Is there a recent update on the mysterious M82 radio source reported in 2010? The Wikipedia page on the "Cigar Galaxy" (M82) has the following summary about an "unknown object":

  In April 2010, radio astronomers working at the Jodrell Bank
  Observatory of the University of Manchester in the UK reported an
  object in M82 that had started sending out radio waves, and whose
  emission did not look like anything seen anywhere in the universe
  before.

A subsequent article in New Scientist dated 13 December 2010 offers the following update:

  The object is still a puzzle, says co-discoverer Tom Muxlow. “It was
  still there the last time we looked, so its lifetime is now well over
  a year,” he says. “We are continuing to monitor this object.”

I can find a number of other references to this unknown object, but nothing more recent than 2010. The articles suggest it could perhaps be a new type of microquasar, since it shares some similarities; however, it doesn't have the high X-ray output that known microquasars exhibit.

Has the mystery been solved, or is this still an area of active research?

The SIMBAD page for the source in question: EQ J095552.5+694045.4

From the list of references, the most relevant ones to this question appear to be as follows:

Joseph et al. (2011) suggest that the object could be a microquasar similar to SS 433, which has a high radio to X-ray luminosity ratio.
Gendre et al. (2013) argue against a supernova interpretation for the source:

  Muxlow et al. (2010) suggested that this
  object might either be due to accretion around a massive collapsed
  object or be a faint and unusual supernova (Joseph, Maccarone &
  Fender 2010). In the latter case, we would expect the flux density
  to steadily decrease over time, as the source would be too young to
  have reached the remnant phase. However, in the observations presented here, it shows
  a flux density at 5 GHz of ∼0.90 mJy, which
  does not seem to vary significantly over the period 2009 May–2010.
  The supernova hypothesis is thus unlikely.

Mattila et al. (2013) state that the nature of the source is still unclear but they favour the microquasar interpretation:

  The nature of the 43.78+59.3 transient remains elusive, and on
  the basis of the limited data available, we regard an extremely bright
  extragalactic microquasar as the most plausible scenario, perhaps
  from a high-mass X-ray binary such as LS 5039 (Clark et al. 2001),
  but with a high ratio of radio to X-ray flux. A bright extragalactic microquasar was
  proposed as the most likely explanation by
  Muxlow et al. (2010) and Joseph et al. (2011). We concur with that
  conclusion although it would mean that the NIR luminosity is a
  factor of about 30 higher than Galactic microquasars.

Varenius et al. (2015) note that the source was not detected with LOFAR.

So from these, the nature of the source hasn't yet been conclusively shown, though the most plausible scenario is that it is a somewhat extreme example of a microquasar.


How do you calculate the lookback time distance to a given galaxy? By lookback time distance I mean the same as Light travel distance.
Can you calculate it by just knowing its redshift?

No you can't. Other information is required.

For low redshifts - let's say smaller than 0.1 - and by that I mean the wavelength increases by 10 percent, you might get away with using Hubble's law to estimate the distance and then get the look back time by dividing by the speed of light.
$$ t \simeq \frac{\lambda - \lambda_0}{H_0 \lambda_0},$$
where $H_0$ is the present day Hubble parameter of about 70 km/s per Mpc, $\lambda$ is the measured wavelength and $\lambda_0$ is the rest wavelength.

So far so good, you just need to know $H_0$. However for larger redshifts it gets horribly complicated because the Hubble parameter changes with time in a way that depends on the curvature of the Universe and hence on the cosmological parameters defining the matter density and dark energy density.

There is indeed a highly complicated, non- linear formula involving integrals that I will look up a reference for. But possibly the better way to proceed is to use a simple look-up table produced from such calculations with certain assumed values for ratios of matter and dark energy densities with respect to the critical density; a.k.a. $\Omega_M$ and $\Omega_{\Lambda}$.

The plot below is an example taken from http://www.astro.caltech.edu/~eran/MATLAB/Cosmology.html which shows look back time versus redshift for two different cosmologial models (but with the same value of $H_0$). The curves are very different at high redshifts, but converge at small redshifts.

Here is a cosmological calculator that can do the job for you. Enter the redshift and your assumptions about the Hubble parameter and other cosmological parameter and it will tell you the age of the universe at that redshift as well as the lookback time.


Is there any real evidence to prove or disprove the existence of alien civilizations? Is there any real proof that alien civilizations exist in outer space? What if someone were to say that Earth is the only inhabited planet in the whole universe? How would you respond?

I need to write a five-paragraph argumentative essay to prove that alien civilizations exist. I need to write an introduction, three paragraphs supporting my statement that aliens exist, and a conclusion. In each of the three "body" paragraphs I need to describe in detail a piece of evidence supporting my claim.

I don't even know where to start. I mean, I want to find out the opinion of some scientists who think that we are alone, and then disprove their claim in my essay. I need to know the latest scientific evidence on the existence of alien civilizations. I don't mean like alien abduction stories or any of that science fiction stuff. I need some real proof that will be convincing and not undermine my argument.

How can you know for sure if there are alien civilizations in outer space? What do they look like? On which planets do they live? What kind of technology do they use? Why are we not alone in the universe? What evidence disproves the claims of Earth chauvinists?

Is there any REAL proof that alien civilizations exist in outer space?

No.

  What if someone would say that Earth is the only inhabited planet in the whole Universe, how would you respond?

Depends on what their goal is. If their goal is to convince me of that, I'd just try to avoid the conversation. If their goal is to come to an understanding of the current body of scientific knowledge on the matter, I'd talk about how we don't know that that statement is true.

  I need to write a 5 paragraph argumentative essay to prove that alien civilizations exist. I need to write an introduction, 3 paragraphs supporting my statement that aliens exist, and a conclusion. In each of the three "body" paragraphs I need to describe in detail a piece of evidence supporting my claim.

That will be difficult, given that there is no such proof. Best you could do is argue that alien civilizations likely exist, given a certain set of assumptions.

  I don't even know where to start. I mean, I want to find out the opinion of some scientists who think that we are alone, and then disprove their claim in my essay.

That is not a good goal for an essay. Don't start with the goal of disproof. Start with the goal of understanding.

  I need to know the latest scientific evidence on the existence of alien civilizations. I don't mean like alien abduction stories or any of that science fiction stuff. I need some real proof that will be convincing and not undermine my argument.

Look at the Drake Equation and recent research that concludes that there are many planets in the galaxy.

  How can you know for sure if there are alien civilizations in outer space?

You would need observational evidence.

  What do they look like? On which planets do they live? What kind of technology do they use?

All unknown.

  Why are we not alone in the Universe?

This question presumes the conclusion that we are not alone. We might be alone.

  What evidence disproves the claims of Earth chauvinists?

None.


Is there a legend for these 88 HST images? Is there a legend for these 88 Hubble Space Telescope images?

I have a 6000px x 4800 px version of this image (reduced & rotated to 480x600 here) that I use for an image slide show.  I would like to add the description of each image into the slide show software.

On this system the file name is HubbleGallery.jpg but I suspect I coined that title (it was a long time ago it was downloaded).  The original image is 5,150,779 bytes in size.

Searching on the smaller, rotated version of the image points to pages like Astronomy Printshop, but there is nothing closely related to NASA in the first pages of the search, and the pages listed did not have much detail (certainly no legend).

It might possible to search on the clipped, individual images, but I was hoping to avoid having to do that 88 times.

To answer my own question: no, not that I can find.

Here is an attempt to identify them by image search.  Let me know if there is an image/legend match that seems wrong.  I can upload a higher resolution (466x426 px) version of the image.

The images were cropped from the original image when it was oriented so the the 'HUBBLE SPACE TELESCOPE' message read from the bottom to top of image, and I will try to identify them in the same order.  As such, we do 1 column at a time from top to bottom, starting from the right most.  The column is marked in red above/below.

But first, here is the captioned video created from combining the captions with a video slideshow of the images!

Column 1

Gravitational lensing of distant galaxies.
Red Rectangle Nebula.
The Sombrero Galaxy: Messier 104.
Doradus Nebula.
Stingray Nebula.
Elliptical Galaxy NGC 4881 in Coma.
Galaxy NGC 4214.
Glowing Eye of NGC 6751.
Collection of galaxies of different types, from different ages in the universe.
Uranus: Springtime in the Southern Hemisphere.
The Lunar Aristarchus crater imaged using ultraviolet and visible wavelengths.

Column 2

Supernova remnant N132D, in the Large Magellanic Cloud.
Ring Galaxy AM 0644-741.
Bug Nebula NGC 6302.
Planetary Nebula M2-9.
The Cone Nebula in NGC 2264.
Young planetary nebula MyCn18.
A stellar jet in the Trifid nebula.
Galaxy Pair NGC 3314. A face-on galaxy lying precisely in front of another spiral galaxy.
Comet LINEAR dispersing after breaking up.
A 5000 light year long jet from the giant elliptical galaxy M87.
The Massive Stars of 30 Doradus as seen in the star cluster R136.

Column 3

NGC 346 is an open cluster with associated nebula located in the Small Magellanic Cloud.
The Cat's Eye Nebula or NGC 6543, is a relatively bright planetary nebula in the northern constellation of Draco.
This infrared view of Uranus reveals that the planet is surrounded by its four major rings and by 10 of its 17 known satellites.
Galaxy NGC 4013. An amazing \"edge-on\" view of a spiral galaxy 55 million light years from Earth.
The inner ring of NGC 1512.
Detail of the Omega Nebula.
This close-up of the red planet Mars was taken when it was just 55,760,220 km away.
Stellar Spire in the Eagle Nebula
Interacting galaxies NGC 1409 & 1410.
NGC 1850 - Star clusters in the Large Magellanic Cloud.
Io, Ganymede, and Callisto casting eclipse shadows on Jupiter.

Column 4

Crab Nebula Mosaic
N44F's central star is wrapped within a roughly spherical shell of cool orange gas, with fingerlike columns of cool gas and dust protruding from the interior wall.
The Whirlpool Galaxy (M51) and companion galaxy.
Aurora at Jupiter's north pole as seen in ultraviolet light.
Bow shock near young star LL Ori.
The Black Eye Galaxy (also called Evil Eye Galaxy; designated Messier 64, M64, or NGC 4826)
A color-composite image of the Pleiades, or Seven Sisters (Messier 45 or M45), an open star cluster.
Orion nebula mozaic.
The Eskimo Nebula (NGC 2392).
Galaxy collision in NGC 6745.
Rich star fields and glowing hydrogen gas silhouette dense, opaque clouds of interstellar gas and dust in a bright star forming region in Centaurus

Column 5

Aristarchus Plateau on the Moon.
The Tadpole Galaxy features a trail of stars about 280,000 light-years long and massive, bright blue star clusters.
Infant Stars in the Small Magellanic Cloud.
The Ghost-Head Nebula: NGC 2080.
The Pencil Nebula (NGC 2736) is a small part of the Vela Supernova Remnant, located near the Vela Pulsar in the constellation Vela.
Flash from star V838 Monocerotis echoes through space.
Rampaging Supernova Remnant N63A.
The center of globular cluster Omega Centauri.
The Bubble Nebula (NGC 7635), also called the Sharpless 162, or Caldwell 11, in the constellation Cassiopeia.
The center of NGC 4414, an unbarred spiral galaxy about 62 million light-years away in the constellation Coma Berenices.
NGC 3132, also known as the Eight-Burst Nebula, the Southern Ring Nebula, is a bright and extensively studied planetary nebula in the constellation Vela.

Column 6

Messier 22 (also known as M22 or NGC 6656) is an elliptical globular cluster in the constellation Sagittarius, near the Galactic bulge region.
Star forming region N11B lies in the Large Magellanic Cloud.
The Homunculus Nebula is a bipolar reflection nebula surrounding the massive star system Eta Carinae.
Hoag's Object: a Ring Galaxy.  On the outside is a ring dominated by bright blue stars, while near the center lies a ball of much redder stars.
Detail of the Keyhole Nebula. The small nebula to the upper left has been nicknamed \"finger of God\" or \"God's birdie\", due to the gesture it appears to be making.
Reflection nebulosity near Merope (IC 349).
NGC 1569: Starburst in a small calaxy.
NGC 1300 is a barred spiral galaxy about 61 million light-years away in the constellation Eridanus.
The Lagoon Nebula
Planetary Nebula NGC 6543, the \"Cat's Eye Nebula\" is thought to have formed one one star of a double system went supernova.
NGC 1999 is a dust filled bright nebula with a vast hole of empty space represented by a black patch of sky.

Column 7

This glowing region reveals arcs and bubbles formed when stellar winds - streams of charged particles ejected by the Trapezium stars - collide with material.
Auroras at Saturn's north and south poles.
Sheets of debris from a stellar explosion (N 49, DEM L 190).
The Horsehead Nebula.
The Retina Nebula: dying star IC 4406 has a doughnut shaped nebula seen edge on.
The center of NGC 1275 (also known as Perseus A or Caldwell 24), which is a type 1.5 Seyfert galaxy
NGC 4622 has bright twisting arms containing newborn stars and lies 111 million light years away.
Double Bubble' of gas and dust in the Large Magellanic Cloud.
Saturn displays its familiar banded structure, with haze and clouds at various altitudes.
Comet-like filaments along the inner rim of the Helix Nebula's gas ring.
Galactic nebula NGC 3603 shows various stages of the life cycle of stars.

Column 8

Thousands of stars are forming in the cloud of gas and dust known as the Orion nebula.
A wheel galaxy around 500 million light-years from the Earth.
Stellar spire of the Eagle Nebula
The Circinus Spiral Galaxy
The Egg Nebula
Gaseous streamers flutter from a group of stars.
The Seyfert's Sextet.
Bubbly ocean of glowing hydrogen, oxygen, and sulphur gas in the extremely massive and luminous molecular nebula Messier 17.
North Eastern region of NGC 6369: The Little Ghost Nebula.
Double galaxies of NGC 4676 in collision.
Star clusters born among the interacting galaxies of Stephan's Quintet.


How do I apply a velocity shift to a wavelength array with uniform logarithmic spacing? Suppose I have a wavelength array for a spectrum in units of Angstroms. Suppose further that the wavelength has "uniform logarithmic spacing" such that if I just take the difference in Angstroms between neighboring array elements i and i+1, this won't be the same as the difference in Angstroms between any other two neighboring elements j and j+1. However, if I first took the log10 of the wavelength array, then the difference between any two neighboring logarithmic array elements i and i+1 would be equal to the difference between any other logarithmic array elements j and j+1.

Now, I know a priori that this wavelength array has a velocity offset of X km/s, which I basically want to remove. How do I apply a velocity shift of -X km/s to this wavelength array which has uniform logarithmic spacing?

If you have bins of equal log wavelength increment then a velocity shift corresponds directly to a pixel shift. That is the reason for binning in equal log wavelength increments.

See  this question for a short proof. All you need to do is work out what each pixel corresponds to in terms of a velocity.
$$\Delta V  = c\, \Delta \log \lambda$$


How close would an LED have to be to your eyes to be as bright as the looking at the sun during midday? I wonder if it is possible to have an LED screen bright enough to simulate the brightness of our sun from Earth, and if so if it could be made small enough to fit on a laptop screen.

Typical sunlight has a power of a few hundred watt per square meter. A typical laptop screen has a power of about 4-5 watt. So obviously there is a vast difference in how bright they are.
To turn this into an astronomy question we can use magnitudes to answer it. The sun has apparent magnitude −26.7. A laptop at an arms length is about 50 lux, which would be about -18.4 apparent magnitude. So to get a solar apparent magnitude I need it to be 8.25 magnitudes brighter. That corresponds to a brightness increase of $100^{8.25/5}\approx 1995$. So if each pixel shone about 2000 times brighter, you would feel like the screen produced sunlight.
You could of course try looking at the screen more closely, making it brighter that way. However, the $1/r^2$ law for how brightness changes with distance stops being valid when your eye get closer to the pixel than the pixel lateral size. Worse, the amount of photons your eye can receive will also be limited by your pupil aperture: you cannot see all of the screen light. So this is why pushing your face against a screen doesn't make it feel like the sun (trust me, I tried this right now with my screen and some sample sunlight).
If it is a smaller region that is very bright surrounded by black it may feel brighter: good visual design can often fake a bit of extra brightness that is not physically there.
I don't see a physical reason for true sunlight intensity screens being impossible, but they are not practical. They would drain batteries like crazy. A visually convincing spectrum may still be doable with less power than a full blackbody spectrum (that would also emit a lot of infrared - nice feeling on the skin, but would heat up the screen a lot) just like LEDs faking white light with just a few frequencies.


Why is carbon so rare on the Moon and on Mars? Carbon is the fourth most common element in the universe and in the Solar system. It is about the ninth most common element in Earth's crust. It doesn't seem to be part of any of the ten most common compounds in the Earth's crust.

So where is all the carbon hiding?
Why isn't it more common in planetary and lunar crusts?

Is it maybe mostly occurring in volatiles together with hydrogen and oxygen, and has left bodies which lack atmosphere or freezing temperatures? Is it primarily biological processes which binds carbon to solid molecules? Having a look around on Earth's natural surface today, there's lots of solid carbon compounds everywhere, so the contrast is interesting.

There is not much doubt that the abundance of carbon in the protosolar nebula was not abnormal. We can tell that by looking at the carbon abundance in the atmosphere of the Sun - it has an abundance of 8.43 on a logarithmic (base 10) scale where hydrogen has an abundance of 12 (Asplund et al. 2009). This is typical for stars in the solar neighbourhood.

However carbon in the solids of the solar system - i.e. in the Earth, moon, meteorites etc. is underabundant with respect to this by something like a factor of 10. Nitrogen is similarly underabundant in the Earth and oxygen to a lesser extent (a factor of two). (Lodders et al. 2009)

The reason for this is that C,N,O are all capable of remaining in the gaseous phase at the kinds of condensation temperatures that are typical for forming most of the "rocks" that make up the Earth and other solids in the solar system. In particular, carbon monoxide is readily formed at high temperatures and cannot easily be captured into terrestrial planets and other rocky materials.


Why does fusing iron in a stellar core use more energy than it releases I know larger stars can fuse heavier and heavier elements up to iron where it stops because fusing iron requires more energy than it releases, causing a collapse and supernova.

Why does fusing iron in a stellar core use more energy than it releases?

An atomic nucleus contains protons and neutrons. There is a balance of forces: the (residual) strong nuclear force attracts protons and neutrons together. The electro-magnetic force pushes them apart.

When nuclei fuse, energy is required to push them together, as you have to push against the electromagnetic repulsion. But when the nuclei come close enough, the nuclei are attracted by the strong force, and energy is released as the nuclei fall into each other. For small nuclei, the energy released in fusion is greater than the energy needed to overcome electromagnetic repulsion. For large nuclei (which have a larger positive charge) the reverse is true, more energy is required to push the nuclei together than is released.

Iron is at the balance point, at which the energy required to overcome electomagnetic repulsion just exceeds what is given out on fusion. So iron is the smallest nuclei to absorb energy when it engages in fusion. And so when substantial amounts of iron form in the core of a star, the core will no longer be able to heat up to maintain the balance with gravity, the core collapses and the star explodes. See Why is iron responsible for causing a supernova?

Iron is therefore one of the most stable nuclei (not the most stable: that is Nickle-62, but for various reasons this is not likely for form inside stars) Iron acts like the "ash" of nuclear fusion.

A more detailed answer would require understanding the quantum mechanics of how binding energy in nuclei works, which can explain why it is Iron that has close to maximum binding energy.


How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes? I just stumbled across the following answer "update" from 2013:

This just in, recent observations have ruled out the possibility that SMBHs gain mass only through merging with other black holes. For more, see this Astronomy.com article.

Question: How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes? I can't figure it out from the linked Astronomy.com article. In fact I can barely read it behind all the blinking advertisements that it now displays (and probably didn't back in 2013)

Potentially related:

Why can't supermassive black holes merge? (or can they?)
How do two supermassive black holes reach "the last parsec" in merging galaxies?
Why do we believe that the super massive black holes at the centers of two merging galaxies would themselves merge?
What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes?
Why would the merger of spinning black holes within the accretion disk of a supermassive black hole cause them to "shoot straight up" out of the disk?

The idea behind the paper (Shannon et al. 2013) that article is based on is to measure the gravitational wave background (GWB) produced by mergers of supermassive black holes, and determine which models of SMBH merger histories can replicate the SMBH population and the corresponding gravitational wave background. In this paper, measuring the GWB is done using pulsar timing - in particular, using the Parkes Pulsar Timing Array and some supplementary data from Arecibo. A pulsar timing array (PTA) detects (or attempts to detect - we have nothing definite yet) gravitational waves by measuring when individual pulses from pulsars arrive at radio telescopes. Gravitational waves should change the times of arrival (TOAs), so if you have enough TOAs from a selection of millisecond pulsars, you can constrain the GWB spectrum's amplitude $A$ and its associated energy density $\Omega_{\text{GW}}$.
The authors applied the aforementioned methodology and then considered four different models of SMBH population synthesis to try and replicate the observational results. The one which involved mass growth entirely through black hole-black hole mergers at low redshifts was ruled out at the 91% confidence level by the pulsar timing array measurements, while the others were more favored. These other models generally included SMBHs growing by accreting gas. No single one is a clear favorite, but the authors were able to rule out substantial regions of parameter space, and these three models were certainly favored more than the pure black hole-black hole merger model.
I want to emphasize something here that wasn't emphasized in the article: That merger model is focused on SMBH growth at low redshifts, and in particular on the SMBH population evolution at $z<1$. Why? It's because this is where the dominant component of the GWB we detect today is expected to come from (see e.g. McWilliams et al. 2013), so it's much easier to say something about low-redshift populations based on PTA data. This is also why the group is interested in how the other models fare at low redshifts for doing things like replicating the quasar luminosity function at $z<1$.
I feel like saying that the pure merger model has been entirely "ruled out" is maybe a bit strong; fortunately, pulsar timing arrays have made major advances since 2013, and we have stricter limits on $A$ and a better understanding of SMBH populations. The 2013 paper used only 20 pulsars, but PTAs have easily more than doubled that number in the interim, and that's only going to increase in the years to come. At the same time, the other models are certainly strongly preferred. I can look into whether this particular analysis has been replicated with newer data.


Units in optical and radio spectral data cubes: Flux vs Brightness I have noticed that the units used in spectral data cubes coming from optical IFU instruments are usually in $erg/s/cm^2/Ang/pixel$. To my understanding this is a measure of Flux. Right?

However, in radio astronomy, spectral data cubes coming from interferometers is usually in $Jy/beam$. To my understanding, this is a measure of surface brightness. Right?

Why optical and radio spectral data cubes don't use the same kind of unit (either flux or brightness)?

The answer is partially historically and partially scientifically. Flux, i.e. energy of radiation integrated across a solid angle (steradian) is basically counting all photons coming from a solid angle, while specific intensity (units of specific intensity are $erg/s/cm^2/Hz/sr$), is the energy per frequency along a line of sight! A surface brightness is flux divided by solid angle, i.e. the mean intensity within this solid angle. This evenly smears out all photons in this solid angle.

As a consequence, flux is the preferred unit if one observes a point source, i.e. a source with a solid angle smaller than the angular resolution of the detector. For point sources, the beam size does not matter. If we observe the same point size with two instruments with different beam sizes, they both detect the same flux because they just count all photons coming from the source. A larger beam does not change this quantity. On the other hand, the brightness (mean intensity) of the object goes down with a larger beam size because we average the flux over a larger area.

If you observe an extended (larger than the beam!) object, e.g. a molecular cloud in radio, then surface brightness is the preferred quantity. Assume it is equally bright across its surface, then the surface brightness becomes independent of the beam size because the flux is proportional to the beam size. A telescope with a larger beam (smaller dish) observes a larger flux (always assuming the object is even larger) compared to a larger telescope with a smaller beam. Comparing the two measurements becomes difficult. The surface brightness of both observations is the same though.

Since optical astronomy was primarily observing stars, i.e. point sources, they used flux. Radio astronomers observed the interstellar medium, i.e. extended clouds of gas and dust, so they used surface brightness.


How long do planetary rings last? I'm surprised that this question hasn't been asked before (here or on Physics), to the best of my knowledge. It's one that I might have asked when I was a bit younger, and one that I think other people will ask.

Anyway, it's clear that Saturn's rings won't form a moon, and the same is likely to be true for other ring systems. However, I'm guessing that they won't last forever (it's just a guess).

How long do planetary rings in general last? What mechanisms could cause them to dissipate/fall apart/end? I'm guessing the Poynting-Robertson effect could come into play, but I'm not sure.

And for anyone curious, yes, I checked just for the fun of it, and Yahoo Answers had a bunch of really, really bad, unsourced and most likely inaccurate answers (given that there was no consensus), ranging from '3 million years' to '13-18 billion years' to 'forever'.

It appears (and I am no expert) that Saturn's ring evolution is governed mainly by "viscous spreading" - collisions between ring particles; and also by interactions with Saturn's moons (resonances); and bombardment with meteoritic material.

There appears to be no consensus on how old the ring system is. Most theories of their formation have this taking place early in the life of the solar system, but there are observations that suggest little contamination by meteoritic material and hence that the rings are young.

According to Salmon et al. (2010)  the viscosity in the ring is critical to its evolution and therefore estimates of ages and lifetimes. As the disc spreads due to viscosity, the viscosity itself will reduce, slowing the evolution and thus the ring system may not empty for many billions of years. On the other hand, comparing viscous spreading models with the current density profile of Saturn's rings suggests they are much less than a billion years old. However, it should be noted that other authors have used similar models to argue that Saturn's rings did indeed form 4.5 billion years ago and were much more massive (by a factor $>100$) then they are today! (Charnoz et al. 2011)


Why does Astronomy still use the Anno Domini system for Time Synchronization Since Time is such a valuable variable in the calculation of Astronomic movement why does the Astronomy Community still rely on the the 6th Century Anno Domini system developed by Dionysius Exiguus to establish Time on Earth.
Obviously it is not 2013 on Earth, wouldn't it be more technically accurate to determine the actual time on Earth or get as close as possible to it than to use a time that is clearly wrong and developed by someone 15 centuries ago?

What makes you think that it is "obviously not 2013 on Earth"?

In actual calculations, astronomers use the Julian day, which is a decimal representation of time. A Julian year is exactly 365.25 days of 86,400 seconds each. Astronomical coordinates are usually written in the J2000 epoch, which allows us to compensate for Earth's axial precession.

Our ability to measure and record time is limited only by the accuracy of our most precise clocks. There's nothing fundamentally wrong or imprecise about it.


Build a high quality telescope from scratch I am interested in building a high quality portable telescope from scratch in an attempt to better understand how telescopes work. What kind of materials do I need? How would I go about putting the telescope together? Are there any online guides that you can recommend?

Amateur telescope making (ATM) is a popular past-time for astronomers.

You can grind your own mirror, or you can buy ready-made mirrors.

You can coat your mirrors with silver using chemical solutions yourself, or have a coating of aluminium applied using a vacuum chamber by an outfit offering such a service.

The main materials are a thick round slab of glass, usually Pyrex, and a smaller elliptical flat mirror. Using this you can make a Newtonian reflector. The support tube or structure for the mirrors can be one of a huge variety of designs, or you can be inventive yourself.

There are websites to guide you in this, such as Mel Bartel's, Stellafane, and AmateurTelescopeMaker.com.


Would an ultra bright moon indicate that the world is ending? There is a sci-fi television episode that shows a man looking out the window into the night sky. There up in the sky is the moon, and it's extraordinarily bright.

The man spends the rest of the night trying to do all the things he wish he had done in his life. A race to experience life before the morning sunrise.

His conclusion; A giant solar flare has erupted, and the side of the Earth facing the sun has been burned away. The world is ending....

How plausible is this story?

Could a solar flare burn the Earth? and if you happen to be on the dark side when it happens. Would the moon be a reliable indicator?

I remember that well. I presume you're referring to an episode of "The Outer Limits," which was based on a short story by Larry Niven. Having looked into the idea back when I read the story in the 1990s, I can say it's plausible in the sense that it's not strictly prohibited by any law of physics.

That said, a solar flare would have to be of precisely the right size, coming in at precisely the right trajectory for precisely the right duration, and with the moon in precisely the right spot for the scenario in the story to play out. More likely, the entire planet would be completely roasted, and all higher life forms extinguished.

Of course, most solar flares are not nearly that powerful. Typically, we experience transient disturbances in global telecommunications, spectacular aurorae, and little else when a flare hits. In the late 1800s a more powerful flare hit, and some people speculate that a flare that size could affect not only telecommunications but also power grids. However, the planet would not even be singed.

There are in fact several ways to burn the Earth, as you put it. Astronomer Phil Platt describes these in wonderful detail in his book Death from the Skies. In addition to a solar flare, a gamma ray burst from a nearby supernova (< 20 ly) would do it. I won't mention the others since I don't want to spoil the fun of the book.


What is star composition after formation Most descriptions of star ignition go something like... The star reaches a critical mass and ignites, blowing away the surrounding material. The most obvious question would be "Why are stars not all the same size?", given that the critical mass for ignition would appear to be the same, but I understand that stars can accumulate mass from collisions from other stars, planetary bodies etc.

So my question is also based on the fact that larger stars also burn more energetically, I assume blowing away more of the the lighter elements from the surrounding cloud after formation and gathering more heavy elements,... do larger stars have a significantly different composition from smaller stars? --- or is the simple description of star formation misleading?

The star reaches a critical mass and ignites, blowing away the surrounding material. (OP emphasis)

I think the misunderstanding is that it's not that explosive when the star "lights up". The process is in some sense continuous, so the nuclear reactions start slowly and gradually increase in strength as the star continues to contract, grow, and heat up inside.  Once the amount of radiation from the surface becomes sufficiently hot, we can speak of "blowing away" material, but even then it's not violent. The absorption of light in the gas creates a pressure that pushes the gas away. It picks up pace as the star settles down (and depending on how massive the star is), but it's broadly a consistent stream. (Astronomers are probably unwise to use the word "ignite" because all we mean is that the nuclear reaction rates become non-negligible. It's not like a rocket taking off or something.)

Generally, star formation is a poorly-understood process and a hot topic for research.  For stars larger than about 7 solar masses, we think that nuclear reactions begin before all the material has gathered in the star. i.e. while some is still flowing on. If this happens through a disk, then the light can stream out everywhere but the equator without much affecting the growth of the star.

That said, at very high masses, you do run into a problem of blowing mass away. For this reason, we don't really expect to see many stars over a few hundred solar masses, and those that are can be violently unstable (like Eta Carina, which threw off about 10 solar masses of material in the mid-19th century).

And as for stars genuinely "blowing up", this does happen in other circumstances. For example, in the core of a solar mass red giant, matter becomes nearly isothermal.  It eventually gets hot enough to ignite helium, and it nearly all lights up at nearly the same time in a helium flash.  This briefly produces as much light as a small galaxy! But only very briefly, and it doesn't make it to the surface for us to see. Incidentally, this is also the same mechanism as Type Ia supernovae: an isothermal carbon-oxygen white dwarf accretes matter until the carbon lights up, and the whole white dwarf basically ignites at the same time.


Factors included in the average density of planets When the average density for a planet is stated does it incorporate the mass of the atmosphere, when a planet has an atmosphere, or is the mass of the atmosphere ignored?

Whether of not you include the mass of the atmosphere doesn't matter much. For instance, the mass of Earth's atmosphere is roughly $10^{-6}$ of Earth's mass, corresponding to the uncertainty in the gravitational constant $G$ used to calculate the mass in the first place.

But the volume that you use for calculating the average density could potentially matter, depending on your definition of the height of the atmosphere. For Earth, the lowest-orbiting satellites are at a height of $\sim100\,\mathrm{km}$, so if you include that, your volume will be 5% larger. This number can be much lower if you define the height of the atmosphere to be the scale height (0.3%), or much higher if you include the thermosphere (1600%).

Because the height of the atmosphere is quite arbitrary, this is rarely if ever included in the average density of a (rocky) planet.


Astrochemistry - what does freeze-out mean? I was recently watching a video on the physics and chemistry in planetary accretion disks, and the speaker at a certain point mentioned 'freeze-out and UV dissociation'. Is freeze-out a purely chemical term, or is it specific for this process? Is it very important in planetary formation?

Freeze-out is indeed a chemical term. It happens in the midplane of accretion disks where density is high enough to block UV ionization/chemical dissociation and limit heating from the central star. No one seems to have gotten around to writing up a layman friendly article on the subject yet, but these refs give the gist:

Protostars and planets V

Accretion Disks 1: During Star Formation

Organic Matter in Space (IAU S251) Evolution of Organic Matter

From ref 2:


Which approximations are required for the synodic period formula? The formula below is used to calculate the synodic period between two bodies orbiting a same third one:

 $$\dfrac{1}{T_\mathrm{syn}} = \dfrac{1}{T_{1}} - \dfrac{1}{T_{2}}$$

This formula can be used, for instance, to deduce the period of a planet in the Solar System, given that $T_\mathrm{syn}$ can be measured (period between two consecutive observations of a planet in the same relative position in the sky) and taking $T_1$ as 365.26 days.

I have read that this formula is derived from:

$$w_{\mathrm{syn}} = w_{1} - w_{2} = \dfrac{2\pi}{T_\mathrm{syn}} = \dfrac{2\pi}{T_{1}} - \dfrac{2\pi}{T_{2}} \therefore \dfrac{1}{T_\mathrm{syn}} = \dfrac{1}{T_{1}} - \dfrac{1}{T_{2}}$$

My question is: which approximations are required for this result to hold true? Must the orbits be circular? Must these two bodies be in the same plane around the third? Also, considering the case of elliptical orbits but still assuming that the mass of the third body $M ≫ m_{1}$ and $M ≫ m_{2}$, would the synodic period of body 2 as observed from body 1 be indeed constant?

The formula :-

\begin{equation}\tag{A}
\frac{1}{T_{syn}} = \frac{1}{T_{1}} - \frac{1}{T_{2}}
\end{equation}

for the synodic period between two planets orbiting the Sun is derived in a similar way to the derivation of the Moon's synodic period  :-

\begin{equation}\tag{B}
\frac{1}{T_{syn}} = \frac{1}{T_{sid}} - \frac{1}{T_{e}}
\end{equation}

where $T_{sid}$ is the sidereal perod of the Moon and $T_{e}$ is the sidereal period of the Earth.

Formula (B) is derived by making the following simplifying assumptions about the motions of the Sun, Earth and Moon :-

Earth has a circular orbit around the Sun of constant speed and a fixed period $T_{e}$
Moon has a circular orbit around the Earth of constant speed and a fixed period $T_{sid}$. (Thus the moon's path wrt the fixed Sun frame is an epicycle, ie a small circle whose center moves along the circumference of a larger circle, in this case the Earth's orbit).
The above two orbits are confined to a common plane (the Ecliptic Plane)
From the direction we are viewing (ie `North') both Earth and Moon orbit in the anti-clockwise direction

and (A) is derived with similar assumptions about the planetary orbits.

(See this answer for the derivation of formula (B) for the Moon - for the planets the derivation of (A) would be virtually the same).

In reality the orbits are not perfectly circular and they are not in the exact same plane. The circular orbit approximation means the speed of the planet is constant, but in the case of an elliptical orbit the speed varies.


Does Universal Time really track mean solar time? At innumerable places on the net you can find the claim that leap seconds are inserted into UTC in order to make it track UT1 which again tracks mean solar time at the reference meridian.

However, when I tried to locate an actual definition of UT1, the closest I could find was IAU resolution B1.8 from 2000 which seems to define it implicitly from the following relationship with the Earth Rotation Angle:
$$ \theta = 2\pi\bigl(0.7790572732640+1.00273781191135448({\it JD}_{\rm UT1}-2451545.0)\bigr)$$

Here the magic constant 1.00273781191135448 appears to be derived from the ratio between the Earth's rotational period and the sidereal year -- but no provision for adjusting it for observed changes in that ratio seems to be made. (The least significant several digits of the constant must be wrong already).

As the Earth's rotation slows down, is UT1 therefore going to drift out of sync with mean solar time at the IERS reference meridian? Or am I missing something?

This question asks in effect: Is there a discrepancy, perhaps progressive, between the official time-measure UT1, and some other credible measure of mean solar time? This is harder to answer than it might first seem.

UT1 is the current official representative of mean solar time: it is doubtful that any independent measure of mean solar time (other than direct relatives of UT1 such as UT0 or UT2) has been recently defined or computed to any precision. (The method of the equation of time as normally carried out is only roughly approximate, because it ignores the many solar perturbations discovered since the early 18th-c. and its results may vary from accurate mean solar time by up to about 3 seconds: see Hughes et al. (1989), "The equation of time",  http://adsabs.harvard.edu/abs/1989MNRAS.238.1529H .)  But as the question indicates, one reason for questioning UT1 is that solar time naturally depends on two independent variables, related to the angular rates of the earth's axial rotation and of the earth's orbital motion around the sun:  In contrast, the official UT1 calculation appears to condense these into a single constant ratio, thus losing one of the independent variables of the natural physical model. This may well raise suspicion that UT1 deviates or will deviate from a more physically-defined mean solar time.

There is in fact a (no-longer-official) alternative and physical basis for calculating mean solar time. This can be gathered from the history of 19th-century mean solar time definitions and calculations. At the same time, there is also some documentation from the history to indicate the origin and nature of some of the steps taken in the 20th/21st-century, to change the old 19th-century physical model to the current (and very non-transparent!) basis of calculation for UT1, which is described in: (N Capitaine et al. (2003), "Expressions to implement the IAU 2000 definition of UT1", Astronomy & Astrophysics 406, 1135-1149, http://adsabs.harvard.edu/abs/2003A%26A...406.1135C ).

Classical method for determining mean solar time (Nautical Almanac, 19th-c.):  (Not discussed here is the method of the equation of time, which grew more cumbersome as perturbations of the apparent solar motion were successively discovered during the 18th-c.; all of them had to be included in precise calculations.)  A simpler equivalent method for mean solar time, to a suitable astronomical precision for its period, was provided by Nevil Maskelyne's tables of 1774 (found at http://docs.lib.noaa.gov/rescue/rarebooks_1600-1800/QB12M31774.pdf), especially his table of the Sun's mean longitude converted from degrees to hours and given to a precision of 0.01 time-second. His instructions showed how to use this to convert observed sidereal time to mean solar time. It may be unclear whether Maskelyne intended this method to make a standard out of Greenwich mean time, but that seems to be what happened. Thus initially, according to J J Lalande's 'Astronomie' (3 editions, Paris, 1764, 1771, 1792) methods for mean time still relied usually on the equation of time, but Lalande's third edition of 1792 (vol.1, https://archive.org/details/astronomielalande01lala , see art.1014-5, p.361) added a mention of Maskelyne's table, commenting that 'in England' it was used to find the mean time of an observation. The Nautical Almanac eventually began (from 1833) to tabulate the relation between sidereal and mean solar time (Sidereal time at (Greenwich) mean noon), and during the rest of the 19th-c. such tabulations provided what became standard observatory methods of finding mean solar from sidereal time (see J C Adams 1884, "On the Definition of Mean Solar Time", Observatory 7, 42-44, http://adsabs.harvard.edu/abs/1884Obs.....7...42A ).

The method has a simple physical basis in spherical astronomy, using a division into two parts of the arc of the celestial equator that extends from the equinoctial point eastwards to the point culminating at the meridian -- shown in the diagram below.

(The upper part of the diagram represents the celestial equator above the horizon (marked E W) for an observer in the northern hemisphere looking south. M is the culminating point where the equator intersects the (south) meridian.  Q is the equinoctial point 'first point of Aries'. QS represents the sun's mean longitude reckoned from the equinox: it has been (in concept) transferred from the ecliptic to the equator where it now represents the mean right ascension of the fictitious mean sun. For convenience of arrangement, the diagram represents a position in early afternoon (S somewhat west of M) in about June (S is nearly 90° east of Q). (N) is below the horizon.)

The time-definitions and explanations of J-J de Lalande (1792, 'Astronomie', 3rd ed. vol.1, art.1014, p.361, https://archive.org/details/astronomielalande01lala) state clearly (expressing right ascensions in degrees, according to Lalande's custom):-

"The mean longitude of the sun, plus the mean time converted to degrees, gives the right ascension of the mid-heaven."   That translates, in terms of the diagram here, to:  QS + MS = QM .

Lalande also pointed out (art.1015) that "The Sun's right ascension, or that of the star used to find the right ascension of the midheaven, should be counted from mean equinox like the mean longitude of the sun ....". Modern readers have to note further that (a) observed and tabulated 'sidereal time' up to about the end of the 19th century always included the effect of nutation in right ascension, i.e. it was 'apparent' sidereal time (see explanation in Nautical Almanac 1864, pp.515, https://babel.hathitrust.org/cgi/pt?id=mdp.39015068159329 ), and (b) also in those days tabular mean longitudes of the sun still included the effect of mean aberration: Newcomb's tables of 1895-8 were the first to attempt to remove it.

Lalande (art.1015) continued:  "it follows that the right ascension of the mid-heaven" {i.e. sidereal time} "added to the complement to 24h of the Sun's mean longitude gives the mean time", and "this method is used in England to find the time of an observation: Mr Maskelyne has given in his tables the Sun's movement in time to hundredths of a second"."  In terms of the diagram, this translates as:   mean time MS = QM + (24h - QS) = QM - QS .

A modern version of the foregoing classical method for defining mean solar time: During the period when methods like the foregoing were current, sources of the solar tables used to compute the annual almanacs were occasionally updated, when newer and more correct theories and tables became available.  Thus, for the (UK) Nautical Almanac, Delambre's mean elements for the Sun were used with minor corrections down to 1833, then Bessel's mean elements from 1834 to 1863, and Leverrier's from 1864 to the end of the century. The principle remained unchanged although slight discontinuities in the tabulations occurred at the points of changeover.  Accordingly, a modernized version of the same method can be implemented using recent accurate data for the sun's mean longitudes. Two modern sources of mean elements/longitudes for the sun (both in terms of ephemeris/dynamical time TT) are available: in J L Simon et al. (1994) "Numerical expressions for precession formulae and mean elements for the Moon and the planets" (derived from the JPL ephemeris DE200 which was the official basis of the Astronomical Almanac from 1984-2002), Astronomy & Astrophysics 282, 663-683, http://adsabs.harvard.edu/abs/1994A%26A...282..663S ; and in J Chapront et al., "A new determination of lunar orbital parameters ..." (derived from the JPL successor ephemeris DE405 in official use 2003-14), Astronomy & Astrophysics 387, 700-709, http://adsabs.harvard.edu/abs/2002A%26A...387..700C .  The solar positions in those two ephemerides hardly differ from each other for present purposes.  Mean solar longitudes reckoned from mean equinox of date are given in Simon et al. The data of Chapront et al. are for a fixed equinox of J2000 to which general precession in longitude has to be applied. Both sets of modern mean longitudes are geometric, leaving mean aberration to be applied as -20".4955 in arc.  With that adjustment they can then be used like the old mean longitudes, converted to time, treated as right ascensions of the fictitious mean sun, and applied in the formulae given above to relate mean solar time with mean sidereal time.  The results may be compared with current values for UT1. {Some example calculations will be added if requested.}

Modern IERS zero meridian: It is worth noting that it appears no allowance is needed for any difference between time at the old transit instrument at Greenwich observatory and time at the new IERS reference (zero) meridian. The ground track of the IERS reference meridian is located about 102m to the east of the meridian line through the old transit instrument. A paper of S Malys et al. (2015), "Why the Greenwich meridian moved" (J Geodesy 89, 1263-1272, at http://adsabs.harvard.edu/abs/2015JGeod..89.1263M ) accounts for the difference of ground track as an effect of local gravitational deflection of the vertical. Thus, while the ground track of the IERS zero meridian is appreciably east of the old Airy transit instrument, by a distance which would make a rotation of about 5.3" arc in longitude or about 0.35 time-seconds if it represented a geocentric rotation on a spheroidal earth, there has in fact been no rotation of the reference frame (within measurement error): the deflection of the vertical makes the difference a parallel displacement.  With the nominally meridian (but off-geocentric) plane of the old transit instrument practically parallel to a geocentric meridian plane through the more easterly ground track of the IERS reference meridian, a transit instrument mounted in the IERS geocentric meridian plane would 'see' stellar transits at the same times as those occurring at the Airy transit instrument.

Modern changes in mean solar time/UT1:  The 19th-century Nautical Almanac standard for mean solar time was altered in several successive ways towards the current standard for UT1. (Not discussed here are the corrections for observed polar wandering and seasonal effects leading to the differences between UT0, UT1 and UT2.)  But there was a history of 19th-c. controversy about one of the changeovers mentioned above from older to newer solar data, worth brief description because it foreshadows and may possibly have influenced some of the 20th/21st-c. departures from the use of mean solar longitudes.

At one of the changeovers (from Bessel's data down to the end of 1863, to LeVerrier's data used from 1864 on), a discontinuity of a little over half a second was generated in the tabular relation between sidereal and mean solar time  (shown e.g. by the daily tabular values of sidereal time at mean noon in the Nautical Almanac for '1863 Dec 32' and 1864 Jan 1) (data for Greenwich mean noon 1864 Jan 1 in the 1864 volume, at https://babel.hathitrust.org/cgi/pt?id=mdp.39015068159329 , and for the same physical day in the 1863 volume where it was designated '1863 Dec 32',  https://babel.hathitrust.org/cgi/pt?id=mdp.39015068159006 ).

There was a brief explanation and apology in the 1864 preface: the step-change arose because LeVerrier's theory had identified new long-term solar periodic perturbations: as long as these remained undiscovered, their effect had been implicit in the old mean elements, and so they had to be removed from the new; the largest of them due to Mars and Jupiter was near its peak at the start of 1864 and responsible for nearly all the time-discrepancy of the changeover.

This changeover and discontinuity attracted little attention for nearly twenty years; but then E J Stone, an astronomer of some repute and director of a university observatory, was trying in the 1880s to account for growing discrepancies between observed places of the moon and positions calculated by Hansen's 1857 'Tables de la Lune'. Stone seized (quite mistakenly), as the cause of the lunar errors, on the 1864 discontinuity of about half a second in calculated mean solar time arising from the changeover from Bessel's mean solar elements to those of Leverrier (E J Stone, 1883 MNRAS 43, 335-345 and 401-407).  The two effects were unrelated and Stone had also made a mistake that exaggerated errors by a factor of about 365. Stone's mistakes were pointed out in print by no less a quartet than John Couch Adams (1883 MNRAS 44, 43-47; 1884 MNRAS 44, 82-84), Arthur Cayley (1883 MNRAS 44, 47-49; 1884 MNRAS 44, 84-85), Simon Newcomb (1884 MNRAS 44, 234-5; 1884 MNRAS 44, 381-3; 1894 MNRAS 54, 286-8) and G B Airy (1883 Observatory 6, 184-5), but Stone stubbornly stuck to his position.  He also wrote a vehement paper for the Royal Society, based on his faulty analysis of the lunar errors, in which he insisted that no revision should ever again be allowed in future to create any discontinuity in the tabular measure of mean solar time (E J Stone (1883), Proc. R. Soc. Lond.  35, 135-7, http://rspl.royalsocietypublishing.org/content/35/224-226/135 ). This was of course a demand incompatible with the use of corrected solar mean longitudes whenever new theories and observations would show the need for correction. But strangely enough, Stone's principle of requiring continuity has actually been followed in the alterations to UT1 made explicitly in 1984 and 2003 (and implicitly in 1960). It is hard to tell whether or not this arose from some descendant of Stone's influence.

There have been four main occasions of alteration away from the 19th-c. practice described above: in 1900, with Newcomb's formula for time quantities; in 1960, and the introduction of Ephemeris Time into many calculations but not into the time quantities; in 1984 with a fresh official definition of UT1 adopted from Aoki et al (1982), "The new definition of Universal Time", http://adsabs.harvard.edu/abs/1982A%26A...105..359A ; and in 2003 with the introduction of the current basis of UT1, described briefly in IAU 2000 resolution B1.8 and more fully in N Capitaine (2003) "Expressions to implement the IAU 2000 definition of UT1", http://adsabs.harvard.edu/abs/2003A%26A...406.1135C .

1900:  Newcomb's time formula made a very small alteration in the time-equivalent of the sun's mean longitude, by increasing the quadratic term. The motivation and effect go back to the work and practice of LeVerrier, summarised by A Gaillot in 1886: "Sur la mesure du temps", Bulletin Astronomique, Ser. I, v.3, 221-232, http://adsabs.harvard.edu/abs/1886BuAsI...3..221G .  LeVerrier's analysis of precession supposed that the earth's axial rotation was perfectly uniform. He also found that his theory of the precession of the equinoxes in right ascension led to a quadratic term discordant with the quadratic term for the sun's secular acceleration in longitude. By incorporating an increment in the quadratic term in the sun's longitude to remove the 2nd-order discrepancy, the effect was to make increments in mean time closely proportional to those in sidereal time (and to transmit the supposed uniformity of earth rotation into a proportional uniformity of the derived (but altered) calculation for mean solar time).  Newcomb mentioned the point briefly in his book "The elements of the four inner planets and the fundamental constants of astronomy" (1895, at  https://archive.org/details/cihm_16774 , see p.188). This appears to be the historical origin of the still-current practice of making the measure of mean solar time an exact linear function of earth rotation, a point specifically raised by the current question. No-one now supposes that earth- rotation is uniform.

1960:  With the introduction of Ephemeris Time (https://en.wikipedia.org/wiki/Ephemeris_time) into the official almanacs from 1960, the solar data was effectively corrected, not as had been usual by change of numbers, but by time-shifting it by a displacement DeltaT, the difference (Ephemeris Time - Universal Time). The numbers were maintained unchanged, but they related now to a different timescale. No alteration at all was made in the calculation of nominally mean solar time. The effect was to retain the buildup of the time errors constituted by the difference between the new best estimate of the sun's mean position and the old.

1984: The definition of UT1 was revised in a way that maintained continuity of value and rate at the moment of changeover, the start of 1984 (see S Aoki et al., (1982), link above).

2003: A further revision to UT1 was made, maintaining continuity at the moment of changeover, the start of 2003 (see N Capitaine et al., (2003), link above).

Summary/conclusion:  The description above shows something of how the current calculations for UT1 have departed in principle from classical methods used to define and calculate mean solar time. But it seems no longer to be claimed that UT1 is mean solar time: thus the IAU 2006 NFA glossary calls UT only "a measure of time that conforms, within a close approximation, to the mean diurnal motion of the Sun". Current values of UT1 can be compared with results of a modernized version of an old standard method for mean solar time from sidereal time. It seems that differences between this and current UT1 are still well under a second, though they can be expected to increase with increasing DeltaT, the difference in seconds between ((dynamical) TT - UT1). Thus the differences probably are remaining less than allowable differences for example between UT1 and UTC. The IAU gloss seems broadly justified, although it may be of continuing interest to have a method of calculating a more traditional form of mean solar time to check on any accumulating discrepancies.

{Notice of any mistakes in all of this will be gratefully appreciated!}


What are the correct positions or alignment of planets in relation to each other and the Sun, is it vertical center, baseline or completely random From a lot a illustrations and videos, the planets seem to align in vertical center to each other like this:

Is this really true? I like to imagine that the planets are actually in weird alignment to each other, for example, maybe Venus is 50,000Km below earths bottom etc

It is impossible to have the planets line up like in your image. This is done for illustration purposes only. If such an image was to scale, either the bodies would be extremely small and impossible to see, or the image would be extremely large and impractical to use.
For example, the Sun is about 1,392,000 km in diameter. The Earth is about 12,756 km in diameter (so roughly 110x smaller), and orbits at an average distance of roughly 149,600,000 km. If the Sun was a 40 cm ball, the Earth would be about 3.6 mm in diameter and located roughly 43 m away from the Sun-ball. At that same scale, Neptune is about 1,3 km away. (Before COVID, I regularly hosted “Solar System Walks” for the city of Montréal, and that was the size of my scale model.)
Now to get back to “lining up” planets… Let’s define a reference plane: the ECLIPTIC is the plane of Earth’s orbit around the Sun. All the other planets have orbits tilted with respect to that plane. For example, Mercury’s orbit is tilted 7° to the ecliptic, and Venus’ orbit 3.4°.
So there IS a point on a planet’s orbit where it lies on the same plane as the Earth—it’s called a NODE, and there’s an ASCENDING NODE, where the planet crosses from “below” (south of) the ecliptic to “above” (north of) it as well as a DESCENDING NODE where the planet goes from above to below.
HOWEVER…
1-The Earth is not necessarily in line with the Sun and the planet at that moment (explaining, for example, how the passage of Venus in front of the Sun is such a rare event [last ones were in 2004 and 2012; next ones will be in 2117 and 2125]);
2-Planets don’t [necessarily] cross the ecliptic plane as the same time as other planets; and
3-The imaginary line between a planet’s two nodes is not [necessarily] coincident with that of another planet. For example, Mercury’s ascending node is at an ecliptic longitude of 48.33° while Venus’ ascending node is at 76.68°.
4-Finally, depending on other specifics of each planet’s orbits (its so-called “orbital elements”), a planet’s orbit is not necessarily divided exactly between “above” the ecliptic and “below” it.
So… To answer your question specifically… The planets are indeed in “weird” positions and not in line, but their specific positions “above” or “below” the Earth–Sun line change with time, and it’s basically impossible for them to be in the plane of the ecliptic at the same time, even roughly speaking.
(Addendum: It applies to planetary alignments “seen from above,” but Jean Meeus has an excellent explanation of the impossibility of aligning more than 2 planets at the same time in his book Mathematical Astronomy Morsels [chapter 31].)


What phenomena was this video from SOHO actually showing? My friend showed me a video of a CME that looked as if it was swirling into a gigantic circular "spaceship" or "UFO" and he explained it as "an alien ship potentially refueling". I did a bit of research and I know assumed the images he was showing me were from SETI SOHO and they were yellow, meaning it was obviously only a single or a couple wavelengths of light shown (possibly UV?).

https://youtu.be/YnzZLNsSGyY

disclaimer: this clip is overly-liberal in its subtitles...

If yoy habe the original source that would be awesome.

Making a total BLIND guess- I'd say the plasma was swirling into the filament of the CME before it reached a critical mass to be pushed away from the sun? Once departed from the energy of the sun under certain conditions it would no longer show because it wouldn't have enough energy to emit the correct wavelength of light?

Am I even remotely correct? Obviously the space ship explanation has no merit- it's just from some dude. Hoping to get a clarification on the video, I do not believe it is a "UFO refueling" heh. Im not a conspiracy nut, and neither is my bud- so we're looking for a science answer :).

It's a polar crown cavity or coronal prominence cavity. Here is another Youtube video explaining it. It references A new look at a polar crown cavity as observed by SDO/AIA

  Conclusions.
  We propose to define a polar crown cavity as a density depletion sitting above denser polar crown filament plasma drained down the cavity due to gravity. As part of the polar crown filament,
  plasma at different temperatures (ranging from 50000K to 0.6 MK) is observed at
  the same location on the cavity dips and sustained by a competition between the gravity and the curvature of magnetic field
  lines. The eruption of the polar crown cavity as a solid body can be decomposed into two phases: a slow rise at a speed of 0.6 km/s,
  and an acceleration phase at a mean speed of 25 km/s

The video on this page shows one forming nicely.


How does the cepheid variables help in finding distance How did Edwin Hubble find distances using Cepheid variables as 'standard candles'?

Why not read his original paper? It's fairly short by today's standards, despite being such a monumental turning point in our understanding of the size and nature of the Universe.

In short though, Hubble measured the distance to Cepheid's the same way anyone does. There is a distinct relation between the luminosity oscillation period of the star and the star's luminosity. If he can measure the star's oscillation period with good accuracy, he can derive the luminosity. If he can also then measure the brightness (i.e., how bright it appears to be to him) then he can calculate the distance (since luminosity, distance, and brightness are all related).


What is a thermal distribution of eccentricities? The abstract of the new (open access) paper in Nature A statistical solution to the chaotic, non-hierarchical three-body problem says:

  The three-body problem is arguably the oldest open question in astrophysics and has resisted a general analytic solution for centuries. Various implementations of perturbation theory provide solutions in portions of parameter space, but only where hierarchies of masses or separations exist. Numerical integrations show that bound, non-hierarchical triple systems of Newtonian point particles will almost always disintegrate into a single escaping star and a stable bound binary, but the chaotic nature of the three-body problem5 prevents the derivation of tractable6 analytic formulae that deterministically map initial conditions to final outcomes. Chaos, however, also motivates the assumption of ergodicity, implying that the distribution of outcomes is uniform across the accessible phase volume. Here we report a statistical solution to the non-hierarchical three-body problem that is derived using the ergodic hypothesis and that provides closed-form distributions of outcomes (for example, binary orbital elements) when given the conserved integrals of motion. We compare our outcome distributions to large ensembles of numerical three-body integrations and find good agreement, so long as we restrict ourselves to ‘resonant’ encounters (the roughly 50 per cent of scatterings that undergo chaotic evolution). In analysing our scattering experiments, we identify ‘scrambles’ (periods of time in which no pairwise binaries exist) as the key dynamical state that ergodicizes a non-hierarchical triple system. The generally super-thermal distributions of survivor binary eccentricity that we predict have notable applications to many astrophysical scenarios. For example, non-hierarchical triple systems produced dynamically in globular clusters are a primary formation channel for black-hole mergers, but the rates and properties of the resulting gravitational waves depend on the distribution of post-disintegration eccentricities.

If I understand correctly, the paper looks at scattering events where an incident third body interacts with an existing pair and after some interaction leaves with one member, the other being ejected. The eccentricity distribution is that of the surviving pair.

The results have important implications in astrophysics; so this question just focuses on one aspect.

Question: What exactly is a thermal distribution of eccentricities? Is there something like a temperature here somewhere and for a given value why would the expected distribution of eccentricities (for some large ensemble of initial states) be a straight line with $$\frac{d \sigma}{d \epsilon_B} = 2 \epsilon$$

Fig. 4 Marginal distributions of binary eccentricity and orientation: Click for full size

  a, c, e, dσ/deB against eccentricity eB. b, d, f, dσ/dCB against the cosine of the binary inclination, CB. Line styles represent ergodic outcome distributions with the same ensemble angular momenta as in Fig. 3. The data points are binned outcomes from the same numerical scattering ensembles as in Fig. 3, with each row corresponding to the same cuts on Nscram. The eccentricity outcome distributions are notably super-thermal (the thermal distribution, dσ/deB = 2e, is shown as a black dashed line). The inclination distributions exhibit anisotropic bias towards prograde binaries aligned with L0 (the isotropic distribution is shown with a black dashed line). Horizontal error bars show bin sizes and vertical error bars indicate 95% Poissonian confidence intervals.

The thermal eccentricity distribution was first calculated by Jeans 1919. The probability density function is indeed $$f(e) = 2e.$$

See this blog post for a nice derivation of this beautiful result, which is independent of the "temperature".

The derivation relies on a small 'swindle' by assuming a population of only binaries and not any single or trinary systems; it also has divergent energy since it allows too many arbitrarily tight or loose binaries - it can only be an approximation to real systems.


Formulae for gravitatitional equilibrium I am trying to calculate at which point gravitational equilibrium sets in for various bodies (planets stars neutron stars etc.) assuming they are perfect spheres. However, the radius I get is not equal to what it should be according to Wikipedia (I tried it for the Sun, a planet and a neutron star, but all of them were off by quite a bit).

Below is the example of the neutron star radius I'm trying to get:

My result (factor) is 1 at:

Radius: $2228588\mathrm{\ m}$

Density: $1.2768744892680034 \times 10^{11} \mathrm{\ kg/m^3}$

The Wikipedia article about neutron stars says the radius of a neutron star is about 12 km and the density is about 10 times higher than mine. My result is 2228km, which is quite a bit off, so I have a second question:

Am I calculating all the forces I need?
Are the formulae I'm using correct?

I took them from Wikipedia and university slides and I found multiple variations of almost all of them (most of which I actually didn't list below), so I'm quite confused as to what is correct.

I know that I'm using iron as the element and I should split it up and convert the protons to neutrons, but that would only increase the degeneracy pressure and result in an even bigger radius, if I'm not mistaken.

Here's how I calculated it:

Mass of an electron: $M_e$

Standard atomic weight of hydrogen: $M_h = 1.00782503223$

Mass of a neutron: $M_n$

Atomic mass of ${\mathrm{Fe}^{56}_{26}}$: $55.934936 \mathrm{\ u}$

Number of atoms: $7.938 \times 10^{31}$

Number of neutrons per atom: $(56-26) \times \text{number of atoms}$

Number of electrons per atom: $26 \times \text{number of atoms}$

Total number of particles: $\text{number of atoms} + \text{number of electrons}$

Mass: $\text{atomic mass} \times \text{number of atoms} = 4.4400513610370694 \times 10^{30} \mathrm{\ kg} \approx 2.3M_☉$

Radius: $2,228,588 \mathrm{\ m}$

Temperature: $9.452 \times 10^{-7}° K$

Force of gravity: $\frac{GM^2}{r}$

Volume: $\frac{4\pi r^3}{3}$

Particle density: $\frac{\text{number of particles}}{\text{volume}}$

a: $\frac{4\sigma}{c}$

Radiation pressure: $\frac{a T^4}{3}$

Gas pressure: $\text{particle density} \times k_B T$

Electron degeneracy pressure: $\frac{\pi^3 \hbar^2} {15 M_e} \times \frac{3 \times \text{number of electrons}} {V \pi}^{\frac{5}{3}}$

Electron degeneracy pressure 2: $\frac{\pi^2 \hbar^2} {5 M_e \times M_h^{\frac{5}{3}}} \times \frac{3}{\pi}^{\frac{2}{3}} \times \frac{\frac{M}{V}}{1}^ {\frac{5}{3}}$

Neutron degeneracy pressure: $\frac{\pi^3 \hbar^2 }{15 M_e} \times \frac{3 \times \text{number of neutrons}}{V \pi}^{\frac{5}{3}}$

Neutron degeneracy pressure 2: $\frac{3^{\frac{10}{3}} \hbar^2 }{ 15 \pi^{\frac{1}{3}} \times M_n^{\frac{8}{3}} \times r^5} \times \frac{M}{4}^{\frac{5} {3}}$

Total pressure: $\text{gas pressure + radiation pressure + neutron degeneracy pressure + electron degeneracy pressure}$

Total pressure force: $\text{total pressure} \times \text{volume}$

If total pressure force = force of gravity, then the body is in equilibrium.

Mass = $\text{atomic mass} \times \text{number of atoms} = 4.4400513610370694 \times 10^{30} \mathrm{\ kg} \approx 2.3M_☉$

Herein lies the problem: your units and exponents are messed up. The mass of your iron isotope is $55.934936 \mathrm{\ u}$, and there are $7.938⋅10^{31}$ atoms. Do the math, and you'd get a total mass of $4.4400513610370694 \times 10^{33}\mathrm{\ u}$. That converts to approximately $7372878 \mathrm{\ kg}$. Nowhere near the mass of the Sun!


How/where to check where the Sun is (constellation)? I would like to check in which constellation the Sun was in at some time in the past, any recommendations how to do that?

There are plenty of online planetarium sites which should be able to do this for you. This one is the first hit for a Google search for "online planetarium software"


How do focal length, angular magnification and field of view relate? The larger the focal length the larger the angular magnification The smaller the focal length the larger the field of view.

I don't understand why this applies. I'm talking specifically about telescopes.

Wouldn't a greater angular magnification imply a greater field of view too?

Magnification and Field of View are directly opposite. A low magnification will give a large field of view, and a high magnification will give a small field of view. (Field of view indicates how "wide" is the portion of sky that is viewable in the eyepiece.)

Think of looking at the Moon with low power. Let's say the Moon fills 1/4 of the view, meaning that you could get 4 Moons lined-up side by side across the view. When you increase the magnification, the Moon looks bigger, so it fills more of the field of view. And therefore, you can get fewer Moons lined-up side by side. If you double the magnification, the Moon fills 1/2 of the view, meaning you can now only get 2 Moons side-by-side in the view.

Note that the above example is unusual for a telescope, depending on the design of course. Most astronomical telescopes used by amateurs will show an area of sky slightly larger than the Moon when using "low" power. Being able to fit 4 Moons is a bit of an exaggeration, for easy of explanation.


Is it possible that Titan is a kuiper object captured by Saturn? I think Titan may be just captured by Saturn, not formed with Saturn together, because:

It has so much nitrogen and methane, which is similar to Pluto and Triton
Except Titan, other moons of Saturn are very small, which other moons may be ejected by Titan during capturing process, similar to Triton and Neptune
According to Nice model, Neptune swapped position with Uranus, while Saturn pulled Jupiter away from the Sun, so I think the orbit of Neptune and Saturn is quite close at past, so if Neptune can capture kuiper object, Saturn also can

Is it possible that Titan was a Kuiper object?

Titan is roughly ten times more massive than Pluto or Eris, the most massive known Kuiper Belt objects (KBOs). (Titan is in fact more massive than any other moon in the Solar System except Ganymede.) It would be a rather strange coincidence if the object that was by far the most massive of the KBOs was in orbit around Saturn, well interior to the Kuiper Belt.

Titan's orbit is prograde (it orbits in the same direction that Saturn rotates) and barely tilted with respect to Saturn's equator (less than half a degree), the rings, and most of the other moons. That strongly suggests it formed out of an accretion disk around Saturn, as the Galilean moons formed around Jupiter.

Titan also has an extremely dense atmosphere, unlike any of the KBOs.

So it's rather unlikely that Titan is a captured KBO.

(Neptun's moon Triton, by the way, is thought to be a captured KBO. But it is only slightly more massive than Eris or Pluto, and it has a peculiar retrograde orbit that is very difficult to explain if it formed around Neptune, but easier to explain if it was captured. And since Neptune is the furthest out of the (known) major planets, it's not that surprising that it could have captured a KBO.)


How would I calculate the length of the day of this planet? Take the following planet:

Same distance from the main star as Earth
Main star is the same size as the sun
Planet is the size of Venus

How would I calculate the length of the day on this planet? If you need more information, please ask.

It's almost completely unconstrained. There's a limit to how fast a planet can spin before it starts to break up, which is $p = 2 \pi / \sqrt{m G / r^{3}}$ (where $m$ is the planet's mass and $r$ is its radius). For something like the Earth (and Venus is pretty similar), this corresponds to about 1.4 hours. So the day has to be longer than that.

But there's no limit to how slowly the planet can rotate. If it's rotating slowly enough so that one rotation period = one year, then as seen from the planet's surface the star would never seem to move in its sky and the solar day (time from one noon to the next) would be infinite. This is what you get if the planet is 1:1 tidally locked to the star.


What values can a color index variable get? I am a statistician with love and passion for astronomy. I try to explain some outliers I found in the values of quasars - color indexes. I found four (4) values close to twenty (20). Is that scientifically possible or there is an error in the CSV file? What values can a color index get?

Ty in advance.

The only physical mechanism I can think of for quasars to have such a large difference in two bands would be at very high redshift ($z\gtrsim6$) where you have the Gunn-Peterson trough completely erasing everything blueward of 912 Å (restframe). But with typical magnitudes of $\sim20$ or so at these redshifts, a color of $20$ that would then mean that the magnitude in the bluest band would be of the order $\sim 40$, which no instrument is able to detect.

So effectively, there would be no detection on the bluest band, in which case the color would be infinite, or rather undefined.

In other words, I think those values must be errors.


What is a good way to start learning Astrophysics I am a junior in high school and throughout my life I have always had a fascination with space. I use to watch "Space's Deepest secrets" and stuff like that but after a while I stopped learning much and became bored. Then I took an astronomy class in sophomore year which was fun until it ended. After that there wasn't much for me to do, but now I want to go from the beginning to the end. I want to learn and everything I can all over again, but this time I want to go farther. This is where my problem comes from, I have absolutely no idea where to start and would really appreciate any help that could be given.

This question is very general/broad and there is no clear answer possible (neither is there a single good way to start learning Astrophysics) so you will get a lot of subjective answers. I imagine that it may not be the typical Q&A style on this platform. But anyway, here's my subjective answer:

The first book that comes to my mind is:

The Anthropic Cosmological Principle by John D. Barrow and Frank J. Tipler

This is the highest level astronomy book that I read during middle/high school and made me decide to study astronomy. It is relatively easy to follow the physics and it has a nice story in the background (the anthropic principle) which makes it a page turner

My introductionary freshman's University book

An Introduction to Modern Astrophysics by Bradley W. Carroll and Dale A. Ostlie

This contains the basic stuff. It is easy to read and covers a broad range of topics.

What helped me understand relativity, but I imagine it is a great source for much other science topics:

Feynman Lectures by Richard Feynman

When I did not understand my course note the story from Feynman helped me through it. I actually only read the special relativity part when I needed to pass my examy, but it covers a much larger range (it is a shame I have still not read the rest while that chapter about relativity had such great impact).

Understanding quantum mechanics was much more difficult for me than relativity. I had to pass the barrier of accepting the principle rather than keep challenging it.

Introduction to Quantum Mechanics by David J. Griffiths

This is a very standard introductory text. Clearly written with simple mathematics.
Inleiding tot de golvings- en de quantum-mechanica by Adriaan D. Fokker

I like reading old-fashioned stuff. It keeps me stick to the hard facts and potentionally makes me understand better why, but espescially how, we got to construct the theoretic ideas (This particular one is hard to find; another more available good classic is Ernst Mach's the science of mechanics).

This work is an early description of the application of wave-mechanics. You can sort of see the birth of the idea of quantum mechanics

Nice works to read:

From quantum fluctuations to cosmological structures: Proceedings of the First Moroccan School of Astrophysics

There must be as many books as stars? This is one of those books. I got it from some second-hand store. This sort of stuff is what keeps me connected to astronomy after quitting my studies in astronomy.

I like to read in it since it covers a wide range of topics and is not too much in-depth. It is pretty old and a bunch of articles, but it is nice to read from this moment in time (I hate it that I can not set back the clock on the internet and get an old snapshot from it. We have the wayback archive but it isn't working sufficiently to generate an old snapshot).
Introduction to cosmology J.V. Narlikar

.... to be continued

Stuff on relativity

Space - Time - Matter Paul S. Wesson
Spacetime and Singularities an Introduction Gregory L. Naber

.... to be continued

Some other stuff that worked on me

Hydrodynamic and Hydromagnetic Stability by Subrahmanyan Chandrasekhar

This was my reference work for my Bsc thesis
an amateur radio telescope by George W. Swenson

When I was a teenager I though I could easily make a radio telescope (not).


The anomaly of spiral galaxies and the need for dark matter Please could some explain this to me - I understand that some galaxies are rotating with a speed that the stars on the outer reaches should not be held within the spiral given the observed matter and consequent gravitational field it forms. As a consequence of this, we are looking for dark matter that would compensate for this imbalance. But my question is this, how do we know that a black hole found at the centre of many galaxies is not providing the ‘extra’ gravitional curved space time required to hold the spiral galaxy together ?

Supermassive black holes are really small, compared to the mass of the galaxy.

We know how big the black hole is, because we can see the stars in the centre of galaxies orbiting it. So in a typical spiral galaxy the black hole has a mass of a few million suns.  But the galaxy has a mass of a few trillion suns.

The black hole dominates the gravity in the cluster at the very centre of a galaxy, but out by us, it is entirely negligible

You don't need to use general relativity for this.  Relativity is significant when you are close to a black hole, but if you are far from it, then ordinary Newtonian gravity is an excellent approximation.  The force of gravity drops in inverse proportion to the square of the distance, which means that something as small as a supermassive black hole is much too tiny to have any effect on the the orbital speeds of most of the galaxy.

To model the orbital speeds there has to be something with a mass of trillions of suns, which entirely surrounds the disk of the galaxy, which the stars can pass through without friction and is completely invisible.  Finding that is the challenge of dark matter.


When a fast-moving star goes supernova, how will the resulting remnant evolve? Let's take a massive star that has zero velocity. When this star goes supernova, the resulting supernova remnant will expand in a sphere. But, let's take a runaway star travelling at 200 km/s. I am predicting that the remnant will form a teardrop shape, with the rounded edge compressed a little with the rear end elongated.
My reasoning is that the material ejected "forwards" relative to the star's velocity vector will gain some velocity, leaving the dead star behind. However, the material ejected backward will lose velocity, meaning that it will stream out over a longer distance. So to a viewer it would look like a "cosmic fireball" in some sense.
Is this correct, and would there be anything that could change this result? If I am wrong, what would be the actual way that the remnant would expand?

A spherical supernova in free space will be a sphere no matter the velocity of the star (ignoring relativistic flattening). Just consider the explosion in the rest frame of the star.
If there is a medium a lot depends on the actual fluid dynamics, which can get rather involved (especially since there is plasma and magnetic effects). The initial shock front moves faster (>10,000 km/s) than the local speed of sound and would be spherical, but soon Rayleigh-Taylor instability sets in and things curl up. Still, momentum conservation means that a particle ejected "upwind" will be slowed a bit more than a particle going "downwind".
However, the relative velocity difference is small (400 km/s) compared to the ejecta velocity so the effect is going to be small in the early stages. It will begin to matter once the ejecta has slowed to such velocities, about the time it has swept up its own mass ahead of it. However, at this time the hydrodynamic instabilities will also have turned the sphere into a fractal. It might well be roughly elliptic with the star at one foci, but the overall shape is still likely close to spherical.


Why is only one side of the Moon visible from Earth? Why do we only ever see the same side of the moon?

If this is to do with gravity are there any variables which mean we might one day see more than we have before?

The reason for this is what we call tidal locking:

Tidal locking (or captured rotation) occurs when the gravitational
gradient makes one side of an astronomical body always face another,
an effect known as synchronous rotation. For example, the same side of
the Earth's Moon always faces the Earth. A tidally locked body takes
just as long to rotate around its own axis as it does to revolve
around its partner. This causes one hemisphere constantly to face the
partner body. Usually, at any given time only the satellite is tidally
locked around the larger body, but if the difference in mass between
the two bodies and their physical separation is small, each may be
tidally locked to the other, as is the case between Pluto and Charon.
This effect is employed to stabilize some artificial satellites.

    Fig. 1: Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. (Source: Wikipedia)

Fig. 1, cont.: Except for libration effects, this results in the Moon keeping the same face turned towards the Earth, as seen in the figure
on the left. (The Moon is shown in polar view, and is not drawn to
scale.) If the Moon were not spinning at all, it would alternately
show its near and far sides to the Earth while moving around our
planet in orbit, as shown in the figure on the right.

                         Fig. 2: Lunar librations in latitude and longitude over a period of one month (Source: Wikipedia)

Libration is manifested as a slow rocking back and forth of the Moon
as viewed from Earth, permitting an observer to see slightly different
halves of the surface at different times.
There are three types of lunar libration:

Libration in longitude results from the eccentricity
of the Moon's orbit around Earth; the Moon's rotation sometimes leads
and sometimes lags its orbital position.

Libration in latitude results
from a slight inclination between the Moon's axis of rotation and the
normal to the plane of its orbit around Earth. Its origin is analogous
to how the seasons arise from Earth's revolution about the Sun.

Diurnal libration is a small daily oscillation due to the Earth's
rotation, which carries an observer first to one side and then to the
other side of the straight line joining Earth's and the Moon's
centers, allowing the observer to look first around one side of the
Moon and then around the other—because the observer is on the surface
of the Earth, not at its center.

All quotes and images from Wikipedia on Tidal locking and Wikipedia on Libration.


Understanding the theory of an expanding Universe How did Edwin Hubble determine the Universe was expanding?

Was he looking in a particular direction?

And where does this Galaxy sit in the expansion.

Hubble discovered that he could measure the approximate distance to some galaxies by observing a type of variable star. The absolute brightness of these stars is known, so by measuring their apparent brightness Hubble could find how far away a galaxy was.

It was also discovered that most galaxies were "red-shifted", which was interpreted as meaning that the other galaxies were moving away from the Milkyway. Hubble plotted a scatter graph with distance on one axis and speed on the other, and noted that the more distant galaxies were moving faster, and the relationship was approximately a straight line. This meant that all galaxies were moving away from all other galaxies.

This expansion could be predicted by general relativity, which allowed for  the space between galaxies to expand. It also suggests that the galaxies were closer together in the past, suggesting that the early universe was hot and dense.

The Milkyway does not have a special position in this expansion, all galaxies are moving away from each other. There is no implication that the Milkyway is central to the universe.


What cosmic disaster is most likely to happen? Of all the different ways that the Earth can be destroyed what ways is the most likely that could happen?

Asteroid impacts are a possibility, but a killer asteroid typically affects the Earth every few dozen million years. This would probably destroy all life as we know it, but life will probably nonetheless persist. Statistically, by 500,000 years from now, we should have been hit by an asteroid 1 km in diameter, and 100,000,000 years from now, we should have been hit by one that is the size of the one that killed the dinosaurs. Statistically it's very unlikely we will get one that completely obliterates the Earth, causing it to be completely and utterly destroyed.

Gamma-ray bursts may irradiate the Earth to the point that complex life will no longer be able to survive, and only very resilient organisms like waterbears may live. At some point in the next 300,000 years, WR 104 is expected to go supernova, and if the poles are somewhere around 12 degrees or lower  we could be in the path of the supernova. We aren't 100% sure what its axis is angled at though. Either way, it won't actually destroy the Earth, but just irradiate it.

In roughly 7.5 billion years, the Earth will probably have been destroyed by falling into the Sun, as it engulfs the inner planets. This is nearly certain, but if somehow, the Earth-Moon system were to survive, then 50 billion years from now, the Earth and Moon will have become tidally locked. Then the Sun would extract angular momentum from the system as a white dwarf, and will cause the Earth's spin in accelerate and the Moon's orbit to decay. If this happens, then roughly 65 billion years from now the Moon will crash into the Earth, which would probably destroy it.

Keep in mind that there are many more ways that life as we know it could end, but these are not cosmic disasters, but are geologic and biological disasters, so they are not included. Generally speaking, it's very hard to completely destroy the Earth.


Are there only a set number of atoms in the whole universe? As per the title, are there only a set number of atoms in the universe? Or does the universe make more atoms, and if so, how does it work?

I'm a high school student that just had a question during science class, I didn't ask my teacher, since he has more knowledge in biology than anything.

There are several processes at work:

Fission (of unstable elements) increases the number of atoms.
Fusion (occurs in stars and supernovae) decreases the number of atoms.
you could claim the formation of neutron stars decreases the number of atoms: it reduces atoms to just neutrons
you could claim the formation of black holes decreases the number of atoms: it moves atoms out of reach of the 'normal' universe.

The next question is which of these processes produce/annihilate the largest number of atoms. Haven't found data for that yet. As Peter Erwin pointed out, any atom undergoing fission must have been built up from hydrogen and helium to start with.