the star I think we'd notice that the Earth seemed "out of place" in our solar system. As it is, its iron content agrees quite well with it forming where it currently resides. Again, none of this is bullet-proof, but it cast's doubts on the validity of Earth forming elsewhere.

Rob Jeffries - Planet capture in the nascent cluster also nullifies arguments 2 (since the Moon would have to be formed after capture) and 4.
jwenting - The moon argument isn't a valid one. The moon could have been formed out of the earth after capture. Which doesn't make that capture a more likely scenario of course.

I wouldn't say nullifies. In fact, I would think it makes them a bit stronger. The process of the Earth forming elsewhere, being flung out of its stellar system and captured by ours (and put in its proper place), then forming a Moon and life, requires that these events happened extremely quickly. While not impossible, it is improbable, especially when you add in your argument for the Earth being in a very precise orbit unlikely to have been achieved if it had been captured. If you argue that the Moon/Life formed after capture you're really limiting the time and chances that the Earth didn't form in our system.


What is high redshift? I'm curious about the difference between low redshift and high redshift universe. Is there any defined limit of redshift beyond which we call things high redshifted?

There definitely is no definition of what is low and what is high redshift — it is always seen in relation to something. An astronomer working in, say, the interstellar medium of the Milky Way ($z\sim0$) might talk about the ISM of a $z\sim2$ galaxy as being high redshift, whereas another astronomer working the reionization of the Universe (which took place around $z\sim10$) would consider $z\sim2$ to be low redshift. A cosmologist working in the CMB or inflation, however, would probably call $z\sim10$ low redshift. It all depends on the context.

Nevertheless I think that most people would consider $z\lesssim0.1$ to be low redshift.


How bad could we reasonably expect a solar flare to impact earth, and what can be done to mitigate the impact? According to numerous articles, solar flares have done damage to the power grid on Earth in the past: http://science.nasa.gov/science-news/science-at-nasa/2008/06may_carringtonflare/

How bad can we expect a severe solar flare to impact the power grid / electronics?  Would a bad flare also break electronics (e.g. computers), or "just" the power grid?
Is there anything that can be done to reduce the effects?

Space Weather is the field within heliophysics that tries to understand this Sun-Earth relationship. When a Solar Flare occurs multiple things may happen and how it interacts with the Earth and our technology depend on different factors.

The first effect, the X-ray radiation produced by an Solar Flare affects the ionosphere and therefore the radio communications. This effect is almost immediate and there's no way to attenuate or prevent it. We can forecast the likelihood of a flare to happen on the day but still we don't have the knowledge enough to exactly predict when it will happen. The radiation from the flare it takes around 8 minutes to get to Earth, and at the time we see it the radio communications have already been affected. This effects mostly to VLF bands.

After a solar flare high energetic particles and coronal mass ejection could be produced, these two events have different effects on Earth and whether interacts with Earth or not depends on where it happens on the Sun and the properties of the solar wind at that time. High energetic particles (also known as Solar Energetic Particle events - SEPs) could arrive around 30 minutes after the flare happened, and Coronal Mass Ejections (CMEs) between 2 or 3 days. These times depends on the properties of the events, not all are the same.

CMEs could be somehow predicted, and are these the ones that could affect, between others, the power grid. It won't affect your computer. The reason of why it affects the power grids is due to a induced electric current happened by the interaction of the CME's magnetic field produces in the Earth's magnetic field. The electric current finds it easier to travel through the power grid than through the Earth itself (the cables are better conductors). In simple words, when that happens the electric grid "thinks" that there is an increase of demand and tries to compensate it generating more electricity and that can burn them - as it happened in Canada in 1989.

To reduce it effects there are a lot of engineering work around these systems and I'm not an expert in such topic to be able to explain them properly. However, a warning to the power grids of a couple of days in advance helps. The difficult bit is how to be exact in the forecasting. CMEs sometime are deflected by the solar wind and just a flank of it "touches" the Earth, other times small ones get accelerated and have a larger impact than expected. So, to be able to reduce the effects we need to have a better understanding of their origin and surroundings, understand their physics and be able to model what they are doing. There are few models out there (e.g., CCMC) but at the moment we need to keep researching to improve these further.

Answering to "how bad" a solar storm (not just a flare) can affect Earth is hard to quantified, in UK it has been classified as one of the highest priority natural hazards in the National Risk Register, and similarly in other countries. An event like Carrington's (the one you linked) could have a lot of impact in our current technological civilization and it's been estimated in many years and trillions of US dollars to restore the damage (see Royal Academy of Engineer report on Space weather).


How to calculate the orientation of planets at current epoch For an application I am working on I am creating a simulation of the solar system. In this simulation, a user needs to be able to 'move' to any position on any planet, and have the sky be accurately simulated around him.

Using the examples & calculations provided at http://stjarnhimlen.se/comp/ppcomp.html and the data provided at https://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf I am able to compute the POSITIONS of the planets at any date, however, what I am still looking for is the ORIENTATION of these planets at that position (i.e. which way is Long 0 Lat 0 pointing, and which way is Long 0 Lat 90 pointing (with respect to the vernal equinox)). I need this orientation in order to figure out what the rotation (quaternion) of this planet needs to be..

I have been looking around, but have so far not been able to find a source for any kind of information like this. The closest I've found was https://en.wikipedia.org/wiki/Poles_of_astronomical_bodies , but that only gives me an up-vector, not a 'forward' (and is for J2000, not the current Epoch).

Can anyone be of assistance?

The IAU WGCCRE (Working Group on Cartographic Coordinates and Rotational Elements) seem to be the authorities on this.
Table 1 in their 2015 report gives the following information for the major planets:

α0, δ0 are the ICRF right ascension and declination of the planet's north pole. If the planet's axial precession is known, these are expressed in terms of T, the number of Julian centuries since epoch J2000.0.

W is the rotation angle eastward from reference point Q to the planet's prime meridian at time d, where Q is the intersection of the planet's equator and the ICRF equator at RA α0 + 90°, Dec 0°, and d is the number of days since epoch J2000.0.

They define a planet's north pole as the one pointing north of the solar system's invariable plane.
This means that W decreases with time for Venus and Uranus.
Their figure 1 appears to indicate Q's RA as 90° - α0, in conflict with the text. I believe the text is correct.
When you are ready to validate your calculations, the S&T Mars Profiler shows which side of Mars faces Earth at a given time.


Trying to use Canon 60D DSLR with Skywatcher 130P. Can't focus image I have a Skywatcher Heritage 130P FlexTube Dobsonian Telescope. It works well, I can see lovely stars, planets, moons of Jupiter etc.  It came with two eye pieces (1.25 inch): 10 mm and 25 mm.
I also have a Canon 60D camera.
I've been trying to attach the camera to the telescope using a Celestron 93640 1.25 Inch Universal Barlow and T-Adaptor which I originally bought for a Celestron Travelscope.
I have never been able to get any kind of decent image out of the camera when attached to either  telescope. Am I doing something daft? (This is all new to me).
Steps I take:

Set up and focus the Skywatcher scope on some nice stars using the 25mm eye piece.
Remove the eyepiece.
Insert the camera with Barlow and T ring where the eye piece was.
Diddle around with the focus. Try and take some photos. Get nothing remotely in focus, not ever a blurry star.

I suspect I have the same issue as described in Astrophotography - Unable to achieve focus with current setup If so, please could someone advise the adapter I would need for the combination of camera and scope I have? I'd prefer to use the DSLR than buy a digital camera if possible. (Many thanks, and sorry if this is a daft question.)

As you suspected, the problem is probably that, as with most Newtonian telescopes, the image plane is too close to the outside of the tube, so you can't physically get your camera sensor close enough to the image plane.
There may or may not be adapters that will solve this problem for your specific telescope and camera. Barlow lenses help but aren't always enough. Luckily your telescope is collapsible, so you should be able to bring the image plane far enough out by collapsing the telescope a little. Attach your camera to the telescope as before and slowly collapse the telescope. You should see the image gradually come into focus.


What accounts into calculating the Hubble constant? From my understanding, the Hubble constant $H_0$ calculates from observed redshifts $z$ of distant galaxys against their proper distance $D$. The current value appears to be 67.80(77) $\frac{km}{s}Mpc^{-1}$

Calculating the Hubble constant via the redshift, I assume one only wants those velocity contributions due to the expansion of the universe, and not those from the real movement of the galaxies within in cluster (peculiar motion) or so. This, I also read in various online resources (which I can't recall right now).

On the other Hand, if your galaxy cluster is far enough away (eq. ~ 1 Gpc), one can neglect peculiar motion, which is in the order of 1000 $\frac{km}{s}$ (1000 $\frac{km}{s}$ / (1 Gpc $\times$ 67.80(77) $\frac{km}{s}Mpc^{-1}$) $\approx$ 1.4%)

Nonetheless, how would you try to correct for the peculiar motion, or is it really just neglected? Calculating all the gravitational components in each cluster? Another idea could be to assume that the galaxies within the cluster move randomly in respect to each other and the peculiar motion cancels out over the average? Any other possibilities?

Note that this question is partly a copy of another question of mine at physics.stackexchange.com, which wasn't fully answered but commented to ask it here.

I think this might answer your question, I have quoted the important paragraph below.

  If peculiar velocities could have any value, then this would make Hubble’s law useless.
  However, peculiar velocities are typically only about 300 km/sec, and they very rarely exceed
  1000 km/sec.
  Hubble’s law therefore becomes accurate for galaxies that are far away, when H0d is much larger
  than 1000 km/sec.
  Furthermore, we can often estimate what a galaxy’s peculiar velocity will be by looking at the
  nearby structures that will be pulling on it.

So it seems that in answer to your question, if the galaxy is far enough away then the peculiar motion can be neglected, or if its close by, a reasonable approximation found using estimates from nearby galaxies (in a cluster for example).


DSO carthesian coordinate estimations I'm more in the CS based stack exchange, but this question prompted me to come in here.

I've been compiling a large set of ephemerides for solar system, stars with mag > 6.8, exo planets etc, and I am now working on the DSO catalog.

The app actually holds celestial objects in 3D space, and I made conversions from RA/DEC/dist(parsec) for various objects, but I am having trouble with DSO objects.

I know of a red shift time base formula, but heard the conversion is not very good. Also, I understand that most estimations cannot be counted as precise, but I would like to have these objects within 3D space.

Can anyone tell me what features I should look for in a catalog and what formula I can base myself on for this conversion, if any?

thanks

There are many cosmological distance measures and what you need depends on what you want to do. If you want to position the object in 3D space where it is today, you need its proper distance. If you want to e.g. display the object with a certain angular size on the sky, and you want to figure that size out from how far away the object is, you need the angular diameter distance.

To calculate any of the measures, you need the object's redshift $z$. You will probably need to store the comoving distance (= proper distance today) and redshift so that you can convert to other distance measures quickly without redoing the integral.


What star is the closest to Betelgeuse? Based on this answer, I should be able to figure out this question, if I know the distance between Betelgeuse and its nearest neighbor. So what is Betelgeuse's nearest neighbor?

I do mean the nearest neighbor in 3D space, not in the plane of the sky. Since Betelgeuse is fairly close to the Sun, I'm guessing we should know its neighborhood with a pretty good precision.


What is the share of stars in total radiation input of Earth? Earth gets its radiation input primarily from Sun, then from reflected sunlight from Moon, and stars. Among these, what is the share of stars (or sources outside solar system) to this total radiation input of Earth. Is it negligible compared to solar radiation? (I am referring to top-of-atmosphere situation.)

It is negligible.

The amount of energy per square metre per second is called the "flux". The flux of the brightest star in the sky, Sirius, is one ten billionth of the sun. That mean the energy the earth receives from the sun in one second is the same as the energy that it gets from Sirius in about 300 years. And Sirius (at magnitude -1.47) is by far the brightest star. All the others add next to nothing.


How to figure out what something in the sky is? A while ago I saw something in the sky early in the evening. At first, I thought it was a plane since it seemed far too bright to be a star - no other stars could be seen. However, it didn't move, so I ruled out that possibility.

It seemed to be relatively close to the horizon so I considered if it was a very high pole with some sort of light at the top, but after I walked for a few kilometres it was still at the same location.

During the few hours I looked at it, it moved a bit across the sky.
I considered a satellite, but it didn't seem to move quite enough of what I anticipated a satellite would move.

Since then, I have seen it every evening when I remember to look for it.

Apart from this object only the moon could be seen. To me, it seemed like the moon moved in the same arched way across the sky as the object, but I could be wrong on the.

I have seen several questions on this site where someone asks "what was it?" which has been closed with "too broad". So I am not going to ask what it was.
Instead I am curious about what I can do to figure out what it is? Which observations is relevant to make when looking at such objects? As someone who is curious about the sky, but does not have any instruments for doing any actual measurements can I actually do anything?

The first picture shows the object in the sky.

The second picture shows the same object, but where I have indicated the arch the object moved in as well as the moon and how that moved.

It's hard to say 100% for sure without more info.  What is your location and what date/time was this?  How fast did it seem to move?

To me, it looks like Venus.  If it looked like a bright star, but much bigger and brighter than other stars, in the west around sunset, and moving slowly toward the horizon, then it was most likely Venus.


First spectroscopic detection of Uranus' rotation? As an undergraduate exercise quite a while ago, we placed the equator of Jupiter's disk across the slit of a high resolution grating spectrometer and then measured the tilt of the resulting line in order to estimate Jupiter's rotational velocity.

When was an experiment of this type first performed for Uranus?
What was the result? Did it make sense at the time? Was Uranus' 98 degree axis tilt appreciated?

Related: Uranus' axis of rotation-when discovered?

Slipher 1912 and
Lowell 1912
used a one-prism spectrograph on the 24-inch refractor at Lowell Observatory in this way.
Their 1911 observations of Uranus yielded a period of 10.8 hours, which made sense given the polar flattening previously observed by others.

They knew the obliquity of Uranus's satellites' orbits and assumed that its equator was in a similar plane.
This affected their observations in two ways.
First the solstice of 1901-02 presented an unfavorable pole-on aspect; after a negative result in 1903, Lowell waited until 1909 to try again.
Then Uranus required an unusual orientation of the spectrograph, so Slipher took special care to avoid instrument flexure bias.

Moore and Menzel 1930 credited Slipher and Lowell with the first such observation and obtained a similar result themselves.


Does the Moon Have Enough Water for Robert Zubrin's Mars settlement Plan To Work? In Robert Zubrin's "The Case for Mars" he outlines a plan to settle Mars by(among other things) finding water and using electrolysis to break up water into Hydrogen and Oxygen. This provides fuel for vehicles and air for the crew to breathe. Can a similar plan work to settle the moon? Are there enough resources on the moon for a settlement to be self-sufficent? Is there enough water on the moon for this to be viable?

Self-sufficiency is an incredibly broad term. We could argue that yes, there is water on the Moon, and that yes, there are viable ways to produce required electricity in self-sustainable ways, but the real question is, are there areas on the Moon that would be viable for both at the same time.

You see, the most likely place where surface or near subsurface water could exist on the Moon and be suitable for mass extraction are its polar, permanently dark regions. Indeed, the ISRO (Indian Space Research Organisation) Chandrayaan-1 spacecraft has detected evidence for water locked in surface lunar regolith minerals in lunar south polar region, water that likely originates from asteroid and comet impacts embedding it deep within the lunar core and released as magmatic water closer to the surface. Any free-form water in other regions of the Moon that are exposed to sunlight and Solar radiation would sublimate to its gas form directly and with ionisation lose hydrogen atoms, so while hydrogen and oxygen atoms might still be present to some extent embedded into the surface layer minerals, extraction would likely be too elaborate there.

But, wherever you'd find your water source, you would still require a great deal of electricity to power your extraction plant, later use electrolysis to separate molecular water into its constituent atoms, and compress it in cryogenic conditions to their diatomic liquids that are suitable as propellant components, diatomic liquid oxygen (or LOX) as your oxidizer, and double as much in molecular quantity of diatomic liquid hydrogen (or LH2) as your rocket fuel. Problem with electricity is, unless you brought your own and a great deal of it on you to power your plants, you will likely want to use be it solar power, or tap into the in lunar regolith embedded helium-3 (or 3He) and power your third-generation Helium-3 fusion reactor. See for example this answer of mine on Space Exploration on how that could be done.

So the main conundrum to exploiting Lunar resources, for the time being, remains finding sufficient and viably mineable resources of water where there is also self-sustainable ways of generating required electricity. One option that I can think of is staying on the most exposed to the Sun lunar equator and extracting deuterium and tritium hydrogen isotopes, as well as helium-3 from lunar regolith, all of them embedded there from Coronal Mass Ejections (CME). Required oxygen could be produced by crushing oxidized minerals and letting them sweat with the presence of hydrogen isotopes into ionized water, and helium-3 could be used as previously mentioned to sustain a fusion reaction producing required electricity to later break water molecules into its constituent atoms of hydrogen and oxygen by electrolysis.

How much of these hydrogen and helium isotopes are actually embedded in the lunar regolith, and how long these deposits persist in it, possibly staying there for at least some time due to the static charge of the regolith as it is bombarded by the Solar radiation, this is however a whole different question and one we can't currently yet answer. The study of the Lunar exosphere and dust environment is the sole purpose of the LADEE (Lunar Atmosphere and Dust Environment Explorer), that we barely just launched there. We will know in roughly one year, if it will be able to provide conclusive scientific evidence for these theories I've just mentioned.


How are gravitational lensing and dark matter related? In a lecture which I attended at my institution on gravitational lensing, the lecturer linked gravitational lensing and dark matter research. Can anyone explain what is the relation of both?

Gravitational lensing of background galaxies and quasars is used to probe the amount of gravitating matter in foreground objects such as clusters of galaxies.

Here is an example, taken with the Hubble Space Telescope.

The foreground object is a cluster of galaxies and the lensed background objects, some of which are marked, are galaxies and quasars that are much further away.

The shapes of these lensed objects can be used to deduce the gravitational potential in the foreground cluster. This potential is due both to the galaxies that can be seen, hot intra-cluster gas (that can be seen in X-rays), but chiefly is due to dark matter that pervades the cluster. Thus a combination of optical, X-ray and lensing observations can map the distribution of dark matter and hot gas in the cluster.

An example is shown below for Abell 1689, where the diffuse, blobby emission is a false-colour representation of the density distribution of dark matter inferred to be in this cluster from the many lensed images of background galaxies that can be seen.

http://hubblesite.org/news_release/news/2010-37

A particularly important example that is a landmark in dark matter research is the Bullet Cluster. This is actually two clusters that appear to have collided and passed through each other about 150 million years ago. The distribution of galaxies in the clusters can be seen in optical images; the hot gas can be traced by X-ray observations and the dark matter is traced by looking at the lensed images of distant background galaxies.

The key image is shown below - it illustrates the distribution of optical galaxies, the X-ray-emitting hot gas (pink/red) and the inferred distribution of dark matter (blue).

As the clusters move through each other, the hot gas interacts and piles up into bow-shocks that are displaced from the distribution of optical galaxies. On the other hand, the dark matter, which is thought to consist of non-baryonic, non-interacting particles remains centred on the visible galaxies and each dark matter "cloud" has apparently passed through the other with no interaction occuring.

The displacement between the hot gas and the majority of the gravitating matter, that is dark, is very strong evidence for a non-baryonic particle description of dark matter, rather than some of the alternatives to dark matter like Modified Newtonian Dynamics.


"Turn on point" for fusion? I've read that when a body reaches about 13 times the mass of Jupiter that deuterium fusion starts and the body becomes a brown dwarf instead of a planet.

I have a multiple question.

Are there any other factors besides mass and possibly metal abundance (composition) that would determine this "turn on" point ?
If the "brown dwarf" was detected indirectly and no spectroscopic information was available, could we assume that metalicity would be the same as the larger star ?
Would a higher metallicity body "turn on" at a lower mass? (Would a difference between [Fe/H] = -0.19 and [Fe/H] = +0.07 make much of a difference in "turn on mass"?)

*I did miss the related question when I posted this one, but I do have a question here that wasn't answered. It was about how stellar metallicity effects the onset of fusion. I think it may be as simple as whether or not adding "metals" makes the core hotter or colder for a given mass. I also wanted to know if I could assume that both "stars" in a binary system should have the same metallicity.

*Now that I've read Rob's excellent (as usual) answer, I would be curious as to how long deuterium fusion lasts in brown dwarfs from 13 to 80 Jupiter masses and what happens (in models) when it stops.

I don't have detailed model calculations to hand and I'm not sure they have been done for the small range of metallicity you mention. However, in handwaving terms.

(1) Mass and composition are the most important variables. Helium abundance, in addition to metallicity, could play a role. Rotation could be a very second order effect. Very rapid rotation would lower core temperatures at the same mass.

(2) This is the general assumption if a star plus brown dwarf companion formed in a binary or even if they formed in the same cluster.

(3) Higher metallicity leads to larger opacity in the atmosphere and a larger brown dwarf. This is a small effect in fully convective objects like these, except perhaps when they are very young and have superadiabatic surface layer. However, as this is when D burning takes place, possibly there is a small effect - higher metallicity leads to a larger radius for the same mass and a lower core temperature. So the mass threshold would be slightly higher.

(4) The duration of D burning is mass dependent. The graph below, from Burrows et al. (1997), shows luminosity vs time for brown dwarfs, planets and stars. The D burning phase is the early plateau in luminosity, lasting a few million years for things which are almost stars to maybe one or two hundred million years at 13 Jupiter masses. When D fusion ceases, then cooling recommences and the luminosity will continue to fade (forever), unless the object is massive enough to ignite hydrogen (i.e. a star: the difference between the green lines and blue lines on the plot).


Could Planet Nine be much smaller than thought previously? In 2016, Caltech astronomers began testing a Planet Nine hypothesis to explain the eccentric and inclined orbits of distant dwarf planets such as Sedna, 2015 TG387, Niku, and 2012 VP113.  The hypothesis stated that these objects are being shepherded by an undiscovered ice giant planet called "Planet 9" about 3.5 times larger and 10 times the mass of Earth.

Recently, an alternative hypothesis has surfaced suggesting that the Planet Nine object does not exist and that the orbits of said dwarf planets are being tugged by the collective mass of other, low-eccentricity objects. But this hypothesis also has it's downsides. The combined mass of the entire Kuiper belt of dwarf planets and asteroids is positively puny compared to the mass of Planet Nine, so it wouldn't be able to effect dwarf planets like Sedna.

But could it be both ways? The majority of these high-eccentricity plutoids are suspected to be in the inner Oort Cloud, a theorized ring of comets and possibly minor planets in the outskirts of the solar system. The combined mass of the inner Oort Cloud (or Hills Cloud) would be roughly 2.37 Earth masses if an estimate of 5 Earth masses is taken for the entire Oort Cloud. If the theorized Planet 9 was only about 7.63 times the mass of Earth, the gravitational pull of it and the Hills Cloud combined would be enough to change the orbits of Sedna-like objects.

This hypothesis more or less worked out in a Universe Sandbox 2 simulation I ran to test the theory, but would it actually be true, considering the inner Oort Cloud extends far beyond the orbit of Sedna?

It could. As posted by the original authors on their blog pages findplanetnine.com it might be ~5 earth masses

https://arxiv.org/abs/1902.10103 and the answer to this thread


What's the composition of Ceres? Strangely, there isn't a lot of information on Ceres on the internet. I want to know the specific percentages of its composition.

Thanks in advance!

The details are not known for certain, however it is generally thought to have a rocky core, formed of silicates (aluminium and magnesium silicate for example) and a large mantle of water ice, perhaps 50% by volume, 25% by mass. The surface crust is formed of various carbonaceous and iron minerals: siderite, dolomite, cronstedtite. There may be a small iron core.


Exoplanet hunters ignored or ridiculed in the 1980s or 1990s? I recently came across this youtube video, in which a few people are singing about the history of exoplanet discovery.

It suggests that, in the 1990s, exoplanet hunters were largely ignored or ridiculed. Is there any truth to this?

Everyone expected exoplanets to exist - there was no reason to suppose that the conditions that existed when the sun formed were unique, and we knew that multiple star systems were common. Newton wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design"

Observations in the 80s were not ignored, they just lacked sufficient evidence. I.e, they were considered and properly rejected. Better observations have since confirmed some of the planets that were "discovered" in the 80s, but denied others. Scientific scepticism means that you don't accept something without evidence.

We knew that finding planets would be hard. The technology of sensitive ccd devices that could be used for constant monitoring of multiple stars didn't exist in a form that could be put on a space telescope, and there were other priorities for space science: solar system exploration.

Knowing that planets would be impossible to observe with contempory technology meant that "planet hunting" wasn't a thing. The first planet to be detected was around a pulsar, as the detection depends on changes in the timing of the pulsar, not on measurements of variations in light intensity during a transit, or doppler shift as the star is wobbled.


Proxima as Supernova Follow this question. If a star at the same distance from us as, say, Proxima Centauri had exploded as a Supernova 4 years ago (I know, it can't explode as supernova, let's say it can)...

How bright will it be?

It will impact us (life on earth and/or technology)?

What about Sirius?

Proxima Centauri can't explode as supernova as it is about 2 orders of magnitude too small and light.

But if a supernova were to happen from that distance? How bright would it be?

You can use the formula:
$$\text{Apparent magnitude} = \text{Absolute magnitude}+5log_{10}(distance)-5$$
where the distance is in parsecs.

Proxima centauri is 1.3 parsecs distant.

A supernova typically has an absolute magnitude between -15 and -19 (although they can go brighter or dimmer in the case of hypernovas etc). So Let's use -15 as the low end and -19 as the high end.

Plugging into the equation, we get an apparent magnitude of between -18 and -23.

For comparison, the sun as seen from earth has an apparent magnitude of about -26, so the supernova would be between 1500 and 15 times dimmer than the sun from earth.

Perusing the list in the Wikipedia article on apparent magnitude, we find the brightness of the supernova would be between the brightness of the Sun viewed from Jupiter and the Sun viewed from Pluto. In any case, that is plenty of light to read by, and plenty of light to see colour by.


Variable Types of Stars I'm trying to decode the Variable Types of stars in the Hipparcos Library.  I see there are a number of stars for example :-

EA/AR - Does that mean the star is both a EA(Eclipsing Binary Star) and AR (Detached System of AR Lacertae) type variable star ? e.g. HIP112009

Whats the difference between EA (Delta Orionis) and EA: (Omicron Puppis) type stars ?  What does the colon signify?

Edited - So on line, 2575 of hip_va_1.dat, the following line has a Variable Type of EA/AR, what does the slash signify.

112009| |F5V         |P|EA/AR | | 9.016| | 9.65 |  -1.7|   2.96850  |  -4.1|8501.4698|4|A| |W_Gru       |   2.96850|2430132.15| 9.40|10.00|P|R

What does the (+) sign signify in a Variable Type e.g. SR+ZA (HIP36623)

I really like to understand what the significance of the +, / and : mean

It seems to me that you're mostly asking about star naming conventions, which is unfortunately a difficult thing to master because there are many many conventions. What makes this process difficult is that the conventions you seem to be referencing don't come from Hipparcos at all. Likely you're seeing the names of these stars as they're referred to by some other, more popular convention. I'll try to go through the things you have listed and describe how those conventions work.

Hipparcos Names

A star in the Hipparcos catalogue is given a number. That number is preceded by HIP and you cite a good example of HIP 36623. The number itself is more or less meaningless for the star. Some catalogues provide numbers to stars based on their RA and/or Dec, but for the Hipparcos catalogue the number is the same as the that in the Hipparcos Input Catalog (all HIP stars were pre-chosen for observation and defined in an Input Catalog). The star entries are, with a few exceptions, ordered by increasing HIP number, which basically follows the order of the object's right ascension (Equinox J2000) independent of declination.

You can find a pretty good list of Hipparcos variable conventions on this page.

Bayer Convention

You also cite the names Delta Orionis and Omicron Puppis. These names come from the Bayer naming convention. The general scheme for this convention is to group stars by the constellation they're in. Then you rank all of them by brightness. You give each star a name which includes a Greek letter then the Latin name of the constellation it exists in. The brightest star is alpha (equivalent to A), the next brightest beta (equivalent to B), then gamma (equivalent to C), and so on. There is a convention about what to do once you've run out of Greek letters, but honestly, I've never seen anyone use the Bayer convention for such stars. It's generally only used for the brightest stars in a constellation.

From the examples you gave, Delta Orionis is the 4th brightest star in the Orion Constellation (since delta is the fourth letter in the Greek alphabet). For Omicron Puppis, that's the 15th brightest star in the Puppis constellation. In this convention, you can sometimes see the greek letter written as a Greek letter, rather than spelled out, and occasionally the constellation name is shortened. Thus, for your examples, you might see them written as $\mathrm{\delta}$ Ori and $\mathrm{\omicron}$ Pup. Note that lower-case Greek letters are used (first), not upper case.

Variable Star Type Designations

Variable stars are complex and come in many many different flavors. As such, there's a whole slew of different names and schemes to be used when referring to a variable star. A good reference site is the International Variable Star Index page. You can refer here to answer most of your questions, but I'll post the descriptions of the non-alphabetic characters.

  A pipe character ( $|$ ) between two different types signifies a logical OR; the classification is uncertain and all possible types are indicated. An example of this is ELL|DSCT, where the star may be an ellipsoidal binary system or a DSCT-type pulsating variable with half the given period.

  A plus character ( $+$ ) signifies a logical AND; two different variability types are seen in the same star or system. An example of this would be ELL+DSCT, where one of the components of an ellipsoidal binary system is a DSCT-type pulsating variable.

  A slash character ( $/$ ) indicates a subtype. In the case of binary systems (eclipsing, ellipsoidal or reflection variables) it is used to help describe either the physical properties of the system (E/PN or EA/RS), the luminosity class of the components (EA/DM), or the degree of filling of their inner Roche lobes (EA/SD). This is the GCVS classification system. In cataclysmic variables, slash characters are used to indicate some properties of the system, as in the degree of polarization (NA/DQ) or the nature of their components (UG/IBWD).

This site doesn't specifically indicate the meaning of the colon, but research elsewhere indicates that the colon is used to "indicate the uncertain nature of the classification". It looks to me that within variable star designations, and particularly by the General Catalogue of Variable Stars, a colon used anywhere (not just on the variable type designation) implies uncertainty.


How do astrophysicists handle the topic 'boundaries'? The naive perception, at least mine, is that in astrophysics the objects of investigation evolve in an infinitely large environment. Since the scenery is so much larger than the events unfolding in it, astrophysicists should never have to take into account that the universe may have walls/envelopes/surfaces limiting the going of things. I wondered to which extent this vision is naive.

How do astrophysicists handle the topic of 'boundaries' in space?
This is a very general, open, `soft' question, almost philosophical. Handle is a jolly word to mean think of, take into account, represent, model, simulate, even avoid, and so on and so forth.

Clearly I am neither an astronomer nor an astrophysicist.

The literal shape of space became a topic with Einstein's theory of general relativity. Before that, there had certainly been arguments about whether space was unbounded or had some limit, but they were mostly philosophical. In general relativity space-time is a mathematical manifold whose geometry is linked to observable features such as gravity, the expansion of the universe - and also lightcones, event horizons and singularities. This meant that (1) the theory now included various forms of boundaries set by the geometry, and (2) that the theory clearly leaves certain things "outside".

The first category of boundary are benign in the sense that they denote either regions that cannot influence each other but have the same kind of physics (we can never observe beyond a certain distance, we can never see the interior of a black hole from the outside), or represent parts of physics where the theory clearly breaks down (and "knows" it) like gravitational singularities or the big bang singularity. How to resolve those breakdowns is of course a rich frontier for theoretical physics, astrophysics and cosmology.

The second category is subtler. Einsteinian gravity is strongly based on the profound insight in Gauss' Theorema Egregium that the geometry of a manifold can be described only using properties found in the manifold, with no recourse to embedding it into a bigger outside space. The perennial question "what is outside the universe" is simply removed: general relativity does not need an outside, doesn't talk about an outside, and doesn't benefit from having an outside. That doesn't mean one cannot construct theories where space-time manifolds are embedded in other spaces but they are theories that need to be motivated rather strongly - there is no obvious evidence making them necessary, so Ockham's razor wants to shave them off.

Similarly quantum mechanics also adds boundaries but in the information domain: certain things are measurable only in a partial sense, and while the part that cannot be measured can still be described nicely mathematically the theory shows that we cannot know its state (in a pretty specific way).

To sum up, there are some things we know we cannot know or interact with but are not particularly worrisome: in a large accelerating expanding universe there are visible galaxies we can never reach, and there are galaxies that will forever be beyond our horizon. We however expect astrophysics to work the same over there. There are also limits to knowledge that are more annoying: the interior of black holes, what is beyond the CMB radiation. Then there are the theoretical boundaries where quantum gravity or other new insights we do not have yet are needed to continue inquiry. And finally there are the "boundaries" where we simply have no information, like "outside the universe" or some multiverse theories. Different people work on different things - philosophers and theoretical physicists are having fun in the last category (and arguing about how one can rigorously argue about such things), while most astrophysicists prefer to work in the data-rich regions and use it to extrapolate that is likely true for the other regions.

(James K. cited Wittgenstein's adage about being silent, but it is worth noting that this is usually interpreted more as a challenge than advice by many theoreticians.)


Time dilation on an object circling earth How would a receiver on Earth hear a radio transmission from an object circling earth at 99% of the speed of light for 24 hours. The transmission from the object circulating would be non-stop.

Because time dilation would occur, would the transmission be slowed to the receiver? The object would travel so near earth that the signal would not have any delay reaching earth.

At 99% the speed of light the behaviour would be almost completely determined by special relativity. The scenario is well-investigated for synchrotrons. In principle a synchrotron or a storage ring, e.g. around the equator of Earth, could be built.

At 99% the speed of light the frequency $f_s$ of the circling object should occur red-shifted by a factor of a little more than 7 to an observer in the center of the circle due to the transverse relativistic Doppler effect:
$$f_o = f_s/\gamma=f_s\cdot\sqrt{1-v^2/c^2}=f_s\cdot\sqrt{1-0.99^2}=f_s\cdot\sqrt{0.0199}=0.141067 f_s.$$
For an observer immediately near the ring, the frequency $f_o$ is the same as for the observer at the center for the signal emitted, when the particle was diametral on the other side of the ring. When approaching the observer along the lign of sight, the signal of the particle is blue-shifted to
$$f_o=f_s\cdot\sqrt{(1+v/c)/(1-v/c)}=f_s\cdot\sqrt{1.99/0.01}=f_s\cdot\sqrt{199}=14.1067\cdot f_s.$$
When leaving the observer along the lign of sight, the signal of the particle is red-shifted to
$$f_o=f_s\cdot\sqrt{(1+v/c)/(1-v/c)}=f_s\cdot\sqrt{0.01/1.99}=0.070888\cdot f_s.$$

By this we now have calculated the observed frequencies for three positions of the circling particle/radio to give some idea about the oscillation of the observed frequency.

More details about the relativistic transverse Doppler shift, see e.g. Ives-Stilwell experiment. Close to the experiment with the observer in the center of the circling particles are Mössbauer rotor experiments. In this cases ions or atomic nuclei emitting or absorbing at known wavelengths are used as "radios".

This paper describes a slower version of the transverse Doppler shift, as observed by using the GPS satellites moving with just 4 km/s. In this slow case the gravitational frequency shift, as predicted by general relativity to be induced by the gravitational field of Earth, plays a relevant role, relative to the (in this case) small transverse Doppler shift. Here the GPS satellites are the moving radios.


How do we measure the mass of a star? I recently came across the book intro to cosmology by j.v. Narlikar, where it mentions about quarks and other star and inadvertently uses mass of sun as reference. I want to know how me measure mass of a star?

I guess from the parallax method we could calculate distance?

In general one can only measure the mass of a star if it is orbiting another body or is orbited by another body.

In essence, the method exploits Kepler's third law, though the exact technique depends on what observations are available, on what timescale and with what resolution (spectral and angular) is available.

Other techniques can be calibrated using such objects as exemplars - for instance there are reasonably well-calibrated mass-luminosity or mass-color relationships.


If we've only observed Oumuamua as a single pixel, how can we know its shape? On a separate Oumuamua question (Oumuamua alien space junk?), I was confused how shape could be inferred from a single pixel, per another user's (@Uwe) comment. It was suggested I ask this question here.

If all we've observed Oumuamua as is a single pixel, how can its elongated shape be deduced from that? How can any non-uniform shape be deduced from a single pixel?

If we've only observed Oumuamua as a single pixel, how can we know its shape?

Nobody "knows" the shape of Oumuamua. However there are standard ways of deducing the objects likely shape based on assumptions, and this methodology has been tested agains asteroids for which their shape has been checked by other methods, including delay-Doppler radar (see Is delay-doppler radar imaging of NEO asteroids possible only if it spins fast enough? for example) and measurements from deep-space spacecraft that have approached and even orbited individual asteroids.

This answer to Could asteroid 'Oumuamua actually be round? shows a preliminary light curve and a simulation based on some assumptions of albedo and rigid-body rotation dynamics. It's not likely we will ever know its shape for sure, at least not with current capabilities of space travel, so this kind of analysis is the best that can be done.

For a more thorough analysis and modeling of Oumuamua's light curve see for example Modeling the light curve of `Oumuamua: evidence for torque and disc-like shape.


Did atoms in human body indeed come from stars? I think I am not alone who saw videos about that we (humans) are made of same atoms which someday were in stars. In other words, some atoms in our bodies are from stars which exploded billions of years ago.

I wonder if it is indeed true. I mean human's life begins when sperm cell fertilize egg cell. Now does that sperm cell or egg cell indeed contains some of the exact atoms from those stars?

I know little bit strange question, but would be interesting to hear if it is indeed true, that atoms in our body are same which someday were in stars.

In case you are wondering that's the video I am talking about: http://www.youtube.com/watch?v=9D05ej8u-gU

The chemical elements in our bodies are inherited from the Earth. The Earth was formed in a disc of gas and dust swirling around the protosun 4.5 billion years ago. The material that formed the Earth was a selection of the material from that protostellar nebula that was itself once part of a larger molecular cloud.

So the atoms in our body were once part of this molecular cloud, so we need to understand how they got there.

After the first ten minutes or so, the universe contained mainly hydrogen, helium and some traces of lithium, deuterium and tritium - and that's all. No oxygen, iron, carbon etc.

Almost all of the heavier chemical elements are made inside stars. We could stop there - the atoms of carbon, oxygen, calcium etc. in our bodies must have been made in stars, and since these atoms/nuclei are stable, they must survive unchanged (you could argue about whether their electrons get swapped about in chemical reactions etc., but since electrons are indistinguishable this hardly matters).

But how do they get into a molecular cloud and what sort of stars make these elements? A couple of answers correctly identify massive stars that explode as supernovae as important. But they are by no means the only contributor, or even the most important contributor for some elements.

If we take carbon and nitrogen, these are manufactured in nuclear reactions inside stars of even a bit less than a solar mass during the horizontal branch and asymptotic giant branch stages. These stars may be less massive and produce less C and N than massive stars, but there are many more of them. The central material is mixed to the surface during thermal pulses and the outer envelope, enriched in a variety of chemical elements, is gradually lost into space via a slow wind. This is a major source of carbon, nitrogen, fluorine, lithium and a number of heavy elements - Ba, La, Zr, Sr, Pb and many others - produced in the s-process. About 50% of the elements heavier than iron are made in the s-process, which can occur in both massive stars that explode (mainly isotopes with $A<90$) and the less massive AGB stars with slow, massive winds (elements up to lead and bismuth).

Iron, nickel and many other elements such as sulphur and silicon are also produced during type Ia supernovae. This is the detonation of a white dwarf, the end stage of a low-mass star, after mass transfer or merger. Milder novae explosions caused by the ignition of material accreted onto a white dwarf also enrich the interstellar medium.

All these different processes produce distinctive patterns of element abundances.

The enriched material is swept up by neighbouring supernova explosions, by interactions with spiral arms and other molecular clouds. It cools, condenses and collapses to form a new generation of stars.

Analysis of "presolar grains" found inside meteorites tells us what our solar system formed from. These analyses tell us that all of the above processes were important in making the chemical elements that made up the Earth and hence those in our bodies.

[Further details on the production of elements heavier than iron (including supernovae, low-mass AGB stars, colliding neutron stars etc.) can be found in my Physics SE answer to this question. ]


How was this interstellar fireball observed and its trajectory analyzed? The short, interesting paper Discovery of a Meteor of Interstellar Origin describes a proposed discovery of a second interstellar object. This one hit the Earth, and was discovered not by telescope, but by "data mining" the CNEOS fireball catalog https://cneos.jpl.nasa.gov/fireballs/

It is not hard to find in the table based on the date given in the paper, but the information listed is sparse, there's no ID number or information how this was obtained:

peak brightness UTC   lat     lon    alt(km)  v(km/s)  vx     vy     vz
2014-01-08 17:05:34   1.3S   147.6E   18.7     44.8   -3.4  -43.5  -10.3

total radiated kJ   calc total imact kt
     3.1e10                0.11

Question: With what instrument was this particular fireball observed and recorded, and how was it analyzed and processed into a trajectory and state vector to be entered into this table?

  Figure 1. Trajectory of the January 8, 2014 meteor (red), shown intersecting with that of Earth (blue) at the time of impact, ti = 2014-01-08 17:05:34.

  Figure 2. Size distribution of interstellar objects based on the detection of ‘Oumuamua and of the meteor detected at 2014-01-08 17:05:34 UTC. Red lines indicate the envelope for possible power-law fits (slopes of -1.9 to -3.8), given 95% Poisson distribution confidence intervals on each number density (based on a single detection for each). The range of possible power-law slopes is consistent with that inferred for small bodies in the Kuiper belt, -2.5 to -3 (Kenyon & Bromley 2004).

"US Government sensors is a euphemism for "nuclear weapon warning early warning satellites", which is why we won't know details of the actual sensor: It is a military satellite and secret.

This particular event occurred in the Bismark sea, North of Papua New Guinea, it probably could have been seen from Papua or parts of Indonesia, if the sky was clear (which it rarely is, in the middle of the tropical rain band), but if it was seen, it wasn't reported to the American meteor society (reports from that time)

The satellites are designed to look for rocket launches, and to track the missiles as the climb through the atmosphere to determine the likely target. Thus they are able to get accurate position and velocity vectors of meteors. If you know the position and velocity vector of the meteoroid just prior to impact, it is an exercise in Newtonian gravity to work out the orbit of the meteoroid around the sun. (It is a three body problem as the meteoroid is moving in both the Earth's and the Sun's gravitational field)


Is there a strong galactic magnetic field? My main question is: Is there a strong galactic magnetic field, perhaps driven by the supermassive black hole at the center of our galaxy?  I am also wondering if this field would be strong enough to make it so that the galaxy rotates in the way it does (with the outer stars moving faster than would be expected), and if this would be an alternate explanation for dark matter.

The thing that led me to ask this question is reading about Jupiter's magnetic field interactions with the plasma emitted by IO.  Jupiter's magnetic field forces the plasma to orbit Jupiter about as fast as Jupiter spins, and I am wondering if likewise, the supermassive black hole at the center of our galaxy "herds" the rest of the galaxy in a similar manner as per the article and image below.

http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter#Role_of_Io

No, the galactic magnetic field is very weak, about 0.1nT.
It is able to bend the trajectory of highly-energetic charged particles and also to align dust grains across the magnetic field.

However, is too weak to affect the rotation of a galaxy.

Although the origin of galactic magnetic field is not clear yet, the supermassive black holes do not significantly influence the galactic magnetic field far from them.


Why is the statement "black holes make up 1% of the mass of the Milky Way Galaxy" unknown? a) We cannot detect all black holes and therefore don't know the percentage of the galaxy's mass they make up.

b) We do not know the mass of the Milky Way's central black hole.

c) According to our understanding of stellar evolution, black holes should make up a much lower percentage of the galaxy's mass.

d)According to our understanding of stellar evolution, black holes should make up a much higher percentage of the galaxy's mass.

In my own understanding, I feel like it should be a). Since the black holes are forming all the time, I think it is hard to detect all of them. But I am not quite sure, could someone give any insight?

(a) We can't detect ALL of anything, but we can certainly makes estimates of their integral properties. For example, we estimate that there are about 100 billion stars in the galaxy, but we can observe only a tiny fraction of these. We have also observed a tiny fraction of the stellar black holes in our galaxy - those few examples that have revealed themselves through their binary companions.

(b) Yes we do. It is just over 4 million solar masses and thus a negligible fraction of the Galaxy's total mass.

(c) and (d) It is not just stellar evolution that is involved, it is the distribution of stellar birth masses (the initial mass function) that needs to be known, possibly as a function of time.

The stellar evolution part has a major uncertainty in that although it is reasonably certain that all stars above about 10 solar masses end their lives in supernovae, it is not certain what fraction will leave black hole remnants. This is probably a strong function of mass - higher masses favour a black hole and progenitor metallicity; low metallicity stars lose less mass during their lives and are more likely to directly collapse to a black hole - see Heger et al. (2003).

You can do a back of the envelope calculation. Suppose the IMF has the Saltpeter form $N(M)\propto M^{-2.35}$ between 0.1 and 100 solar masses. Further assume that: because a 1 solar mass star has a lifetime of 10 billion years, the age of the Galaxy is 10 billion years and the stellar lifetime is a very strong inverse function of mass $(\propto M^{-2.5})$, that the 100 billion stars that we deduce exist in our Galaxy (by counting up the local stars and measuring stellar densities elsewhere and extrapolating) are predominantly all the 0.1 to 1 solar mass stars that have ever been born. This allows us to then estimate how many stars have been born in any mass range - even if they have long-since died.

So making the above assumptions, I will make the further assumption that all stars from 20-100 solar masses die and leave a black hole remnant.Then if my maths is correct then there are about $10^{11}/1400 \sim 7\times 10^{7}$ black holes in our Galaxy. That is there is 1 black hole for every 1400 stars of 0.1-1 solar mass.

The remaining parts of the puzzle are what is the mass of a typical black hole and what is the mass of the Galaxy? A good number for the former is about 10 solar masses, since the black hole binaries appear to cluster at this value or a little lower (Ozel et al. 2010). The latter is still a topic of investigation. It appears to be an order of magnitude bigger than all the stars, gas and dust due to some form of dark matter. Let's use the number $7\times 10^{11}$ solar masses (Eadie & Harris 2016).

So my final number is that black holes form $7\times 10^{7}\times 10/7\times 10^{11} = 0.1$ percent of the Milky Way mass.

You could push this number up by a factor of a few by assuming that the IMF contained more higher mass stars in the past, or that the threshold mass for black hole production was lower in the past for low metallicity stars. Both seem theoretically possible and are active topics of investigation.