Is "Giga-hertz Astrophysical Transient Emission" distinct in any way from "Fast Radio Burst"? In this answer I have just read about The University of Texas Rio Grande Valley's The Spacecraft Tracking and Astronomical Research into Giga-hertz Astrophysical Transient Emission (STARGATE).

While Fast Radio Bursts (FRBs) seem to fit that description spot-on, I am wondering if the phrase was chosen to be in some way broader or more general than FRB to include other phenomenon, and if so, have there been any Giga-hertz Astrophysical Transient Emissions observations that are not also considered FRBs?

$\hspace{4cm}$

above: Logo of The University of Texas Rio Grande Valley STARGATE program.

I am going to float this as a tentative answer. I'll appreciate feedback, either validating it or letting me know it's wrong, in which case I'll delete it. If it is part of a better answer, please run with it and I'll also delete this.

X-ray navigation XNAV is being look at as a source of navigational data for spacecraft in deep space that does not require measurements and telemetry from a deep space facility on Earth. While a spacecraft could not easily receive radio pulses from a pulsar without a very large antenna to overcome the thermal noise in the receiver's front-end, a suitable X-ray detector could potentially build up timing data from several pulsars.

Since a spacecraft is usually moving without thrust under gravitational forces from known bodies, an instantaneous fix is not needed and data could be collected for several minutes or even longer.

While radio pulses from pulsars are generally periodic, there are transient phenomena that can be related to infall of ionized material interracting with the magnetic field causing bumps and wiggles of individual radio pulses, which may have a bearing on X-ray time structure as well.

And so I am speculating that these transient deviations in a Pulsar's otherwise regular behavior would be of relevance to technology development for both deep space exploration and business development.

However, while intermittent download of data from spacecraft could be considered Transient GHz Emissions, it would not be considered Astrophysical.

NOTE: The variability is in the production of electromagnetic radiation in a pulsar's incredibly intense magnetic field (radio) and at the surface (X-ray) produced during the infall of matter along the field lines towards the magnetic poles. The underlying rotation of the neutron star and it's field are still quite regular.


Telescope lens problem when cold We are having a problem with our scopes on cold nights.  (-10F is about average here.) One poorly aimed breath means that the eyepiece lens fogs.  We have to remove the lens and keep it under our arms to warm up to clear it.
The scopes are wrapped with heating tapes which does seem to keep the large lens from fogging/steaming up.    It is the eyepiece lenses that we are having issues with.
I searched Orion & Celestron's websites, but they just offer heating tapes which we already have.  I am concerned that frequently wiping the lens free of moisture, even with a soft cloth, could eventually scratch the lens.
Does anyone have a solution?

Dew-heater bands (straps) do come in sizes even down to 1.25" and 2" eyepiece. Multiple vendors make these.  Dew-straps are something of a standard in that they are resistive heat-straps designed to run on roughly 12v power.  The typically use the (RCA) tuplip type plug.  The dew-strap and dew-controller typically do not need to be made by the same vendor to be compatible.
Check vendors such as AstroZap, Dew-Not, Kendrick Astro Products, and others.
Depending on your dew controller, you may need a controller with additional channels.  Multi-channel dew-controllers allow you to set different power levels for each channel (or at least for groups) since the amount of power needed to heat, say, the corrector plate on a large telescope is much more than the power needed to heat a tiny 1.25" eyepiece.
If you have adequate power, you can use a hair-dryer to defrost an eyepiece (although I prefer the dew-heater strap to prevent fogging.)


Where do asteroids impact upon the Earth most often? Where do asteroids impact upon the Earth most often?

Are impact zones primarily along the equator?

Is there an average area where impacts are more likely to occur?

If there is a zone, is there a way to determine if it's changed? More to the point, if there is an impact zone, are there any similar areas that might indicate the zone has shifted?

The occur all over the world.  On this image each yellow circle is an recent impact of a small asteroid the size of the circle is related to the energy of the fireball (the sizes of the circles don't represent crater areas). You will note that there is no region that get more impacts, and similarly, nowhere is safe.

The very large yellow circle is the Chelyabinsk meteor in Russia. It was the biggest such impact over land in recent times.


What is the cut-off point of $\frac{\mathrm{d}F}{\mathrm{d}r}$ beyond which no tidal locking will occur? I am a beginner, please pardon my non-use of proper astronomical terms. I would appreciate proper edits to my questions.

I have learned that when the $\frac{\mathrm{d}F}{\mathrm{d}r}$ is a very big negative number, that would mean that the far side will have much less gravitational tug than the near side of a satellite. The differential gravity or $\frac{\mathrm{d}F}{\mathrm{d}r}$ will be as follows

$$\frac{\mathrm{d}F}{\mathrm{d}r} = \frac{-2 GMm}{r^3}$$

If we compute $\frac{\mathrm{d}F}{\mathrm{d}r}$ of Earth-Moon system we get $-1.308 \times 10^{-5} \ \mathrm{M_o^2G/AU^3} $

I was surprised to know that Ganymede, being almost 3 times as far away from Jupiter as our Moon is from Earth, still is tidally locked.

If we compute $\frac{\mathrm{d}F}{\mathrm{d}r}$ of Jupiter-Ganymede system we get $-1.565 \times 10^{-6} \ \mathrm{M_o^2G/AU^3}$

I then wondered what is the $\frac{\mathrm{d}F}{\mathrm{d}r}$ of Sun-Earth that our Earth is not tidally locked to it and I found $-2.422 \times 10^{-8} \ \mathrm{M_o^2G/AU^3}$

Which is a very small negative number so the difference in tug is not significant.

So what is the minimum $\frac{\mathrm{d}F}{\mathrm{d}r}$ (or I guess "maximum") value or a boundary value at which the object can be tidally locked and beyond that, the tidal locking won't occur?

I can't comment here because I don't have enough reputation, but this is very related to your problem.

https://en.wikipedia.org/wiki/Tidal_locking#Timescale

Tidal locking will always occur albeit potentially not in a Hubble time. In order for there to be no tidal locking, you would need:

$$\frac{\mathrm{d}F}{\mathrm{d}r} = 0$$

Which any extended object will not have. Problems like this always involve timescales and not definite yes/no answers. I hope this helped.


Why is this video showing radio waves transmitted from a radio telescope? I saw this video at Space.com and I noticed that at 00:05 the animation illustrates waves propagating from space into The Dish at Parks Observatory, but at 00:50 the direction of propagation is reversed! Would there be some reason for transmitting - for example to create an artificial "star" for wavefront correction analogous to adaptive optics at visible wavelengths, or could it be simply a mistake in the animation?

Here are some GIFs from screenshots. You can check the video at the indicated times.

circa 00:05

circa 00:50

I guess it's a mistake in the animation. At radio wavelengths turbulence doesn't affect the measurments anymore and above 200 MHz the ionosphere doesn't play a role yet, so I guess the notion of a guide star is unnecessary here.


What's the proportion of Mars' surface compared to Earth's surface uncovered of water? How does Mars surface compare to Earth's surface? Mars is quite smaller than Earth, but most Earth's surface is covered with water. I thought making the math could be relatively easy but Earth isnt a sphere and I dont know about Mars?

What's the proportion of Mars' surface compared to Earth's surface uncovered of water?

The surface area of Mars is 144798500 km2

The land area of Earth is 148940000 km2 (figures from wikipedia)

Thus, the land area of Earth is slightly (about 3%) larger than the land area of Mars. However much of the Earth's surface is suitable for human habitation, but none of Mars is suitable for humans.


"What was that spot in the sky?" - How to identify satellites First of all sorry for my immense ignorance in this matter and for all the stupid things I'll say here.
Yesterday, May 22nd 2017, I removed the telescope from its dusty box1 and decided to watch Jupiter. After about 2 minutes of observation I spotted something across the sky [X-files music in background], it was a little less bright as Jupiter's "Galilean" moons2.
Below you can see an accurate [Windows Paint] reconstruction of the fact I saw in my telescope. The red line indicates the direction of the thing (from right to left outside of the telescope).

Available data to help identify the object:
Time: May 22nd, 2017 @ 22:12 UTC
My position: {
    Lat: 41.720194062015096
    Long: 12.713427287843729
    Sea level: 540 m
    }
Direction: Probably South // The direction of Jupiter from that location at that time (didn't have a compass)
Telescope: {
    Focal length: 700 mm
    Diameter: 60 mm
    Lens: H 20 mm
    }

I only have data of the ISS and Iridium insights on an Android app3 so I am here to ask if there is some tool with which I can identify by myself as many satellites as possible across the sky.
Notes:

Winter is not nice with observers from here
Not visible at naked eye
I can almost certainly say that the "thing" was not the ISS or one of the Iridium satellites

P.s: I will post here what the object was (with reference if possible) as soon as I identify it for curious people like me.

This will be hard to track down. I'd suggest looking at something like SATVIEW Time Machine. I've entered the location and time you provided (I had to deviate by a few minutes to find something so maybe your time is not 100% accurate) and I can see there were a few satellites flying overhead. With enough work, you may be able to figure out which one was most likely the one you saw based on the Alt-Elev information detailed in the table.

There's plenty of other online satellite tracking websites out there, but keep in mind, most of them aren't going to have everything. And possibly, what you saw is in no database because it's a classified satellite in which case you probably won't find any record of it.


Are trojans in L5 more likely than in L4? Mars has much more trojans in L5. Does it reflect a common pattern?

There is a symmetry in the rotating gravitational field, which means that capture of an asteroid to the L4 is just as likely as to the L5 Lagrange point.

In the case of Mars the split is 1:6, and a simple binomial model suggests a probability of 0.0625 (the probability of a 1:6 or 0:7 split given the hypothesis that they are distributed randomly is 0.0625) This doesn't give a reason to suppose there is anything unusual happening, and as noted in the comments, there is no favouring of L5 when other planets are considered.

As noted by UserLTK: "Earth has an L4 asteroid, none in L5. Uranus also has an L4, no L5 and Neptune has more L4 than L5 objects."

The conclusion is that No, the greater number of L5 martian trojans is just a random effect.


Why is the Boomerang Nebula colder than the CMB? An earlier answer on temperature mentioned that the temp of the Cosmic Microwave Background (CMB) is $2.4\,{\rm K}$ and the temp of the Boomerang nebula as ${\rm 1\,K}$. How did the nebula cool faster than the CMB?

The Boomerang Nebula (or Bow Tie Nebula) is a cloud of gas being expelled from a dying low-mass star, at $164~\mathrm{km}~\mathrm{s}^{-1}$ (cf. Raghvendra Sahai and Lars-Åke Nyman: The Boomerang Nebular: The coldes region of the universe?). In general, when a gas expands, it cools (see extended explanation below). If the gas were optically thin to the CMB — that is, if it were sufficiently dilute that CMB photons could easily penetrate — it would quickly get reheated to the temperature of the CMB, i.e. 2.7 K. However, the Boomerang Nebula is optically thick (dense), so the CMB hasn't yet had the time to heat it. The temperature in its outer parts, however, is higher, and as the nebula expands further, it will eventually (on timescales of, say, 1000s of years) be heated not only by the CMB, but also by the central white dwarf, i.e. the remnant of the star that produced the nebula.
Why does an expanding gas cool?
The usual approach to explain this is to consider a gas in a piston. When the volume is increased, the gas molecules do work on the piston, and hence lose energy, so temperature decreases. However, in the case of the Boomerang Nebula, there are no walls on which the gas can do work.
Cosmologically speaking, the nebula expands rather quickly (it has "only" been expanding for ~1500 yr). Assuming that it doesn't have the time to exchange energy with its surrounding, the expanding is thus adiabatic. For an ideal gas undergoing a reversible adiabatic expansion (or contraction), we know that
$$P V^\gamma = \mathrm{constant},$$
where $P$ and $V$ are the pressure and volume of the gas, respectively, and $\gamma$ is the adiabatic index. For a monoatomic gas, $\gamma = 5/3$, but here there are probably also molecules, so it's probably somewhat higher. At any rate, it's higher than $1$, which is the important thing for us, as we shall see below.
Now the temperature $T$ of the gas can be obtained from the ideal gas law:
$$P V = N k_\mathrm{B} T.$$
Here, $N$ is the total number of particles, and $k_\mathrm{B} = 1.38\times10^{-16}~\mathrm{erg}~\mathrm{K}^{-1}$ is a constant (Boltzmann's, to be specific). Even for non-ideal gasses, this relation is usually a pretty good approximation. Combining these two equations, we see that
$$T V^{\gamma-1} = \mathrm{constant},$$
and since $\gamma \gt 1$, it is evident that if $V$ increases, $T$ must decrease.


How do I see more than just points in the sky? I just bought a new telescope with these specifications:

Aperture: 203 mm
Focal Length: 1200 mm
F/ratio: f/5.9

I usually use a 28 mm eyepiece, giving me about 42.5x magnification, but I also have a 20 mm and a 10 mm eyepiece.

I was trying to find Messier objects, but I only found the Orion Nebula, and even then it wasn't very impressive. I was wondering, what types of objects are see-able in my telescope? I read that Charles Messier found the Messier objects using a four-inch refractor, and I have an 8 inch reflecting telescope.

When I tried to find things like the pinwheel galaxy, or the crab nebula, I only saw points of light. So basically, how do I see things like nebula and galaxies?
For reference, I live in the midwestern United States.

Dark Skies are your friend.

When Messier was searching for comets, it was before electric lighting and the nights were dark in a way that is rarely seen today.

Take a look at the Dark sky map, and try to get somewhere in the blue or black region.

Then allow your eyes to become dark adjusted, and to look with the edge or your vision, or learn to do astrophotography.

And limit your expectations. Messier was looking for comets (little fuzzy blobs) and he made a list of the fixed little fuzzy blobs that might confuse him. Most Messier objects do look like fuzzy patches. You might take an account with telescope.org to see what can be done with a telecope 16 times larger than yours.


Converting rectangular to polar coordinates I'm following the steps in this paper on how to calculate geocentric longitude. In section 3.4.2 it says the
Geocentric rectangular co-ordinates of Mercury then are:
Xg = Xh−X0 = 0.990 A.U.
Yg = Yh−Y0 = −0.259 A.U.

Converting these into polar form, we get the geocentric distance and longitude as:
rg = √(X2g+Y2g) = 1.024 A.U.
λg = tan−1(Yg/Xg) = 345.3◦.

For the life of me I cannot get the inverse tan of (Yg/Xg) to give me 345.3. This is what I'm doing in python:
Xg= 0.990
Yg= -0.259
math.degrees(math.atan(Yg/Xg)) = -14.66091789971255

Any help greatly appreciated.

You miss the simple fact that a circle is 360° periodic. Your result is correct:
360°-14.66° = 345.34°.
Thus you miss that the arctan usually returns -180...+180° and you don't correct for the fact that the longitude is positive defined in the range of 0 ... 360°. Thus add +360° when your result is negative.


What is the ultimate fate of a cluster of galaxies? We're fairly aware clusters of galaxies drift apart due to space expansion, which will drive them out of each other's cosmic event horizon eventually, leaving them separate, alone, each a single "body" within its local observable universe with no means to communicate or influence other clusters.

The clusters themselves are held together by gravitational binding that keeps them from being internally torn apart by space expansion, at least assuming density of dark energy doesn't increase drastically eventually.

What will happen to the cluster though? At first I assumed its black holes will swallow all its available matter, and connect into one single, lonely black hole. But then, neither are black holes ultimately eternal, due to Hawking Radiation, nor do they swallow all the stars, supposedly most stars escaping their gravity well.

Well then, what current theories say a local cluster of galaxies will transform into as t→∞ ?

As you point out, in an accelerating Universe, large scale structures will become more and more isolated. So at a certain point you will have gravitationally bound superclusters separated by very large voids and less and less filamentary structures.

Once isolated, we can then study the dynamics of these independent superclusters. On very large time scales, galaxies will collide and merge. After collisions you will tend to form elliptical galaxies. So I think that you will end up with a big single elliptical galaxy.

Then we can be interested in the future of the stars of these galaxies. First we currently see that the star formation rate has already peaked some billion years ago. So as the number of galaxies collision that usually targets star formation, the star formation rate will slowly continue to decrease. Moreover, as heavy elements (all elements apart from hydrogen and helium) are formed in stars, the future generation of stars will have more and more heavy elements. From the nuclear point of view, the most stable element is iron, so on very very large time scales light elements will be converted into iron, whereas heavy elements will decay into iron.

This is a little speculative, but I think that on large time scales, more and more interstellar gas and stars will fall at the center of the gravitational potential of super elliptical galaxy. So as the density will increase at the center, you will mechanically form a heavier and heavier supermassive black hole. Another interesting point is that we do not currently know whether proton are stable or not. So on time scales larger than $10^{30}$ years (see more details here) protons may naturally decay into lighter subatomic particles.

So maybe as $t\rightarrow\infty$ we will end up with supermassive black holes and light particles. But as you mentioned, black holes will themselves slowly lose mass by Hawking radiation. In the same time, the expansion rate may have increased significantly leading ultimately to isolated particles in an expanding Universe.

Note: this is an hypothetical scenario and there are a lot of unknowns


What programming languages does SETI (Search for ExtraTerrestrial Intelligences) most commonly use for signal processing? Please give preference to those most widely used in the field, if possible.

A significant portion of SETI's work is done through a program known as SETI@home, which takes signals that have been received and processes them for responses, such as recurring patterns. Here is a page on how to download its source code. I grabbed the most recent nightly tarball and it turns out the program is coded in C++. I offer this as evidence that C++ is the most commonly used programming language used to process SETI's workload.


What type of star does theory predict should be the most massive? The most massive known stars are Wolf-Rayet stars. However, as Wolf-Rayet stars do not appear to be the first stage of any star's lifecycle, I infer that whatever these Wolf-Rayet stars used to be must have been even more massive. My guess is that the most massive stars are Wolf-Rayet because of some combination of 1) their luminosity and/or 2) the potential short life of prior stages of the star's evolution. Assuming this is correct, to what group would we expect the most massive stars to belong (even if we don't know of specific examples)?

There is no particular type. Yes Wolf Rayets were once more massive and have suffered extensive mass loss. Other examples of evolved massive stars are very luminous red supergiants and luminous blue variables (LBVs). The evolutionary tracks through these processes are somewhat poorly understood, as are the exact magnitudes and durations of the various mass loss phases.

However, they all started out as massive main sequence stars with very short hydrogen burning lifetimes. There are a number of examples of O3V main sequence stars in the Galaxy which probably have masses of around 100 Suns.

Hence the most massive stage of a massive star's life is (briefly) as a main sequence star of type early-O.


Will universe's lakes get boiled by the CMBR? From what I can understand, Cosmic Microwave Background Radiation (CMBR) is microwave now, but was initially visible light emitted long time ago.

The microwave is producing a wave that’s about 12 centimeters long.
Now CMBR has right now this (time changing) distribution (peak at 2mm):

By Quantum Doughnut - Own work

This diagram was created with gnuplot., Public Domain, Link

So it looks to me that in some time, when its wavelength falls at around 12cm), some of its energy should be transferred to water molecules the way it is in a microwave oven.

Is it the case? Will some of the CMBR energy actually transferred to water molecules?
Will this energy be too faint to matter at our scale?

Not really: microwaves heat mainly by using a lot of radiation, the particular wavelengths matters only for the ability to (somewhat) heat evenly.

You probably know of the Sun, a thing in the sky which shines most brightly in the optical, that is in wavelengths much shorter than the ones in a microwave. Yet it is clearly very capable of heating you and everything around you far above the ambient temperature of space.

So the wavelength of radiation is not crucial for heating (it matters a bit depending on the material), it is the amount of incoming radiation that matters. If the CMB could start to boil water because it was redshifted from 2 mm to 12 cm, it would need to be at such high intensity that water would already be boiling, because water absorbs more radiation of 2 mm wavelength than of 12 cm:


Does the cosmic microwave background change over time? Does the cosmic microwave background pattern change over time?  I would assume it is getting "cooler" as in more redshifted as time passes, but I am more interested if the pattern on the background, as depicted below changes.  I would expect we would see something like the cosmic microwave background retreating one light year (or possibly further due to inflation) every year, and it would be neat if we could make a "3D map" of the background as time passes (e.g. take an image of it every year).

http://lambda.gsfc.nasa.gov/product/map/dr5/map_images/wmap_planck/wmap_planck_ilc_300uK_med.png

The CMB patterns do indeed change over time, although statistically they remain the same, and although it will not be noticeable on human timescales.

The CMB we observe now comes from a thin shell with us in the center, and with a radius equal to the distance that the light has traveled from the Universe was 379,000 years old and until now. As time passes, we will receive CMB from a shell with an increasingly larger radius. As that light has traveled farther through space, it will, as you say, be more redshifted, or "cooler". But it will also have been emitted from more distant regions in the early Universe that, although statistically equivalent, simply will be other regions and hence look different.

The patterns that change the fastest are the smallest patterns we can observe. The angular resolution of the Planck satellite is 5-10 arcmin. Since the CMB comes from a redshift of ~1100, the angular diameter distance defining the physical distance spanned by a given angle — is ~13 Mpc, so 5 arcmin corresponds to a physical scale of roughly 19 kpc in physical coordinates, or 21 Mpc in comoving coordinates (that is, a structure spanning 5 arcmin today were ~19 kpc across at the time of emission, but have now expanded to a size of ~21 Mpc, with 1 Mpc = 1000 kpc = 3261 · 10³ lightyears).

Assuming an isotropic Universe, if the smallest observable parcels of gas were 19 kpc across perpendicular to our line of sight, they are also on average 19 kpc across along our line of sight.

So the question of how fast the CMB changes comes down to how much time did it take light to travel 19 kpc when the Universe was 379,000 years old. This is not simply 19 kpc divided by the speed of light, since the Universe expands as the light travels, but it's pretty close. Hence, it took light roughly 62,000 years to traverse such a patch.

Since we see events at redshift $z$ time dilated by a factor $1+z$, we will have to wait $62\,\mathrm{kyr}\times1100$, or roughly 70 million years (assuming that Planck will not get replaced by better instruments within that time which is, um, dubious).

So you're right, you could make a 3D image of the CMB, but since the patterns are much larger than a light-year, you don't have to take a new picture every year.


Can it be assumed that every star in an amateur astrophotograph has been cataloged? While I was looking at a wide-angle photograph of the Milky Way I wondered: Is it possible that some of the stars in the image have yet to be identified and/or cataloged?
My answer to this is no, but I would like confirmation or counterexamples for my reasoning:

The usual cited number of stars in the entire night sky that you can see with the naked eye (up to magnitude 6) is from 2500 to 10000 [1]
With a telescope and long-exposure/stacking on a camera, stars of magnitude 10 to 14 can be seen, perhaps even fainter
The Gaia star catalog (although not fully released [2]) contains data for 1.7 billion stars brighter than magnitude 20.7

Assuming that a telescope/camera combination can detect 100 times as many stars than the naked eye can in the entire night sky, if we calculate the proportion of visible to cataloged stars: $$ \frac{n_{visible}}{n_{cataloged}} = \frac{100 \cdot 10e3}{1.7e9} \approx 0.05\% $$
I would consider this a low enough value to conclude that every star above a certain magnitude has been detected and cataloged. Even seeing a thousand times more stars than the naked eye would give you 0.5% - a negligible proportion of the star catalog.
Is this justifiable? Can these assumptions be challenged?
[1] - https://earthsky.org/astronomy-essentials/how-many-stars-could-you-see-on-a-clear-moonless-night
[2] - https://irsa.ipac.caltech.edu/Missions/gaia.html

Variable stars might challenge this.  A distant Mira type variable or recurrent nova could have been at its minimum when the (for example) Gaia catalogue was being assembled, but appear in an amateur photographs.
These stars undergo very large variation in brightness.  Mira can increase in brightness by 8 magnitudes, so it is not inconceivable that an uncatalogued Mira type variable could be found in an amateur photograph. Yet have been omitted from other catalogues by bad luck.
Recurrent novae are also "variable stars". They also undergo large changes in brightness. As they spend many years at minimum brightness and then suddenly flare, there is a greater chance that they could have been missed in the Gaia and other catalogues.
However, these are rather special cases.  In general, any star that an amateur could image with reasonable equipment will have been photographed, identified and catalogued by one of the sky surveys. Automated image processing makes this possible. Many of the things that amateurs did in the 1980s and before are now done by robots and computers.
Supernovae have often been found in other galaxies by amateurs, with no catalogued progenitor (though supernovae might not be considered a "star")


Are there any studies that prove/disprove the Oscillating Universe Theory? Is there any evidence that supports or disproves the Oscillating Universe theory? What kind of observations could provide this evidence?

The main obstacle with the Oscillating Universe model is the pesky second law of thermodynamics — that entropy always increases within an isolated system, and never the reverse. Thus, as time progresses, entropy increases, until it (as far as we know) reaches its maximum — the heat death or Big Rip.

Consider the Big Bang, when entropy was at its minimum, or at least incredibly low. As the Universe evolved, entropy increased, as dictated by the second law. If the Big Crunch were to happen, we'd see a decrease in entropy as conditions in the Universe recede back to how they were at $t = 0$. As we've stated, the Universe's entropy can never decrease.

But say we ignore the whole "Big Crunch problem" and instead focus on the cycles. As stated before, the entropy always increases with time. I'll let Baum & Frampton (2006) explain the problem:

  ...one principal obstacle is the second law of
  thermodynamics which dictates that the entropy increases from cycle to cycle. If the cycles thereby become longer, extrapolation into the past will lead back to an initial singularity again, thus removing the motivation to consider an oscillatory universe in the first place.

As they described, the entropy in the Universe would increase with each cycle. At this point we realize that each cycle in the Universe's history would be different — and if we extrapolated back into the past, we would find that at some point, entropy must have been at its minimum (the Big Bang!) The fact that we would be back to the Big Bang would remove any need for an Oscillating Universe hypothesis — which tries to explain what preceded the Big Bang.

That problem led many astrophysicists to abandon the Oscillating Universe hypothesis. But nonetheless, the idea that the Universe is reborn is quite attractive, and we still know very little about cosmology — including dark energy, which we think has a vital role in the Universe. Some cosmologists continue to research the Oscillating Universe model in hopes that, as we learn more about dark energy and the Universe, we may be able to avoid the problems with the second law of thermodynamics.


Of the stars in the Gaia / Hipparchos catalogs, which one is the most isolated? This is just a matter of curiosity; I was thinking about a scifi story.

By "Isolated", I mean that if for every star (or binary) we found the distance to its closest neighboring star, then which one would be the furthest from its closest neighbor?

From a comment by barrycarter:

  In our galaxy, it might be CX330, which is 1000 light years from any star-forming region, much less any star. However, the article describes it as "Loneliest Young Star", so there may be older stars that are even more isolated. This source contains the phrase "CX330 [...] has been described as the galaxy's loneliest star.", but this may just be bad reporting.


Why not send a Gaia-like mission to Mars? This answer to the question why we didn't send Gaia to Neptune's orbit raises the question of why we don't send a Gaia-like mission to the orbit of Mars. It seems like it doesn't have the problems mentioned in the answers, and it would make it possible to measure parallax for stars 50% further.

orbital speed: waiting 6 extra months to get better precision seems like a better trade off than waiting 9 years to get the same precision as Gaia;
partial coverage: after two years, the entire sky could be covered. Far from the 168 years for Neptune;
telemetry: sending back data from a Gaia-like mission can't be that much harder than sending back data from any other of the tens of missions to Mars;
power: the solar panels don't need to be 900 times bigger;
radiative environment: contrary to Jupiter, Mars doesn't have a magnetosphere, so the spacecraft wouldn't be disturbed by it.

The panels need to be about 2.3 times bigger to generate the same power.

The telemetry requirements are limited by distance to some power, assuming that the communications beam cannot be infinitely narrow. Mars is much further away than the L2 point - somewhere between a factor of 50 and 250.

It wouldn't necessarily make it possible to measure parallaxes 50% further. Gaia works by taking approximately 70 (on average) measurements of a stellar position over 5-6 years. This amounts to sampling the 2 au baseline about 10 times, thus reduing the error bar on a single baseline parallax by about $\sqrt{10}$ and making the "equivalent baseline" about 7 au.

To get similar precision with Mars you would need to be able to complete the 3 au baseline about 5 times, which would still take 5 years. So you don't obviously win in precision terms.

Overall I think it is the telemetry problem that is the real killer. Gaia is currently telemetry limited - that is why there is a limit on the number of stars it will measure and on the number of stars it will get spectroscopy for.


Which kind of strategy should we take to discover Planet 9 and other KBOs? What are the differences between the discoveries of Sedna, 2012vp 113, and the supposed Planet 9?

All need deep infrared surveys at the ecliptic plane, but P9 needs a much deeper survey?

This question already discusses the challenges of discovering planet 9

The main problem of the hypothetical planet 9 as compared to Sedna and 2012 VP113 is:

Sedna and 2012 VP113 have orbits that reach far out but currently they are near their perihelion so it was possible to observe them in the visible light spectrum. Planet 9's orbit is far out or at least it is currently near its aphelion. Otherwise it probably would have already been discovered.

Also the planet's orbit is probably tilted towards the ecliptic and it moves very slowly compared to the stars in the background.

At this position the maximum of its radiation would be emitted in the far infrared and it moves before the background of the Milky Way.

So to summarize:

We would need a long term observation with an far-infrared capable, spaceborne, telescope (because our atmosphere absorbs far infrared radiation) which has to include the background of the Milky Way. The telescope has to be able to detect faint infrared sources. We could be lucky and planet 9 just happens to pass in front of a star as we are observing it. Then we could get our first hint with an optical telescope. But that's pure luck and we can't plan towards that.

WISE already tried to find planet 9 but did not find it. The James Webb Telescope may be our sole instrument capable of discovering planet 9 directly. But it's not built to permanently survey an area of the sky although it could confirm a finding.

Gaia could probably detect planet 9 if it happens to pass in front a star.


Where do we have it from that the Moon is migrating away from Earth? I know Neil Armstrong placed a mirror on the Moon and people shoot lasers there which get reflected, by this, measuring the time the light is needing, they can conclude the current distance of the Moon. But the Moon is once in a time moving away from the Earth (when annular eclipses are possible) and once in a time getting closer (when total eclipses become possible). Sometimes we hear of records like "the Moon is as close as it wasn't for decades". I remember about one year ago I saw a very close and bright full moon, almost outshining most stars.
So how is it concluded that the Moon would constantly move away from the Earth? Is it just based on the physical prediction of celestial mechanics, like calculation that a day on Earth would get longer and the Moon's orbit slower?

There are, I think, at least four parts to this argument: the first being the theoretical argument that ties it all together and the remainder being observational evidence for the Moon's orbit increasing in size.
1. The underlying theoretical argument. This, of course, is the idea that tidal braking causes the Earth to slow down in its rotation and the Moon to move further out in its orbit. (Once upon a time this also caused the Moon to slow its rotation, until it became tidally locked, with its rotation period = its orbital period.) This predicts two things: the Moon should be gradually getting further away from the Earth over time; and 2) The Earth should be slowing down in its rotation (days getting longer) over time.
There are at least three sets of evidence supporting this.
1. The increasing distance of the Moon, as derived from lunar laser ranging. The key point here is that it's the average distance that's increasing. Since the Moon's orbit is elliptical, the distance varies over the course of one lunar orbit, and there are additional variations introduced by the gravitational influence of the Sun, other planets, the not-perfectly-spherical shapes of the Earth and Moon, etc. What this means is that if you trace the Moon's distance over time, you will see it increasing and decreasing, but you will also see that the average distance is gradually getting larger. (And this is what has been measured with the laser ranging experiments; e.g., this 1994 article by Dickey et al. which found that semi-major axis of the Moon's orbit was increasing by about 3.8 cm/year.)
2. The slowing down of the Earth's rotation. As required by basic physics, if the Moon is getting further out in its orbit, and thus gaining angular momentum, there has to be a compensating loss of angular momentum, and this happens via the Earth gradually spinning more slowly. This has been measured in at least two ways:
A. Timing of historical lunar and solar eclipses. There are Chinese records of eclipses going back to roughly 400 BC, and Babylonian records back to almost 800 BC, as well as more recent Greek, Arab, and European records. These can be used to estimate changes in the length of the day, in two ways. First, some records include an approximate time of day, so we can run our calculations backwards and figure when an eclipse visible at, say, Babylon in 200 BC should have occurred. And even when we only know the day (and location) of an eclipse, we can predict where it should have been visible. As explained by this article describing a 2016 study by Stephenson et al.: "The oldest event in the catalog, a total solar eclipse that occurred in 720 B.C.E., was observed by astronomers at a site in Babylon (now modern-day Iraq). But, working backward, today’s astronomers would have predicted that the eclipse should have been seen a quarter of a world away, somewhere in the western Atlantic Ocean. The discrepancy means Earth’s rotation has gradually slowed since the 8th century B.C.E."
B. Geological and paleontological measurements of the length of the day.* It turns out there are some geological records which can be used to derive the number of days in a lunar cycle, or the number of days in a year, millions or hundreds of millions of years in the past.
One way is via the analysis of "tidal rhythmites", which record alternating effects of ocean tides. Since tides have cycles of twice per day (high and low tides) due to the Earth's rotation and twice per lunar orbit (spring and neap tides), you can use the combination to work out how many days there were in a lunar orbit. There can also be yearly variations, which allows you to work out how many days there were in a year when the tidal rhythmites were formed.
Another way is by studying certain fossils, where the growth of part of the organism is recorded in both daily and annual variations: for example, the growth of a shell can vary over the course of a day, but also over the course of a year (e.g., more growth in summer, less in winter). Putting this together allows you to figure out how many days there were in a year when the organism was alive. A fascinating recent example is this careful study of a 70-million-year-old fossil of a rudist (an extinct type of mollusc), which reveals that a year had 372 days back then, so that each day was 23.5 hours long instead of 24 hours.


How do I calculate the velocity dispersion of a galaxy given an integral field spectrograph? I couldn't find on the web how to get the velocity dispersion and velocity maps of galaxies from the 3D data cube that I get from the integral field spectrograph.

If the 3D data cube is sufficiently highly resolved, try to find Doppler shifts in the spectra by looking for typical absorption/emission lines. Doppler shifts correspond to radial velocities. Calculate the mean radial velocity, and e.g. the standard deviation of the radial velocities, as velocity dispersion.

The velocity dispersion of galaxies leads to a widening of spectral lines in comparison to a reference star. A couple of methods are discussed in this paper, mostly some kind of Fourier fitting, but direct fitting is used, too.

Redshifts may contain a gravitational component.

There exist data reduction software tools.

A paper describing data reduction and velocity analysis using the XOASIS tool.


Could supermassive black holes form in dwarf galaxies? Supermassive black holes often exist in large galaxies (e.g. the Milky Way, Andromeda). They make up a fair portion of the galaxy's mass. Is it possible for a supermassive black hole to form in a smaller galaxy - a dwarf galaxy (i.e. the Sagittarius Dwarf)?

The prediction was that dwarf galaxies have SMBHs, but with smaller masses than regular galaxies.  This was based on the idea that regular galaxies were built from dwarf galaxies and the BHs need to have started growing early on.  Also the correlation between bulge mass and BH mass requires the BHs to come from the building blocks.  A recent paper at arxiv detects 28 AGN in dwarf galaxies with BHs in the $10^3 - 10^4 M_{\odot}$ mass range.


Current topics on Radio Astronomy and looking for advice Background and Question: I'm an undergraduate student interested on Radio Astronomy. I consider myself enthusiastic about it and I project myself following this "path" so I'd like, apologizing myself if this is a naive question, to ask you, more experienced astronomers, what are the current topics being part of research in Radio Astronomy.
My thoughts:

I know Cosmology and CMB Measurements are something that will keep its route for a long time as huge projects continue being developed.
Also I'd say Radio-loud Galaxies are something that Radio Astronomers are putting lots of work into.
Interferometric techniques, data reduction algorithms and image synthesis.
Astro-Chemistry and the study of spectral lines using data cubes such as provided by ALMA.

More questions:

Am I wrong with any of the topics listed? What else is being done now?
How do you see this in the next 5 - 10 - 20 years ?
Considering I want to work on my thesis project this year, what would you recommend (or to other undergrad. students) ? What would be a "good route" to follow after graduating? (my grades to this date are at ~ 80%).

Here are some examples that you can read about within Astronomy SE and Space SE:
Time Domain Astronomy (link)
e.g. LOFAR and (as it turns out) CHIME. Think better FRB detection gravitational lensing effects on pulsar timing (needs several questions of its own)

Why did it take five years to "figure out" how to use astrometric calibration sources to deblur LOFAR images?
Open dataset from antenna array

and

How tightly clustered are the CHIME-detected FRB repeats?
What are the challenges for the building and data analysis of the CHIME telescope?
How does Canada's radiotelescope CHIME manage polarization-dependent apodization of the fast f/0.25 optics?
What will succeed the Arecibo Observatory?

also

What exactly is interplanetary scintillation; what was the Interplanetary Scintillation Array looking for? Did it successfully observe any?

Space based, and Satellite VLBI

Far Side of the Moon stuff, e.g. Chang'e 4: The NCLE on the orbiter and the LFS on the lander work in synergy performing low-frequency (0.1–80 MHz) radio astronomical observations.
What can be learned from low frequency radio astronomy available outside of Earth's ionosphere?
Has VLBI been done using any space-based receivers besides Spektr-R? (Space SE)
What's the status and timeline for Millimetron? (Russia's 10m Deployable Antenna cooled to 6 K Earth-Space VLBI) (Space SE)
What are Spectr-R's major contributions to radio astronomy that could not have been done from Earth? (Space SE)
How is Spektr-R doing these days? (Space SE)
Has VLBI been done using any space-based receivers besides Spektr-R? (Space SE)
What kind of ground-based radio astronomy is NASA's DSN used for? Who are the PIs? (Space SE)
Are images of exoplanets' surfaces technically possible? (Space SE)
Could a radio telescope orbit Earth-Moon L2 so that it is in the Moon's radio shadow?
Rotating liquid mirror on the Moon: this answer to Why can't we build a huge stationary optical telescope inside a depression similar to the FAST?

Fancy, complicated focal plane arrays and techniques

How would a Fast Fourier Transform Telescope work without a mirror, dish or lenses?
How do ASKAP's focal plane phased array feeds interact with the entire array phasing?
What is the highest granularity focal-plane array on a dish radio telescope? Or is this the ONLY ONE?
Would Adaptive Optics be Useful in Radio Astronomy?
Has radio astronomy ever been done on objects that appear very close to the Moon? Is this avoided?

Radar Astronomy (link)

What are recent discoveries with Earth-based radar telescopes?
What will succeed the Arecibo Observatory?
What causes "North-South ambiguity" when doppler radar imaging a planet surface equator on?
https://space.stackexchange.com/q/22493/12102
What is the physical geometry of this apparent "eclipse" of a tiny moon of Asteroid Florence?
Farthest distance to a solar system object that's been measured by radar? (Space SE)
How did Arecibo detect methane lakes on Titan, and image Saturn's rings? (Space SE)
Why is Saturn invisible in this radar image of its rings? (Space SE)
Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit? (Space SE, not exactly Astronomy but pretty cool from an instrumentation and signals point of view)
Why are Titan's lakes “black” in radar images rather than transparent? (Space SE)
Is delay-doppler radar imaging of NEO asteroids possible only if it spins fast enough? (Space SE)
Separating fact from fiction, how to get from this radar image to this GIF? (Space SE)


Why don't we use amateur astronomers' telescopes to create a huge interferometer? Some telescopes in space have been proposed to function as an interferometer.
Being placed several hundreds or thousands of kilometers apart they could actual image exoplanets directly. To my knowledge none of these space telescopes have yet been deployed.
So I was wondering why we don't connect all the several thousand amateur telescopes on Earth together to create a huge interferometer?
That sure would require some efforts (especially software to coordinate it all) but compared to sending telescopes into space it seems outright simple.
Are amateur telescopes simply not strong enough, even when linked together? What would be the best we could achieve?

You can't make an interferometer by combining photographs. You need to combine the light sources so that the light waves can "interfere", which means you need to have not only the intensity of the light, but also the phase of the wavelength. A CCD only detects the intensity.
You also need to position your telescopes to sub-wavelength accuracy. For Radio telescopes, with long wavelengths, it is possible to detect and then combine the radio signals from several telescopes electronically. That's not possible with light. Visible wavelength interferometers capture the light and combine it as light. It requires very high precision.
Finally, we still have the atmosphere to contend with. Amateur observers know that, beyond a certain size (about 10in), larger telescopes don't have greater resolution, because of the blurring due to the atmosphere1. Bigger mirrors mean more light, mean dimmer objects can be seen, but no more detail is visible. There are ways of getting round this, (Lucky imaging) but these are not suited to interferometery.
So since placing amateur telescopes at nanometer accurate positions, and linking their output with single strand optical fibre is beyond the budget of amateurs, and since the results would be limited by the distortion of the atmosphere, this is not normally done.
1Large observatories like the Very Large Telescope use Adaptive optics to obtain dramatically improved resolution through Earth's atmosphere but that kind of technology is sophisticated and out of reach of amateur instruments.


How does a neutron star emitting magnetic-dipole radiation lose angular momentum? Is the ultimate reason that the neutron star loses angular momentum that the magnetic-dipole radiation is not spherically symmetric and photons have momentum?

Just for the loss of angular momentum, particles emitted from the neutron star surface are not necessary, right?

Radiation carries a momentum per unit volume given by
$$ \vec{p} = \epsilon_0 \vec{E} \times \vec{B}$$

It therefore carries an angular momentum of
$$ \vec{L} = \vec{r}\times \vec{p} = \epsilon_0 \vec{r} \times (\vec{E}\times \vec{B})$$

For plane-polarised transverse electromagnetic waves propagating in the $\hat{r}$ direction, then $\vec{L}=0$. However, if the axis of a magnetic dipole is inclined to the rotation axis then the radiation is not a single pure transverse wave; the dipole moment is the sum of two oscillating magnetic dipole moments that are perpendicular and oscillate $\pi/2$ out of phase. These produce two transverse waves, with arbitrary amplitudes, perpendicular polarisations and a $\pi/2$ phase shift between them - a.k.a. elliptically polarised light. Elliptically polarised light carries angular momentum.


Meaningful theories of the shape of the universe Normally, when people say "the universe is infinite" they generally mean something like "the observable universe is locally flat, it is easy to assume that it is flat way beyond the observable universe, I don't know what lies beyond that, I won't talk about that, that is enough for practical purposes" (or any other proposition implying infinity in any direction and assumed for simplicity's sake while actually focussing on a small region of the space, the observable universe). So, "the universe is infinite" does not seriously mean that physical space goes on forever because this simply makes no sense at all, "infinite" is not a number, it is not a magnitude, it is not a property of real things (if you disagree with this premise please explain how the notion of infinity can be seriously applied to any direction of physical space, i.e. how you can seriously think to extend, sic et simpliciter, such a property of your mathematical model to the real world). (Personally I don't think this should be counted as a theory of the shape of the universe at all).

On the contrary, "the universe is finite and unbounded, compact, closed in all directions" is indeed a meaningful and in principle acceptable theory because it can be thought, described, discussed for what it actually implies. Of course, we don't know what lies beyond the observable universe but the idea of a compact universe makes sense and it seems to me this is the only meaningful explanation currently available of what "could" be (it is not necessarily the right, true explanation, of course).

Is there any other meaningful alternative (which can be thought, described, discussed)? Is there any alternative to the dichotomy "finite universe" vs "infinite universe"? Is there anybody out there working on models that overcome this dichotomy?

I disagree with your premise. First, the universe being infinite does not imply anything at all about its spatial curvature without additional assumptions. Second, even with the usual cosmological assumptions of large-scale homogeneity and isotropy, being infinite would not imply spatial flatness, as negative curvature would still be possible.

  On the contrary, "the universe is finite and unbounded, compact, closed in all directions" is indeed a meaningful and acceptable theory.

This this alternative still makes grand assumptions about the structure of the universe beyond what we can observe, I don't see why you attribute more 'meaning' or 'acceptability' to it. At the end of the day, all we really know is that the bit of the universe around us is very close to being flat. Any statement whatsoever about the larger structure of the universe is going to introduce some assumptions not backed by observations, and you haven't provided much guidance to what kind are 'meaningful and acceptable'.

  Is there any other meaningful alternative?

You seem to be using that word in an idiosyncratic sense I'm not familiar with. In any case, you're mistaken about what "the universe is infinite" implies.


What if a white dwarf is less massive than her partner? For example: a system with a 0.2 solar mass white dwarf orbiting a 0.25 solar mass red dwarf. Does the red dwarf steals mass from the white dwarf or vice versa.

If they are close enough to exchange mass, the Red dwarf will always lose mass to the white dwarf.  That doesn't mean there won't be some exchange going back the other way, but the white dwarf will always win the exchange rate and it won't even be close.   It makes no difference whether it's heavier or lighter.   What matters primarily, is how tightly held it's outer mass is to each star and how strong the tidal forces are.

The white dwarf's mass is very tightly bound to it by gravity and being so small, it's not going to experience strong tidal variation.  The red dwarf's outer layers are far less gravitationally bound to it, and being much larger, it experiences greater tidal bulging.

How much mass is exchanged would depend on how close they are, but around any star, and if they are close enough, the white dwarf will always steal mass from the main sequence star.

A curious thing happens with main sequence stars.   As their mass goes up, their surface gravity goes down.   A star with 0.25 times the mass of our sun, extrapolating from this chart would have a radius about 32-33% of our Sun, and being 1/3rd the size, using the inverse square rule, it's surface gravity would be about 9/4ths or 2.25 times the gravity on the surface of our sun - roughly speaking.  There would be other factors like rotation rate and solar storms, and, as mentioned above, how close they are, but there's no way that a main sequence star of any size could steal more mass than it gives up from a densely packed White dwarf.


When were the horizontal and equatorial celestial coordinate systems invented? When were the Alt-az system and the RA-dec system first used?

I remember answering a question on the origin of declination last year (Why is declination positive in the northern hemisphere?) which led me to discover this paper on the origin of celestial co-ordinate systems. In it they seem to indicate that the ancient Greeks were the first to use Right Ascension to describe locations in the sky around 300 B.C.
Whereas Alt-Az co-ordinates (or variants of) have been around much longer because the horizon is an easy point of reference to start from. The first documented use comes from Babylonian times but the ancient Egyptians are also thought to have used a similar system as far back as 2000 B.C.

Reference paper: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1942PASP...54...77W&db_key=AST


Could a planet form without a molten state? Based on what I know, the Earth accreted out of the solar nebula, had a period when it was entirely molten, and then slowly cooled. Could a planet form without ever passing through a molten phase?

If you accrete a mass of $M$ the total binding energy will be $3GM^2/5R$, which will at least partially turn into heat. If the melting temperature is $T$ and specific heat capacity $C$ we get the condition $$\frac{3GM^2}{5R}=CTM,$$ or $M/R<5CT/3G$. So if we use a low silicate rock melting point of 600 C and basalt thermal capacity then $M/R<1.8313\cdot 10^{16}$ kg/m, while the upper range 1200 C gives $3.0899\cdot 10^{16}$ kg/m. So if we consider a basalt planetoid it will start to have some melt when cohering into one ball of radius larger than 1000 km. So, not much of a planet.


Oscillation in tilt of the solar system I know that the solar system is tilted 62.6° to the plane of the galaxy.  I'm curious whether this angle changes over time, and what are the extents and time frame of any such oscillations if they exist?

Upon reflection, I'm assuming that since such an oscillation would require a change in the overall angular momentum of the entire solar system, and that would necessitate a collective re-orientation of the orbit of every planet, dwarf planet, SSSB, etc. related to the Sun that it's unlikely that any such mechanism exists?

The tidal field of the Galaxy does lead to the oscillation of the plane of very wide binaries.  The mechanism of this oscillation is identical to the Kozai-Lidov mechanism (the only difference is that in the case of KL oscillations the tidal field is generated by the averaged orbit of a tertiary stellar companion).

However, if you run the numbers, the timescale for these oscillations is extremely long for any of the planets (I forget how long exactly, but much longer than the age of the Universe).  It's somewhat shorter for comets in the Oort Cloud, only a few billion years, because their orbits are much larger and so would tend to experience larger a larger tidal effect.


Hydrogen burning vs Hydrogen fusing Does the term "Hydrogen burning" mean the same as "Hydrogen fusing" in astronomy? If not, then what is the product of "Hydrogen burning"? Assume the product of "Hydrogen fusing" is helium.

In stellar astrophysics, "burning" means nuclear fusion, not chemical combustion. So a star burns hydrogen to helium. (Incidentally, normal chemical burning of hydrogen in air produces water).
This might seem to be confusing terminology, but it's not an issue in practice because the regions in stars where fusion takes place (the core, and shells surrounding the core, in big old stars) are far too hot for molecules to exist, so chemical processes simply cannot occur there.

Hydrogen fusion in a star core is not a single nuclear reaction. There are two (known) sets of reactions: the proton–proton chain, and the catalytic CNO (carbon-nitrogen-oxygen) cycle. From Wikipedia:

The proton–proton chain, also commonly referred to as the p-p chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium.
It dominates in stars with masses less than or equal to that of the Sun, whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun.

Hydrogen burning is the main way that young stars produce heat and light. (Older stars can burn heavier elements, if they have sufficient mass). But hydrogen burning is also important because it's the main way that neutrons are produced: the burning of heavier elements generally does not change the number of neutrons. (Neutrons are also created when a heavy star collapses into a neutron star).
In a Sun-like star, hydrogen burning is a very slow process. It takes a lot of heat and pressure to overcome the electrical repulsion between two protons and fuse them into a diproton. However, the diproton is extremely unstable, with a half-life of much less than a nanosecond. Usually, a diproton just splits up into two protons, but sometimes one of its protons transforms into a neutron, converting the diproton to a deuteron (a deuterium nucleus). The probability of diproton to deuterium conversion is very small, in the order of $10^{-26}$. Various p-p chain reactions convert deuterons to helium nuclei.
The slowness of deuterium production is a good thing: it means our Sun will continue to burn hydrogen for billions of years. OTOH, it also means plain hydrogen will never be a practical fuel for an artificial fusion reactor.
The density in the Sun's core is ~150 g/cm³, which is much greater than any terrestrial material (eg, the density of lead is only 11.3 g/cm³). So a cubic metre of solar core matter has a mass around 150 metric tons. But the rate of hydrogen fusion is so slow that it only produces around 276.5 watts of power, which is similar to the heat production of a cubic metre of compost, as mentioned on Wikipedia. Of course, a 1 m³ compost heap won't keep pumping out heat for billions of years. ;)

Wikipedia has a good collection of articles on the various stellar nuclear fusion processes, including:

The triple-alpha process, which burns helium to carbon.
The carbon-burning process, which converts carbon to oxygen, neon, sodium, and magnesium.
The neon-burning process
The oxygen-burning process
The alpha ladder
The silicon-burning process


Strip Brightness in a Spherical System My question concerns the following problem, published in Galactic Dynamics by Binney and Tremaine and based on a work by Schwarzschild.
Specifically, I am having difficulties in deriving the given result in part (c), which I suppose is due to the confusion surrounding the definition of strip brightness. Could someone please explain, with the aid of a diagram if possible, how $x$ is related to the projection, $R$, of the  radius $r$ onto the plane of the sky? I am having trouble visualising what $x$ is measuring here.

It is the luminosity in a straight strip like this:

(Credit: West 29 / CC BY-SA from here)
where the center of the strip passes a distance $x$ from the center of the circle at closest approach.  So starting from the center and moving out, each successive strip would have a progressively larger value of $x$, but $x$ is a constant for a given strip (with $x = 0$ for the center strip).
The only differences from the picture are that (a) the strips have an infinitesimal width $dx$, and (b) they are infinite in length rather than the finite circle shown here.


Retrograde motion of planets: reason and time? As beginner, I would like to understand how planets exhibit retrograde motion.
When does the retrograde motion start for a planet like Mars and how long (time limit) planets will appear to be in such motion?
Any good URL or simple explanation will be helpful.

The Wikipedia article on apparent retrograde motion seems to have a table containing exactly what you are looking for:

The apparent motion is observed when the projection of a planet is compared to the star background. There are no real change in the planetary orbits, but when for example the Earth catches up with Mars, it seems to move retrograde relative to the background because of our own change of view. (Like how it appears that you are scrolling further down on this page when the background moves upwards.)


How to convert celestial Coordinates with a radius, to max and min celestial coordinates? I am writing a piece of python code to convert right ascension and declination with a given radius to minimum and maximum right ascension and declination. I firstly can't understand why the min and max is not always just the given coordinates +- the radius, and secondly how I would write a piece of code to make this conversion. Cheers.

If you were working with a rectangular coordinate system as shown in the figure below (that also had no bounds), then it is correct that the right ascension would be $RA=RA_p\pm d$ (points 1 and 3) and the declination would be $Dec=Dec_p \pm d$ (points 2 and 4) where d is the radius of the circle.

The sky is spherical, so the lines of RA are not a constant separation. (It also has limits such as declination less than $\pm 90$.) Therefore you need to use an equation for spherical trigonometry. The angle d between two points i and j is

$$\cos(d) = \sin(Dec_i)\sin(Dec_j) + \cos(Dec_i)\cos(Dec_j)\cos(RA_i - RA_j)$$

For points 1 and 3 on the circle, $Dec_i=Dec_j=Dec_p$ in the above equation, and you can solve for the difference in right ascension $RA_i - RA_j$ to be added to or subtracted from $RA_p$. For points 2 and 4, $RA_i=RA_j=RA_p$ and the above equation simplifies to $Dec=Dec_p \pm d$. Of course, keep in mind that declination is between -90 and +90. For any other point on the circle, you can assume either an RA or Dec (within the bounds of the above 4 points) and solve for the other unknown.


What abbreviations, letters, or symbols do we use to denote highly degenerate stars? We have OBAFGKM to denote the strength of hydrogen lines.

Apparently we added S, N, C, and the W classes when we learned of new kinds of stars.

We use T, L, and Y to denote brown dwarfs.

The D classes refer to electron-degenerate stars (white dwarfs).

I've looked around quite a bit and I haven't seen anything regarding neutron stars and black holes. Does this mean there's no common symbol or abbreviation? What do they use in star catalogs? (I've never found a catalog with stars of all degeneracies in it.)

Neutron stars and black holes do not have assigned spectral types since they do not have a measurable optical/IR spectrum - which is the basis for assigning a spectral type.

There are many classifications for systems containing black holes and neutron stars. They are not related to the letter-based spectral types of "normal" stars or white dwarfs, but more to do with their X-ray, radio or inferred physical properties (e.g low- or high-mass X-ray binary systems).


How can you determine the date using nothing but a star chart from the day and the time? I'm doing a science olympiad event called Reach for the Stars, and one question that I've encountered provides a star chart and time of day, and you must figure out what month of the year it is from. The strategy that I developed includes figuring out which direction is west (usually a star chart from the night and the Sun sets in the west), then going in that direction estimating which zodiac constellation in Sun is in (and in turn approximately what month it is).
However, I realize that this method would not work if it was taken late at night, much after the Sun sets, because although I am fairly sure that each hour the sky rotates about 15 degrees, I can't estimate what constellation the Sun would be in.
Is there a more definite way to figure this out?
Edit: The time in the example chart is 8:30 PM in the time zone of the observer
Example star chart:

First determine the North (South) direction by drawing a line from the center through Polaris (Sigma Octantis). The other principal directions follow from that.
You need to obtain the coordinates of a well-recognized star in equatorial coordinates (right ascension and declination), not too near the celestial pole. Given your chart, it's no problem as it shows the sky for a location near or even exactly at the equator.
As it's a question from a competition I will only sketch the path briefly:  The stellar coordinates allows you to calculate the local hour angle of it and from that the sidereal time. You then have to find the date where the sidereal time matches your given mean solar time. Depending on accuracy and possibly further details given in the task, you will also have to consider the equation of time.
OLD ANSWER where the time was not yet given:
A star chart is not unique, it's ambiguous for date vs. time. Just as you state: it could be from 22h today. Or it could be the sky at 21h in 14 days or 20h in a month. Generally every half month you have to remove one hour from the clock and it is identically valid.
Obviously a star chart is of limited use for times where the Sun is over the horizon or for the early twilight time where you still don't see any stars.
So what you can do is figure out for what date / time combinations the star chart is valid. In year books usually several of those are given for the star charts printed. Assuming that people usually observe in the early evening hours, one can get an estimate for the intended month though (stating the observation time).
Thus practically I'd use a programme like Stellarium and try find a month where for the chosen observation hour the night sky looks like shown. Mind also it is WAY more accurate to find North (South) direction in a star chart by drawing a line from the centre through Polaris (Sigma Octantis). All other directions follow.
You can break ambiguity of time and date, if your chart shows solar system objects like planets which change their position with respect to the stars.


Difference between luminosity, luminous flux, and radiant flux? I've looked this up, and was wondering the differences between them. Luminosity is the total energy emitted by a source, in watts. However, I'm confused about luminous flux and radiant flux. Since radiant flux is the total energy emitted by an object, would that make luminosity the same as radiant flux? Then how does luminous flux relate to luminosity? Is luminous flux just radiant flux, but only showing the visible light? And this website, https://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/inversesquare.html, says lumens are a unit of luminosity, but how if lumens are candela times steradians?

Photometric units can often seem complex at first sight.

Luminosity

Luminosity refers to the total energy released by an object per unit time. This quantity is dimensionally equivalent to power, and is therefore measured in joules per second, or watts $(\mathrm{W})$. Another common unit is the solar luminosity $\mathrm{L}_{\odot} = 3.846 \times 10^{26}$ $\mathrm{W}$.

Luminous Intensity

Luminous intensity refers to the wavelength-weighted power emitted by a source in a particular direction. Wavelength-weighted is important here. Because the human eye is more sensitive to some wavelengths than others, this weighting is designed to set a standard for perception. This is set at $555$ $\mathrm{nm}$, the wavelength to which the human eye is most sensitive. This unit is called the candela $(\mathrm{cd})$, and is equivalent to a source emitting at $555$ $\mathrm{nm}$ with a power of $\frac{1}{683}$ $\mathrm{W}\cdot \mathrm{sr}^{-1}$. Essentially this is taking into account the fact that concentrating the same amount of light into a smaller solid angle will cause it to appear brighter. Finding the luminous intensity of a source with multiple wavelengths is a little more complicated. Let us define the following:

$I_v$ is the luminous intensity in candelas,

$I_e$ is the radiant intensity in watts per steradian, and

$\bar{y}(\lambda)$ is the standard luminosity function, which does the wavelength-weighting.

Then we can define the luminosity intensity of a multi-wavelength source to be:

$I_v = 683 \int_{0}^{\infty} \bar{y}(\lambda) \cdot \frac{dI_e(\lambda)}{d\lambda}d\lambda$

Luminous Flux

The luminous flux is equivalent to the total wavelength-weighted power emitted by a source. It is similar to luminosity. It is measured in the lumen $\mathrm{lm}$, which is equivalent to the candela-steradian $(\mathrm{cd}\cdot\mathrm{sr})$. One can imagine this as the human-useful amount of light emitted by a source.

Radiometric Units

The radiometric units are the same, except they refer to the true emitted power, with no wavelength-weighting.


How easy is it to mine water on Ceres? It has been suggested by some futuristic or sci fi leaning thinkers, that Ceres' surface might be mined for water to support human exploration and settlement of space. But NASA's Dawn mission and other observations show that it is very dark and seems to consist of "hydrated minerals" in an environment warm enough to make water ice sublimate. Does this mean that mining water on Ceres is really hard? Requiring deep drilling or excavating through thick hard surface layers before useful water ice is encountered.

Could a lander on Ceres just heat up the surface and collect water sublimating from it?

Comments converted to community wiki

Ceres' density (2.08 g/cc) suggests it has abundant water/ice below its presumably mostly dry surface. That and the shiny spot (salts left after subsurface water has sublimated) suggests it wouldn't too difficult. Low gravity, keep the dig in shade to prevent the ice from sublimating in direct sunlight.

We could not do it now, but with improved technology and continued space exploration, it should be very possible. Ceres should be a comparatively easy place to build a space elevator too. It seems a pretty good mining option. Only potential issue might be the occasional asteroid impact.

Ask the asteroid mining companies "Planetary Resources" and "Deep Space Industries". Actual and serious companies. See the websites www.planetaryresources.com and www.deepspaceindustries.com


What is the distribution of dark energy? Dark matter is not distributed evenly. It forms clouds, it concentrates in galaxies, it forms hairs around celestial bodies.

A google search for dark energy distribution floods me with dark matter distribution results.

How is dark energy distributed? Does it cover the space uniformly, at even density, or does it create areas of concentration or other irregular forms?

The actual model says that dark energy is distributed uniformly around all the universe and as you probably know it expands the universe with him so it stays distributing uniformly, there are no clouds or irregular forms because that wouldn't explain the actual universe model. Maybe in the future that change.


Computing the Sérsic profile of a galaxy from jpg images I am trying to calculate the Sérsic profile of various galaxies from the SDSS based on the images provided by the galaxy zoo site. I am doing this as part of a kaggle competition on using machine learning to predict galaxy morphology. I have no chance of a high rank in this competition so I'm not hesitant to ask for help.

I used the R contourLines function to identify the isophotes of the galaxy and then fit ellipses to each isophote. This seemed to work well, the isophotes are almost always well fit by the ellipses and the ellipses are nearly concentric. Then letting I be the pixel intensity of an isophote and R be the length of the semi-major axis of the corresponding ellipse, I need to fit an equation of the form

log I(R) = log I_0 - k * R^(1/n)

The simple approach seemed to be to take the log of both sides and use OLS regression, so I fit a linear model in R of the form

log(log(I)) ~ log(R)

The resulting graphs showed a good fit but the resulting Sérsic indices n are almost always less than one and never as big as two. This doesn't seem right since indices of 4 or higher seem common in my reading. I don't get anywhere near 4 for an image of M87.

Possibly taking log log flattens things out too much and the index is not responsive enough. I tried using nls to work with just the log but it didn't move the indices much.

Is there any standard software or algorithm for computing the Sérsic index from an image? Are there reference images I can work from that would let me check if my algorithm is reasonable? Any recommendations on how to proceed would be welcome.

UPDATE: I have found the programs GALFIT and GIM2D which look like they might be useful. Any other software that is commonly used for this?

Is there any standard software or algorithm for computing the Sérsic index from an image?

I don't think it is standard but Vika et al (2013) have used a modified version of GALFIT to extract Sérsic profiles. EDIT> But I see you already found it ;-)

  Are there reference images I can work from that would let me check if my algorithm is reasonable?

There are references in literature that provide images with Sérsic profiles, but I haven't been able to find a database which I think is what you are looking for.

  Any recommendations on how to proceed would be welcome.

For your purposes it is not really necessary to calculate the Sérsic index so that you can compare it with literature. If your Sérsic index, which is just one feature for your classification algorithm, is distinctive enough to be able to distinguish between different morphological classes than that is fine.

I imagine that it might be difficult to calculate reliable Sérsic indices from the images provided by GalaxyZoo. The images I remember are often low resolution images of galaxies.

So I would proceed by calculating your Sérsic index for your training and test sets and see how well this feature performs in a classification task.


Why is it impossible to estimate the inclination of an exoplanet with the Radial Velocity Method? The formula of the radial velocity is:

$$v_i= V - K(\sin(f_i+\omega)\ + e\sin\omega),$$ with
$$K = \frac{m_p}{m_s+m_p}\frac{na\sin i}{\sqrt{1-e^2}}.$$

Being $V$the systematic velocity of the system,
$m_p$ the mass of the planet, $m_s$ the mass of the star, $n = \frac{2\pi}{P}$ the mean motion and $P$ is orbital period of planet, $a$ the length of the semi-major axis of the planet, $i$ the inclination of the orbital plane with the ecliptic,
$e$ the eccentricity of the planet, $f_i$ the true anomaly at time $t_i$ and $\omega$ the longitude of periastron.

It's well known the usual procedure. You have data about the instances $t_i$, the radial velocities $v_i$ at time $t_i$ and then you also have the measurements uncertainties $\sigma_i$. With that data, you can estimate 6 parameters: $V$, $K$, $e$, $\omega$, $T$ (the periastron passage time) and $P$. After you estimate these parameters, you can calculate (if you know the mass of the star and you assume that $m_p<<m_s$), $m_p\sin i$, which is known as the minimum mass. So, as far as this way shows, you got no way to estimate the mass of the planet and the inclination separately.

But what if instead of estimate this set of parameters, we introduce another one? Just one with all the parameters in the equation: $V$, $m_p$, $i$, $e$, $T$, $\omega$, etc.? Couldn't that be possible? But nonetheless, it's always assumed that it's impossible to know the inclination with the RV data. Is there some reason why?

Essentially the problem comes down to geometry: radial velocity provides a 1-dimensional view of a 3-dimensional system. See the diagram below:

The observer sees only the component of the velocity vectors along the line from the observer to the star. That means that they cannot distinguish between the two blue velocity vectors: they will see only the purple (radial) component. This means a larger velocity vector that is tilted further away from the line-of-sight can masquerade as a smaller vector in the line of sight.

No matter how you define your variables, you will always run into this issue because it is inherent to the geometry of the situation.

This does break down in the case of planet-planet interactions, which can break the degeneracy here (a good example of this is Gliese 876).


How to calculate the frequency of a gas giant eclipsing the sun from a moon? Say you have a moon around a gas giant which goes around a star. If the moon has an inclination of around 0° relative to the gas giant's orbit, the gas giant will eclipse the star every orbit of the moon. However, if the inclination is somewhat larger than 0°, not every orbit of the moon will contain a planetary eclipse. How would you calculate the amount of moon orbits in a row with a planetary eclipse?

Let’s start with an easy example and say the orbit of the planet with radius $r_p$ around the sun with radius $r_s$ is circular, the orbit of the moon around the planet is circular, the inclination of the moon's orbit is 90 degrees from the planet's orbital plane, and the orbital period of the moon is 1/360th the orbital period of the planet. Let's call the vector from the sun to the planet $v_1$ (with length $d_1$), the vector from the planet to the moon $v_2$ (with length $d_2$), and the vector from the sun to the moon $v_3$.  Let’s assume the moon is so small we don’t have to worry about orbital passes in which there is a solar eclipse visible from one side of the moon, but not from the other.
Every consecutive time the moon ascends through the planet's orbital plane, the angle $\theta$ between $v_1$ and $v_2$ will increase by 1 degree.  A little trigonometry (does everyone remember SohCahToa?) will give us the angle between $v_1$ and $v_3$ as $\phi = \arctan (d_2 \sin\theta/(d_1+d_2 \cos\theta)$.  So from the perspective of the moon, the angle between the sun and the planet in the sky will be $\alpha=\theta-\phi$.  The planet will be at least partially eclipsing the sun in the lunar sky whenever $\alpha<\beta_p+\beta_s$ where $\beta_p= \arctan(r_p/d_2)$ is the half angle of the planet in the lunar sky, and $\beta_s= \arctan(r_s/\sqrt{(d_1+d_2 \cos\theta)^2+(d_2 \sin\theta)^2}$ is the half angle of the sun in the lunar sky.
Here is a diagram looking at the planet's orbital plane when the moon passes through the plane:

Clear as mud?  Let’s do an example with Jupiter (which we will now assume has a circular orbit) and our Sun!  Jupiter’s orbit is 4333 days, so our moon’s orbital period should be 4333/360 =~12 days.  Kepler’s third law says the ratio of the cube of the SMA (semi-major axis) to the square of the orbital period is constant for moons about a body, and we know Jupiter’s moon Callisto has a SMA of 1.883e6km and an orbital period of 16.69 days, so we have the equation $d_2^3/12^2=(1.882e6)^3/16.69^2$.  Solving yields $d_2=1.51e6km$ is how far our moon is from Jupiter.  Jupiter has a radius of $r_p=6.9911e5$km so the half solid angle of Jupiter from our moon is $\beta_p = \arctan(6.9911e5/1.51e6)=0.5radians$ or 28.6 degrees.  The solid angle of the sun from our moon will average about $\beta_r = \arctan(6.957e6/7.8e8)=0.00892radians$ or 0.551 degrees, but it varies a bit in the third significant digit.
Plug it all into the above equations and you get an orbit without an eclipse, followed by 59 orbits with an eclipse and then another orbit without an eclipse.  Here is the matlab code for posterity:
theta = 0:1:359;
d1=7.8e8;
rs=6.957e6;
d2=1.51e6;
rp = 6.9911e5;
phi = atan(d2*sin((pi/180)*theta)./(d1+d2*cos((pi/180)*theta)));
bp = atan(rp/d2);
bs = atan(rs./sqrt((d1+d2*cos(pi/180*theta)).^2+(d2*sin(pi/180*theta)).^2));
alpha =pi/180*theta-phi;
sum(alpha<bp+bs)*2-1

Notes:

This was an easy example because I got to choose all the orbital geometries.  Want to make it harder? Choose a planetary orbital period that isn’t an integer multiple of the lunar orbital period. Want to make it harder? Choose an inclination less than 90 degrees so that some eclipses occur when the moon is outside the planetary orbital plane. Want to make it harder?  Choose highly elliptical orbits for both the planet and the moon, so that $d_1$ and $d_2$ both vary as separate functions of time.  Want to make it harder?  Specify a large moon so that some eclipses can only be seen from some points on the moon, but not others.

I provided the solution for partial eclipses.  If you want the solution only for full eclipses, use this equation: $\alpha<\beta_p-\beta_s$ instead of this equation: $\alpha<\beta_p+\beta_s$ above.

I did not provide a closed analytical solution for the general problem of counting consecutive solar eclipses from a moon given the orbital elements for the planet and moon and the radii of the sun and planet.  However, in describing the solution for a particular class of orbits, I think I've clearly detailed the calculation method that could extend to any particular orbit. The key is in understanding that even though a moon's orbital plane is fixed in an PCI (Planet Centered Inertial) coordinate system, it appears to precess in a sun-centered, planet-fixed coordinate system. A similar example of this phenomenon is the four seasons here on earth due to the Earth's axial tilt.  The Earth's axial tilt is nearly constant, but from our perspective here on Earth, it appears to precess.

Classical orbital mechanics is disgustingly hard and unforgivingly unintuitive. It's much easier to use Satellite Toolkit or Universe Sandbox to plug in your initial orbits, propagate them, and count the eclipses as others suggest.


Why don't astronomers use meters to measure astronomical distances? In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as these are the SI unit for distance? Since the meter is already used in particle physics to measure the size of atoms, why couldn't it be used in astrophysics to measure the large distances in the Universe?

For example:

The ISS orbits about 400 km above Earth.
The diameter of the Sun is 1.39 Gm (gigameters).
The distance to the Andromeda Galaxy is 23 Zm (zettameters).
At its furthest point, Pluto is 5.83 Tm (terameters) from the Sun.

Edit: some have answered that meters are too small and therefore not intuitive for measuring large distances, however there are plenty of situations where this is not a problem, for example:

Bytes are used for measuring gigantic amounts of data, for example terabytes (1e+12) or petabytes (1e+15)
The energy released by large explosions is usually expressed in megatons, which is based on grams (1e+12)
The SI unit Hertz is often expressed in gigahertz (1e+9) or terahertz (1e+12) for measuring network frequencies or processor clock speeds.

If the main reason for not using meters is historical, is it reasonable to expect that SI-unites will become the standard in astronomy, like most of the world switched from native to SI-units for everyday measurements?

In addition to the answer provided by @HDE226868, there are historical reasons. Before the advent of using radar ranging to find distances in the solar system, we had to use other clever methods for finding the distance from the Earth to the sun; for example, measuring the transit of Venus across the surface of the sun. These methods are not as super accurate as what is available today, so it makes sense to specify distances, that are all based on measuring parallaxes, in terms of the uncertain, but fixed, Earth-Sun distance. That way, if future measurements change the conversion value from AU to meters, you don't have to change as many papers and textbooks.

Not to mention that such calibration uncertainties introduce correlated errors into an analysis that aren't defeatable using large sample sizes.

I can't speak authoritatively on the actual history, but solar system measurements were all initially done in terms of the Earth/sun distance. For example, a little geometry shows that it's pretty straightforward to back out the size of Venus's and Mercury's orbit in AU from their maximum solar elongation. I don't know how they worked out the orbital radii of Mars, etc, but they were almost certainly done in AU long before the AU was known, and all of that before the MKS system existed, let alone became standardized.

For stars, the base of what is known as the "cosmological distance ladder" (that is "all distance measures" in astronomy) rests on measuring the parallax angle:
$$\tan \pi_{\mathrm{angle}} = \frac{1 AU}{D}.$$
To measure $D$ in 'parsecs' is to setup the equation so that the angle being measured in arcseconds fits the small angle approximation. That is:
$$\frac{D}{1\, \mathrm{parsec}} = \frac{\frac{\pi}{180\times60\times60}}{\tan\left(\pi_{\mathrm{angle}} \frac{\pi\, \mathrm{radians}}{180\times60\times60 \, \mathrm{arcsec}}\right)}.$$
In other words, $1\operatorname{parsec} = \frac{180\times 3600}{\pi} \operatorname{AU}$.

Astronomers also have a marked preference for the close cousin of mks/SI units, known as cgs. As far as I can tell, this is due to the influence of spectroscopists who liked the "Gaussian units" part of it for electromagnetism because it set Coulomb's constant to 1, simplifying calculations.


Why is the metallicity of elliptical galaxies low? As far as I understand, metallicities of old stars are usually low and those of new stars are usually high.

If elliptical galaxies form after merger of galaxies, then surely that means the elliptical galaxy passed through a stage where it was an irregular galaxy. Now, irregular galaxies are said to be sites of rapid, new star formation.

Why then do we say that elliptical galaxies have low metallicity, if the star formation went on for a long time there?

[New to astronomy, so a detailed explanation would help]

It isn't true that elliptical (or early-type) galaxies are metal-poor. This sounds like a pop-sci myth being propagated. The first line of the abstract of Pipino & Matteucci (2006) states that "Elliptical galaxies probably host the most metal rich stellar populations in the Universe".  Thus your suspicion of the idea that "ellipticals are metal-poor" is well-founded.

Measurements of galaxy metallicities and metallicity gradients have been around for decades and show that ellipticals obey fairly simple scaling relationships between mass, luminosity, velocity dispersion and metallicity.

In terms of metallicity, massive ellipticals with bigger velocity dispersions are a bit more metal-rich than the Sun, whilst smaller galaxies may get down half the solar metallicity. The range is much larger for spiral galaxies. In addition, there is a tendency for ellipticals that are older to have higher metallicities. Examples of these trends can be seen in Graves & Faber (2010) and Li et al. (2018).

The central plot here (taken from Graves & Faber 2010) shows how the metallicity of galaxies varies with their stellar velocity dispersion (an indicator of their mass). [Z/H] is a logarithmic base 10 scale where 0.0 means the metallicity of the Sun and -1 would a tenth the metallicity of the Sun. You can see that the elliptical galaxies (red) have a relatively narrow range of metallicities, that most are as metal-rich as the Sun, that less massive ellipticals have lower metallicity and that spirals (blue) have a much bigger range of metallicity.

The reasons behind these trends are still debated. But as you rightly point out, ellipticals likely form from mergers of objects that have already been actively forming stars. A low metallicity is therefore not necessarily expected. Much depends on how much enriched gas from the ejecta of previous (higher mass) stellar populations is retained within the galaxy. Higher mass galaxies with deeper gravitational potentials are able to retain more of that enriched gas.


Time taken for journey to the moon India is going to launch a spacecraft to the moon on 16 July which will consist of a lander, an orbiter and a rover. I found an article in today's newspaper regarding   this. If you zoom in a bit, you will be able to see the list of other lunar missions that have taken place over the years. A common difference that I found in lunar missions of 20th century and lunar missions of 21st century was the time. Earlier, the spacecrafts reached moon in a day or two but recent missions take over 2 months.

With developed technology, it should take even less time, but it's the other way round. Can anyone explain the reason behind this.

Some trajectories need less energy than others, so to save on fuel, scientists often choose the lengthy, fuel efficient journey rather than a shorter one. In the case of the Apollo astronauts, it was important to get them to the moon in the shortest possible time, so a short trajectory was chosen.


How do we know that comets definitely mase and not just fluoresce? Wikipedia's Astrophysical_maser#; comets mentions some anecdotes of notable masers associated with comets, and Maser emissions from comets begins:

  The 18-cm lines of the OH radical are the only well-documented masers in comets. They have been observed in more than 65 comets since 1973. The good knowledge of their excitation mechanisms and their linear regime allow us to estimate the water production rate and its variation with heliocentric distance from radio observations of OH in comets…

I'm guessing that OH around comets starts as water and is dissociated by UV light from the Sun. Either it left in a metastable excited state or it's excited (pumped) up to one by other photons from the Sun.

What is it about the 18-cm lines that allows one to conclude that this is masing; that there is microwave amplification by stimulated emission rather than just strong fluorescence?

I'm not sure if this is what you're asking but:
fluorescence:   excited electrons randomly emit photons.
Superfluorescence:  excited electrons emit photons, and there is some self-stimulation, leading to a strong short pulse but little coherence.
Superradiance:    strong, shorter pulse,  much self-stimulation , high coherence in output.
Lasing/Masing:  almost always requires a cavity of some sort, leading to directional coherent output.
I cannot tell exactly what the authors of the article consider qualifying as Maser vs. Superradiance.  possibly the  material volume in question (on the comet) is elliptical enough to lead to a more-or-less directed beam.


BIg Bang Happened everywhere According to the  Friedmann equations big bang happened everywhere, but if we account for inflation theory, big bang did not happened everywhere. How ? Anyone please explain. Please see the video for reference.

Okay, I think I know what Max Tegmark is talking about in the video. He is referring to the fact that, when you observe the cosmic microwave background radiation (CMB) — i.e. the radiation that was "released" when the Universe had expanded and cooled sufficiently to allow protons and electron to combine into neutral hydrogen without immediately being ionized again — you find that it has almost (to within $1$ in $\sim10^5$) the same wavelength everywhere, no matter in which direction you look, meaning that the temperature of the Universe was almost the same when it was emitted.

In a universe like ours, but without inflation, this is puzzling. The reason is that, using the observed expansion rate and densities of the universe, you can calculate that at early times such a universe would simply expand so quickly that it would be impossible to reach thermodynamical equilibrium on such large scales as the Universe we observe today. In fact, when we look at the sky, any two points separated by more than roughly 1 degree would never have been in causal contact, i.e. been able to exchange information. Thus, there would be no way for one of these points to know e.g. what is the temperature of the other points, not to mention the rest of the universe.

This puzzle is called the horizon problem.

Map of the CMB sky. The color coding shows the temperature of the radiation, and red is roughly $18\,\mu\mathrm{K}$ hotter than blue.

Credit: ESA/Planck Collaboration (plus my own annotations).

What Tegmark points at is that although we're told that "Big Bang happened everywhere", this doesn't make sense, because how can one part of this "everywhere" know about the other parts of "everywhere"?

One solution, obviously, is pure luck. After all, the regions on the sky are a little bit different. But this is a very unsatisfying solution, known as fine-tuning, which we loathe.

Enter inflation.

If you assume the our Universe was much, much denser in the beginning, but then went through a period of extreme (!) expansion, then in the very first tiny fraction of a fraction of a second, before this epoch which started when the Universe was $\sim10^{-36}$ seconds old, everything was so close to each other that a region much, much larger than the part of the Universe that we can see (the "observable Universe") could exchange information and reach thermodynamical equilibrium.

The reason that many people believe in this scenario, I think, is that it simultaneously solved two other puzzles, known as the flatness problem and the magnetic monopole problem (and that, so far, no better scenario has been proposed).

So, to recap, Tegmark is not saying that Big Bang didn't happen everywhere, but rather that, before Alan Guth proposed the inflation scenario in 1979, but after we first observed the CMB, this didn't make sense.