How to find the local azimuth of the highest point of the ecliptic I would like to use pyephem or skyfield1 to find the degree of the local horizon (starting from South = 0°) at which is located the highest point of the ecliptic.
Note that I am not trying to find the culmination of a planet. It could well be that, when looking South, there is no planet on the ecliptic by that time/latitude, but there will still be a highest point of the ecliptic. Can anyone help?
1Skyfield is preferred over PyEphem for new projects for several reasons

Edit:
Getting the exact value winds up being simple, but it was only later in the day that the answer occurred to me. All we need to do is ask for the altitude and azimuth of the Ecliptic’s south pole! The ecliptic itself will be highest above our horizon in exactly that direction, at an altitude that is exactly 90° above its pole.
Thus the answer is given by the coordinates of a single vector:
from skyfield import api, framelib
from skyfield.positionlib import Apparent

ts = api.load.timescale()
t = ts.utc(2021, 2, 16, 22, 52)
bluffton = api.Topos('40.74 N', '84.11 W')

f = framelib.ecliptic_frame
r = api.Distance([0, 0, -1])
v = api.Velocity([0, 0, 0])

p = Apparent.from_time_and_frame_vectors(t, f, r, v)
p.center = bluffton

alt, az, distance = p.altaz()

print('Altitude of highest point on ecliptic:', alt.degrees + 90.0)
print('Azimuth of highest point on ecliptic:', az.degrees)

The answer:
Altitude of highest point on ecliptic: 67.54977465183501
Azimuth of highest point on ecliptic: 162.61593086793783

— is far more exact, and faster to compute, than my first try. Enjoy!
Original answer:
While it’s likely that someone will soon jump in with a closed-form solution that uses spherical trigonometry — in which case it’s likely that Skyfield or PyEphem will only be used to determine the Earth orientation — here’s a quick way to get an answer within about a degree:

Generate the ecliptic as 360 points one degree apart across the sky.
Compute the altitude and azimuth of each one.
Choose the highest.

The result agrees closely to what I see if I open Stellarium, turn on the ecliptic, and choose a field star right near the point where the ecliptic reaches the highest point in the sky.
import numpy as np
from skyfield import api, framelib
from skyfield.positionlib import Apparent

 = api.tau
ts = api.load.timescale()
eph = api.load('de421.bsp')
bluffton = api.Topos('40.74 N', '84.11 W')

t = ts.utc(2021, 2, 16, 22, 52)

angle = np.arange(360) / 360.0 *
zero = angle * 0.0

f = framelib.ecliptic_frame
d = api.Distance([np.sin(angle), np.cos(angle), zero])
v = api.Velocity([zero, zero, zero])

p = Apparent.from_time_and_frame_vectors(t, f, d, v)
p.center = bluffton

alt, az, distance = p.altaz()

i = np.argmax(alt.degrees)  # Which of the 360 points has highest altitude?
print('Altitude of highest point on ecliptic:', alt.degrees[i])
print('Azimuth of highest point on ecliptic:', az.degrees[i])

The result:
Altitude of highest point on ecliptic: 67.5477569215633
Azimuth of highest point on ecliptic: 163.42529398930515

This is probably a computationally expensive enough approach that it won’t interest you once you or someone else does the spherical trigonometry to find an equation for the azimuth; but at the very least this might provide numbers to check possible formulae against.


Can black holes be a subset of quark stars? If not, what are black holes made of? Can stellar black holes be a category, or subset, of quark stars?
<img src="https://i.stack.imgur.com/QpE2v.jpg" alt="
" />

Strange stars are so called not because they're weird (though they
are) but because the extraordinary pressures within them should turn
some of their constituent quarks into a heavy type of quark named
"strange." Neutrons and protons are made of two lighter types called
"up" and "down." (The names were chosen whimsically and have no
particular meaning.)
"It's possible that all the stars we have been calling neutron stars
are in fact strange stars" or hybrids of the two, said Turner. If
strange matter does convert everything it touches into more strange
matter, these stars would consist of the stuff right up to their
perfectly hard-edged surfaces, with nothing but vacuum above. If it
doesn't, the stars could be hybrids with strange-matter cores and
neutron-star outer layers.
from https://skyandtelescope.org/astronomy-news/quark-matter-stars-said-found/

The visible quark stars such as the one depicted would be quark-neutron hybrids. This would conveniently answer the question "what is black hole substance made of".
If this is certain to not be the case, what (if any) known substance can black holes de made of?
Note, I said a subset - "...strange stars exhibit a low-luminosity but high-temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disc, may be the key signature to differentiate massive strange stars from the black hole." in https://academic.oup.com/mnras/article/400/3/1632/962273
https://physicsworld.com/a/calculations-point-to-massive-quark-stars/

Black holes are not made of anything... at least nothing in our current understanding of reality.
We understand black holes in terms of General Relativity, the theory of gravity.  In this theory there are some "vacuum solutions"  These are solutions that have no matter at any point in spacetime.  One solution is "flat space", but another solution is a solution with a singularity in time and space.  This solution was discovered by Schwatzchild, and for long was thought to be a mathematical curiosity but not physically real.
But it was later discovered that when a star collapses under gravity it reaches this state.  All the matter falls towards the singularity and we can't describe what happens later than the singularity in time. From the outside it seems as if all the mass of the star is concentrated at one point, but there is actually nothing inside, just the distorted spacetime.
In General relativity nothing can exist in a black hole. For anything falling beyond the event horizon, all directions lead to the singularity.  It is as unavoidable as "next Wednesday". No ball of matter could exist. In normal space, two particles can remain separate. Inside a black hole this isn't possible, since both particles end up at the singularity in a finite (and short) amount of time.
Now we aren't certain if this truly describes reality, because we lack a theory of gravity that takes account of quantum mechanics. It might be that the singularity is real. It might be that something happens (with strings or something) that prevents the ultimate collapse to a singularity. Whatever it would be outside our current models of reality.  Not "quarks".


Why does iron consume more energy in the fusion process than it produces? I understand that once a star starts fusing iron, it's doomed to collapse because iron fusion requires more energy than it releases in the process, allowing the opposing gravity of the star to cause it to collapse.

But why?  What makes iron special in this way?  It seems to occupy a pretty inconsequential spot in the middle of the period table among the transition metals.  So, why is it that iron breaks the rule in play for all the elements before it when it comes to fusion?

I noticed that on another question someone said that it wasn't iron, but nickel that was the first element that required more energy to fuse than it released- but every documentary and book I've read claims that it's iron.  So, if you're answer is "iron isn't the first element that requires more energy..." please explain why every other source I've ever heard is wrong!

It would be good if you referenced your sources, because you may be misunderstanding them. We'd be able to see what they actually say, and help you understand them.

Nucleosynthesis of iron does not use more energy than it produces.

It is, however often referred to as the heaviest element created in fusion that results in more energy produced than consumed.

However, that isn't quite true. Heavier elements can be produced by fusion that produce more energy than is used, except these fusion reactions don't occur in stars. (Eg 40Ca + 40Ca)

Also, it is possible for heavier nuclei to be fused in stars that result in more energy being produced than is used, but these are unstable isotopes and they decay quickly.

So, more accurately, iron is the heaviest element produced in stellar nucleosynthesis in any significant quantity that produces more energy in fusion than the fusion consumes.

This is called the alpha process ladder. Keep adding alpha particles to the newly generated nuclei, until you stop getting more energy out than you put in.

The last step in the alpha process that does produce energy is 52Fe + 4He => 56Ni(excuse the rubbish notation; if this answer is considered helpful at all I will try to tidy up the notation)

56Ni + 4He => 60Zn uses more energy than it produces.

56Ni has a very short half-life of just 6 days, decaying to 56Co which has a half life of 77 days, which decays to 56Fe, which is stable. So when a star is on the limit of collapsing, it will be producing a lot of iron in it's final stages - some of it from decaying heavier radioactive isotopes.

Why does the alpha process stop producing energy at this point? It is because that is where the peak binding energy is. More.


Neutron star size in different reference frames What is the estimated size of neutron stars observed in their reference frame and in our reference frame?

That is, how bent is space-time around neutron stars?

If we ignore the rotation issue (as Pela pointed out, the correction is <1%), then we can approximate the spacetime curvature inside the neutron star using the interior Schwarzschild metric and use the exterior one for the outside. The $r$ coordinate is not the radial distance as one would expect, but defined in terms of $r=$ constant circles having circumference $2\pi r$.

A neutron star of radius $R$ in these metrics has a circumference measured on its surface as $2\pi R$ as expected. But the distance to the core (that is, the length of a hole from the surface to the core) is: $$d(R) = \int_0^{R} \frac{dr}{1-Kr^2} =  \frac{1}{\sqrt{K}}\tanh^{-1}\left(\sqrt{K}R\right).$$
where $K=r_s/R^3$ and $r_s$ is the Schwarzschild radius of the star (all of this assumes constant density in the interior). If we plot this for stars of different $R$ (but same mass, say one solar mass, producing a constant $r_s$) we get the following:

As the neutron star approaches becoming a black hole it gets "deeper": there is more volume than one would expect. Less dense neutron stars have depths that scale linearly with their radius... except that it is $r_s/3$ larger! This odd result comes from the assumption that we look at stars of the same mass even though we make them much larger. A big constant mass object will curve spacetime too, and this produces this effect. (Yes, this means that the sun is $r_s/3 = 984.73$ meters deeper than it looks!)

If we instead decide that we use a constant density (say nuclear density) and plot the depth, the collapse to a black hole instead happens to the right as the neutronium sphere becomes too large. Here I still use the solar Schwarzschild radius as a length scale to keep things comparable. Now, for small spheres the coordinate and measured depths converge:

In actual neutron stars things are complicated by rotation, the core pressure diverging as one approaches $(9/8)r_s$, and of course that it is hard to dig a hole through superfluid neutrons.


What would we find if we could travel 786 trillion light years in any direction I've sometimes heard of galaxies that are millions or billions of light years away but if we could instantly travel 786 trillion light years in any direction, what are we likely to find? And what if we continued on for another 786 trillion light years?

PS: I'm not a physicist.

I'm afraid the only correct answer to this question is that nobody knows.

Cosmology is based on a number of simplifying assumptions (sometimes called principles) - the most basic of which is that the universe is "isotropic and homogeneous on large scales". Isotropic means that is (or looks) the same in any direction; homogeneous means that what we see does not depend on our position in the universe.

This "cosmological principle" is reasonably well tested and would certainly make sense if the universe were very much bigger than we are capable of seeing. The most distant thing we can "see" in our universe is the cosmic microwave background (CMB), the light of which has been travelling for about 13.6 billion years. However, if we were to think about a parcel of gas that emitted the CMB and where it is now, we could calcuate that it is about 46 billion light years away due to the expansion of space in the universe. Thus the "edge of the observable universe" is about 46 billion light years.

What we can say, is that on these scales the Universe is very isotropic, and homogeneous, because the CMB is very smooth and looks the same in all directions to a very good approximation (but see below). This suggests that if you were to travel 46 billion light years in any direction, that you would probably have a very similar view to what we have now (that is, if you were able to miraculously travel instantaneously; if you travelled more slowly then of course the universe would continue to get older).

You are talking about travelling more than a factor 10000 further. On these scales we do not know anything outside of speculation. The inflationary model for the big bang might suggest that our observable universe is just a tiny part of a much greater entity and that within this very much larger entity everything will be homogeneous and isotropic at a given cosmic time. However, the recent very precise Planck measurements of the CMB, suggest there may be small, but real, anisotropies on scales as big as the observable universe, which would then suggest that any extrapolation by a factor of 10000 might be wishful thinking.


How is time defined in astronomy? I was amazed when my teacher told me that in space, time is define as a coordinate. It means we can define something  with a four coordinate system with "negative time" $-t$ as easily as we can say $-x$. Assuming that we can, what will this suggest?

Edit:) yesterday I saw so many answer on it, create one more doubt in me, that how do I define a origin in space, because there nothing to which I compare, suppose I have two events A and B, how can I say A occur before B, if I say this, from which reference I am comparing it....

$t$ signifies time; see the Wikipedia article for spacetime, and then the subsection for 4-vectors.

The basics are pretty natural to understand. Suppose something happens, an event, like an apple falling off of a tree.

In order to tell someone else about it you need the three space coordinates $x, y, z$ and the time coordinate $t$. Without all four, you won't be able to see it happen.

update: The question was clarified to focus on the meaning of "negative time" or $-t$. That's no big deal. There's no problem with $-x$ for example. We don't think of that as negative distance or negative space. It's just a coordinate relative to some origin $(0, 0, 0)$, and we can put that anywhere, arbitrarily.

So you should really thing of the time coordinate as $t-t_0$ where $t_0$ is some arbitrary origin in time. It might be midnight yesterday, or midnight next Tuesday. That would make $t_{Now}$ positive in the first case and negative in the second case, but either way it's still Now.


What percent of all stars that have lived (up to now) are stellar remnants? According to this post, I found that about 90% of all stars are main sequence stars. So I am assuming that the other 10% consists of protostars, older giant stars, and stellar remnants. So I'm wondering how much of the stellar population is dead (i.e. turned into a remnant, excluding supernova progenitors that blow apart the whole star without leaving anything behind)?

Most of the remnants are white dwarfs, because their progenitors can be much shorter lived than the lifetime of the galaxy and are much more numerous than the higher mass progenitors of neutron stars and black holes.
A census of the local neighborhood suggests that about 10% of "stars" are white dwarfs. This number is low because (a) only stars more massive than about $1M_\odot$ have had time to become white dwarfs and (b) the birth stellar mass distribution is heavily weighted to stars less massive than this.
Extrapolation of this birth stellar mass function to estimate the numbers of stars born with $M>8M_\odot$ (almost none of which are still on the main sequence now) suggests there are a further $\sim 1$% of neutron stars and $\sim 0.1$% black holes, though these fractions remain to be observationally confirmed and the latter number is very uncertain even as a theoretical calculation.


Is Ceres in the inner solar system or outer solar system? Do we consider the dwarf planet Ceres to be in the inner solar system or outer solar system (or possibly neither / in between).  It is located within the asteroid belt, but is the asteroid belt part of the inner or outer solar system?

Inner. The entire asteroid belt is in the Inner Solar System (now). The definition of "Inner" vs. "Outer" is non-arbitrary, based on the current "frost line", approx. 5 a.u. radius.

https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29

https://en.wikipedia.org/wiki/Solar_System#Inner_Solar_System


What happens if the Moon moves twice as fast? If the Moon would rotate around the Earth with double its current speed.
What would happen then?
Would it get further from the Earth or it would be possible to keep its distance from Earth?

No.  The orbital speed is related to the distance of the moon by the equation
$$v=\sqrt{\frac{GM}{r}}$$
Where G is a constant, M is the mass of the Earth (adjusted for the mass of the moon) and r is the average distance of the moon(wikipedia).
So there are two interpretations of your question: firstly what would happen if we gave the moon a big push that doubled its speed? Well this would cause the moon to change its orbit from a roughly circular to an open orbit.  The moon would sail off into space (slowing down as it is dragged back by the Earth, and end up orbiting the sun). The escape velocity is $\sqrt2$ times the orbital velocity, so if you are in orbit, and double the speed, you are sure to escape.
If you gave it a more gentle push it would enter an elliptical orbit.  The speed would then be non-constant. If you increased its speed by 10% it would move 10% faster at perigee, but slower at apogee and on average its speed would actually be less than before.
The only way to increase the speed is to decrease the distance to the moon.  If you reduced the distance by a factor of 4 (to about 95000km).  This would cause the moon to double its speed.


How long does an over contact binary star system last? I read recently about VFTS 352, an overcontact binary star system where both stars have roughly equal mass. All of the reports I've read (in mass-media type publications) have said that the system has one of two fates: either the two stars will merge, or they'll supernova. But when will this happen?

The wikipedia page for contact binaries says that they have a lifespan of millions to billions of years, but doesn't say if that's different for overcontact binaries. It also says that they're often confused with common envelopes, which have a lifespan of months to years, and I'm not sure where in that spectrum an overcontact lies (or really what the distinction is, since the page for contact binaries says they share an envelope, which sounds the definition of a common envelope). I'm also not sure whether the fact that both stars have roughly equal mass affects the lifespan.

The mass-media articles I've read have implied that the merger-or-supernova is happening soon, but I don't know if this is on a human scale (months) or galactic scale (millions of years).

Short answer: $t \lesssim 10^5\ \mathrm{years}$ (maybe)

An "overcontact binary" is just another way of saying "common envelope binary". The two phrases are exactly the same and it's frustrating that the authors on the VFTS 352 paper decided to create their own convention - as if astrophysical classifications weren't confusing enough!

A contact binary exists on timescales predominantly dependent on stellar evolution, so figuring out how long a contact binary will exist is heavily dependent on the mass, metallicity, and rotation of the primary star among other things.

Deriving the timescale:

Let's keep the scope to systems like VFTS 352, where the primary is massive and the binary has an orbital period less than 4 years (2.5 AU separation). In order to have a common envelope event, the stars must have overflown their Roche lobes. The radius for the Roche lobe of two point masses is
\begin{equation*}
r_L = \frac{0.49 q^{\frac{2}{3}}}{0.6q^{\frac{2}{3}}+\mathrm{ln}(1+q^{\frac{1}{3}})}a
\end{equation*}
where $a$ is the separation. For close binaries, the general observed trend is a high mass ratio $q=M_2/M_1$. So, if we assume $q=1$, then $r_L = 0.38a$. Hence, for a binary with $a<2.5$ AU,
\begin{align*}
r_L &\lesssim 1\ \mathrm{AU}\\
r_L &\lesssim 215\ R_{\odot}
\end{align*}
since $q=1$ is an upper bound on the Roche lobe radius. Now, performing some trivial rearrangement of the blackbody luminosity equation $L=4\pi\sigma_{SB}R^2T^4$, we find that
\begin{equation*}
R \approx 3.31\times10^{7} \bigg(\frac{L}{L_{\odot}}\bigg)^{\frac{1}{2}}\bigg(\frac{1\ \mathrm{K}}{T}\bigg)^2 \ R_{\odot}.
\end{equation*}
Massive stars typically have roughly constant luminosity, so we will choose $L\approx10^5\ L_{\odot}$. Hence,
\begin{equation*}
R\approx 1\times10^{10}\bigg(\frac{1\ \mathrm{K}}{T}\bigg)^2 \ R_{\odot}
\end{equation*}

The massive star needs to evolve until its radius is equal to that of the Roche lobe radius, so we find that the star reaches the common envelope phase for
\begin{equation*}
T \gtrsim 7000\ \mathrm{K}
\end{equation*}
Taking a peek at an HR diagram, this star varies from about $30000\ \mathrm{K}$ to $4000\ \mathrm{K}$ from ZAMS to end of main-sequence. Thus the primary spends roughly 3/4 of its time on the main-sequence not in the common envelope phase. Hence, this binary's common envelope phase lasts for, at most, 1/4 the primary's total lifetime, which is on the order of $10^6$ years. Thus, the upper bound for the timescale of a common envelope event with massive stars with negligible rotation is $\sim10^5$ years.

Please note that this derivation does not take into account the bulging effect that occurs as the separation decreases. This will certainly lower this upper bound, but by how much I'm not sure. It could lower it by 1 year, or $10^5\ \mathrm{years}$.

Lower bounds to this timescale are entirely ambiguous and not particularly helpful in any physical context. The stars could be spinning really fast, have high or low metallicity, the binary could have a different mass ratio, there could be another binary close by, and there may be magnetic interaction (?). The list goes on! I'm sure there's something I left out.


Are Barlow Lenses Good For Deep Sky Observing? I'm using a 750mm (focal length) by 150mm (aperture) newtonian reflector and I've been curious about using Barlow lenses on deep sky objects.

Some of the astronomers at my local observatory say that if you get the lens selection correct so you don't lose any light through the eyepiece, you don't need a Barlow lens. While others say that the Barlow lens doubling the focal length of the telescope allows me to see deep sky objects better than without it.

So, is a Barlow lens an effective tool for deep sky observing? If not, what should I use it for?

Note: I already have a 2x Barlow lens.

The Failures of High Magnification

Higher magnification doesn't help you observe deep sky objects better. Deep sky objects unlike stars are extended objects. They subtend a finite solid angle on you. This ensures that the surface brightness(brightness per unit solid angle) of extended objects remains constant. Hence, a higher magnification would not make it brighter for you to see.

It is worse for another reason. After a certain magnification, the apparent angular size of the extended object becomes comparable to the field of view, or sometimes even greater. Your eye won't be able to distinguish the object against the background since most of the background is the object itself.

Good Uses of Barlow

Some of the uses that you can put your Barlow:

Observe the Moon, Solar System objecs
Some of the clusters which seem like point objects really "explode" when using a Barlow. The best example is Omega Centauri.

Experiences

I have a 8" Dobsonian Newtonian Reflector. The viewfinder is 20x80. I really get a better picture of deep sky objects like Ring Nebula, which have a small angular size, when using Barlow. But for cases like Lagoon and Trifid Nebula; they are visible to me from my viewfinder but not through the eyepiece(even in the lowest magnification).

Conclusion

Barlow really helps if you have a deep sky object with small angular size. For larger objects, a binocular may do better, especially for city conditions.


Cosmological deflation? Wikipedia tells us:

  In physical cosmology, cosmic inflation, cosmological inflation, or just inflation is the exponential expansion of space in the early universe. The inflationary epoch lasted from 10-36 seconds after the Big Bang to sometime between 10-33 and 10-32 seconds. Following the inflationary period, the Universe continues to expand, but at a less accelerated rate.

I've no idea about astronomy, I just wondered whether somebody ever published a theory about cosmological deflation, which would make the universe shrink in a fraction of a second.

In a SFI novel you could travel by locally causing a deflation of space in the direction of your spaceship; the unshrunken space of a lightyear would (only locally in front of the ship) measure 1000 km for the fraction of the second. Or maybe a technically very advanced civilization could keep its solar system inflated by the time of a deflation of the whole universe and thus survive, which leads to the interesting question, whether this solar system could reenter the space of the next universe after its new cosmolocical inflation and if yes, does somebody look out for stellar systems at places in space where you wouldn't expect them?

However: cosmological deflation?

There are a number of flaws with your idea (despite the relativity breaking transatlantic journey times that would be possible).

Inflation is a popular theoretical model put forward to solve three main issues with the Big Bang cosmology: the horizon problem, the flatness problem & the magnetic monopole problem (see the wiki for details). It consists of a scalar field called the inflaton, a physical field that pervades all space. During inflation it undergoes a phase transition to a lower energy state, releasing huge amounts of energy that drive the expansion. To force the field back to a higher energy state would require inconceivable amounts of energy, and have to be applied across the entire universe. Not only is this impossible, but also annoyingly prevents its use for local galactic travel.

  Or maybe a technically very advanced civilization could keep its solar system inflated by the time of a deflation of the whole universe

I assume you mean contraction, as opposed to the current expansion that we see today due to dark energy. Again this would be unfeasible due to the reasons outlined above, and if contraction was to proceed to it's conclusion, i.e. a singularity like the Big Bang, then no information from this universe could be carried through that singularity, even if there was another universe on the other side.


Internal heat and planetary differentiation I was reading an article on planetary differentiation, and apparently internal heat production plays a major role. There are several sources of such heat described, such as tidal heat, radiogenic heat, etc. What I did not understand however is how, on chemical level, does warming up a system "heterogenize" it. Logically, wouldn't an addition of kinetic energy mean that molecules bump around more violently, therefore becoming more mixed up?

Or does this 'internal heat' story mean that the heat is actually escaping the planet, therefore kinetic energy is lost, therefore the planet is actually cooling down?

source: http://geology.isu.edu/wapi/Geo_Pgt/Mod03_PlanetaryEvo/mod3_pt1.htm

Your argument (large temperature leads to greater mixing) is correct so long as there are no other large scale forces acting on the system.  This isn't true in planet formation, because gravity plays a very important role.

I'm not an expert on planet formation, but I think the argument goes something like this:  As a planet forms from material from the protoplanetary disc it will begin very homogenous, something like an asteroid, just rock and metal all the way through.  If the planet is heated enough in the core, some material will melt.  Buoyancy (due to gravity) will drive lighter material "higher" in the planet, away from the core.  The more heat you generate the more melting can happen and the more material will start to separate by density.

Of course, there's a lot more to the story.  Fluid material obeys the laws of hydrodynamics, so large scale convective flows can form in some regions, mixing those areas quite well.  Planetary rotation adds centrifugal and Coriolis forces to the mix, which pull more material to the equator.  The surface of a planet is exposed to space and can radiate excess heat, cooling to a solid (like the Earth's crust).  If the core is magnetic and rotating, metals in the planet may be pushed by the magnetic field.

All this is just to say that planetary formation is a balancing act of a lot of competing forces.  These forces can differentiate (or not!) different types of material, but first (for rocky planets at least) they need to be freed by internal heating.


Mercury mapping attempts before Mariner 10? Mercury's surface was not mapped up-close until Mariner 10 passed by in 1974. This was followed by MESSENGER in 2008. I have seen several attempts of mapping Mars that came before space exploration, such as Schiaparelli's maps from 1877.

With regards to Mercury, were there any documented attempts to map its surface with ground-based observations prior to unmanned space exploration? If so, however unlikely, did any observed features from Earth match up with Mariner or MESSENGER close-ups?

It's very hard to see anything on the surface of Mercury. It is always seen against the glare of the sun. It is always low in the sky, so we see it through a lot of atmospheres, the seeing is worse than it would be for an object higher in the sky. With CCD imaging technology you can see some details, by taking very short exposures and combining them(source). You can't do that with conventional photographic plates.

Lowell produced a map at the start of the century showing supposed linear markings that he thought were the result of cooling.

The best that could be done in the middle of the twentieth century, direct observation images from the Pic du Midi telescope

It wasn't until radar-based observations in the 60s that the rotational period was established. There were some mapping attempts at mapping in the 70s https://www.sciencedirect.com/science/article/pii/0019103572900231 (behind paywall)

It would be hard to identify any features on these drawings with know regions or features on the Mariner images.


Is there an upper limit to the mass of terrestrial planets? The exoplanet Kepler-10c has a mass between 15 and 19 times the mass of the Earth (making it comparable in mass to Neptune), and yet is thought to have a density of about 7g/cm3 and to be a terrestrial planet, with a substantial proportion of "hot ice"

Is there an upper limit to the mass of terrestrial planets, or can rocky planets form that are larger than Kepler-10c?

This, older, article in Universe Today suggests that terrestrial planets can't form more than 5-10 Earth Masses, substantially smaller than Kepler 10c.

I'll shamelessly reference an answer I wrote on Worldbuilding to an almost identical question. Lammer et al. (2014) suggested that "super-Earths" with masses of $2$-$5M_\oplus$1 could retain massive hydrogen/helium envelopes, up to $\sim10^{25}$ kilograms. Above this, up to about $10M_\odot$ or more, "mini-Neptunes" exist, possibly composed of volatiles and having significant gas envelopes2. In other words, there's a transition region between terrestrial planets and gaseous planets (gas dwarfs included) that depends significantly on whether the planet can hold onto an envelope for a significant amount of time.

Part of this depends on the environment. For instance, you cite Kepler-10c as a possibly upper limit to terrestrial planets. The reason Kepler-10c can't be a gas planet, as Dumusque et al. (2014) explain, is that it's way too close to its parent star to retain a hydrogen envelope. It would likely lose a lot of such an atmosphere via Jeans escape, the same mechanism that leads to chthonian planets. It's possible, then, that Kepler-10c would have been an ice giant if it had formed farther away from Kepler-10. It has a similar mass and potentially similar composition; it just happens to be much closer to its parent star than Neptune.

This is something of an unsatisfying answer, but there simply isn't an exact line that separates terrestrial planets and gas planets, just like there isn't a good line between massive gas giants and, say, sub-brown dwarfs.

1 There are, of course, exceptions, like Kepler-138a, which has a mass similar to Earth yet is a good gas dwarf candidate.

2 For more information on the transition region, see e.g. Lopez & Fortney (2013).


What happens if you fail to reach escape velocity for the galaxy? I was listening to Skeptics Guide to the Universe the other day, and Steve Novella mentioned that the escape velocity for the galaxy is 544 km/s.

If an object fails to reach escape velocity on Earth, it will return to Earth.  What will be the fate of an object, say the Voyager spacecraft which is currently traveling at 17 km/s, that fail to meet the 544 km/s.

I'm having a hard time wrapping my head around what galactic escape velocity means.

Does this mean that Voyager is incrementally slowing down by the pull of the galaxy?  Given enough time, will it end up in the supermassive black hole in the center?

If you fail to reach escape velocity from the earth, you won't break free of its gravitational well, in other words, you will keep orbiting the center of mass of the earth. If the path of your orbit intersects the surface of the earth, then you will, you know, crash.

If you fail to reach escape velocity for the galaxy, you'll keep orbiting the center of mass of the galaxy, just like the other couple hundred billions of stars in the galaxy. The galaxy has no hard surface to crash into, so you'll keep orbiting until you come close enough to another orbiting body to perturb your orbit (which could, in the right circumstances, leave you with escape velocity and kick you out of the galaxy.


Can eternal inflation and gravity explain dark energy? First of all, I have absolutely no background in astronomy and physics. I just watch a lot of "i-hope-scientific" youtube channels talking about physics and astrophysics and I can kind of follow the maths and concepts.

So, I was thinking, with absolutely no physics to support this:

REAL QUESTION STARTS HERE:

Say, the big bang happened everywhere, followed by inflation that never stopped. This is wat eternal inflation described (if i am correct)

Then, as eternal inflation goes on and more and more universe outside of the observable universe is created. Then there would be an unlimited amount of mass outside our observable universe.

Can this mass create an gravitational effect on the galaxies in our universe?

We are at the center of our observable universe, so the gravitational effect of all the mass outside our observable universe would never affects us. But from the perspective of another galaxy, it would be affected by a bit of universe that is outside our observable universe.

Then you might explain the effect that galaxies that are farther away from us move faster away from us. And that galaxies closer to use move slower away. Then you can explain this by saying that the galaxies farther away are closer to the "unlimited" mass outside our observable universe. And thus are more affected by this. And that galaxies close by are less affected.

The theory with dark energy claims that dark energy causes an outward push from the inside of our universe. Wouldn't this be indistinguishable from a gravitational pull from the outside?

Also, could this explain the red-shift we see in light from the far end of the observable universe? If I remember correct, gravity can red-shift light if the light moves away from the source of the gravitational pull (as seen near black holes). Light form the far end of the universe has moved trough the gravitational pull away from the "infinite mass" of the outside universe.

This was my idea, and I would like to see where I am thinking wrong.

One thing where I obviously might be thinking wrong is that this effect would be homogeneous throughout the universe and it should cancel out. But is this also true when you throw some relativity into the mix?

We need to cover some basic concepts: inflation in our universe ended 13 billion years ago. Inflation occurred for just a short period of time after the big bang. Eternal inflation is different. It hypothesises that spacetime is infinite and keeps on inflating. In some regions of spacetime, the inflation stops and the energy from this inflation is converted into energy and matter. This region where inflation stopped becomes sort of like a 'bubble' in the ever inflating spacetime. This bubble is then a universe. This there can be multiple universes accelerating to this hypothesis.
Okay. Our current theory of gravity is General Relativity, and for now all you need to know is that it seats that gratuity thanks time to propogate and has only a finite reach unlike Newton's description of gravity.
So even if our neighboring galaxies have a slightly different observable universe, the gravity from galaxies far apart don't even affecter them at all. In fact, that is why or universe is expansion by without any noticable obstruction from gravity.
Gravity takes time to propagate and has a limited access, so far away galaxies don't even affecter each other. That is why expansion is actually happening.
Another thing is I think your understanding of dark energy is somewhat misleading. You say that dark energy pushes the universe from the inside. That is the exclamation it the universe. The expansion does not happen because of dark energy. Dark energy accelerates the expansion. We have observed that expansion of the universe is getting faster and faster as time passes. For this you need energy. That energy is dark energy, failed so because we haven't detected it yet.


How does this comet have alcohol in it? Recently I read about comet C/2014 Q2 (Lovejoy).The thing got me amazed was that scientists found it was dumping ethyl alcohol all along its way, at a rate of around 300-500 bottles per second. So I want to ask how common is alcohol in outer space, and how did it form? I usually imagine space as an inert mixture of dry dust and light gases but it turns out there are sometimes alcoholic compounds.

Getting atoms together into molecules requires two basic things - getting the reactants together and them providing the means to overcome the "activation energy" that is due to the mutual repulsion of the electrons that surround them.

Astrophysical chemistry is a rich and complex field of study. Most molecules are produced by a wide variety of processes. For ethanol in solar system objects, a cartoon of the process might be this:

Grains of dust (basic silicates and soot) form in the protosolar nebula; cosmic rays and high energy radiation from the protosun ionise atoms (remove outer electrons), which makes them "sticky"; these ionised atoms are concentrated onto the surfaces of dust grains, increasing their densities to the extent that reactions between them become likely; hydrogen ions are able to hydrogenate carbon and oxygen to produce CH and OH radicals; these then encounter each other in the presence of more hydrogen to form ethanol; dust grains with ethanol are then incorporated into solar system bodies like comets.

But as I said, for any individual ethanol molecule, other pathways are possible.


What exactly is wrong with this plaque in Neil deGrasse Tyson's "back yard"? Has it been fixed? The Wikipedia subsection Observable Universe; Misconceptions on its size shows this image of a plaque, with the caption:

  An example of the misconception that the radius of the observable universe is 13 billion light-years. This plaque appears at the Rose Center for Earth and Space in New York City.

According to Wikipedia:

  The Rose Center for Earth and Space is a part of the American Museum of Natural History in New York City. The Center's complete name is The Frederick Phineas and Sandra Priest Rose Center for Earth and Space. The main entrance is located on the northern side of the museum on 81st Street near Central Park West in Manhattan's Upper West Side. Completed in 2000, it includes the new Hayden Planetarium, the original of which was opened in 1935 and closed in 1997. Neil deGrasse Tyson is its first and, to date, only director.

That last line is the operative part. Dr. Tyson is a world-renowned stickler for truth and accuracy, and so a problem in is own back yard needs to be investigated.

What exactly is wrong with this plaque photographed in 2011?
What should it have said?
If something is indeed wrong, has it been fixed?

above: Incorrect plaque at the Rose Center for Earth and Space, April 2011

below: Rose Center for Earth and Space Source which literally contains the famous Hayden Planetarium.

Update: I just noticed that even in 2011 the plaque had already been amended (at least) once!

I see 2 candidates:

the universe is 13.8 billion years old, so should be rounded off to 14
the cosmic horizon is 46 billion ly away


Does a continuous deformable mirror cause diffuse reflection? We have all seen images like this of deformable mirrors:

Continuous deformable mirrors are known to have a higher quality over segmented mirrors, but it seems to me that a continuous mirror (or even a segmented tip tilt mirror) would cause diffuse reflection.  Here is a figure showing a segmented and a continuous mirror

Since the light is coming in normal to the telescope (let's just say space telescope so we can avoid all possible atmospheric non-normal light), the purpose of the deformable mirror is to adjust the phase of the light.  It would appear to me that a continuous deformable mirror would also diffuse the light as seen from my rough drawing (I know that most mirrors are placed at a 45 degree angle, but you get my point).  Here is a figure showing the diffuse reflection at a 45 degree angle.

It is well known that a segmented mirror causes light scatter (at each segmentation location), but I have not been able to find any literature on light diffusion on a continuous deformable mirror.  This must be true, but how is it not a problem and how do they address it? (pointing towards scholarly resources would be appreciated)
UPDATE
I will accept the answer posted below, but I think it is missing something important.  In the fourth bullet it is stated,

I would conjure that the diffusion reflection at segmentation locations is not significant enough compared to the overall sub-mirror reflection

This is still an opinion (though it could be correct), which is why I specifically asked for scholarly resources.
I reached out to Professor Thomas Bifano from Boston University who specializes in deformable mirrors and his answer made sense and was readily verified in scholarly papers

Diffuse reflection occurs when the reflective surface is rough: scale of surface asperities is large in comparison to the wavelength of light reflecting from it. In diffuse reflection, incident light at a fixed angle of incidence reflects (actually, scatters) at many different angles. If the local roughness of the surface is <<wavelength, there no diffuse reflection.

Given your reply it seems your definition of diffusion is slightly different from my sense. But the phenomenon is true that at segmentation locations wavefronts are being deflected not in a uniform manner, causing the diffusion issue posted in this question.
Before addressing this issue, let me briefly describe how adaptive optics works in practice. In a typical AO system, a wavefront sensor measures the incoming wavefront distortion (usually it's slopes), given which a deformable mirror corrects for. The deformable mirrors consist of multiple actuators, which enable the mirror deformation. These actuators are controlled by commands.
The input/output are:

Input: wavefront slopes $\mathbf{y}$
Output: actuator control commands $\mathbf{x}$

These two are usually related through a so-called interaction matrix $\mathbf{A}$:
$$
\mathbf{y} = \mathbf{Ax}
$$
This matrix models all system artifacts, including the diffusion issue in this question. Since we would like to solve $\mathbf{x}$ from $\mathbf{y}$, we would also like the pesudo-inverse of $\mathbf{A}$, denoted as $\mathbf{A}^{-1}$, so in real-time the AO system can solve the actual commands as:
$$
\mathbf{x} = \mathbf{A}^{-1} \mathbf{y}
$$
$\mathbf{A}$ and $\mathbf{A}^{-1}$ are usually calibrated before running the AO system.
Now consider this question:

Yes, there are diffusion reflection from the mirror. But this issue contributes in part to the interaction matrix $\mathbf{A}$. As long as people calibrate it carefully it does not really affect AO performance.

For normal AO systems both measurement and correction wavefront lateral resolutions are low: typically at thousands of measurements, i.e. $\mathbf{x}\in\mathbb{R}^{1000}$ or so. In such a situation, I would conjure that the diffusion reflection at segmentation locations is not significant enough compared to the overall sub-mirror reflection, i.e. the fill factor of the sub-mirror is high enough to ignore this effect.


Relation between intensity of Sun and the light that is emitted from surface of sun please correct me if I am mistaken, I was in the middle of a thought process. We take a unit area dA of sun,, which is emitting light directly to our line of sight. But there are also numerous other unit areas throughout sun's surface whose light does not reach our eyes directly. Then why do we still see the sun with same intensity?

We take a unit area dA of sun,, which is emitting light directly to our line of sight

You are, I think, imagining the light as if it was directed out from a point, with directions of photons outward from the center.

What actually happens is that the light is emitted from every point on the surface and what's more every point on the surface can send light in any and all directions.

So the Sun is a diffuse source of light, not a point source of light.

  But there are also numerous other unit areas throughout sun's surface whose light does not reach our eyes directly.

So because of what I have described - light being emitted in all possible directions from all over the Sun's surface - we do see light from all parts of the surface.  And that light does reach our eyes directly.


When did telescopes become available? The History of Science and Mathematics stack exchange has a question "When Did Telescopes become Affordable?" asking when telescopes became available to members of the general public instead of wealthy people, professional astronomers, sea captains, and army officers.
And I think that telescopes have become more affordable to the general public over centuries.
So do any amateur astronomers at this site have any information about the history of making telescopes to sell to amateur astronomers?

This is an opinion based question and so I don't think there is an objective answer.  For example what constitutes a wealthy person?
However, in his book "Celestial Objects for Common Telescopes", first published in 1859, the author, Thomas Webb, writes: Instruments quite sufficient for the student's purpose are far less expensive than formerly; a trifling outlay will often procure them, of excellent quality, at second-hand
This suggests to me that telescopes had become reasonably available by the mid 19th C.


In what units to quote the thermal Blackbody temperature Just a quick question relating to the thermal Blackbody temperature of a celestial object. In the c.g.s regime of astrophysics, is it more 'sophisticated' to quote the thermal Blackbody temperature in units of Kelvin ($\mathrm{K}$) or in units of $\mathrm{keV}$? That is to say to use $k_{B}T_{\mathrm{bb}}$ and convert to an energy?

I'm not entirely sure what you mean, but the (Planck's) formula for blackbody radiation is given by

$$S_{\lambda} = \frac{8 \pi h c}{\lambda^5} \frac{1}{e^{hc/\lambda kT} - 1}$$

where $h$ is in $\mathrm{J \cdot s}$, $c$ in $\mathrm{m/s}$, $\lambda$ in $\mathrm{m}$, $k$ in $\mathrm{J/K}$ and $T$ in $\mathrm{K}$.
So, the temperature is just in kelvin, not in energy.
This formula, with these units, gives $S$ in $\mathrm{W/m^2/m}$, which describes the amount of energy per temperature and wavelength. There is no need to convert a given temperature to energy.


How to create unique 3d star maps? I have been trying to create a calculator which finds the distance between two stars that the user chooses. But I'm having a hard time visualizing where these stars are in real space. Do you know of any software that could actually help me crate a star map with only the stars in my calculator? I know of some star maps like Celestia, but I would like to be able to create my own map that only contains certain stars. If there isn't anything, I can find my own solution. Thanks in advance for any answers!

I hope the following doesn't come across as self-promotion, but you can have a look at the source code for my near-space 3D map:

http://beyondproxima.com/

Just View Page Source.

It's not perfect - the 3D projection is not geometrically accurate, and some brighter stars are duplicated. But it might give you a starting point?


Questions about convolving/deconvolving with a PSF A point spread function (PSF) has many different uses. Consider for instance the following quote:

  To  extract  the  maximum  information  out  of  an  observation, even the smallest details of the PSF are important. Some examples include: deconvolving the PSF from an observed image to remove the blurring caused by diffraction and reveal fine structure; convolving a model image by the PSF to compare to an observed one;

My question about this is the following:
If we know the PSF of a system, and use this to deconvolve the raw image (purpose 1 from the quote above), why would we convolve a model with the PSF to compare it to an image (purpose 2 from the quote)? That is, can't we just compare the original, unconvolved model, with the deconvolved image? In terms of maximizing the information that seems like the way to go.

Convolution is not a uniquely invertible process in the presence of random noise in your image. Deconvolving a noisy image can give misleading results, even if you have perfect knowledge of the PSF.

In general, when you are fitting models to data, it is far better to compare the models and data in the observational space of the data, where the uncertainties are reasonably well understood.


Could dark matter be wrong? As I understand, the evidence for dark matter is the observation of the gravitational dynamics of objects around the size of galaxies and above.  They move in a way such that the equations imply there is more mass than what we can see.  There has never been any direct observation of dark matter, and our theories tell us it cannot be composed of any elementary particles from the standard model.  Furthermore, there is no observation which suggests that dark matter is unevenly distributed, that some galaxies consisting of roughly the same amount of normal matter have varying amounts of dark matter.

I wonder if therefore an alternative explanation for the phenomena is that we have the wrong dynamical equations for gravity.  Just as Newtonian gravitation is a good approximation to general relativity at low masses/energies, perhaps general relativity is a good approximation to the true gravitational theory at sub-galactic distances.  Maybe when distances become very large we start seeing that gravatational force is described by slightly different curves than what general relativity predicts.

Is there any good evidence speaking against this alternative approach?

The reason this question is different from the "possible duplicate" is that the other question asks if the source of the effect attributed to dark matter is parallel universes, rather than different dynamical laws.  However, the answer there does mention these alternative gravitational theories.

I realize now that this is not a new idea, and I thank the commenters and answerers for pointing me to existing research.  However, this part of StackExchange does not bill itself as an "experts only" forum, so I don't think the downvotes are fair.  In mathematics for example, there is one forum for experts, and another for everyone.  I'm an expert in something, but not astronomy.  Thanks.

Multiple theories and hypotheses have been proposed as an alternative to dark matter (DM). The most popular are, arguably,

MOND (MOdified Newtonian Dynamics):A term for various theories where the gravitational force falls off less steeply than $1/r^2$ at large distances.
TeVeS (Tensor–vector–scalar):A relativistic generalization of MOND.
f(R) gravity:A generalization of general relativity, where the so-called Ricci scalar — which describes the curvature of space(time) — may take different forms than just a number.
Entropic gravity:A string-theory-based theory describing gravity not as a fundamental interaction, but as a so-called entropic force.

Note, however, that DM is not just "a fudge factor to explain weird dynamics". What makes most (but not all) physicists believe that DM exists is, I think, that several independent observations yield consistent results, not only qualitatively, but quantitatively. In addition to rotation curves of spiral galaxies, or velocity dispersions in stellar clusters, elliptical galaxies, and galaxy clusters, you have for instance

Gravitational lensing:Massive clusters of galaxies "bend" space around them, causing the light from background sources to diverge from a straight path. Measuring this divergence tells us the mass of the foreground object, which can then be compared to the visible mass..
Cosmic microwave background:The patterns in the inhomogeneities of the background radiation is sensitive to the amounts of both dark and "normal" matter.
Structure formation:DM explains how matter may collapse and form galaxies a few 100 million years after Big Bang, before the Universe expanded too much.
Supernova observations:The way that the brightness of distant supernovae decreases with redshift depends on the expansion history of the Universe, which in turn depends on the densities of the various components of the Universe. A certain amount of matter is needed to explain the expansion, and this amount is larger than the observed amount by the same factor that the other experiments yield.

Some of the above phenomena are explained by the alternative theories as well, but in general several ad hoc hypotheses have had to be made. In my view (but I am no expert on this!), DM is simply the simplest explanation, and so, by Occam's Razor, is favored by most physicists. I think the theory, unfairly, has a reputation for being "some magic that astronomers pulled out of their hat to explain what they don't understand, and which we made sure can't be observed so you can't prove us wrong". But firstly, postulating an unknown object to explain some phenomenon is quite normal in physics — this is how e.g. Neptune and several elementary particles were found. And secondly, there are several candidates for what DM might be, and at least some of these make falsifiable predictions. If DM is a particle, then that particle might produce a signal that can be observed. There are many efforts to search for a DM annihilation signal, which is expected to be in the (tens of) GeV region (e.g. H.E.S.S Collaboration 2011 and Cui et al. 2017). There are also efforts for direct detections — see e.g. Liu et al. (2017) for a review.

One of the most compelling pieces of evidence yet for DM came, I think, with the observation of the Bullet Cluster (and subsequently many other clusters), where lensing clearly shows how unaccounted-for mass follows the collisionless galaxies, but not the collisional gas, as seen in this beautiful image:

The Bullet Cluster. Pink shows X-rays from the gas, whereas blue shows where the mass is concentrated. Credit: NASA/CHANDRA.


Is the Earth going to evolve towards Mars' fate or Venus' fate? Upon reading on this site (and many others), one can think that Mars might have supported life in a distant past (discovery of liquid water, valleys, mountains...).

On the other hand, Venus is the perfect example of the consequences of the greenhouse effect. A planet where life is simply impossible because no heat can escape the planet's ultra-thick, CO2 filled atmosphere.

So, with a lot of exageration, one can say that both Mars and Venus could be the Earth's distant future.

Since the Earth is actually facing global warming and greenhouse effect, I was wondering if the Earth would one day be like Venus ? Or if it is going to be like Mars, with no more magnetosphere and almost no atmosphere left ?
Or if it is not going anywhere near those two planets' fate ?

Either or neither. It's impossible to tell from the present.

If runaway climate change occurs, then yes, the conditions on Venus could be a potential analogue for the kind of environment on Earth due to the greenhouse effect.

Mars's atmosphere is assumed to have been much thicker in the past, otherwise it could not have sustained liquid water on the surface; it would have evaporated away. This water is required to explain the gorges and riverbeds we see on the Martian surface today. Researchers are still unsure where the atmosphere went, but the prime candidate is through top-loss to outer space, rather than sequestration through minerals on the surface (see here).

The likelihood of such an atmospheric escape process becoming dominant on Earth is small due to the Earth's greater mass and strong relative magnetic field, which prevents ion escape. In fact, the dominant loss process on the earth is sequestration. Some estimates put the reservoirs of sequestered carbon from original CO$_2$ at 250000 times the size of the existing atmosphere.

The alternative is that the Earth remains in its current steady state, or close to it, far in to the future. There are many mechanisms to enable such an equilibirum, such as oceanic sequestration of C0$_2$ highlighted above. There are also more contested theories such as the Gaia hypothesis. This proposes that the global biological ecosystem on the Earth is self regulating, helping to maintain life enabling conditions on the Earth. It's pretty much a dead-cert that life doesn't exist on either the Martian or Venusian surfaces, so we can accept that if the Gaia hypoethesis is true, the Earth will never reach these conditions.


Is the resulting light from a supernova a product of photons bouncing around in the Sun? I was thinking about how photons formed in the centre of the Sun take quite a long time to exit the surface. And it occurred to me that at the end of a star's life (during a supernova, given that it's big enough) all the photons that had built up in the radiation zone would be emitted at that point. Is the resulting emission of all those built-up photons what produces the great source of light seen during a supernova, or is it simply a contributor?

First, the Sun will not end up as a supernova - only a star $>8$ times the mass of the Sun will end its life in that way.

You also have the wrong idea about "trapped light" (photons bouncing around and gradually working their way to the surface). Photons are constantly emitted and absorbed again and don't travel very far (compared to the radius of the Sun), except at the photosphere, where by definition, they can escape.

Thus there is no storage of photons. What we see in a supernova is just a photosphere that is much hotter and much larger than the Sun. That is why it is many orders of magnitude more luminous than the Sun.

The heating (and explosive expansion) of the supernova envelope is ultimately derived from a small fraction of the gravitational potential energy released by the collapse of the stellar core.

Edit: The photosphere of a core collapse supernova is in fact about the same temperature as the Sun during most of the decay phase. Type Ia supernovae are a bit hotter. The huge luminosity is because the photosphere is huge.


Seasonal effect based on Earth position I know that Earth rotates around the Sun in an oval route:

And I know the seasons happen, because the axis of the Earth is not vertical. I think it is 23 degree slope.

How come the distance from the sun when the earth at the closest point to the Sun has no effect and if it has effect what is it?

I can see that when it is summer on the northern hemisphere it is winter in the southern and vise versa.

Shouldn't be it is hotter time for both sides when earth is closest to sun and colder when it is furthest?

The amount of insolation (the solar irradiation a planet receives) follows the inverse-square law: half the distance to the Sun results in four times the insolation for a given surface area. However, Earth's orbital eccentricity is very low – currently just 0.0167 – which means our orbit is nearly circular, so the change in insolation is never so dramatic as a quadrupling: our perihelion is 147.1 million km and our aphelion 152.1 million km – a difference of just 5 million km – which means the Earth receives $\left(\frac {152.1}{147.1}\right)^2$ = 1.069 times (i.e. 6.9%) more insolation at perihelion.

Another effect of orbital eccentricity is that under Kepler's second law, Earth's orbital velocity is greater at perihelion than at aphelion: Earth spends about 4.7 fewer days in the seasonal quadrant at perihelion than in the opposite quadrant at aphelion. In other words, the northern winter and southern summer are slightly shorter, and the northern summer and southern winter slightly longer.

These variations do indeed affect the seasonal weather: the southern hemisphere currently has slightly hotter, shorter summers and colder, longer winters, and the northern hemisphere has slightly milder, longer summers and milder, shorter winters. 13,000 years ago, when the Earth's axis was pointing in the opposite direction, it was the reverse: slightly hotter but shorter northern summer, etc.

However, there are many other things that affect insolation and its impact on Earth, including variations in the Sun's radiation (for just one example, see the Maunder Minimum in sunspot activity), variations in albedo, scattering and absorption (e.g. greater northern hemisphere land mass means more heat absorbed; loss of Arctic ice in summer reduces surface albedo so more heat absorbed), variations in Earth's axial tilt, etc. It's very complicated!

Some of these variations are cyclic in nature. The combined impact on Earth's insolation from cyclic variations in orbital eccentricity, axial tilt (obliquity of the ecliptic) and axial direction (precession of the equinoxes) are described by the Milankovitch cycles.


Formation of our Universe So I got this information from a magazine , Scientific American issue Physics at the Limits. The theory is that our Universe came about, when in the "Bulk Universe", which is four dimensional, a four dimensional Star collapsed and formed a black hole which became our 3 dimensional "Brane Universe", in which we live. My question is, how could this one Star have contained all the material that is in our Universe ?

I didn't read the SciAm article, but: Any cosmological theory compatible with "the" mainstream big bang theory doesn't assume material to have been contained in "Our Universe" (needs to be defined) from the very beginning.
Instead a very hot, unified state, is assumed, which then underwent several symmetry breakings (https://en.wikipedia.org/wiki/Symmetry_breaking, https://en.wikipedia.org/wiki/Higgs_mechanism), before "material" came into existence, as we observe it by now.

So, the very hypothetical, presumed four-dimensional star just needs to trigger an initial state causing the big bang. Whether "Our Universe" is a brane universe embedded in a higher-dimensional universe, is another mere hypothesis. Whatever "star" or "black hole" in a higher-dimensional universe would mean, the physics would be very different from the physics we can observe. For instance, inverse square laws wouldn't hold due to the additional dimension(s), hence orbital dynamics, electromagnetism needed for atoms, etc., all would either fail, or work rather different. Presuming the existence of stars in such a hypothetical universe is very daring, already.


Thermal Doppler broadening How to plot Doppler profile corresponding to the spectral line? The spectral line was plotted from data: flux with respect to wavelenght and I know the temperature.

These two function are Doppler profile? What should I substitue for $\Delta \nu_{th}$, $\nu_0$ and $\Delta \nu$. These values are from wavelenght according to $\lambda = \frac{c}{\nu}$?

https://en.wikipedia.org/wiki/Doppler_broadening

Or could you recommend me another formula for plotting Doppler profile?

$\nu_0$ is the frequency you would expect for the absorption/emission line in the absence of any broadening. i.e. It is the centre of the line profile.

$\delta \nu$ is just $\nu - \nu_0$. (i.e. it is the separation in frequency from the centre of the line profile and is equivalent to $f -f_0$ in the second expression).

$\Delta \nu_{th}$ is a measure of the broadening due to (I guess) thermal motions in whatever gas/plasma is emitting/absorbing the radiation. It is similar to the Gaussian "sigma" term in the expression
$$ f(x) = \exp(-x^2/2\sigma^2)$$

If you have a gas that follows Maxwell-Boltzmann statistics, then $\Delta \nu_{th}$ will be related to the speeds of atoms/ions/molecules in the gas and hence to the temperature of the gas.
$$ \Delta \nu_{th} \simeq \sqrt{\frac{2k_BT}{m}}\times \frac{\nu_0}{c} \,$$
where $m$ is the mass of the particle emitting the light.


Was is the angle needed to see a glory (a rainbow-like effect) from space off the Earth? What positions would the Earth, Sun and viewer need to be to see a glory (a rainbow-like effect) from space?

What position would they need to be in to reproduce this picture?

Source: Gizmodo (NASA via Universe Today)

Related:

https://earthscience.stackexchange.com/questions/16308/rainbow-dynamics
https://space.stackexchange.com/questions/34357/rainbow-space-probe

A glory is a rainbow-colored bullseye appearing directly opposite the Sun due to diffraction by water droplets of a certain size.
From an airplane, you can sometimes see this effect surrounding the airplane's shadow.
As the NASA article explains, the polar orbiting Terra satellite's scan pattern transformed the circular rings into elongated ovals.


Two lights roughly 5 minutes apart about as bright as Mars or maybe Saturn I'm in the Minneapolis/St. Paul area and around 9:30 on August 4th 2017 I saw a bright light that got dimmer moving from the tree line to the east up and to my left fairly slowly, and steadily getting dimmer. There were no flashing lights like an airplane. Around 5-10 minutes later I saw another light exactly like the first in nearly the same place going the same direction, they both faded and disappeared. I grew up in the north and have spent many nights out in hay fields staring at the sky but sadly I have never really studied astronomy and my guess at brightness is based on Google images. My first guess would have been a satellite but seeing two on the same path makes me think otherwise. Thoughts?

According to CalSky (https://www.calsky.com), there were two Iridium flares visible low in the south-east from Minneapolis at 21:18 (Iridium 60) and 22:28 (Iridium 55) on that evening. Both would have been about 15 degrees elevation and both in the right ballpark for brightness.

Iridium flares occur when the rising / setting Sun's rays bounce off the antennae or solar panels of one or other of the Iridium communications satellites. By their very nature they occur around dusk or dawn. They'll flare up, persist for a few seconds and fade away.


When is optical refraction important in astronomy? What are commonly important astrophysical systems/models, where optical refraction is important or necessary to account for?

I would kindly ask you not to consider refraction in Earth's atmosphere or inside instrumentation in this question.

Comment: Gravitational lensing is different from optical refraction even though it affects light paths. I would like to ask the authors to avoid mentioning it in the following.

Optical refraction is related to the change in direction of a light ray when the refractive index changes. Excluding Earth's atmospheres and instruments, I think that refraction has little/no impact in astronomy.

The only cases that comes to my mind where we can (probably) have some important refraction is in eclipsing star binaries or near edge on planetary systems.
Let's imagine a planet transiting behind his star. Some of it's light passes through the stellar atmosphere and gets refracted. As the atmosphere is curved and likely changes the refractive index with height, it acts like a lens dispersing (intuition says so) the planet light.

Edit A similar description holds in general for any object passing behind an other one that have atmosphere.

And there is Gravitational Lensing (if you allow me), which has much larger impact on observations. This is caused by gravity bending light rays when passing near galaxies/cluster of galaxies(/stars/...). One of the differences of gravitational lensing with respect to standard lenses is that there is no change in refractive index, so it's achromatic (all wavelengths get bent by the same angle).

The effective index of refraction can be described as ( source: Narayan and Bartelmann(pdf) ):
$$ n = 1 + \frac{2}{c^{2}} |\Phi|$$
where $\Phi$ is the gravitational potential and is generally a function of position of the object.

Gravitational lensing is canonically divided in three groups:

Strong lensing, usually observed in galaxy clusters or around massive galaxies. The gravitational potential is so strong that the the image of a background galaxy is heavily distorted into arcs and rings, like in this striking image of Abell 2218 from HST:

(source: hubblesite.org)
Weak lensing. The light of a galaxy encounters matter (and a lot of dark matter) travelling to us and gets refracted. This doesn't have a dramatic effect as in strong lensing, but distorts the shape of the galaxy. And this distortion can be used to study, e.g., the dark matter distribution around some object or the content of the universe.
Micro lensing. Imagine to observe a star and somehow know that a blob of dark matter is going to pass in front of the star. The blob is not big enough to distort the star shape, but for sure it will increase by a small amount the luminosity of the star.


How do astronomers detect gases that are in the atmosphere of exoplanets? Exoplanets are planets that are located outside our solar system - whether that be orbiting a star or drifting past one. Now, the closest star to us is Alpha Centauri which is just over four light-years away. So, how can astronomers detect gases in the atmospheres of planets over this distance with any degree of accuracy (or at all)?

It is only possible to detect gases in transiting Exoplanets. The spectrum of the star is taken when the Exoplanet is not in transit and again when in transit. The differences between the spectrums is due to a small amount of light from the parent star passing through the upper atmosphere of the Exoplanet and being absorbed by gases in the atmosphere.
The effect is VERY small and difficult to detect.


Is Universe Sandbox 2 realistic? Universe Sandbox 2 is the second installment of Universe Sandbox that is available on Steam. It is an educational simulation game based on astronomy and anything space, it ranges from supernova animations to climate and atmospheric simulations (and of course collisions) but do any of these features hold water? Do they truly resemble the dynamics of space?

Not sure if these kinds of questions are allowed here, but I may as well answer it.

Universe Sandbox 2 is mostly realistic. Its accuracy degrades as you increase the flow of time. The slower the time, the more accurate the calculations. This
means that if you want to simulate the Solar System over a long period of time, it won't be very accurate. If you want to simulate day-long events, like planets crashing, it might be better.

However, Universe Sandbox 2 uses 300-year old Newtonian physics, rather than general relativity. Newtonian physics just requires n-body mechanics, so it is much easier to implement. General relativity requires simulating the spacetime itself. That is, taking your simulation space, discretizing it to a hi-res 3-D grid and checking the effect that each and every point in that grid has on all neighboring points at every timestep.

Instead of simulating N number of bodies, they would be simulating a huge number of points. They would start with some initial data of the shape of spacetime and then see how it evolves according to the Einstein equations, which are 10 highly non-linear partial differential equations. Accurate general relativity simulations require supercomputers.

Still, Newtonian physics can get the job done for the most part. The developers seem interested in incorporating some GR concepts, like gravity propagating at the speed of light, spinning black holes and ways to better approximate GR.


In a reflecting telescope (Cassegrain), is there a difference between using a concave or convex secondary mirror? These are two diagrams of Cassegrain reflecting telescopes.

One uses a concave secondary mirror, and the other uses a convex one.

Is there any difference between the two?

For example, does one cost more? Does one produce a better image?

Thanks!

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EDIT: These images came from:

http://astro-canada.ca/les_telescopes-reflecting_telescopes-eng
http://www.a-levelphysicstutor.com/optics-telescs.php

===

There are many flavors of reflecting telescopes based on the original Cassegrain design which has been attributed by history to Laurent Cassegrain:

  The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25, 1672 Journal des sçavans which has been attributed to Laurent Cassegrain. Similar designs using convex secondaries have been found in the Bonaventura Cavalieri's 1632 writings describing burning mirrors and Marin Mersenne's 1636 writings describing telescope designs. James Gregory's 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments. (from here; emphasis added)

However, a Gregorian Telescope does indeed use a concave secondary mirror, placed after the primary focus. The diverging beam is then re-imaged behind the primary. According to Wikipedia, examples of Gregorian telescope designs in use include:

The MeerKAT, the Green Bank Telescope, the Arecibo Observatory, and the Allen Telescope Array are all radio telescopes employing off-axis Gregorian optics.
The Vatican Advanced Technology Telescope, the Magellan telescopes, and the Large Binocular Telescope use Gregorian optics.
The Giant Magellan Telescope will also use Gregorian optics.
The Solar Optical Telescope on the Hinode satellite.

Advantages of the Gregorian include the option of putting a field stop at the primary focus, helpful when viewing near bright, interfering sources of light, and providing an upright, non-inverted image.

In either the convex/Cassegrain-type or concave/Gregorian type of system, the curved secondary mirror provides several important advantages, including:

Slowing the convergence of the rays, which allows for the diameter to be much smaller than the primary (so as to block less light and be much lighter weight) while still bringing the focus beyond the hole in the primary and out to where an eyepiece and/or other optics can be conveniently located.
The length of the telescope can be quite short while the effective focal length seen at the eyepiece is still quite long, allowing higher magnification for a given eyepiece.

The cartoon illustration shown in the Canada under the stars website shows a meniscus with it's concave side used as a reflector. I think this is just an error by the illustrator. It wouldn't make sense to show a Gregorian telescope in a paragraph about Laurent Cassegrain, and the primary focus does not fall in front of the secondary.

I've tilted the drawing so that the optical axis is horizontal, and highlighted the incoming and reflected rays of light striking the top edge of the secondary. You can see that the reflected ray is more parallel to the axis, which would only happen if the mirror were convex. The very large distance between the lens of the eyepiece, and the actual location of the exit for one's eye confirms that some artistic license has been taken with the optics here.

For reference, look at the realistic drawing of a Cassegrain optical system from Wikipedia shown below.

The drawing from the "A-level Physics Tutor" website is also problematic, showing the primary focus far in front of the primary mirror of an optical telescope, followed by an eyepiece lens with the same diameter as the secondary mirror. Again this is artistic license, but there's no excuse for it in a proper physics tutorial.

There are examples of optical telescopes where the secondary mirror is so strongly convex that the light then becomes divergent, expanding toward the primary before being refocused (made convergent again) by a tertiary mirror, such as the (crazy/beautiful) Large Synoptic Survey Telescope. See also LSST Optical Design.

above: Drawing of a Cassegrain reflecting telescope, from here.

above: Illustration of the optical system of the Large Synoptic Survey Telescope, from here (rotated).


Can a Y dwarf star undergo a supernova? Y dwarf stars have sub-zero temperatures, so I became curious how a star like these dies out and if is could go supernova.

Stars of spectral class Y are dim brown dwarfs, with temperatures of about 300-400 K (For comparison, Earth's average temperature is around 288 K, give or take, the surface of the Sun is 5,800 K, and the core of the Sun is 15 million K). Their temperature range means that they don't fuse hydrogen in the same way that "true" stars do. They may fuse deuterium ("heavy hydrogen"); Y dwarfs may not fuse lithium, as other brown dwarfs do, because they are relatively low-mass.

The fact that Y dwarfs (and brown dwarfs, as a whole) don't undergo normal stellar nuclear fusion means that they'll never undergo core-collapse supernovae, which typically happen when an extremely massive star gets to the point of iron fusion and then can no longer generate energy from fusion. Brown dwarfs work completely different; this is not at all a possibility.

Instead, they will slowly radiate away their heat until they become extremely dim, via the Kelvin-Helmholtz mechanism. For more information, see the basic example evolutionary tracks on this page; if you're really feeling ambitious, you can try going through these notes to get a better feel for brown dwarf evolution.


What was the bright orange glow in the sky? It moved then disappeared. What was it?

This is low on the horizon and there is glow from distant lights.
I have seen stars or planets suddenly disappear when fairly low, and it happens when clouds that were not visible at night move in between the object and myself.
If there is some hazy glow already in the sky, and the cloud is darker, it could be completely invisible and undetectable until it blocks something.
If it move just a tiny bit, it could be that the viewer moved and the the movement relative to the branches is what was actually seen. Assuming its a star or planet it would not have moved quickly in the sky, but relative to foreground objects for reference if one's head moves slightly it could have seemed to move.


Rotation and relativity When a planet is spinning around its own axis, it has an effect on the trajectory of its satellites. I believe it is called frame dragging.

Spin increases the kinetic energy of an object, contributing to its mass-energy, but some other effect must come into play as well.

How does one differentiate between spin and orbit velocity?

What is the effect of the spin? Lets say that Earth suddenly started spinning faster, what would happen to the moons trajectory?

The moon's rotation plane would precess (more) in the same direction as Earth's rotation, if the Earth spun faster.


How do we know that black holes are spinning? How is it possible to know if a black hole is spinning or not?

If a planet is spinning, you can see it clearly but you can't really see a black hole.

Next thing would be that matter interacts with adjacent matter and we could see in which direction the matter surrounding the BH spins (like if you spin a ball on water, the water around would spin too in the same direction) but matter can't interact from inside the event horizon to the outside, so matter right at the event horizon would just be interacting with gravity (like the BH has no friction).

Now gravity. I would think that you could measure the differences in gravity if a large object is not perfectly uniform but I think a BH has the same gravitation pull on all sides.

What am I missing here? How can one even detect or determine by observation that a black hole is spinning, or better yet, measure how fast?

The gravitational field of spinning matter, or a spinning black hole, causes matter around it to start spinning. This is called "frame dragging" or "gravitomagnetism", the latter name coming from the fact that it's closely analogous to the magnetic effect of moving electric charges. The existence of gravitomagnetism is tied to the finite speed of gravity, so it doesn't exist in Newtonian gravity where that speed is infinite, but it's present in general relativity, and for black holes it's large enough to be detectable.

Also, for purely theoretical reasons we expect that all black holes are spinning because a non-spinning black hole is the same as a spinning black hole with an angular velocity of exactly zero, and there's no reason why a black hole's angular velocity would be exactly zero. On the contrary, because they are so much smaller than the matter that collapses to produce them, even a small, random net angular momentum of the collapsing matter should lead to a rapidly spinning black hole. (The classic analogy for this is an ice skater spinning faster when they pull their arms in.)


How is it determined that the X-ray and radio intensity come from a magnetic field bridge between two clusters of galaxies? Gizmodo's Astronomers Spot Mysterious, 10-Million-Light-Year-Long Magnetic Field Connecting Two Galaxy Clusters shows the image below, and Space.com's A Weird 'Radio Bridge' 10 Million-Light Years Long Links Colliding Galaxy Clusters discusses it as well.
They both link to the new paper in Science A radio ridge connecting two galaxy clusters in a filament of the cosmic web (also arXiv).
What I've gathered so far is that seen between two known and characterized clusters of galaxies, there is a region with some excess continuum radio power at 140 MHz and some excess continuum X-ray power.
Neither of these are spectral features so their distances can't be associated with the galactic clusters by doppler shift.
Question: How do they know that the X-ray and Radio continuum are both really coming from the space between these two clusters of galaxies, and not in front of or behind it?
"bonus points" how does Planck's "y parameter" figure in to this mix?

“A radio ridge connecting two galaxy clusters in a filament of the cosmic web”, F. Govoni et al. 2019, Science. Optical: DSS and Pan STAARS (insets) — Red, X-rays: XMM-Newton — Yellow, y-parameter: Planck satellite — Blue, radio 140 MHz: LOFAR. Image credits: M. Murgia - INAF
The two galactic clusters and the intervening radio ridge. Image: DSS and Pan-STARRS1 (Optical), XMM-Newton (X-rays) PLANCK Satellite (y paramter), F. Govoni, M. Murgia, INAF

I think the answer is that it's primarily a matter of association on the sky, though there are some (weak) additional constraints.

The first thing to note is that there are three observations that are associated on the sky: the X-ray emission (which has been known about since the mid 1990s), the newly detected 140 MHz LOFAR emission, and evidence for a Sunyaev-Zel'dovich effect in the same region, from Planck data (e.g., Bonjean et al. 2018). From the caption to Figure 3 in the new paper: "The Planck data show a bridge of gas between the pair of galaxy clusters Abell 0399 – Abell 0401, in the same location as the LOFAR ridge." and from the main text: "The radio ridge is located along the filament of gas connecting Abell 0399 and Abell 0401 detected with Planck (9, 10)."

(The "Planck $y$-parameter", also known as the "Compton parameter", refers to the measurement of the Sunyaev-Zel'dovich effect due to inverse Compton scattering of cosmic background radiation photons, derived from Planck satellite data. I believe it's proportional to the line-of-sight integral of the gas pressure.)

The Suzaku X-ray spectrum of this region (e.g., Figure 2 of Fujita et al. 2008; see below) does have some spectral features, including the Fe K emission line. Due to the low spectral resolution, these features don't constrain the redshift of the X-ray-emitting gas very strongly (and I think Fujita et al. fixed the redshift at the mean of the two clusters in their analysis), but it would probably be noticeable if it differed by more than $\sim 0.1$ in redshift.

From Figure 2 of Fujita et al. (2008): Suzaku X-ray spectrum of part of the "bridge" region between the two clusters. Black is observed data, red is the combined model, including Galactic and extragalactic background (dashed blue), and emission from the inter-cluster gas (green).


How does the sun move over the year in the sky of the Moon? Wikipedia's section on the Moon's inclination says it is always 1.5° to the ecliptic. Does that mean that as seen from its surface, over the year the sun at zenith would move 1.5° to the north, then to the south, varying by 3° overall?

That's right. Unlike the Earth, which has a substantial inclination of 23.4 degrees to the ecliptic, the Moon is inclined by only 1.5 degrees. This means that there are no substantial seasons. The moon also spins much more slowly than the Earth. Each "day" lasts for one Earth month. It also means that at the poles there are craters that are completely shaded from the sun. In these craters, there may be useful amounts of water ice.

Now as noted by the Wikipedia site, this is due to two motions being linked. The moon's orbit is inclined by 5.1 degrees to the ecliptic, and the moon's equator is inclined by 6.7 degrees to its orbit, however the direction of the pole moves to keep these two angles always in opposite directions to each other (There can be no pole star on the moon) The result is that the inclination of the moon to the ecliptic is fixed at 1.5 degrees.


Does the observed period of a pulsar change with the time of year? Over in the Physics SE a question was posted asking about the difference in the time dilation of the Earth between perihelion and aphelion:

Does Earth experience any significant, measurable time dilation at perihelion?

Rather to my surprise it turns out that because of the changes in the Earth-Sun distance and the Earth's orbital velocity there is a difference of about $60\mu$s per day between the two extremes.

A commentator pointed out that pulsars can be measured accurately enough to detect this difference. However I have never heard of a pulsar measurement having to be corrected for the time of year, and Googling has found me nothing related. I would be interested to know if this is something that needs to be considered.

The difference is slightly over one part in $10^9$, so presumably it depends on whether pulsars can be timed this accurately.

Yes. In terms of pulsar timing measurements, this is a massive effect! A +/- 30 km/s doppler shift changes the pulsar frequency by +/- 1 part in 10000. This sounds small, but the accumulated phase shift over many periods is readily apparent. In addition, the light travel time across the solar system has to be taken into account, as well as the Earth's rotation and some other smaller effects - such as Shapiro delay.

If the question refers to the specific annually varying difference in clock rates caused by the different gravitational potential experienced by an Earthbound telescope on an elliptical (as opposed to circular) orbit -the answer is still yes.

This is item 4 in the list of applied corrections given on p.52 of "Pulsar Astronomy" by Lyne et al. The maximum effect is a rate change of $3\times 10^{-9}$ leading to a maximum lead or lag of 1.7 ms.


Why must all magnitude systems have a reference point? Let $f_*$ and $f_0$ be the observed flux of a star and a reference flux in a particular spectral band, and let $m_*$ and $m_0$ be their respective apparent magnitudes. Then the star's magnitude is given by $m_*-m_0=-2.5\log(f_*/f_0)$.
Encyclopædia Britannica maintains that

All magnitude systems must have a reference, or zero, point.

Why is this? It seems to me that one could just say $m_*=-2.5\log f_*$ and move on, no?

Because you can't take the logarithm of something with dimensions only the log of a number.
Your magnitude would depend on what you measured f in. In other words, by choosing a set of units, you choose a zero point! Whether that unit is Jy, W/m$^2$ or the flux of some other object.


Two body transient solution In the two body problem, if each body is given an initial condition as a momentum vector, what is the transient part of the solution as it settles into the steady state orbit?

Your question is very unclear.  As long as the orbit is bound (i.e. the total energy is negative), then there is a unique closed orbit solution (see Bertrand's theorem).  There will be no 'transient' part of the solution.


What could be the name of a huge asteroid I watched some years ago? Some years ago I was with my family in Venezuela. I'm not sure about the date/hour, but probably it was somewhat around 2007 in the afternoon. We were talking when one of us saw a huge meteor moving "slowly" from East to West. It was visually about 1/4 of a full moon size. It took a lot to get lost from our view (I'd say more than a minute). We could see the fireball burning and leaving the trail.

I never heard about this in the news and wanted to know if this same event is familiar to somebody else or if you've had a similar experience before.

I mean, this thing was huge and we were just waiting for it to crash but it seems we were lucky.

Edit:

It wasn't a satellite, since we could see the "rock". It wasn't like these meteors which you cannot distinguish the trace from the fireball.

I'm not sure about the date.

Meteors don't move slowly.

Large fireballs do occur, rarely, but they move pretty fast, a few seconds at most. Meteors that are bright enough to be seen in daylight are even rarer. When a meteor is falling you can't see the "rock".  Even a large meteoroid is only a few metres across, and they are 50-100km up in the atmosphere, and surrounded by glowing plasma.  Take a look at some of the videos of fireballs. Very few are during the day (because even bright fireballs are not clearly visible) and the few which are visible during the day are very well know (the 1972 daylight fireball or Cheblynsk)

What you saw cannot be a meteor. Because meteors don't look like that.

Re-entering space debris or satellites move more slowly, and can have more of a "burning" appearance than a meteor, but there is no mention of a satellite re-entry in the database: http://www.satobs.org/reentry/Visually_Observed_Natural_Re-entries_latest_draft.pdf  For example on Jan 2007 the rocket for Corot-r (a space telescope launched by Russia) re-entered over the USA and generated many reports.  That would not have been visible from Venezuela (and was in the middle of the night).

If you had an exact date then it would be possible to search more deeply, without it, this will probably remain a mystery.   (I pondered briefly about a rocket launch from the Guiana site, but they always launch to the East, over the sea (for safety and efficiency) so that can also be excluded.


Could the Sun and our Solar System one day leave our galaxy? I have now learned that the Sun is not fixed in place, so I asked myself this question: It is possible, that the Sun and our Solar System will one day leave our galaxy? If yes, how, and if no, why not?

It's certainly possible for a star system to be stripped away if its galaxy interacts gravitationally with a second galaxy. Tidal forces between the two can disrupt stellar orbits and form tidal tails and other possibly transient structures. This won't happen on human timescales - no satellite galaxies are anywhere close enough to us to pose a threat - but in several billion years, the Milky Way will collide with Andromeda in a process that will completely disrupt the structures of both galaxies.
It's been suggested that the Solar System may be stripped from its present galactic orbit during the collision. Cox & Loeb 2008 suggested that there is a  12% chance that we will be swept into a tidal tail >20 kpc from the center of the merger remnant, and a >50% chance that we will end up >30 kpc from the center, will in the outer halo - compare both of these with our current distance of ~8 kpc from the galactic center. Andromeda could even strip us away on its first pass at the Milky Way, though the authors regard this as unlikely.
All of these outcomes, of course, do still end with the Solar System being bound to the remnant. It's certainly true that we could be ejected from the system entirely, but that probability appears low compared the odds that we will simply move into a comparatively wide halo orbit.
If you regard the scenario where we're temporary bound to Andromeda as leaving the galaxy, then yes, that's indeed a possibility. Unfortunately (or fortunately?), we will soon coalesce once again with the Milky Way on timescales of ~1-2 billion years.
As an aside: There are other ways for a star to be ejected from a galaxy. One of the more common involves an encounter with a supermassive black hole. If a binary star approaches the black hole, the resulting three-body encounter will disrupt the system; one star may be captured by the black hole, while the other may be ejected at speeds greater than the galactic escape velocity. We know of several dozen of these so-called hypervelocity stars, including US 708 and S5-HVS1.


Is there a complete edition of the Cambridge Catalogue of Radio Sources (esp. 3C) available digitally? In radio astronomy, many common objects are refered to by their designation from the 3rd Cambridge Catalogue of Radio Sources, e.g. 3C 353.

For hobbyist purposes, I would like to create a catalogue for Cartes du Ciel that shows the location of the 3C objects. However, the catalogue exists in three versions, 3C from 1959 being the original, and 3CR (1962) and 3CRR (1983) updating the original.

Later surveys at different frequency bands used similar designations, e.g. 4C, 5C and so on.

Looking at the databases linked above, however, it seems that both only list the updates to the original catalogue (e.g. 3C 353 is listed in neither of them). Is there a digitized, comprehensive catalogue of all 3C objects? Or, even better, is there an archive where I could combine entries from several C surveys based on, say, 21cm flux?

You can use VizieR to retrieve the whole catalogue (3C and revised 3C).

VizieR is an online astronomical catalogue that allows you to access a quite comprehensive list of catalogues (16,929, as of today); you can easily query and extract all kind of data from this online tool.


Metallicity: What are the metallicity values of various galaxies? What are the various values of metallicity of various known galaxies, like Milky Way, Large Magellanic Cloud, Small Magellanic Cloud, Andromeda and so on?

metallicity is a complicated issue, and giving just one number (usually Z, but also 12+log(O/H), ...) is very rough. But, here are the standard values for the Magellanic clouds:

LMC: Z~1/2

SMC: Z~1/5

with Z=1 being the solar value. Practically, the values will vary across any given galaxy (the Milky Way also has a metallicity gradient).

Reference:

Madden et al. 2014, PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 125:600–635, 2013 June
http://adsabs.harvard.edu/abs/2013PASP..125..600M

The article is also available on astroPH


Where goes original asteroids mass after impact? In relation to possible asteroid mining on the moon, where goes asteroid mass after the impact? Will it be burried, scathered, or evaporated?

All three.  The impact velocity will be huge and the kinetic energy of astroid extremely large. The asteroid will be pulverized into millions of fragments of asteroid. Many of these will be as small as dust particles and will be vaporized from the heat associated with collision. Hence some small fragments will evaporate will other larger fragments will be buried or blown away from impact site. The scatter field is an ellipse about the crater elongated along direction of motion of meteor.