How do we know that other planetary systems aren't composed of antimatter? How do we know that other planetary systems aren't composed of antimatter? If they never touch matter, then they don't annihilate. All their observable physics (spectra) are the same as matter, right?

There are two major points about this, I think.

First, and ultimately most importantly, is that if we have matter here, and they have antimatter over there, then somewhere in between we must have a region that transitions from matter to antimatter.  Even if this region is located in the intergalactic medium, where densities are typically very low, collisions between matter and antimatter would happen often enough that there would be a tell-tale radiation signature from this region.  To date, no confirmed detections of such a region have been made.

Now for the second point, it is not actually true that matter and antimatter have the same observable physics, though the observable differences are hard to detect even within our own laboratories.

Electromagnetism (classical or quantum), gravity, and the strong force all obey C-symmetry, and so you would not see any observable differences between matter and antimatter with these forces.

However, C-symmetry is not preserved in the weak interaction/weak force.  It is also known that parity symmetry can be violated in weak interactions.  Parity symmetry, loosely, means reflecting all particles in a mirror, and violation of it means that something has "handedness": you can tell your left hand from a right hand, and the mirror makes your left look like a right.  Remarkably, there is reason to believe that the weak force also violates the combined CP symmetry.

So the weak force introduces this sort of handedness into particle physics.  It does so with the neutrinos and anti-neutrinos in particular.

Now this handedness of neutrinos is something that could, in principle, let us detect that a mass is composed of antimatter.  Beta decay is a result of the weak force and emits neutrinos or anti-neutrinos, depending on whether the original atom is matter or anti-matter.  So theoretically we might tell a planet is antimatter if it has some radioactive material undergoing beta decays.

Except that will actually be pretty impossible.  The scale of even an entire Jupiter's worth of radioactive mass would just be too small a source of neutrino emissions.  Current neutrino detectors confirm on the order of a handful of neutrinos that probably originated from outside of our solar system every decade.  And those aren't really looking for neutrinos from any particular area; they'll look for any neutrino coming in from anywhere in the universe (well, where you are on the earth matters a bit).

So our current technology basically isn't good enough to detect these weak sector asymmetries from sources outside of our solar system.

Otherwise, assuming some improvements, a lot of time, and/or one heck of a lot of luck, perhaps our best shot at confirming a large source of antimatter comes from supernovae.  Supernovae are very, very bright (they will outshine the entire galaxy they occur in), and therefore very noticeable.  They also emit tremendous numbers of neutrinos very quickly.  So if we chance to notice a supernova, and if we luck into our neutrino detectors having enough neutrino detections at the right time to correlate to that supernova, then if somehow that was an anti-matter star we might be able to tell due to detecting the difference in the neutrinos.

But that's really unlikely.  We're substantially more likely to detect the energy signature of annihilation events at the matter/anti-matter interface, as mentioned in the first point.

See also this Q&A, which proposes using polarized light as a means of detection.


How is the Sun tilted to its orbit around the Milky Way's center? We know how the Sun is tilted in reference to its planets but how is it tilted to Sagittarius A or its orbit around it? Or isn't it tilted at all?

The wikipedia page says

  Obliquity 7.25°[2] (to the ecliptic)
  67.23° (to the galactic plane)

Interesting fact: the sun's layers rotate at different speeds, as well as different rates depending on latitude.


Obtaining stars vector position in relation to the center of the galaxy at a given date How can someone obtain such vector, for stars in the Milky Way?

Input should be star data (from a catalog, eg: Hipparcos) and Julian Date.

Output should be a Vector Position (in pc) in relation to the galactic center.

EDIT: It turns out a major problem with galaxy-center-based coordinates is that we don't actually know how far from us the center of the galaxy actually is. According to https://en.wikipedia.org/wiki/Galactic_Center estimates vary between 24,800 and 28,400 light years. You could arbitrarily choose a value, but the results would be off by at least 2,000 light years in some cases.

I was surprised that I couldn't find something like this
online directly. The closest I found is the file
"heasarc_tycho2.tdat.gz" on
http://heasarc.nasa.gov/FTP/heasarc/dbase/dump/

The format is explained on
http://heasarc.nasa.gov/W3Browse/all/tycho2.html and it
includes the galactic latitude and longitude which are
explained here:

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

Using this information, you'd still have to compute the
positions yourself, but at least all of the data is there.

Of course, as others have noted, you can take any star
catalog with any set of coordinates and convert it to the
format you need. The catalog I list above just makes it a
little easier, since they're precomputed some of the steps
for you.

As you mention above, SIMBAD also includes galactic
coordinates, but I couldn't find an easy way to download
the entire catalog.


Why is magnitude of Beta Lyrae given as 30.0 in Henry Draper catalog? Beta Lyrae (HD 174638) is ascribed visual magnitude 30.0 in the Henry Draper catalog. There are hundreds of other stars in the catalog also given magnitude 30.0. What was meant by these nominal magnitudes? Is there a resource in Vizier or elsewhere that provides corrected magnitudes?
https://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=III/135A/catalog&-out.max=50&-out.form=HTML%20Table&-out.add=_r&-out.add=_RAJ,_DEJ&-sort=_r&-oc.form=sexa

30.0 mean "variable".
If you click on the "Ptm" column header, you get a pop-up window including "Note (2) : codes used for the magnitudes:"; below that is a list of codes, including "30.0 = variable (var. in published catalog)"


How much is the zoom of Sun-Pluto eclipse image The Sun-Pluto eclipse picture was taken from 2 million km from Pluto. At that distance, Pluto will have angular size of just 4'5". Hence this picture must be a zoomed up image. Approximately how much will be the zoom of this image? Also, is it possible to give the power of its camera in megapixel terms? Thanks for your responses.

The "raw" LORRI image is 1024x1024 pixels. The field of view is about 0.29°, see section 3.6 of this paper, or abstract of this paper.
Pluto is a little less than 250 pixels in the raw image, corresponding to 0.29° x 250/1024 = 4'15'', well consistent with your estimate.


How long could an ocean last in space? If all of Earth's oceans were suddenly moved into outer space, how long would it take for them to evaporate?

My understanding is that water evaporates straight away in space... but what about when it's a quantity as large as an ocean?

Would it be a matter of seconds? minutes?

Would it be possible to swim in an ocean, that was floating in space, for a short while before it all evaporated?

Consider you have a capsule with water at $T=300$ K. Water will start boiling right after you remove containing pressure. Evaporating water will take out the heat, required for phase transition into vapor, so remaining water will cool off. It will boil until it will reach $T_0=273$ K temperature. Then part of water will start to transit into ice. The remains are ice and vapors. Solving equation $c_p m_i + c_p m_i (T-T_0) - c_L m_v = 0$, where $m_i$ is mass of ice, $m_v$ is mass of vapor, $c_L,c_p$ --- energy required for phase transitions, you can obtain fraction of ice which will remain. Whole process takes up to few minutes (depending on what size is the hole of container). Quantity of water does not change the rate of evaporation, which mostly depends on the heat transfer. The heat transfer in this case is the exchange between vapor and ice, i.e. between water and water itself, which does not depend on quantity or the size of free surface. Water will boil in every point, over all volume at once.

If the hole of container is small, then process will be slower and remaining water will solidify into a solid brick of ice. If the hole of container is large, then all water will jump out immediately and break down into small drops. Similar to liqiod nitrogen breaking down when you pour it on table.

Atmosphere is the exact reason for oceans to exist on Earth, so without it the oceans have no meaning. Jupiter and Saturn have oceans of their own, but those are hidden beneath very thick atmospheres.


What aliphatic compounds were found on Ceres, and how where they identified? Los Angeles Times's article NASA's Dawn mission finds life's building blocks on dwarf planet Ceres says;

  While the scientists aren’t sure exactly what the compounds are, the fingerprint is characteristic of material containing carbon-hydrogen bonds, and may include components like methyl and methylene.

I've located the mentioned Science article Localized aliphatic organic material on the surface of Ceres but it is paywalled.

What was found? Is it some spectroscopic feature? Aliphatic compounds are hydrocarbons that are not aromatic. Is there a spectral feature that says "I am not aromatic" specifically, or does this just mean that aromaticity was not detected?

OK, I did a bit of research on this.   The LA Times article you quoted says something a little bit useful.

  While the scientists aren’t sure exactly what the compounds are, the
  fingerprint is characteristic of material containing carbon-hydrogen
  bonds, and may include components like methyl and methylene.

Methyl means a carbon with at least 3 hydrogen bonded to it.  Because carbon has 4 bonds, that 4th bond can be part of a chain, so a methyl group offers a very large body of molecules to choose from.

More info

Methlyene is a carbon with 2 hydrogen.

This article, which is a pretty good summary, says:

  The organic molecules that have been detected on Ceres are complex
  aliphatic compounds that seem to be almost tar-like in nature.

Tar is a pretty nonspecific term, tar-like even more so, but tar is made by cooking coal or wood without oxygen, so I would assume this implies it's not Oxygen rich.  More "simple" complex hydrocarbon chains (and because "simple" complex is somewhat confusing, another way to say that it's probably mostly Cs and Hs in a fairly big molecule, probably 5, maybe 6 carbons minimum I would think, otherwise it would be too quickly sublimated into gas.  Tar like aliphatic compounds would still be vulnerable to photo-disintegration by UV light from the sun, which is common enough on Ceres Surface, but would be high enough melting points to not evaporate somewhat rapidly like shorter chains would.   That these compounds were found locally probably implies somewhat recent, localized hydro-thermal activity.

The Aromatic C-H bond and the Aliphatic C-H bond have different spectroscopy.   Source.

  There is a weak alkene CH stretch above 3000 cm-1. This comes from the
  C-H bonds on carbons 1 and 2, the two carbons that are held
  together by the double bond. The strong CH stretch bands below 3000
  cm-1 come from carbon-hydrogen bonds in the CH2 and CH3 groups.

and

  The IR spectrum for benzene, C6H6, has only four prominent bands
  because it is a very symmetric molecule. Every carbon has a single
  bond to a hydrogen. Each carbon is bonded to two other carbons and the
  carbon-carbon bonds are alike for all six carbons. The molecule is
  planar. The aromatic CH stretch appears at 3100-3000 cm-1 There are
  aromatic CC stretch bands (for the carbon-carbon bonds in the aromatic
  ring) at about 1500 cm-1. Two bands are caused by bending motions
  involving carbon-hydrogen bonds.

So, less than 3,000 cm wavelength implies what the LA-Times article said, Methyl and Methylene or CH3 and CH2 groups on fairly long chain carbon molecules, probably tar-like.   But that's hundreds, perhaps thousands of molecules that fit those criteria, some with single bonds between carbons, some with double bonds, some maybe more complex and from all the articles I've read, they don't know which ones.  They can't tell which hydrocarbons by orbiting spectroscopy, they can only see the signature of the CH3 and/or CH2 groups at the end of the chains.

To know which molecules, they'll need to test a sample.    So, a mars-like rover is probably going to go to Ceres at some point.

Why this is interesting is that these molecules aren't that long lived on the surface of Ceres, because UV light form the sun destroys them and they're location implies they came from inside Ceres, probably by hydro-thermal activity.   There's also ammonia rich clay and carbonate rich clay and those are 3 key ingredients to life.    None of this is proof that there's life underground in Ceres, but the ingredients are all there (or mostly there).  Warm water (below the surface - and Ceres has a lot of internal water based on it's density), and the basic building blocks.   It's still a far cry from proof of life as nothing has been found on the surface of Ceres that doesn't exist in comets, other than the implication of underground liquid water.   But having all the ingredients in the same place is interesting.

Small point to add, but the article above mention Ammoniated Clay and Carbonates.  Both of those are building blocks.  Neither is organic.

  This is strengthened by the fact that the molecules are found together
  with carbonates and clays containing ammonia. These have been observed
  in many regions of Ceres

Carbonates are simple.   A molecule or molecules bound to a CO3(2-) is a carbonate.

Ammoniated clay took some looking up, but I found this:

  The orbiting Dawn spacecraft has a more ideal perch to observe how
  molecules on the dwarf planet’s surface reflect various wavelengths of
  light. It was in those wavelengths that Dawn investigator Carle
  Pieters and her colleagues spied the signature of ammoniated
  phyllosilicates, which are minerals similar to clays on Earth, mixed
  in with other materials.

Phyllosilicates are just clay or dissolved rock, with one or more OH- molecules.   Ammoniated Phylosilicates, instead of having an OH- (Hydroxy group) have one or more NH2- (amides) or perhaps NH4+ (ammonium), which is a different charge so that wouldn't replace the OH but might still bond with silicates with hydro-thermal mixing.

From this article.

  The researchers also identified a mixture of minerals on the surface
  of Ceres, which they think are ammonia-bearing clay minerals and
  magnesium carbonate. The clay minerals could have been produced by
  silicates reacting with ammonia ice. However, if Ceres had formed
  where it is now, it would not have been able to pick up any ammonia
  ice to enable such a reaction

Ceres ammonia clay implies ammonia rich waters under it's surface, so Ceres may have formed further away from the sun than it's current position, perhaps in the Kuipebelt where ammonia ice can exist and be part of a planet or moon's formation.  Ceres lower density than most asteroid belt objects is also evidence of that.   While it's important that Ceres has abundant ammonia, where it formed isn't relevant to this question.

Somewhat complex Chemistry happening inside a planet with heat, pressure and liquid water isn't surprising.   None of these discoveries are even close to proof that Ceres has life, but they indicate that Ceres has all the basic building blocks of life.  Bound Ammonia, hydrocarbons, carbonates and liquid water.   It's still a huge step from building blocks to life, but these ingredients suggest it's worth taking a closer look.

Corrections are welcome.   That's all I could deduce from reading the articles.


How big should be a piece of neutron star to reach Earth's mass? Imagine a neutron star could be broke into pieces.

How big would be the piece matching Earth's mass?

The question has no answer. The minimum mass for stable neutron matter is about 0.1 solar masses (see my answer here) and so is much more massive than the Earth.

As neutron stars become less massive they also become less dense, so your question is unanswerable at that level too, since we cannot put a figure on the density of something that cannot even exist in principle.

If your question is merely what radius is a spherical object that has a similar  density to a typical neutron star, can you not do
$$\frac{4\pi r^3}{3} \rho = M_{\rm Earth}$$
to find $r$?


What is a probability of a particle launched from earth hitting any object in the universe? Imagine I pointed a laser into the night sky. How likely is it that a particular photon will ever hit anything?

This question bothered me for a while. I know very little about astronomy. When I asked my roommate he laughed and said that it must be very little, since the universe is mostly empty. I know it is true, but it seems to me that volume of the objects in the universe does not matter. Rather, it should be the area of objects when projected on a sphere that matters.

On one hand, universe is pretty empty, so hitting something outside the Solar System might be hard. But on the other hand, there are a lot of possible targets. These targets, however, appear smaller when the distance increases.

Which one of these, or other, forces will dominate? Would it make a difference if we ignored gravity?

I can't give you an exact number, but the chances your photon will hit anything within the next several billion years is exceedingly small. Nay, infinitesimally small. Almost no chance that it will happen.

What you're really asking, stated as a physics question is, what is the mean free path of a photon in the present universe?

If we consider an object (be it an electron, photon, or baseball) traveling through some medium (be it the Sun, space, or Earth's atmosphere), we can calculate how far, on average, it can travel before hitting something. This distance is known as the mean free path. You want to know how far a photon can travel through our universe before it hits something, in other words, what is the photon's mean free path?

The answer is that a photon's mean free path in our universe is larger than the observable universe. What that means is your photon can travel from Earth to the edge of the observable universe and still not hit anything (on average).

To support this fact, I point you to the Cosmic Microwave Background (CMB) radiation. This is radiation which was produced more than 13 billion years ago in the early universe and has been traveling through the universe since, almost completely unimpeded. In fact, if you tally up all the photons in the universe, you'll find that a very large fraction are CMB photons. This tells you that these photons, despite being produced 13 billion years ago and traveling through the universe since then, have never hit anything since and most are still traveling through space, unimpeded.

You are correct when you say that its the projected area of objects on our sky that matters, but you have to take into account the extremely minuscule projected area the matter in the rest of the universe has, primarily due to how far away it is.


Kinetic energy and galaxy arm rotation rate The higher than expected rotational velocity of stars and gas clouds in the outskirts of galaxies is explained today by invoking dark mater that supplies not only the additional gravitational mass, but explains the amount of gravitational lensing of more distant galaxies.  Furthermore, dark matter distribution about spiral galaxies places it on the outside of galaxies and not so much on the inside.
Certainly the higher than expected velocity on the outside of galaxies also translates into higher than expected kinetic energy. Should the extra kinetic also increase the gravitational stress energy tensor in that region of space.
If so, do our models already factor in kinetic energy and it's gravitational stress energy tensor,  overlook it, or the effect is just too minuscule to be of any importance?
I don't expect the extra kinetic energy to be a replacement for dark matter, and suspect the effect may be too small to be of much importance, but the kinetic energy distribution  around a galaxy, intuitively on the surface, appears to have the right distribution, so I'm asking generically, do astrophysics factor this in their models, or not, and if not, should it?

It is miniscule. General Relativity is not required to understand the dynamics of galaxies. Motions are non-relativistic, a few 100 km/s at most, so the kinetic energy of objects is always much smaller than their rest mass energy.
Another way of seeing this is to calculate $GM/Rc^2$, a ratio which tells you the relative size of errors you will get in ignoring GR in any calculation.
For our Galaxy, we might say there is about $M\sim 10^{11} M_{\odot}$ within $R\sim 15$ kpc, and the ratio is then $3\times 10^{-7}$, which indicates Newtonian gravity is fine for most purposes.


Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used? This newly updated answer to How precise are the observational measurements for the speed of gravity? and this answer to How is the most accurate value of  measured? cites the November 2017 arXiv preprint Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A which says that these measurements:

constrain(s) the difference between the speed of gravity and the speed of light to be between −3×10−15 and +7×10−16 times the speed of light.

What are the major assumptions and other measurements that went into these error bars? Did they use a range of values for the Hubble constant? Was the dispersion of the interstellar medium at optical frequencies included? Were known limits to the variability of fundamental constants also applied or were those assumed to be constant? Anything else?
We don't often see uncertainties in the 10-15 range in Astronomy! :-)

Potentially related:

Have more recent LIGO/VIRGO gravitational wave measurements narrowed down the speed of gravity further?
Have there been studies of "old photons" to see just how constant things like Planck constant has been?
Are photons aged?

What are the major assumptions and other measurements that went into these error bars?

The error bars in the paper are based on the shortest reasonable distance (to the authors) between the source and the Earth and a zero to ten second lag between gravity wave emission and gamma ray emission.
One key assumption is how long it took for the two signals, gravitational wave and gamma rays, to travel from the source to the receiver (the Earth). The 1.74 second difference in timing of the arrival of those signals would mean the speed of gravity and speed of light would be nearly identical if the signals traveled a long time (long distance), but maybe not so little if the signals traveled a shorter amount of time. The authors of the paper intentionally picked what they deemed to be the shortest reasonable light travel time (shortest reasonable distance) so as to magnify the uncertainty bars.
Another key assumption is that the two signals were emitted very closely in time, with the photon burst following the gravity wave by no more than ten seconds. The authors alluded to fringe theories that had the photon burst preceding the gravity wave emission by a non-trivial amount of time, and other fringe theories that had the photon burst lagging behind the gravity wave emission by much more than ten seconds. The paper mentions these only in passing.

Did they use a range of values for the Hubble constant?

The Hubble constant does not come into play here. The authors used what they deemed to be the shortest reasonable distance (the product of travel time and the speed of light) between the source and the Earth, based on luminosity.
Note that at 26 Mpc, the Hubble constant is not particularly relevant.

Was the dispersion of the interstellar medium at optical frequencies included?

Apparently not; this is a simple calculation. Moreover, dispersion at optical frequencies is rather irrelevant as the observations were of gravity waves and gamma rays.


Existence of multiple universe? In this youtube video, Prof. Michio Kaku mentions there could be multiple universes and if there are, then there could be a worm hole to some other universe.

My question is: We are not sure if the Multiverse exists, so how can we believe or have imagined that worm holes could exist and create a path to another universe?

We don't know. The Multiverse Theory is really just a hypothethis; sure, it makes sense, but it cannot be scientifically proven. We really have no reason to doubt that this is our only universe. Wormholes are just as hypothetical, since they would require infinite energy or negative energy to be created, neither of which we can get. Also, keep in mind that String Theory is, for the most part, unprovable. It cannot be confirmed through scientific experiments and is mostly guesswork.

Also, it will take trillions of years before the Universe gets too cold for life, and we will be long gone by then. However way you put it, it's unlikely that humans will ever reach other habitable planets before we go extinct, should our planet be destroyed. The closest potentially-habitable planet is nearly 480 lightyears from us – as in, it would take us 480 years to get there, if we travelled at the fastest speed in the Universe. We surely won't live to see the end of the Universe.


Can a supernova make a new star? Today I was wondering with a friend about the birth of a star. So we know that stars are born from a nebula, but the residue of a dead star (like a supernova) is itself a nebula. So can a star be born from this nebula?

This idea has been around for decades, so I'm not sure who first came up with it. Here's a reasonably sourced article on the involvement of supernova in solar system formation:Exploding Star May Have Sparked Formation of Our Solar System

  The shock wave from an exploding star likely helped trigger the formation of our solar system, according to a new 3D computer model, researchers say.
  The solar system is thought to have coalesced from a giant rotating cloud of gas and dust known as the solar nebula about 4.6 billion years ago. For decades, scientists have suspected a star explosion called a supernova helped trigger our solar system's formation. In particular, the shock wave from the explosion is thought to have compressed parts of the nebula, causing these regions to collapse.
  According to this theory, the shock wave would have injected material from the exploding star into the solar nebula.

So yes, material from supernova can end up triggering the formation of new suns, and material from supernova do end up mixed in with the nebular material that makes the new stars.


What is the right ascension overhead at Greenwich at noon on 21 march My question is seen in the title: What is the right ascension overhead at Greenwich at noon on 21 march

I'm a little bit confused since the right ascension is set by the vernal equinox and don't understand how this is related to Greenwich.

At the vernal equinox, the sun crosses the celestial equator, and by definition, it has an RA of 0h (at the current epoch).  At mid-day on that day, the sun will be more or less due south, and as the line of zero RA forms a great circle, the RA "overhead" will also be zero.

The sun won't be exactly zero RA except at the moment of the equinox, and also the length of the solar day varies throughout the year, which results in the time at which the sun is due south is not quite 12 noon by clock time.

All the above is independent of whether or not you are observing from Greenwich.


Discovery in Astronomy vs one in Physics - do they differ in required burden of evidence? Discovery in Astronomy/Astrophysics (of astronomical objects) vs discovery of physical phenomena in experimental Physics (as such) - do they differ in required burden of evidence (with the notion, that astronomy/astrophysics deals with real physically existing astronomical objects, while the aim of experimental physics per se is mostly to verify rather somewhat abstract concepts of theoretical physics)?

Perhaps answering my question in some definite way would require some analysis with historical perspective and consideration of precedences ...

This generalized question of mine was motivated by more specific case featured in https://physics.stackexchange.com/q/236107/25575

There is no central authority in science. There is no council that sets the standards. The criteria for a discovery are the same: You publish your findings, and your peers accept your results.

There is the 5 sigma rule in particle physics. Perhaps you were thinking of this. But that is not an official rule, instead it's a convention among particle physics. And it's not applicable to every field.

Things are never cut and dry. What does "Peers" mean. What if only most are convinced. Does it count? Discoveries are usually judged in retrospect. After the dust has settled and the debate s are over.

//edit

Why is the bar set so high in physics?  When "discovering" a particle there is no way to "see" it, instead you observe a mass of data and see a statistical discrepancy. Cern produces masses and masses of data, and it is sifted for anything unusual. And with so much data, the chance of seeing something unusual is actually quite high. (imagine searching for repetitions of '9' in the digits of pi - if you search far enough you can be sure to find a string of 6 9s even though the chance of a random string having 6 9s is very low)

With a mass of data, and the opportunity to repeat experiments it makes sense to set the bar very high.

Compare with an observation, such as the "chirp" at Ligo. We can't repeat it at will, the data is there: something happened that is consistent with a black hole merger. No other theory has been proposed that can explain the observations. The observation is less dependent on a statistical finding after running multiple experiments, but a single direct observation.

In fields in which a result could be explained by chance, then a statistical analysis is done, and published. This contributes to the quality of a finding, and so the number of people you will convince. And convincing your peers is the only criteria that counts in the end.


The contribution of components like dark matter, stars and gas to the mass of a galaxy I am doing an introductory course in astrophysics and astronomy undergrad level.
In it we were taught that the main components of matter in a galaxy are dark matter halo ($M_h$), gas ($M_{gas}$) and stars ($M_*$). Thus the mass of the galaxy can be given as the sum of these three components. I want to read more about this but when I search this on net I don't get any appropriate results or they are very advanced. I guess this is because the actual terminology and nomenclature of these masses is different.
Thus if anyone can give me accurate terminology of these masses it would be helpful.
Additionally we were told that given the mass of dark matter halo the other two masses can have only a certain range of values. I want to know how we can determine these range of values possible for a galaxy.
Also how do these values change as the galaxy ages?
Any links to resources regarding the above topics are also welcome.
Edit 1: Regarding my question above about the change in mass of galaxies over time I am giving my thinking. Please correct me if I am wrong somewhere.
Initially galaxy consists of only dark matter and gas. This initially contracts but later undergoes segmentation to form stars. Thus initially there should be decrease in $M_{gas}$. But slowly some gas is returned as stars die and also gathered from surroundings, hence the gas mass should remain constant or decrease at a very slow rate. Now for the stars initially $M_*$ should increase due to creation of new stars. But after some time as some of the early stars start dying out there will be simultaneous birth and death of stars, hence the $M_*$ should fairly remain constant during this period. Finally as the star formation rate decreases due to depletion of $M_{gas}$ the birth of new stars will be reduced. Slowly more and more stars will start dying and $M_*$ will be depleted leaving behind only heavier elements. Hence $M_*$ should decrease during this period. Finally coming to halo mass, I am still not sure about it. I think it should not change much over the lifetime of the galaxy because dark matter does not seem to interact with the baryonic mass. I know that these changes will be slow but can anyone give me a rough number on the slowness of these changes?

This is a rather complex question, for several reasons.

First, galaxies come in many variations, regarding such diverse
properties as mass, morphology, and environment.
Second, different observational techniques, and different models,
yield different observables — you can observe the same field of the
sky with two different instruments and deduce a different
distribution of galaxies and their properties.
Third, as you mention, galaxies evolve, and you will not necessarily
obtain the same ratio between, say, gas mass and stellar mass, even for a
given galaxy type, at different redshifts.

Nevertheless, some things can be said to be true "in general":
Stellar mass – gas mass relation
The more massive a galaxy is (in terms of stellar mass, $M_*$), the more efficient it is at forming stars. Hence, the gas fraction $f_\mathrm{gas} \equiv M_\mathrm{gas}/(M_\mathrm{gas}+M_*)$ decreases with $M_*$. Moreover, although part of the gas in stars is returned to the interstellar medium (ISM), as time goes, and a galaxy forms stars, it will "deplete" the ISM, further reducing $f_\mathrm{gas}$.
This can be seen in this plot from Magdis et al. (2012), showing the gas fraction as a function of stellar mass today (open circles) and ~10 billion years ago (closed circles):

The galaxies used in this survey are "main sequence" galaxies, and other selection criteria apply as well.
Stellar mass – halo mass relation
The dark matter (DM) component of a galaxy is much more extended and diffuse than the baryons (because DM is collisionless), rather lying in a large "halo" around the gas and stars. Of course, we cannot see DM, making measurements of its mass difficult. Only in numerical simulations, we know exactly its mass.
The larger the DM halo mass ($M_\mathrm{h}$), the more stars the galaxy has. But the relation is not straightforward. In general, $M_*$ increases with $M_\mathrm{h}$ more rapidly for low-mass galaxies, while for $M_\mathrm{h} \gtrsim 10^{12}\,M_\odot$ (roughly Milky Way-sized galaxies) the relation flattens out:
This is seen in the left panel of this plot from Behroozi et al. (2013):

The different colors correspond to different epochs in the Universe. The data is from a cosmological simulation, but the simulation was calibrated to match various observations.
Another way to show this relation is seen in the right panel, where the stellar fraction $M_*/M_\mathrm{h}$ is seen to rise until around $M_\mathrm{h} \sim 10^{12}\,M_\odot$, after which it decreases again.
Why is this? In general it is thought that star formation is suppressed at low masses because gas is more easily expelled from a shallow gravitational potential, while a high masses, active galactic nuclei become very efficient at blowing out gas, thus quenching star formation.
How are masses measured?
There are several techniques to measuring these masses.
Stellar masses are measured using known relations between the amount of stars and the amount of light from some physical process — either a single emission line, or a broader band of light. For highly star-forming galaxies, where there are still many hot O and B stars that ionize the surrounding gas, nebular lines such as H$\alpha$ or Ly$\alpha$ can be used, while for non-star-forming galaxies you can use e.g. the continuum radiation from heated dust.
The conversion depends on the assumed initial mass function of the stellar population.
Likewise, gas masses and molecular masses may be measured knowing how much light a given amount of gas emits (at a given temperature, pressure, …).
Measurements of halo masses are typically done looking at the width of various spectral lines, thus deducing the velocity dispersion $\sigma_V$ of the gas and stars. Then, the total mass $M$ can be calculated from
$$
\sigma^2 = \frac{GM}{CR},
$$
where $G$ is the gravitational constant, $R$ is the radius, and $C$ is a geometrical factor (see this answer for an explanation).


From whence is the Event Horizon Telescope black hole data available for amateur reconstruction? Ralf Vandebergh's tweet shows his own reconstruction of an image of the accretion disk around a black hole, generated presumably from Event Horizon Telescope data.

I'm assuming that the data came from a publicly available source, but I don't know for sure. That would make sense, they used two completely independent groups to reconstruct the images to cross-check each other before they were published, because with so few of the telescopes in the array recording data at any one time, the reconstruction took a bit of artistic license or "filling in".

Since the raw data is huge, boxes and boxes of hard drives with high speed digital recording of GHz rate ADC conversions, I don't think there's an ftp site for the actual raw data. And since calibrations and baselining is pretty complicated, it would be hard to reconstruct.

So I'm wondering what kind of data is available that would allow someone to reconstruct their own EHT image?

FYI Ralf Vandebergh's astrophotography can be found in links within Why exactly do “CMOS astrophotographers” prefer CMOS sensors?

From tweet:

  This is one of my processings of the #blackhole #EHTBlackHole first ever image. @RHdijkgraaf @govertschilling @NASAblueshift @ehtelescope @jordydavelaar @UofA @perimeter @smithsonian @TheNRAO @PhysicsWorld

 click for larger

From whence is the Event Horizon Telescope black hole data available for amateur reconstruction?
So I'm wondering what kind of data is available that would allow someone to reconstruct their own EHT image?

In the CalTech video Imaging the Unseen: Taking the First Picture of a Black Hole - Katie Bouman - 6/7/2019  which I just found in this question, after about 25:45 Dr. Katie Bouman says (edited from closed captions):

So we collected five petabytes of data during that time... and a little less than a petabyte was for M87... but actually what we use for imaging; we don’t work with the petabytes of data.
This is reduced down through a process of correlation and a calibration stage that initially it’s at petabytes, then it goes to terabytes, then eventually what we use for imaging is in the megabytes range, where you can just email the file. And you can actually download it online now. We have all of the data online that we use for imaging and you can try to make your own image...

It seems that the MB sized raw data is available here, found on the EHT website here per @PeterErwin's comment.
More information is available at https://github.com/eventhorizontelescope/2019-D01-02
One can only speculate on how the tweeted image was generated, but luckily that wasn't the question.
A sample of a random selection of the text files begins with:
# SRC:M87 DATE(MJD):57849 FREQ:227.0707GHz
# time(UTC)      T1     T2   U(lambda)        V(lambda)         Iamp(Jy)    Iphase(d)   Isigma(Jy)
00.86805555      AA     PV  -4396973056.0000  -4510950912.0000  0.13778746    178.1494  0.00382148
00.87083334      AA     PV  -4397497344.0000  -4511639040.0000  0.13610664   -179.6852  0.00413867
00.87361112      AA     PV  -4398009344.0000  -4512314368.0000  0.13498885    179.0204  0.00400464
00.87638891      AA     PV  -4398538752.0000  -4513015296.0000  0.13268795    179.9253  0.00369624
[...]


Could a Jovian or Saturnian moon keep its atmosphere if the system was within the habitable zone? Currently the atmosphere of Titan is 1.5x thicker than Earth's atmosphere, but it's also much much colder. It's been said that an astronomical body can keep it's atmosphere if the escape velocity is more than six times the average speed of the molecules.

Does this mean if a moon like Titan was moved to the habitable zone and heated up to livable temperatures, then it's atmosphere would escape?

What is the minimum gravity necessary for a moon to hold onto an earth-like breathable atmosphere within the Habitable Zone?

To your first question... from this paper, we can see that there are 3 types of atmospheric escape:

Jeans Escape: Temperature and escape velocity factors determine the gasses and the amount that's lost. Using the chart provided in the comments, and moving Titan along the x-axis to around below Earth's position, we can see Titan will lose most gasses except Xenon.
Charge exchange: Solar radiation creates electrons and positively charged ions in upper atmospheres by tearing electrons off atoms or molecules. Titan is protected ~95% of the time by Saturn's magnetic field, and the other 5% of the time Titan's atmosphere is magnetized, creating an artificially induced magnetic field for a short period of time. This prevents a massive atmospheric loss. Thus, if Titan is placed in an orbit similar to Earth's Titan will lose a lot of atmosphere due to the solar wind.
Vertical atmospheric escape/impact erosion: Energetic objects that strike a planet erodes its atmosphere by creating a plume of heated gas. If Titan is struck, if will lose a considerable amount of its atmosphere. However, this has little to do with your scenario, so let's not consider this.

So we can see if Titan is moved into the Habitable zone, it will lose a considerable portion, or possibly almost all, of its atmosphere.

Regarding your second question, the major factor to atmospheric loss in that scenario, for a small(relatively speaking) celestial body in the Sun's habitable zone is the solar wind. If we look at the inner planets, we can see that they all have some form of magnetic field. Mercury has a relatively strong one; Venus has an weak, induced one; Earth has a strong magnetic field; Mars has remnants of its dissipated one. The magnetic field provides protection to the planet's atmosphere.

Titan does not have a magnetic field. If placed in the Sun's habitable zone, a celestial body would have a hard time satisfying your criteria. So then, let's suppose the moon we put in the Sun's habitable zone has a magnetic field likewise to that of Earth's.

In the habitable zone, we can say that the average temperature of the body would be around 300 K, similar to Earth's. Then, let's define that an Earth-like atmosphere would be comprised of at least nitrogen, oxygen, H2O vapor, and CO2. Using the same chart, we can start by moving Titan to below Earth, and then moving it up to where H2O vapor can be retained. We find that a celestial body with an Earth-like magnetic field can retain an Earth-like atmosphere if its escape velocity is around 6~8 km/s.


How can I observe Sgr A* with itelescope.net I've been experimenting with itelescope.net and tried to observe Sgr A*.

I was wondering if there are any suggestions how I can best observe the surrounding stars.

I expect that all the itelescope.net instruments work at visible wavelengths. Therefore you have no chance at all to image the stars around Sgr A*, since it is behind about 25-30 magnitudes of optical extinction, caused by dust between us and the Galactic centre.

The published images you have seen were taken by large telescopes working with adaptive optics in the near-infrared (wavelengths longer than 1.5 $\mu$m). At these longer wavelengths the extinction caused by dust is only 2-3 magnitudes.


Venus started with retrograde rotation? I have heard that when our solar system formed that all planets except Venus were revolving around the sun in the same direction because of the normal rotation of their axes. Venus was rotating and revolving backwards or retrograde. There was a collision with Venus that caused it to rotate and revolve normally.

But why would a planet revolve backwards around a star relative to all the other planets in the stellar system?

You've confused its rotation and revolution. Venus revolves around the Sun in the same way as all the other planets, but its rotation is in the opposite direction of theirs. It has retrograde rotation, but not revolution. I think this solves your paradox.


Are sulfur and phosphorus also created in the nucleosynthesis of big stars? There are as far as I know two  fusion reactions by which stars convert hydrogen to helium: The CNO cycle (for carbon–nitrogen–oxygen) and the  proton–proton chain reaction.
The elements created in the CNO cycle are C,N,O and Fluor.
But in this video min:13:00 https://www.youtube.com/watch?v=pxrPaNDvaQE the prof. talks about Phosphorus and sulfur. Is that true and are those element also created in the CNO cycle or is he talking about something else?

It is not true to say that the CNO cycle produces these elements. It is more like a catalytic chain that aids the conversion of hydrogen to helium. Thus pre-existing CNO nuclei are required and all the reactions do is change the balance of these elements because the reaction steps in the chain have differing timescales.

Sulphur is mainly produced by alpha capture (fusion) onto. carbon and oxygen via neon, magnesium and silicon. This requires the high temperatures found near the cores of massive stars (more than 8 solar masses) and occurs fairly late in their lives, not long before the type II supernova that casts a lot of the processed material into space.

Phosphorus is not part of this "alpha chain" (https://en.m.wikipedia.org/wiki/Alpha_process) because its mass number is not divisible by 4 and it has an odd atomic number. However, it is also expected to be produced in massive stars via neutron capture onto isotopes of silicon (see https://arxiv.org/abs/1112.3824 ). This occurs in the neon and oxygen burning shells, late in the lives of massive stars.


Would a satellite that has a retrograde orbit and a shorter orbtial period than its planet's rotational period be tidally accelerated or decelerated? There are 4 configurations to consider. A satellite orbiting a planet that:

Has a prograde orbit and larger orbital period than its planet's rotational period (example: The Moon)

Has a prograde orbit and smaller orbital period than its planet's rotational period (example: Phobos)

Has a retrograde orbit and larger orbital period than its planet's rotational period (example: Triton)

Has a retrograde orbit and smaller orbital period than its planet's rotational period (example: N/A)

For the 1st example, the planet's rotation will slow down and the satellite will receed into a higher orbit. This will keep happening until both the planet and satellite are tidally locked to each other. (Tidal acceleration)
For the 2nd example, the planet's rotation will speed up and the satellite will spiral into the planet. (Tidal deceleration)
For the 3rd example, the satellite will spiral into the planet but the planet's rotation will also slow down. Angular momentum is conserved because the orbit is retrograde. (Tidal deceleration)
What would happen in the 4th example? Is it a "negative times negative gives a positive" situation and we refer to the 1st example?

In the first case, the planet's rotation will carry the tidal bulge ahead of the satellite,  this will tend to accelerate the satellite, causing its orbit to recede.  In the other cases, the planet's rotation will pull the bulge behind the satellite, causing the orbit to decay.  It doesn't matter if the period is greater or smaller in the case of a retrograde orbit. The bulge is behind the satellite, and so the orbit will decay.


Has THC been detected in space? I was wondering if and how astronomers have/had detected the molecule Tetrahydrocannabinol (THC), commonly known for it's effects in marijuana, in space.

I had previously seen a hoax regarding the drug being detected on an exoplanet, but I wanted to know if there was any possibility of it.

The detection of complex organic molecules, often abbreviated as COM in astrochemical literature, has only taken up pace in the last couple of decades since substantial advances were only made possible with the advent of new, high-end observational facilities (Green Bank Telescope (GBT), Very Large Array (VLA), Atacama Large Millimeter/Sub-Millimeter Array (ALMA), etc.) in the radio-regime, where specific fingerprints of rotational, vibrational or ro-vibrational transitions are possible to observe.
An regularly updated list of detected molecular species in space can be found at the Cologne Database for Molecular Spectroscopy (CDMS), maintained by the Universität zu Köln. In the table below, you can immediately see that only four organic molecules have so far been detected that are made out of more than 12 individual atoms. However your molecule of interest, Tetrahydrocannabinol (THC), is constituted by 53 (!) individual atoms as one can infer from its chemical forumlae: $C_{21}H_{30}O_{2}$
Its going to be almost if not certainly impossible to ever detect THC in an extraterrestrial environment through remote observations (i.e. sending rovers or other space exploration mission for in-situ explorations is another story altogether). Furthermore, THC would need to be in detectable quantities in an exoplanets atmosphere or within a molecular cloud in interestellar space. A scenario which is missing any arguments or evidence about a process or mechanism that could provide such a high density. The existence of detectable amounts of THC in interstellar space is hindered by the harsh enviroments, e.g. X-ray and UV-fields from nearby stars that would not allow any high concentrations of very large molecules. Furthermore, I believe THC is a biogenic molecule, i.e. not being created except for biochemical processes in terrestrial lifeforms or - of course - as synthetical variant in chemical laboratories. I am not aware of any non-biogenic process that could provide for high THC concentrations in a planets atmosphere. That would make it a dream candidate as a biomarker, i.e. a molecule whose presence alone would strongly hint at biological/xenological activity.  Anyhow as a consequence, the feasibility of the detection from THC fails on numerous levels, thus making the detection via telescopes virtually impossible, from at least what we know so far about how, where in what typical quantities THC is formed.


How much light does Jupiter project onto the surface of Ganymede? I suppose the answer would be a range as the Jupiter-facing side of Ganymede passed from Jupiter's light side to its shadow side. I'd also like to know how much light the sun projects onto the surface of Ganymede as well. I'm also interested in day night cycle characteristics.

How much? Well how accurately do you need it? How do you want it quantifying? And in what wavelength range?

Jupiter scatters a fraction of its incident sunlight. It also has its own luminosity (predominantly in the infrared).

A quick calculation:

The solar constant (flux at 1 au) is about 1370 W/m$^{2}$. Jupiter is situated about 5.2 au from the Sun (it varies by about +/- 5%) and has a radius of 70,000 km. The albedo is about 0.34.

Thus it receives about $7.8\times 10^{17}$ W from the Sun and radiates about $2.6\times 10^{17}$ W back into space. Assuming this is done more-or-less isotropically into a hemmisphere, then Ganymede, at a distance of 1,070,000 km from Jupiter, receives a flux of only 0.1 W/m$^2$ multiplied by the fraction of the sunlit hemisphere that can be seen.

This compares with the $\sim 50$ W/m$^2$ it receives from the Sun!

This surprising result (to me) puts into perspective all the simulations you see of things in orbit around Jupiter. The planet is still pretty faint compared to the Sun.

I'd be grateful if someone could double-check the sums!

[The intrinsic infrared luminosity of Jupiter is less than a fifth of what it receives from the Sun, so this would increase, but not double the received power.]


Does Uranus have a stronger gravitational pull on its moons than Earth does? I've read that Uranus has a lower surface gravity due to its lower density than Earth. However, does this mean Uranus has a weaker pull on its moons than Earth does for the same reason?

The "strength of gravity" (ie gravitational acceleration) is determined by the mass of the planet and the distance between the planet and the moon. The formula is
$$GM/r^2.$$

For the Earth (mass= 1 Earth, distance to moon=390000km) the acceleration is $0.003 ms^{-2}$
For Uranus and Titania  (mass =14.5 Earth, distance=435000km) the acceleration is $0.03 ms^{-2}$

In summary, the gravitational pull of Uranus on its moon Titania is 10 times greater than the gravitational pull of Earth on her moon.

The same formula applies to surface gravity:

For the Earth, (mass = 1, radius = 6370km) the acceleration due to gravity on the surface is $9.8 ms^{-2}$
For Uranus (mass =14.5 Earths, radius = 25400km) the acceleration due to gravity is $9.0ms^{-2}$.

The surface gravity on Uranus is less that that of Earth, due mostly to the relatively low density of Uranus meaning that you are much further from the centre of the planet when you are at the surface.

(These value vary due to the neither the Earth nor Uranus being perfectly spherical, and the effective gravity is also lower due to centrifugal effects)


Why has Venus's atmosphere not been stripped away by solar wind? This answer on Space Exploration to a question about Mars says that one reason Mars has such a thin atmosphere is because it lacks a magnetic field to protect it from the effect of double solar winds.
Here MBR explains that Venus does not have a magnetic field.

Image credit: ESA
If this is the case, then why has Venus's atmosphere not been stripped away by solar wind like that of Mars?

Venus has a strong ionosphere that protects it against violent solar winds. So, even though Venus has no intrisic magnetic field, it has an effective, induced magnetic field due to the interaction between the solar winds and the atmosphere, that protects it against solar winds.

Venus atmosphere is thick enough to have a consequent ionosphere, that would be the difference between Mars and Venus (and Venus was able to keep a thicker atmosphere due to its greater mass, contrary to Mars).

Sources:

Luhmann & Russell (1997),  on Venus
Luhmann & Russell (1997),  on Mars


Why is molecular hydrogen (H2) so difficult for astronomers to detect? I am reading a great deal about the various forms of hydrogen throughout the universe, and I keep reading that 'molecular hydrogen is notoriously difficult to detect', and other sentiments along those lines...
Why, exactly, is molecular hydrogen gas so difficult to detect?

The light that we receive from space contains a lot of information.  Specifically, because of quantum nature of molecules and atoms, theses small species absorb light at very specific wavelengths.Each molecule, atom or ion has a unique set of absorption features.  Think of it as the molecule own fingerprint.  Hence, if we look at a star or a hot gas, we can deduce which species are present by looking at the very specific emission lines present in our spectrum of light.  Similarly, if the light of a star or of a galaxy passes trough a dust cloud, then we can deduce which species are present in that cloud from the absorption lines present.
Molecular hydrogen doesn't really have absorption lines in the infrared, visible and UV light and so it doesn't emit nor absorb very much light and so we don't see it in our spectrum, i.e. whether there is hydrogen or not, the light that we receive looks the same.


Do we know the exact spot where big bang took place? If you rewind the universe back 14,7 billion years, all matter were in one spot, and then started expanding. Do we know where this is in reference to our own solar system? And is there anything there? Or are all matter in the universe just blown inn all different directions and none is left at the "original" spot where it originated

You're envisioning the Big Bang as a cosmological "explosion" in space where the universe is the resulting material expanding in outward all directions.

The problem is, the universe doesn't work like that.

The universe isn't a region of space that is expanding outward into another thing where you can (even hypothetically) fly out to the border and say, "Yup, that's about all there is. Let's use these borders to find the center of this expanding sphere."

The Universe is literally everything there is: space, time, and everything it contains and ever will contain. And the word contains is even a bit of a misnomer in that it suggests a container with boundaries. But the universe is simultaneously both unbounded and finite at the same time. "Finite" refers to the fixed amount of stuff this universe contains (matter, energy, etc)… and unbounded because this thing we know as "the universe" has no borders in a sense that we point to or even experience. So how would you even define a "center"? There isn't one.

There's a famous way to help visualize an expanding universe that is both finite and unbounded without a center or borders; it's called "The Balloon Analogy." Imagine our friends in Flatland living in two-dimensional space… where everything they know (their entire universe) exists on the surface of a balloon. If you start to inflate that balloon (the expanding universe),  all the little astronomers on the surface will observe that all the surrounding galaxies are moving away from them — and the farther away those galaxies are, the faster they seem to be moving away.

That's pretty much what most cosmologists believe is happening to us.

Going back to that balloon analogy, picture yourself existing on the surface of that balloon-universe and ask these questions again — In my expanding universe (the surface of the balloon), where is it's center? Everything is expanding outward in all directions, so if we rewind back in time, certainly everything should converge somewhere, right? Can we just find the borders of this outward expansion and calculate the exact center? So basdically… "Do we know the exact spot where big bang took place?"

Be careful taking the balloon analogy too far, because it starts to break down in many ways. But the answer to where the expansion of the universe emanates from really is that it emanates from everywhere; everything we know is contained within the origin of our own existence, and there is no center and there are no borders by any definition of existence that we can experience.


Given telescope diameter and focal length only, how can I find the size of a produced image? I'm new to astronomy and I'm struggling to understand this concept. For example, A 20-cm diameter telescope with a focal length f = 2 meters produces an image of the moon 1.7 cm in diameter. How large is the image of the moon produced by a 5-meter diameter telescope, focal length f = 16.7 meters?

To solve this problem, you can replace the telescope imaging system with a simple pinhole (conceptually) and just draw the rays that pass through the center of the entrance aperture. The Moon is about 29 to 34 arc minutes wide or about 0.0085 to 0.01 radians.
If your telescope has a focal length of 200 cm, then at the focal plane it will be 1.7 to 2.0 cm wide. Since the angles are small you don't need to use trigonometry, just use a small angle approximation:
$$\sin(\theta) \approx\ tan(\theta) \approx \theta$$
when $\theta$ is in radians. So the size of the Moon will be about $\theta F$ where $F$ is the focal length.

In other words, ignore the diameter of the aperture, objective lens or mirror. Just use the pinhole camera model for imaging systems. If you have a multi-element telescope (e.g. Cassegrain or refractor with a Barlow lens) make sure to use the effective focal length of the system at the image plane for $F$. A Cassegrain might have a 40 cm focal length objective but a 120 cm effective focal length for the whole system. You might have a 100 cm focal length refractor but with a 2x Barlow lens you effective focal length will be 200 cm.

Source


Proper Terminology For Satellite Going Into Planets Shadow What is the proper terminology for when a satellite or spaceship passes into a planet's shadow or night side? Is it crossing the terminator? Passing into eclipse? If there is a different name for coming back into the light, what would that be as well?

SUMMARY: The word is 'eclipse'

If you visit https://www.skyandtelescope.com/observing/a-jupiter-almanac/ and click on "Phenomena of Jupiter's Moons in 2018" you get to https://s22380.pcdn.co/wp-content/uploads/WebJphenTab2018.pdf . The latter document refers to such events as "an eclipse by Jupiter's shadow".

Since Sky and Telescope is a fairly reliable source, I'd say this is a good answer, though an answer from NASA or IAU would be better.

Note that it is also a lunar eclipse as viewed from Jupiter and a solar eclipse as viewed from the moon being shadowed.

I realize you're talking about Earth's artificial satellites, and this answer involves Jupiter's natural satellites, but I think the same terminology applies.

I happen to be working on Jovian lunar eclipse calculations and ran across this resource incidentally.


Can you identify a star's 'signature' from its spectroscopy? I overheard a discussion between friends:

  If your spaceship was transported to a random location in our galaxy, you could identify nearby stars by their signature, and then triangulate your position.

Another friend replied.

  The signature of a star isn't sufficiently unique. You can't identify a star by spectroscopy.

A third friend chimed in

  There would be enough unique pulsars to work out your position from.

I came away not convinced on the stellar spectroscopy question.

My question is: Can you identify a star's 'signature' from its spectroscopy?

Realistically, no - there are too many stars with the same spectrum.

The pulsar idea is a good one, though. There is a reason our location relative to some bright pulsars was sent out on the Voyager plaques: we are confident that a civilisation finding one could then find our sun, and us, as the pulsar rates are unique enough.


Are there any known asteroids with average density similar to that of Earth's? In this answer I mention that for very low orbits around spherical bodies, the period tends to scale only as the inverse square root of the density, and not the diameter.

For a low orbits where the semi-major axis is close to the radius of the central body, the period is related to the average density of the body and unrelated to it's size.
So a low orbit around a spherical asteroid (which there usually aren't) made of a mixture of rock and iron (which there usually aren't) will be roughly 90 minutes just like LEO (low Earth orbit), even if it's only 1 km in diameter.

I'd like to use an example, but I don't know where to find an asteroid density table, nor even a histogram, that includes something close to 5 g/cm^3.
Question: Are there any known asteroids with average density similar to that of Earth's?
Some example mean densities from Wikipedia (some rocky planets included for good measure):
body      mean density (g/cm^3)

Earth            5.5
Mercury          5.4
Venus            5.2

Mars             3.9
Io               3.5
Vesta            3.4
Moon             3.3
Hygiea           2.8
Pallas           2.7
Ceres            2.1

Apparently 21 Lutetia (~80x120 km) is one of the most dense asteroids known at about 3.4 g/cm$^3$. Unfortunately that is barely over 60% of Earth's density. 21 Lutetia is unique in that scientists believe that it has a primordial core, that is, it has a core from the formation of the solar system (and it hasn't been destroyed after all these years). We believe it is hidden under the surface. It would be rich in metals, explaining the density, and because it's hidden under the surface, 21 Lutetia would look no different from any other asteroid, which is the case. This asteroid is a prime target for mining because of its metal core. I suppose the answer to your question would be no, unless 3.4 is sufficient for your definition of "close".


Will it be dangerous when close stars become Red Giants? What will happen if close stars like Sirius A or Procyon_A become red giants? Is it dangerous for the Earth and life on it?

If for Sirius A we still have 500-800 million years, but Procyon A might become a red giant in 10 to 100 million years, by astronomical scale it is very soon.

Or nothing will happen with the Earth, because it is still too far?

Life on Earth will not be adversely affected when nearby stars become red giants.

Certainly, things will get very messy inside those systems: red giants throw out a lot of gas and dust. Red giants are a lot more luminous than the Sun because they have such a huge surface area, but they don't emit large amounts of dangerous radiation, like X-rays or ultraviolet light. In fact, a red giant is cooler than the Sun, so it emits relatively little UV light.

According to Wikipedia, it's possible that a red giant system could even be habitable, so life in nearby systems should be quite safe.

  Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a 1 M☉ star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 AU out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional 109 years.
  Later studies have refined this scenario, showing how for a 1 M☉ star the habitable zone lasts from $10^8$ years for a planet with an orbit similar to that of Mars to $2.1×10^8$ yr for one that orbits at Saturn's distance to the Sun, the maximum time ($3.7×10^8$ yr) corresponding for planets orbiting at the distance of Jupiter. However, for planets orbiting a 0.5 M☉ star in equivalent orbits to those of Jupiter and Saturn they would be in the habitable zone for $5.8×10^9$ yr and $2.1×10^9$ yr respectively; for stars more massive than the Sun, the times are considerably shorter.

However, the Sirius and Procyon systems mentioned in the question are both binary systems, and both Sirius B and Procyon B are already white dwarfs. That makes the picture more complicated, and definitely more dangerous inside those systems, but neighboring stellar systems won't be harmed.

When Sirius A and Procyon A become red giants their white dwarf companions will get bombarded by a lot of material. If a white dwarf accretes enough hydrogen, it can lead to a runaway fusion reaction, in other words, a nova explosion.

  Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and rapidly converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf surface and produces an extremely bright outburst of light.

A nova is bright, but it only emits a tiny amount of energy compared to a supernova, and poses no danger to neighboring stellar systems.

Actually, it's very unlikely that either Sirius B or Procyon B will have nova events: they are simply too far from their companions to accrete enough hydrogen for that to occur. (Thanks to Peter Erwin for that info).


If I have amateur astrophotography image data that might be of scientific value, what should I do? I have some image data that may be interesting to someone, somewhere, but I don't know who to contact or notify.

I would imagine it would depend on the type of data, but maybe there is a central place people can contact?

It depends, as you say, on the subject matter. For instance, images useful in variable star study can be provided to the AAVSO http://www.aavso.org/ and images relating to asteroids can be provided to the IAU Minor Planet Center. http://www.minorplanetcenter.net/ Other subject matter will be accepted elsewhere.

However, for the images to be useful in scientific fields, you will need to comply with the submission instructions, which a casual snap probably will not comply with. On the MPC guide for beginners there is a list of 44 technical suggestions for submitting scientific data: http://www.minorplanetcenter.net/iau/info/Astrometry.html The AAVSO has a tutorial in six chapters on photometry using a DSLR camera (and another for astro CCD cameras) http://www.citizensky.org/content/dslr-documentation-and-reduction

If you just want to get it out there and have people comment on what is in it, post it to Google+.


How strong is the gravity from half of the distant universe? I would like to know, how strong would the gravity be from the distant universe if we were somehow able to remove half of it.  For example, if everything behind us did not exist, how strongly would the stuff in front of us be pulling?  By distant, let us consider everything beyond 10 billion light years (e.g. remove all the closer stuff).  How many G's acceleration would I experience (discounting the fact that I would be in free fall and wouldn't feel it)?

Not very strong at all. I get a rough figure of $3.725\times 10^{-9} m/s^2$.

To perform that calculation I made a few simplifying assumptions.

Assume that we can ignore everything outside the observable
universe.
Assume that the observable universe is a static homogeneous sphere
with a radius of 46.6 billion light-years, and a mean density of
$\rho = 0.85\times 10^{-26} kg/m^3$, that figure is the critical
density
needed for the global curvature of the universe to be zero (i.e., on
the very large scale spacetime is flat).
Assume that we can use Newtonian physics to calculate the
acceleration due to gravity.

I didn't bother removing all the stuff closer than 10 billion light years.

We're standing at the centre of the flat surface of a hemisphere of radius $r$ and we want to know how strong the gravity is. We can pretend that all the mass of the hemisphere is concentrated at its centre of mass, which is $\frac{3r}{8}$ below us.

The Newtonian formula for gravitational acceleration is

$$a = \frac{GM}{r^2}$$

Mass is volume times density, i.e., $M=V \rho$ and the volume of a hemisphere is

$$V = \frac{2}{3}\pi r^3$$

Combining everything, not forgetting to use $\frac{3r}{8}$ as the distance term in the denominator of the acceleration formula, we get

$$a = G \rho \frac{128}{27} \pi r$$

Plugging in the values I mentioned earlier for $\rho$ and $r$, the Google Calculator says $3.72484086\times 10^{-9} m/s^2$. That's about $3.8\times 10^{-10}$ standard gravities.


Absolute model ages of lunar craters I have been looking at several articles on the dating methods of craters, but I am wondering how exactly the "absoluteness" creeps into it. I came across several methods that link crater "freshness" (i.e. the 7 degrees of freshness by Pohn and Offield (1970), see e.g. Google ebook) to the number of superimposed craters to rock abundance to thermal inertia etc. and these are all supposed to be an indicator of age, but what is the gauge for that? How do you convert that to a number of years?
The literature seems to call these "absolute model ages". I have seen it mentioned that you obtain them from fits to crater production functions.
So how do you obtain absolute model ages, really?

First, Terminology:  Age is how old something is.  Relative age is how old something is when compared to another (older vs younger).  Absolute age is putting a number on that age (I am XX years old).  Absolute model age is an absolute age that is based on a model, but where the age has not been measured directly (based on this cat's tooth, its absolute model age is roughly 3 years old).
Second, the Lunar Crater Chronology, Relative:  A generally known quantity (with some caveats that I'm not going to get into in this answer) is that impactors have been striking surfaces in the solar system since its formation.  So, if a surface has more craters than another surface, on the same planetary body, then it can be assumed to be older than the surface with fewer craters.  So by measuring the spatial density of impact craters on multiple surfaces, you can tell a relative age relationship of those surfaces.
Third, the Lunar Crater Chronology, Absolute Model:  If we are able to tie a specific age, like 3.7 billion years, to a surface that has a specific crater spatial density, like 0.002 craters ≥1 km per square kilometer, then we have an anchor point.  And, to within a factor of ~2 or so, we can say that any surface on that planetary body with 0.002 craters ≥1 km per square kilometer is going to be 3.7 billion years old.  If we are able to anchor several surfaces, with different crater spatial densities, with different radiometric ages, then we can plot them on a graph (crater spatial density on the vertical, age on the horizontal) and fit some function to them.   We can then use that function, or model - which has been calibrated based on absolute ages - to give us the absolute model ages of any lunar surface by measuring their crater spatial densities.  We've done this with data from the Apollo and Luna sample returns.  As the other answerer posted, I did a recent review and re-kajiggering of that function.
Fourth, Dating Individual Features:  That lunar crater chronology can be used to date individual features even if they don't have 1 km craters on them.  We measure smaller craters, and then use another model of how many small craters there are relative to large craters, to figure out how many there WOULD be on that surface if 1 km craters existed on it.  Or, if it's a big or older surface, we have those 1 km craters there and can get the age via that lunar crater chronology function.
Fifth, Rock Abundance:  The DIVINER instrument on the Lunar Reconnaissance Orbiter takes temperature measurements at different times of day and from those can model the rock abundance on the surface.  Basically, a boulder holds onto its heat longer than sand, so the boulder is going to be warmer at midnight than the not-boulders.  What the DIVINER team found, maybe a decade ago, is that young (<1 billion years old) lunar craters have high thermal inertias - large rock abundances - surrounding them.  Rebecca Ghent (et al.) used a few craters that had absolute ages (from Apollo) and absolute model ages (from the chronology curve based on smaller superposed craters) and demonstrated there was a pretty good linear relationship, meaning that for craters <1 billion years old, we now have another method to date those individual features via the DIVINER rock abundance measurements, which, honestly, is easier than counting craters (or at least takes less time).
Sixth, Spectral Slope:  A recent paper demonstrated that young craters also show rays in the far-ultraviolet light, and that removal of those far-UV rays follows a fairly parametrizable relationship, so they were able to get independent ages of young, large craters, too.  Optical maturity studies is not new, but this is the first remember seeing it used in UV for this exact thing.


Oxygen in a methane atmosphere akin to methane in oxygen atmosphere? If we assume a planet the size of Earth with an equally dense atmosphere of nitrogen and methane, rather than oxygen, and it's in the goldilocks zone. Would a hypothetical alien species be able to use oxygen for lanterns and stoves the same way we use methane (or natural gas, etc)?

If so, what primary differences in ratios and/or densities of gasses would have to exist? If not, why not?

Yes, it would work.

The difference between "fuel" and "oxidizer" is rather made for convenience, but it's not a fundamental distinction at the macroscopic level. The only differences are at the microscopic level, related to what the electrons do during the reaction. All such processes are redox reactions (reduction-oxidation), where one participant is the reducer, and the other is the oxidizer. That simply means the reducing agent loses electrons, while the oxidizing agent gains electrons; once the electron balance is final, the two components are bound together and the reaction is complete.

The oxidizer doesn't even have to be oxygen, it can be many elements or compounds hungry for electrons.

The bottom line is - you can burn either gas in an atmosphere made of the other. The gases don't care, as long as they react with each other. There are videos online where you can see chemists burning oxygen in an atmosphere of hydrogen, or burning hydrogen in an atmosphere of chlorine, etc. All these are redox reactions.

As to what's the minimum percentage of methane in the atmosphere that could sustain an oxygen flame, I am unsure. But it must be similar to the situation on Earth, where a low-enough oxygen contents in the air will eventually extinguish any flame. I would guess that 20% methane would work just fine for burning oxygen, but that's just a guess, so take it for what it's worth.

As others have pointed out, a methane atmosphere would indicate a very different chemistry at the planetary level, which may or may not be able to sustain life in general. It would definitely not sustain life as we know it.

Also, the Goldilocks zone for a methane atmosphere is different from the same zone for an oxygen atmosphere - it's further away from the star, because methane retains infrared better than oxygen (methane is a "greenhouse gas").

Fire and Flame 19 - Oxygen Flame in Hydrogen (Royal Society of Chemistry)
Fire and Flame 21 - Oxygen Balloon in Hydrogen (Royal Society of Chemistry)
Fire and Flame 22 - The Planet Jupiter (Royal Society of Chemistry)
Fluorine - Periodic Table of Videos
Exploding Hydrogen Bubbles (Periodic Table of Videos)


Have I just observed a supernova? First of all, I'm from Syria. In Syria we have 70% of the night without electricity. So I love hanging out at this period of time in the night, looking up just as Prof. Stephen Hawking said to the sky, observing the beauty of the night sky. Tonight, when I was looking arbitrarily I saw a star. Suddenly its brightness increased gradually, and then this star released two big sparkles. They were more like distorted yellow or golden rings formed or more like waves.

I don't know how to express this phenomenon! Then those two sparkles disappeared, and the brightness of this star decreased very very fast until it just disappeared. It was the first time in my life I saw something like this! I just don't know what to say! This phenomenon lasted for a couple seconds.
I was shocked as much as you can imagine. I was unable to speak, totally impressed! I just cannot express my feeling! Maybe this is not a supernova! But what is that? Why did this star do that?

The odds of a supernova being visible to the naked eye is slim. Adams et al. (2013) estimated that there is a 20% chance that we will be able to see such a supernova within the next 50 years. That should give you an idea of how unlikely this is.

The vast majority of supernovae that have been discovered lately have magnitudes ranging from +20 to +16 (check out this page, updated daily). The naked eye can see objects of magnitude +6 or less, meaning that a supernova of magnitude 16 is $10^{10}$ times fainter. Powerful telescopes are needed to observe these events.

Supernovae also aren't that quick. They fade over days or weeks or months or even years. You would not see a bright, quick flash of light. I don't want to say that it's impossible . . . but it's impossible.

If this was a supernova, it's even more unlikely that you could have seen such detail. When I say that a supernova is visible to the naked eye, that just means that you can see the light from it. It doesn't mean that the structure of any gas or dust being expelled can be observed. To make out that level of detail, it would need to be quite close to Earth - and so it would definitely have been observed by astronomers and amateur stargazers in quite a few places.

So no, I don't think this was a supernova. I have no idea what you saw, but it was not what you think it is.


Eternally collapsing objects? As has been pointed out by some scientists, that black holes aren't stable bodies but are eternally collapsing objects. How does one come to such a conclusion? Is hawking radiation related to this in some way?

With respect to "eternally collapsing", they are probably referrring to the fact that in the reference frame of an external observer, gravitational time dilation prohibits matter from ever reaching the event horizon (the "surface") of the black hole.

Denoting the radius of the event horizon $r_\mathrm{S}$ (for "Schwarzschild radius"), time runs slower by a factor of $(1 - r_\mathrm{S}/r)^{-1/2}$ for an observer at a distance of $r$. As $r$ approaches $r_\mathrm{S}$, this factor goes to infinity, i.e. the observer will never reach $r_\mathrm{S}$

However, this is only for the external observer; the falling observer would cross the horizon and reach the center in a finite time.

With respect to "not being stable", you're probably right that they are referring to Hawking radiation which presumably makes the black hole slowly "evaporate". Near $r_\mathrm{S}$, pairs of virtual particles are being created and can be turned into real particles by the gravitational field. If they avoid falling into the black hole, energy is taken away from the hole, reducing its mass.


Could the rotation of a black hole cause other planets to rotate? Because everything rotates so perfectly, and we know that some blackholes rotate and they're the biggest things in the universe, so a silly question comes to mind.

If you remove all black holes from a galaxy:

Would it still rotate?
Would the rotation of the planets inside it slow down until there's no more rotation/gravity?

I imagine that the black hole has something to do with the fact that we have gravity since it's always rotating and pulling spacetime fabric inward like a tornado. There are 100 million black holes in our galaxy so I think of them as rotation generators.

Last year I read a research paper that the rotation of a black hole can't cause gravity, but forgot why.

A rotating black hole (or indeed any other rotating object) can cause nearby objects to rotate through an effect called frame dragging. However this effect is really small! Detecting it requires either a a body to be in a pretty close orbit around a black hole, or extremely sensitive instruments.

The fact that the galaxy and almost everything in it rotates in one way or another is not considered to have anything to do with this effect. Instead it is basically the rotation left over as clouds of gas, dark matter and later dust collapse under their own gravity. Inevitably they do not fall exactly straight into the middle of the cloud, but end up aiming off to one side or another and so orbiting around the centre, and the final spin of the collapsed object comes from the more or less random overall total of these spins. Objects spin so fast because very large clouds of gas and dust have collaposed to form fairly small objects like planets and stars, and just like an ice skater speeding up their spin by pulling in their arms, this speeds up the spin of the central object.

If you removed all the black holes from a galaxy there would be no major change. A shift in the orbits of stars, especially near the centre of the galaxy and some changes in the behaviour of the clouds of gas between stars as the radiation from the central black hole stops acting on them.


Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen? I just saw this YouTube video made by PBS Studios. There they explain that $H_2$ can reduce the traslational kinetic energy of regular $H$ atoms when it is formed since the energy can also be stored in vibrational and rotational modes after each collision. They state that the gas was so hot that regular atomic $H$ generated enought pressure to impede gravitational collapse and that, only because of $H_2$, it was possible to lower that pressure and stars could form.
My question is, if $H_2$ were impossible to form (for some reason) wouldn't gravitational collapse eventually happen as the universe expanded and the gas cooled down? Or the expansion itself would be enought to prevent this by lowering the density of the gas before any star could be formed?
Would a universe without $H_2$ had formed any stars in its entire evolution?

The role of H$_2$ is to allow primordial gas to cool down sufficiently to allow the collapse to start and then to hold the gas as a relatively low temperature as it gets much more dense. The formation of H$_2$ is essential because atomic hydrogen simply has no way of cooling itself below temperatures that are capable of exciting the $n=2$ level and then emitting Lyman alpha photons. This means that without H$_2$ the gas can't cool effectively once it gets below about $10^4$ K and it will never reach the densities required to form stars.
The ongoing formation of H$_2$ in primordial gas phase reactions (no dust required) allows a collapsing cloud to cool down to around $\sim 200$ K.
Without molecular H$_2$ you get stuck with large ($\sim 10^4 -10^6M_{\odot}$) pressure-supported "protogalaxies" that won't collapse any further. These are formed at redshifts of around 20-40 from growing primordial density perturbations and have initial temperatures of a few thousand K. The ongoing formation of H$_2$ in primordial gas phase reactions (no dust required) allows a collapsing cloud to cool down to around $\sim 200$ K.
The importance of H$_2$ cooling is outlined in reviews by Glover 2005 and reiterated more recently by Klessen 2018. A telling quote is from the review by Bromm 2013, who says

If the gas were unable to cool, there would be no further collapse, and consequently no
gas fragmentation and star formation. The gas would simply persist in hydrostatic
equilibrium, roughly tracing the density profile of the Dark Matter. Early on, it was realized
that cooling in the low-temperature primordial gas had to rely on molecular hydrogen
(H2) instead (Saslaw and Zipoy 1967).

Conversely, in gas that is enriched with heavier elements, H$_2$ cooling is totally unimportant because there are more efficient routes for a collapsing cloud to radiate away energy (e.g. Glover & Clark 2012).


Where did the nickel in the Earth's core come from? Why is there nickel in the core of the earth? Does it come from collisions of two neutron stars? And how did we know the core of our planet is made of nickel-iron alloy? Thank you.

Nickel is formed in supernovae.
There are several different types of supernova.  Wikipedia suggests that about 70% of nickel was formed in exploding white dwarfs, and 30% in explosions of massive stars.  Unlike say gold, merging neutron stars do not produce a substantial proportion of the nickel on Earth.
We know about the composition of the core by Earth Science techniques (studies of Earthquake waves that pass through the core tell us about its density, we also know about the mass of the Earth, and these are all consistent with an iron-nickel core)  And from meteorites.  Metallic meteorites have come from the cores of disrupted asteroids, and these asteroids formed at the same time as the Earth. They are made of the same mix of iron and nickel.


Do comets have a limited number of orbits? Do comets have a limited number of orbits?

Everything in the universe has a limited number of orbits.   But the way to make sense of your question is to ask how long comets last.  Comets lose ice when they fly past the sun, so they get smaller each time.   How much smaller depends on how close they get to the sun and their composition, or how easily they "melt" or break apart.   (Ice doesn't exactly melt in space, it sublimates when it heats up)

Sungrazers have a very limited number of orbits.   Halley's also is expected to have a limited lifespan of about 100 more orbits before it stops being a comet and basically becomes a rocky body.   Halley's passes closer to the sun than Venus at its perihelion, so it loses a lot of ice, each pass, which helps make it's tail impressive. (when Earth and Halley's are properly aligned that is).

Your question also depends on your definition of a comet.   Will Halley's still be a comet after it loses it's ice and it's tail? (sometimes called a Manx comet).  That's a matter of interpretation.   It will get smaller, but it will likely remain in orbit for some time after it loses it's ice.

Orbiting objects that cross planetary orbits are also, on average, less stable, but I suspect that's another subject.   Halley's might be destined to crash into the Earth, or the Moon at some point.  It's orbit nearly intersects the Earth's orbit in two places.  But don't worry.  That's not going to happen anytime soon, if it happens at all.


Filters for solar eclipse and solar feature viewing I have

a Vixen ED100SF ED refracting telescope and
10x50 6.5 Bushnell Legend Porro Prism binoculars

and would like to outfit both with filters for solar eclipse viewing. I gather the basic setup simply involves finding an appropriate filter, that fits the equipment, but I'm new to this and the range of options is bewildering. Ideally I'd also like something that allows me to see solar features, but I gather that these can be quite expensive.

What are my options (or a good reference on options) for viewing solar eclipses and solar features? Would something like an "eyepiece" filter work on its own for safe viewing, or would my telescope require something different?

You mustn't use an eyepiece filter. It is very very dangerous. In my opinion the best is buying an Astrosolar paper, that isn't expensive and you can see very well an eclipse. If you want to see solar features you must spend more and you need an H-alpha filter, but the view with that is amazing


Draw the celestial sphere for latitude, showing the star Sirius, Estimate the approximate date when Sirius rises with the Sun I'm having trouble understanding how to solve a certain problem.

The problem is from the book Astronomy: Principles and Practice by Roy and Clarke. It is actually a solved example, but I am unable to understand parts of the solution.

  Draw the celestial sphere for latitude 30° N, showing the star Sirius (right ascension
  6h 40m, declination 17° S) at rising and draw the ecliptic. Estimate from your diagram the approximate
  date when Sirius rises with the Sun.

This is the image from the solution. I'll copy the entire solution.

Now, here's what I don't understand. If Sirius is at X, and if X is on a parallel of declination, why isn't this parallel parallel to the equatorial plane? Does the same go for the Sun? And how do we get the RA of the Sun?

The figure doesn't show the -17° parallel of declination.
If it were shown, it would be parallel to the equator as you expected, as would the parallel of the Sun's declination.
PAQ and PBQ are semicircles of right ascension, intersecting at the celestial poles P and Q.

For an interactive aid, try Stellarium.
You can highlight the meridian, equator, ecliptic, etc. in the View > Markings dialog.
Under View > Sky, orthographic projection renders the sky as a celestial sphere, mirror-reversed from the book figure.
To reproduce the problem scenario, set a location at 30° N and a date of...July 24.
The Sun's right ascension on that date is closer to 8h10m.

The authors don't explain how they determined an RA of 8h40m for the Sun, and they're off by half an hour; maybe they eyeballed it as 30° east of Sirius in the diagram.
The Sun's RA is exactly 0h at the March equinox, 6h at the June solstice, 12h at the September equinox, and 18h at the December solstice.
Between those milestones, the Earth's orbital motion makes the Sun's RA increase about 4m per day or 1h in 15 days.


Normalizing the supernova spectrum by photometry I have a SN spectrum here (already reduced) that seems to have some artifacts. Professor tells me that I may have to normalize it using photometry.
As far as I understand, I just have to multiply it by some factor to make the integral over all frequencies equal to bolometric luminosity. Am I right, or is there some more complicated thing here?

I think you understood correctly.

Just to be clear. Your photometric data have enough images including bias, flat, and standard stars, and are reduced + calibrated with standard stars. And, your issue is that you can reduce spectrographs (with bias, flat, and arc lamp) but you don't have standard stars for the calibration. Hence, you want to calibrate your spectra with the photometric data. Is this correct?

The following is for more detailed example how to proceed this.

First, note that a photometric data point is the accumulation of photons within a bandpass. See the response curve shown here for example. In the example, B band has its effective wavelength 4353 A, FWHM 781 A, and with its own shape of the response function.

For simplicity, the following demonstration will assume that the response function of the B band is delta function, i.e., zero elsewhere except at 4353 A. We will extend this example to other response function later. Suppose that you have a spectrum containing 4353 A, and you want to calibrate it with the known B photometry. So you can find a ratio = flux(B; photometry) / flux(B; spectrum), and apply this ratio elsewhere in your spectrum.

Note that the spectrum and photometry must be from the same epoch ideally. Otherwise, use the nearest available epoch as an approximation.

If you have more than one photometric data points to calibrate the one spectrum, ideally the ratios elsewhere will be the same. Realistically, they are not. Hence, you average the ratios by somehow.

Next, if you would like to consider the dispersion of the response function, instead of using the flux at a point, you accumulate the fluxes by considering the shape of the response function.

For simplicity, let's say the B band response function is a Heaviside step function = 1 within [4353-781, 4353+781] A and = 0 elsewhere. So, you accumulate the fluxes weighted by the response function from your spectrum. Then find the ratio, and follow the rest of the process.


How should I think about sunlight for the purposes of analyzing the angles it makes w/ earth? Most diagrams I see of sunlight look like something like this:

.

They show that for basic geometric analyses, the sun's radiation can be thought of as 'thin lines' that strike earth. Further, at an equinox, those rays strike hit the horizon of every point on earth (except the poles) directly from the east. So, the diameter for the earth does not matter at all it terms of the angle at which the rays strike.

I'm curious - how far will this conceptual frame take me? What is a better way to conceptualize how the sun's light strikes the earth?

This is a first-order approximation, and works well for simple models of irradiation of the atmosphere or the Earth's surface.

But, a simple experiment with a pinhole camera or a single lens will show that this model doesn't work in imaging situations.  The sun is an "extended object," meaning it is not a point source, so the image you get is not what you would expect from a point source.  Worse, even if the sun were a point source, the distance to the geometric focal point is different very slightly from the distance if all the sun's rays were perfectly parallel.  See the Lensmaker's Formula,  $ \frac{1}{f} = \frac{1}{p} + \frac{1}{q} $ where p and q are the distances from the lens to the source and the image respectively.   (I'll warn that for a reasonable lens, say focal length $ f = 10 m$, the difference between focal location for rays that are parallel and rays from a point source $150\space million\space km$ away is ridiculously small)

Similarly, if you want to model the umbra and penumbra during a solar or lunar eclipse, you'll need to take the physical extent of the sun into consideration.