How would the night sky look from inside a globular cluster? When the weather is clear, we can look at the stars. And we normally would see several thousands of them, they all being more than a $\textrm{pc}$ away from us.

Now, there are globular clusters, which consist of some $10^5$ stars concentrated is a few $\textrm{pc}$ area. From oustside they look like this:

Now, how do they (and the whole sky) look from inside? Imagine that solar system is inside such a cluster. Would it be a big change? Would there be a significant difference between night and day? Would it be easier or harder to study astronomy there?

P.S. The credits for the question go to Ross Church.

Globular clusters occupy an interesting place in the spectrum of composite stellar systems. As you point out, they are highly concentrated populations of stars, and seem to lack any dark matter component, unlike more massive dwarf galaxies.

Binary interactions become very important in simulating globular clusters, and interestingly enough (maybe unsurprisingly), the one example of a discovery of a planet found in a globular cluster has been around a binary star system (see: PSR B1620-26 b; this circumbinary planet was found orbiting a pulsar and a white dwarf.). This is not to say there are not other examples, however, this was the easiest for me to come across. I would be interested to know how common this situation is, and in addition, how stable it is given the potentially highly chaotic environment it lives in. These speculations don't answer your question, but I thought it interesting enough to bring up as evidence in favor of your question not being an unreasonable one to ask.

From the wiki page:

  Globular clusters can contain a high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster.[26] The typical distance between stars in a globular cluster is about 1 light year,[27] but at its core, the separation is comparable to the size of the Solar System (100 to 1000 times closer than stars near the Solar System).[28]

This seems to indicate to me that location within the globular cluster would matter quite a bit. If at the core the average distance between stars is about three thousand times closer than our nearest neighbor is to our sun (my estimate to give some perspective: a few lightyears to Proxima Centauri divided by 100 is about 3000AU (about 100 times further than Pluto from the sun)), then stable orbits may be shifted inward, or simply may not exist due to two-body interactions.

However, if life were to exist (an assumption we're going to make for the purposes of your question), one would see a very different night sky. According to this paper, the number density profile of stars within the globular cluster M92 follows a Wilson Profile fairly well, which has the form:

$$ f_{W} = A\{ e^{-aE} - e^{-aE_{0}} [1 - a(E-E_{0}) ] \} $$
where $E \le E_{0}$. E is the specific energy of the star:

$$ E = v^{2}/2 + \Phi(r) $$

and where $\Phi(r)$ is the mean-field gravitational potential, determined from the Poisson equation. For each family of models, the constants A, $E_{0}$, and a in the above distribution function define two dimensional scales (a typical radius and a typical mass or velocity) and one dimensionless parameter, the central depth of the potential well (related to the concentration parameter) (all of the information is taken from the paper I've linked).

It seems to be the case that globular clusters are not "simple stellar populations", in that they are usually made of multiple generations (sources: 1,2). However, globular clusters generally consist of population II stars and are older stellar systems when compared to other star clusters. I bring all of this up because in addition to the number density of stars, the stellar distribution of stellar types would certainly be an important factor in how the night sky would look. If you lived a thousandth of a lightyear from a blue supergiant, you could imagine that that would make a huge difference in what you would see on a day to day basis. At the same distance, a supergiant star is on the order of $10^{5}$ times the luminosity of our sun (and therefore is $10^{5}$ the flux, since $L \propto f$ holding $D_{L}$ constant). At the same distance as our sun, the magnitude of a star with $10^{5}$ times the flux would have an apparent magnitude of about -38 (I used Rigel as my test case; this produces a star in our sky which is 12 magnitudes brighter than our sun). Moving this to the average distance between stars at the center of a globular cluster we would get an apparent magnitude of:

$M = -6.43 $ from $m=-38$ at distance of $d=1$AU.
$m=-26.43$ from $M=-6.43$ at a distance of $d = .00326 ly$ (the new average distance between stars at the center of a globular cluster)

In other words, a blue supergiant at an average distance between stars within a globular cluster would appear to be as bright as our sun is to us! This is absolutely crazy. Depending on where it is in relation to the sun, it could effective cause two days, or potentially one day which is greater than half the time it takes your planet to rotate once. I would image that this would certainly interfere with observing in optical (and shorter wavelengths).


Calculate NEO object position with nasa Near Earth Object data I wish to create a 3D visualisation of NEO's by date in relation to earth, but calculating their position at that time has got me stuck.

From this question I was able to fine Nasa's NEO object data:

Previous Question

Listing Of NEO's:

Nasa Date range NEO Data

Detailed NEO Data

Given the information in these JSON streams. Is it possible to calculate the position of the NEO in relation to earth? It doesn't seem to offer any positional information that I can see, although this is not my field.

Or is there a better data set which will assist me in this?

The file you had linked as 'Detailed NEO Data' lists all the orbital parameters needed to determine the position of the object as a function of time. It lists these parameters for the objects orbiting the earth. This should be just what you need. They are located at bottom of file. There are routines to convert the orbit in orbital parameters to cartesian coordinates. This may be easier to use. You can google on orbital parameters conversion to find these routines. I think I have a copy in Fortran but not many people use Fortran now.


How to tell a nebula from a galaxy? Not all galaxies are spiral in shape[1][2], some nebulae are huge[3] and nebula are the nursery of stars[4]. How to tell them apart?

Note

I have already compare the contents for galaxy and a nebula, they are very similar as both have stars, planets and dusts.

Galaxies are a large organised collection of stars (& nebulae) outside of our own galaxy, very distant. They shine by their own light.

Nebulae are clouds of gas & debris from, usually, a stellar explosion (Nova, supernova, etc), within our own galaxy, lit up by nearby stars (maybe internal ones) - but the gas is not glowing with it's own light, generally.

So a galaxy is going to be a bright clump of glowing stars, regular in shape - be that elliptical, barred or other, generally they will appear symmetrical & regular. A nebula will be much more amorphous & patchy, possibly with shapes from the clouds of gas (c.f. Horsehead, Orion), and you will be able to see individual stars in it - unlike a galaxy where they are too tightly packed & distant to make out individual stars.


What is the color of Venus if it has no atmosphere? Mars looks reddish and Mercury looks gray because they lack thick atmospheres and we can see their "real color" easily, how about Venus? Is the surface of Venus really yellowish in color?

According to the webpage from the Planetary Habitability Laboratory of the University of Puerto Rico: The Real Colors of Venus without the Clouds, did the following:

  We combined radar topography and emissivity to generate the surface texture assuming a generalized basalt composition with gray to light brown tones, depending on the iron and sulfur content. In the images we also included the effect of the 'orange sky' of Venus thus given its surface a more dark brown appearance

As in the image below.
Additionally, according to the model the scientists made, they state that

  Under white light the surface should look grayish and more similar to the Moon.

As an aside, the website also shows a fictional map of Venus with 70% surface water.


How many stars are between Milky way and Andromeda? Are there some stars between those two galaxies? In photos is only black.

And if so, how many we could find there?

The stars between galaxies are called "intergalactic stars", and one study (2012) claims to have identified about 675 of them between the Milky Way and Andromeda:

  Now, Vanderbilt astronomers report in the May issue of the Astronomical Journal that they have identified a group of more than 675 stars on the outskirts of the Milky Way that they argue are hypervelocity stars that have been ejected from the galactic core. They selected these stars based on their location in intergalactic space between the Milky Way and the nearby Andromeda galaxy and by their peculiar red coloration.

  -"Rogue stars ejected from the galaxy found in intergalactic space", Research News @Vanderbilt (2012-04-30)


Are any Pluto-sized objects remaining to be discovered in the Kuiper Belt? An object approximately the same size as Pluto, Eris, was discovered only 8 years ago (in 2005). Are there any Pluto-sized objects remaining to be discovered, and if so, how far away from the Sun would they have to be to not have been detected already?

This is a part answer to your question, as it is difficult to answer without speculating, so here are some facts/observations related to your question.

Asides from Pluto/Charon, Eris, Triton (could be a captured Kuiper Belt object), Makemake and the football shaped Haumea, most of the Kuiper Belt Objects (KBOs) are according to the article "Kuiper Belt Objects: Facts about the Kuiper Belt & KBOs" (Redd, 2012):

  thousands of bodies more than 62 miles (100 km) in diameter travel around the sun within this belt, along with trillions of smaller objects, many of which are short-period comets

and is believed to have a total mass of only a tenth of the Earth, according to the article "Forming the Kuiper Belt by
the Outward Transport of Objects
During Neptune's Migration" (Levison and Morbidelli).

Here is a list Of the many Transneptunian Objects that have been documented, detailing their absolute magnitudes.

In regards to one of your main questions - according to Redd (2012), the challenge in their detection is

  Because of their small size and distant location, Kuiper Belt Objects are a challenge to spot from Earth. Infrared measurements from NASA's space-based telescope, Spitzer, have helped to nail down sizes for the largest objects.

I would add, their irregular elliptical orbits* and extreme (compared to the major planets) inclination to ecliptic make it that much more difficult to detect them.  Additionally, according to "The Edge of the Solar System" website, further difficulties include low surface reflectivity.

An example of a possible KBO with an extremely elliptical orbit is Sedna, which is believed to take over 10,000 years to orbit the sun; is smaller than Pluto, but was observed at about 90AU (3 times further than Pluto).

So, there could be many small Pluto-sized 'dark' worlds in highly elliptical irregular orbits in the Kuiper Belt and beyond.  However, beyond those listed, we have not seen that many and the total mass theorised does not support the idea of too many in existence, but that does not mean that they are not out there.


Is Earth slowing down in order to stay on its orbital track? The Sun is losing some of its mass during explosions.
The Gravitational force between Sun and Earth will change (probably decrease).
If Earth maintains its speed, it will change its orbit.
Hence, Earth will decelerate

Is this true?

As the Sun's mass is reduced and it's gravitational attraction is thus reduced the Earth will slowly spiral out from the Sun and yes slow down (to preserve angular momentum). But the effect is ridiculously small (do the math yourself) it will not be measurable directly.


Why can't ptolemaic Venus be fully lit? In wikipedia's article on the phases of Venus it says:

The Ptolemaic system would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the Sun, which is impossible if Venus's orbit in its entirety is between the Earth and the Sun,

But the moon's orbit "in its entirety is between the Earth and Sun", and is "fully lit from the perspective of the Earth" once a month, so how can we conclude that Venus revolves around the Sun?

The Moon orbits the Earth in such a way that it sometimes can be on the opposite side of the Earth as seen from the Sun. In other words, it’s possible to have Sun-Earth-Moon along an imaginary straight line, in that order.
However, by Ptolemy’s time, it had already been noticed that Venus is always seen close to the Sun (maximum angular distance about 47°). So it can never get opposite to the Sun as seen from Earth.
Ptolemy didn’t know why (the reason is that Venus orbits the Sun, not the Earth, which was inconceivable to Ptolemy), but his system made it so that Venus (and Mercury, for that matter) were orbiting the Earth closer to it than the Sun, and in such a way as to always be near it.
So imagine moving your two hands at arm’s length, but one closer to your face than the other. Imagine the distant one is the Sun. Then your closest hand would never get completely lit. Unless you put it behind your back, where Venus (and Mercury) never go.
So that’s why in the Ptolemaic system, Venus (or Mercury) would never get fully lit.


When was it realised that most major moons orbit in the equatorial plane of their parent planets? Inspired by the discussion of the moons of Uranus providing a clue to the planet's axis of rotation in this question, I'm wondering when it was realised that the major satellites are typically located in the equatorial plane of their planets. Our own Moon is a counterexample to this trend, so presumably it would have taken a bit of detective work.

Since the question has sit unanswered for months, I'm trying to give an acknowledgedly incomplete answer.

In summary: By 1700 it would have been a reasonable guess. By 1800 it was becoming an observed trend.

TL;DR:

It's hard to decide when the consensus was created on the typical orbits of satellites, because there are only a few data points (planets with satellites) but we can know when those few data points where known.

Obviously, the only data point known in antiquity was the Moon, but by that time the whole idea of satellites was far from mainstream.

In 1610 Galileo discovered the four biggest moons of Jupiter and that they orbit Jupiter in roughly the same plane. However, it took until the 1660s for Cassini to discover features in Jupiter and therefore be able to realise that its moons orbited in the equatorial plane.

By 1655 Huygens had discovered Saturn's rings and Saturn's satellite Titan, both of them in the same plane. Between 1671 and 1684, Cassini discovered Tethys, Dione, Rhea and Iapetus all of them in the same plane of the rings except for Iapetus.

The rotation of Saturn is a bit harder to detect, and it wasn't measured until 1794 by Herschel.

There is an additional data point even before being able to see Saturn rotating: both Jupiter and Saturn appear elliptical in the 17th century telescopes. Although a causal relation with rotation wasn't established until Newton, as soon as Jupiter's rotation had been discovered it was known that the planet was rotating around the smallest axis of the ellipsoid.

No more data points were added for a long time, because the moons of Mars and the rotation of Uranus were discovered much later, although the first two moons of Uranus had been discovered by Herschel in 1787.

Additionally, it was known from antiquity that all planets were in similar planes, and when sunspots were observed (by Galileo) and the rotation of the Sun could be measured, the Sun's equatorial plane was close to the planes of all planets orbit. Therefore, when the idea of planets and satellites as "little solar systems" arrived, the idea of the central body rotating in the same plane of the system wouldn't have seemed strange.

Then we can summarise what an informed observer at the end of the 17th century would have known:

Jupiter's rotation, Jupiter's oblation, and Jupiter's moons were in the same plane.
Saturn's rings, Saturn's oblation and most of Saturn's moons were in the same plane.
There were only two known exceptions to the rule of "everything around the same planet in the same plane": Earth's Moon and Iapetus, which aren't in the equatorial plane - although they aren't very far away from it.

With all that at hand, I would say that guessing that most satellites orbit in the equatorial plane of its planet would have been a reasonable guess by 1700.

By 1800 the evidence would have increased with:

The rotation of Saturn had been actually observed, confirming that most satellites and the rings orbit on the equatorial plane.
Uranus had two satellites in the same plane.

With that, it seems even more reasonable to suppose that Uranus'satellites were orbiting on its (unknown) equatorial plane.


How does a supernova occur when a massive star collapses into a black hole? My layman's understanding of a supernova explosion is that it's caused by a rebound when a massive star collapses. When the core collapses to a point where neutron degeneracy pressure dominates, the core collapse stops, and the matter falling in from the outer layers bounces back, causing a supernova (please correct me if I'm wrong). However, I've also read that the end result of the supernova can either be a neutron star or a black hole depending on the mass. When the star is massive enough to collapse into a black hole, what causes the supernova? What do the outer layers bounce off of? Shouldn't the whole thing just collapse into the black hole?

You are right. A direct collapse of a heavy star into a black hole can lead to a failed supernova without a detectable explosion.
However, a black hole can also form with an SN explosion. One possibility would be the initial formation of a neutron star and bounce off of the outer layers but subsequent fall back of enough material to collapse the neutron star to a BH.
In rapidly rotation stars the supernova can also be fueled by the energy released during the accretion of matter around the newly formed black hole. This would than be a Collapsar.
There is also the possibility for rapidly spinning stars that a magnetar forms during the supernova. Here, the explosion is not primarily driven by the bounce off and the neutrino wind but by the release of magnetic energy and a subsequent spin-down of the neutron star. As the rapid rotation can stabile a NS against its collapse, this spin-down can also lead to a delayed black hole formation. Here a paper about that.

So, SN explosions can become pretty complex and a black hole can form with and without a supernova explosion. All depending on mass, spin and metalicity of the progenitor star (& whether these is a binary companion).


Are astronomers researching or trying to find signs of worm holes? In many movies and in the popular culture wormholes are often referenced, as well as "faster than light travel", which seems almost to be the same thing, is possible.
Are astronomers seriously considering and researching this phenomenon? What could be the signature of wormholes? Would next generations telescopes as ELT able to detect such signatures?

This has been considered long ago (Here's a paper talking about this).

Wormholes are not forbidden by physics, but the creation of wormholes is iffy ground. THere are two possible paths one can take to create a wormhole:

Choose a pre-existing wormhole in the quantum foam and "expand" it by feeding it exotic matter.
"Tear and sew up" space — we're not sure if this is allowed by physics, as it ventures into the area of physics that we don't have an adequate explanation for.

Whatever it is, the creation and sustenance of a wormhole requires us to have control over exotic matter (in this case, particles/waves with negative mass/energy density). Vacuum fluctuations already have regions of negative energy density, but they're an uncontrollable quantum phenomenon by current technology.

"Faster than light travel" is a different matter, though. While wormholes let one jump to another point in space, one does not attain a speed greater than c whilst in them.


Approximating density of hydrogen in [observable] universe Let universe be completely made from hydrogen. And also we have redshift $z= 6$. with Hubble constant $H_{0} = 2.1941747572815535\times 10^{-18}\:\mathrm{s}^{-1}$. We also know that density of the universe $\rho =\frac{3H^2}{8\pi G}$ ($G= 6.67\times 10^{-11}\:\mathrm{m^3\ kg^{-1}\ s^{-2}}$).

Is the following approach right to find the number density of hydrogen in universe?

First we calculate density $\rho =\frac{3H^2}{8\pi G}$, then we have
$1+z = \sqrt{\frac{1+v/c}{1-v/c}}$. From that we get $v = H\times R$ and from this we calculate $R$, the radius of observable universe. With that, we calculate volume of universe $V = (4\pi/3)R^3$ and we get Mass $M=\rho V$. Then we calculate number of Hydrogen atoms with $N = M/m_{\mathrm{hydrogen}}$. Now we have number density of hydrogen $\rho = N/V$.

Is this solution right? Or we must solve it another way?

The source of question is National Olympiad of Iran which is not available in English so the above is my translation for the question.

So to estimate the answer, you need to convert the mass density to number density by $n_H = \rho_H / m_H$. After getting the current number density you need to redshift it to finite $z$, here $z=6$. Keep in mind the density of hydrogen evolves $\sim (1+z)^3$ due to the expansion of the universe.

That said, the assumption 'let universe be completely made from hydrogen' renders this question highly unrealistic, hence I agree with @James K that this is not real world astronomy. In the real world, only about 5% of the total energy is made of baryons, among which Hydrogen abundance is $\sim 75\%$. Also, at redshift $z\sim 6$, star formation etc. lowers the neutral hydrogen gas density, c.f. recent observations here and here.


Can a black hole "supernova"? In layman terms:

nothing ever escapes the pull of a black hole, not even light
when a super massive star reaches the end of it's life you get a supernova
sometimes the "remains" of these stars can turn turn into black holes

My question is: can a black hole "supernova" the same way a large star does?

What I'm saying is, could this process occur in reverse? I.E:

  black hole -> supernova -> particles form dust cloud -> new star

EDIT: Apologies if any of this sounds stupid (or vague), I'm just curious.

No, it cannot. A black hole is something vastly different from a star. It's vastly different from anything else. It's not a thing, really, but more like a portion of very distorted spacetime. Nothing escapes from it simply because there is no way out - spacetime is distorted in such a way that all trajectories lead to the center.

Now, there is a mechanism where radiation is generated just outside the black hole, sucking energy from its gravitational field and therefore from its mass. This is called Hawking radiation. It does not come from inside the BH, but the way it's created it leeches the BH's energy / mass. In time, this would diminish the mass of the BH.

It turns out, the smaller the BH, the stronger the Hawking radiation. This in turn makes the BH lose weight even faster, so the HR gets even stronger, and so on. It's a vicious cycle. If the black hole is small enough, the process turns into an explosion - HR gets so intense, the whole mass of the black hole is converted into radiation at once. It's not a supernova, not as big as that, but it is a powerful explosion.

The only issue here is - HR is very weak when BHs are big. So regular BHs will take an incredibly long time to slim down, if ever, to the tiny size where HR spirals out of control. Basically this never happens in reality.

TLDR: Not a supernova, just a huge bomb, but not very likely.


Is the vernal equinox always in zenith somewhere on the equator? I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses equator from south to north makes sense, as long as the vernal equinox is always on the equator. And I suppose it usually is. But the Earth's axis precesses--how can the RAAN make any sense, if the vernal equinox is no longer directly above the Earth's equator?

The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator.

Given the various ways that objects in orbit over Earth are perturbed, any set of keplerian orbital parameters that describe the orbit of any object over the Earth (including the Moon!) will need updating long before the drift of the Vernal Equinox along the celestial equator becomes significant.


Moons with curlicue paths around our Sun? I naively believed that since our Moon orbits Earth, and since Earth orbits the Sun, the path our Moon might take around the Sun would be this type of epitrochoid curve:

I was surprised and delighted to find that our Moon's orbit around the Sun actually forms a convex curve which is really darn close to Earth's orbital path: Moon's orbit around the Sun.  Since the Earth's instantaneous velocity around the Sun is much greater than the Moon's instantaneous velocity around the Earth, we don't get any of the closed "curlicues" I expected to see.
Are there any moons in our solar system with epitrochoid paths around the sun that appear to have closed curlicues?  If so, what are they and what do the paths look like?

A moon will describe a path like this if its orbital speed relative to its parent planet is greater than the parent planet's orbital speed about the Sun.  (Assuming the moon's orbit about the planet and the planet's orbit about the sun are roughly coplanar;  I'll ignore Uranus for the remainder of this discussion.)  Going through the planets:

Neptune:  All moons from Proteus inwards (7 total) orbit Neptune sufficiently fast.  However, they're all pretty small;  in particular, all are irregularly shaped, since they're all too small to form into spheres under their own gravity.
Uranus:  (ignored because the paths of the moons are just too baroque)
Saturn:  All moons from Dione inwards (20 total), along with Dione's trojan moons Helene & Polydeuces, orbit Saturn sufficiently fast.  This includes a few large & charismatic "pre-Voyager" moons: Mimas, Tethys, Enceladus, and Dione.
Jupiter: All moons from Europa inwards (6 known), including Io, orbit Jupiter sufficiently fast.

It's possible that there are some satellites of dwarf planets that might also make this list, but I didn't spot any immediately.  In general, a satellite of a dwarf planet would have to be orbiting its parent extremely closely to make this happen, since the orbital speed of a satellite is roughly proportional to $\sqrt{M/r}$, with $M$ the mass of the parent and $r$ the radius of the satellite's orbit.


Is time itself speeding up universally? Time moves more slowly near a mass than in relatively empty space.  For example, I get that an observer would see someone falling into a black hole to appear to move more slowly and get "stuck" on the event horizon, while the falling body would see the observer as aging more rapidly.

Additionally, an observer watching a passenger craft moving near the speed of light (high mass equivalent) might see a passenger who tripped falling in slow motion, because the forward momentum of the passenger + the speed of the craft must never exceed the speed of light, while the passenger would experience no effect of this time dilation, and would simply trip and fall.

Which brings me to my question...

Black holes, even supermassive black holes, are nowhere near the mass of the universe as a whole; so when the universe itself was much smaller and far more dense, then at a universal scale time itself would have progressed more "slowly" than it does today, and since the expansion of the universe is accelerating, doesn't it mean that time is also accelerating?

(Assuming, of course, there was some "outside" to observe this from).

This is not a duplicate of Does time slow down because the universe is expanding at an accelerating rate?.  That question and its answers seem more concerned with local, relativistic time (which is as malleable as space itself).  My question asks more about the effect across a universal scale as it relates to density of mass across that same universal scale.

The rate at which a clock ticks is a local property. There is no universal rate of time which could speed up, so the answer is no.

A clock stationary in the same frame of reference, and the same gravitational field as me will tick at the same rate, (one second every second) It is only clocks that are moving, or in different gravitational fields that that tick at a different rate. If I am falling into a black hole, and I stop the check the time, I wouldn't see my watch slow down.

So there is no standard clock rate. Clocks in the early universe (or at least things that depend on time, such as nuclear decay) ran at the same rate, in their local gravitational field (at one second per second)

If we were to observe a clock from the early universe, it would be receding at great velocity (and so it would be redshifted and time dilated)

Now, the gravitational time dilation does not depend on the amount of mass directly, but on the intensity of the gravitational field. A galaxy has huge mass, but the only place in which the gravitational field is significant (from a GR perspective) is in the neighbourhood of Neutron stars and black holes.

To get gravitational time dialtion you need an intense gravitational field. Just having a lot of mass is not enough. After the period of inflation, the universe was homogeneous, and there were no significant "lumps" to give a net gravitational field. As the gas in the universe collapsed into galaxies and stars, and eventually black holes regions of intense gravity formed, in which clocks would run slowly.

However there is no general speeding up of clocks in the universe.


Who gets to name Planet Nine if it is confirmed? According to Wikipedia, "On 20 January 2016, researchers Konstantin Batygin and Michael E. Brown at Caltech announced calculation-based evidence of a massive ninth planet in the Solar System."

If visual evidence or another researcher confirms the existence of Planet Nine, does the actual discovery warrant naming rights, or would this go to our Caltech friends?

I know from this question's answer, that:

  "Since the beginning of the 20th century the IAU is setting the standards for naming celestial bodies including planets. However, the IAU doesn't set the names, it only sets the standards."

Does the 'discoverer,' or the 'confirmer' get to name (theoretical) Planet Nine?

The cited answer is correct insofar as the IAU is not the body that makes names of celestial bodies official. However, it's authority is implicit because it is recognized by the astronomical community and the public as the foremost authority on astronomical naming, and thus the names the IAU adopts are the ones most commonly used in the scientific community (excluding objects referred to by their catalog numbers).
From one of their information pdfs:

The IAU has been the official arbiter of planetary and satellite naming since its inception in 1919. The IAU’s decisions are officially adopted by the nearly 11 000 professional astronomers who are its members, coming from more than 90 countries.
. . .
The IAU does not consider itself as having a monopoly on the naming of celestial objects — anyone can in theory adopt names the way they choose. However, given the publicity and emotional investment associated with these discoveries, worldwide recognition is important and the IAU offers its unique experience for the benefit of a successful public naming process (which must remain distinct, as in the past, from the scientific designation issues).

The IAU may reach out to the public for naming suggestions, or it may taken case-by-case suggestions by individuals. So while Batygin and Brown can call the ninth planet whatever they want - they've informally referred to it as "Phattie" - public consensus will largely rest on the results of the IAU's decision, which could, of course, end up taking a suggestion of the pair.
Anyone can come up with a suggestion, although sometimes that's a terrible idea.


Gravastar Star With no Horizon with ultracompact and very density possible? Is that possible with some strange Quantum Effect could give rise to a completely new kind of a star? Gravastar(Gravitational Vacuum Star) an object hypothesized in astrophysics as an alternative to the black hole theory.The notion of gravastars builds on Einstein's theory of general relativity and imposes a universal "smallest size" that is known to exist according to well-accepted quantum theory. This size is known as the Planck length and is derived using the speed of light, Planck's constant and the gravitational constant. Quantum theory says that any scale smaller than the Planck length is unobservable and meaningless to physics and physicists. This limit can be imposed on the wavelength of a beam of light so as to obtain a limit of blue shift that the light can undergo. However, in the presence of an event horizon, the blue shift diverges.

In the original formulation by Mazur and Mottola, gravastars contain a central region featuring a p=-ρ false vacuum or "dark energy", a thin shell of p=ρ perfect fluid, and a true vacuum p=ρ=0 exterior. The dark energy like the behavior of the inner region prevents collapse to a singularity and the presence of the thin shell prevents the formation of an event horizon, avoiding the infinite blue shift. The inner region has thermodynamically no entropy and may be thought of as a gravitational Bose-Einstein condensate. Severe red-shifting of photons as they climb out of the gravity well would make the fluid shell also seem very cold, almost absolute zero.

Of course unknown "strange Quantum Effects" could do anything, but this letter reports calculations which suggest that at least one of the LIGO gravitational wave events is consistent with black holes but not with gravastars. That doesn't rule out the possibility of gravastars and black holes existing, but it speaks quite strongly against the idea that all the phenomena attributed to black holes are actually due to gravastars.


finding Dec and R.A of a star by having Dec and R.A of another star How can we find Dec and R.A of a star by having Dec and R.A of another star.

For example in the following picture , we have that for Errai (Dec: 77 , R.A 23H 40M). How can we find Dec and R.A of Gamma Delphinus. Note that we can't use Polaris for example because of cloud or something else. I want answer for when we have telescope and when we don't have telescope.

You can measure the position in the sky of any star; this is its altitude and azimuth. The star's position in the sky depends on its actual position in space, your position on Earth and the rotation of the Earth.

Measuring the position requires some kind of angle-measurer. Such as a quadrant or sextant. You also need knowledge of which direction is North. This can be determined, even if the pole star is not visible, from the location of other stars. A telescope can increase accuracy but the same measurements can be done either with or without a telescope.

You can use your position, and the position of Gamma Cephei in the sky (by altitude and azimuth) to determine the local sidereal time. You can then use the position of Gam Del and the sidereal time to determine the ra and dec of Gam Del.

Other questions discuss the conversion between alt-az to ra-dec Software to convert RA and DEC into ALT and AZ


Telescope collimation issue Recently I purchase Chinese 8 inch reflector telescope from local store. When I observing starts and planets I've been having trouble getting really sharp image from this telescope. I observe Venus and Jupiter with several eyepieces but I got some "coma like" effect all the time with this telescope.

To improve the image quality I decided to collimate this telescope with my laser collimator and cheshire collimators.

Before attaching the collimation device(s) I check primary and secondary mirror alignment through draw tube and it looks like this:

But when I test this setup with laser collimator it show vast miss-alignment in both primary and secondary mirrors. After align both secondary and primary mirrors I've got view similar to this through the draw tube:

I also check above setup with Venus and few starts and I didn't notice any improvements of the image.

I try to collimate this telescope with both cheshire and laser collimator but still I didn't get any clear image with this telescope.

What was the "most likely" to have issue in this telescope?

Is problem in draw-tube assembly alignment can cause problem like this or is this happens because of the problem in primary / secondary mirror of this telescope?

Also I test this telescope with several branded and unbranded eyepieces and I didn't notice any improvement.

First off - good job for paying attention to collimation. For a newtonian telescope, it is as important as changing the oil on your car.

It would help to know a little bit more about the telescope. What make and model? What is the focal length?

EDIT:

Saw your comment below. It's an f/4 parabolic mirror, you're always going to have coma at the outer edge. But the center should be aberration-free if the scope is collimated.

Beware - it's an inexpensive scope, so depending on your luck some residual aberrations may remain even after perfect collimation. But that doesn't mean you should stop trying to collimate it. On the contrary, the better the collimation the better the performance, always.

  I observe Venus and Jupiter with several eyepieces but I got some "coma like" effect all the time with this telescope.

If the image in the center is fairly good, but you get "coma like" effects at the edge, then that is indeed coma (perhaps mixed with other aberrations). It is normal at focal ratios of f/5 ... f/4 or faster. If the focal ratio is around f/6 ... f/8 or slower, coma is not so easily visible, or not visible at all.

  But when I test this setup with laser collimator it show vast miss-alignment in both primary and secondary mirrors.

It is a very common issue with cheap laser "collimators" that the collimator itself is miscollimated. The laser only helps if it is very precisely centered, otherwise it is misleading. Unfortunately, many, many commercial lasers are woefully off-axis.

Plug the laser into the focuser. Tighten the screw only a little bit, so that the laser is sitting there semi-snugly, but you can still turn it around its own axis (rotate it in place in the focuser). Fire up the laser. Now gently turn the laser around, while watching the laser spot on the primary. What does the spot do?

If the laser is centered, the spot should never move as you rotate the laser in place. If the spot moves even a little bit (try to avoid rocking the laser tube sideways, as that would falsify the test), if the spot describes a circle on the primary as you rotate the laser in place, throw that laser in the garbage can.

(Ok, some lasers can be adjusted, provided they have the 2 or 3 adjustment screws on the collar, on the lateral part of the laser tube. Sometimes the screws are hidden under a plastic or metal ring. If that's the case, try and rescue the laser by carefully adjusting those screws. Otherwise dispose of the laser, or destroy it, since it's misleading.)

Other things to try

See if you can get better results with the Cheshire only.

Try and make a very simple collimation cap. It's basically a plastic cap with a tiny hole in the exact center, that's all. The procedure for using it is explained here:

https://www.youtube.com/watch?v=YAVGcGEBmCE

A more detailed procedure is explained here:

http://www.garyseronik.com/?q=node/169

If you're patient, you could collimate by the stars. It's complex and time-consuming, but it's guaranteed to be 100% accurate. It is always a good idea to use a quick star test at the end of whatever other collimation procedure you use - think of it as a reality check. The laser may or may not work properly, but the stars never lie.

http://www.astrophoto.fr/collim.html

Laser collimators can be very good and very quick - provided they are centered very precisely from factory. I use Howie Glatter's collimators, but they are not cheap. If you could borrow a Glatter collimator it would show you what's really going on.

http://collimator.com/

Is your primary center-dotted with good precision? In many cases, mass-produced telescopes come from factory with an off-center dot on the primary, which is as misleading as a bad laser. Even the best laser will not work properly if the primary is not precisely center-dotted. But you can always re-center the dot with good precision. See here:

http://www.garyseronik.com/?q=node/168

(Always use a donut shape to center the primary, instead of a solid dot. The donut allows you to use laser collimators, unlike the solid dot markers.)

More thoughts on collimation below. These articles are written by very experienced amateur astronomers and telescope makers. Highly recommended to read these before you begin.

http://www.garyseronik.com/?q=node/165

http://web.telia.com/~u41105032/myths/myths.htm

Try something simple (like the collimation cap) before you open up the scope and do "major surgery" on it. Then do a star test. Does it look pretty good on the stars? Then you're probably doing something right. The stars are looking pretty bad no matter what? Then perhaps you need to open up the scope, re-center the primary, check if the mirrors are sitting properly in their cells, etc.

Final comments

If you use too much magnification, the image will not look very sharp, no matter what. Stay within 100x ... 200x while testing. It is pretty rare that atmospheric conditions and/or the quality of mass-produced optics allow good performance above 250x.

Always check the results in the center of the image. With most mass-produced optics, performance degrades at the edge, and there's not much you can do about it (other than replacing them with better optics).

Before you do anything, take the scope outside and let it "breathe" for 30 ... 60 minutes. After it reaches ambient temperature, its performance might get better. More details here:

http://www.garyseronik.com/?q=node/55


Can any stars ever form supermassive black holes? I heard that most supermassive blackholes are not formed by stars – in fact, we aren't even sure how they're formed. But could a star, with enough mass and low enough metallicity, form a supermassive black hole? Rather, would there be any stars that fit this category?

As you say, making black holes quickly in the early Universe is a major unsolved problem in astrophysics. There are various hypotheses, of which two roughly correspond to supermassive stars.  All basically involve trying to give the black hole a headstart in mass.  There isn't really enough time to grow a $100\,M_\odot$ black hole to $10^9\,M_\odot$, so the idea is to rather get something more massive than a few $\times1000\,M_\odot$. The most recent review I know of offhand is probably Volonteri (2010), but I'm not up to date on the literature.

Basically, imagine a few hundred thousand solar masses of gas collapsing into a primordial galaxy.  As the gas collapses, it potentially fragments, depending on whether or not it can cool efficiently.  If so, the fragments can presumably form stars, but in a very dense cluster.  Ultimately, either (a) the massive stars in the cluster collapse into black holes that then merge into a larger black hole, which can subsequently accrete its way to supermassiveness (supermass?); or (b) the individual stars first merge in the centre, creating a star of perhaps some thousands of solar masses, which would collapse into a massive black hole that could grow.

If the primordial galactic cloud doesn't fragment, we expect a sort of monolithic collapse.  Somewhere in the middle, gas will start to reach hydrostatic equilibrium: a protostar forms.  But it's a protostar that's potentially accreting several solar masses of material per year (or even faster).  So what happens next depends on whether that rapidly infall of material has to time to reach local thermal equilibrium with the protostar, or if it just piles up on the outside.

If the former, then the protostar can become very large: thousands or tens of thousands of solar masses, which, like the supermerger product above, presumably leaves a massive black hole in the end. If the small protostar evolves independently of the infalling gas, it's probably also big enough to leave a black hole, just a much smaller one: tens, maybe hundreds of solar masses.  But it's a black hole embedded in this enormous cloud of infalling gas, which potentially settles into an envelope around the black hole. This structure has been dubbed a "quasi-star", and the black hole inside can grow very rapidly in this cocoon. Eventually, the envelope will evaporate/disperse, leaving the now massive black hole to continue to accrete its way to supermassiveness.

Note that these formation mechanisms are expected to be particular to the early Universe.  Once you add even a small amount of metals to the gas, then star formation is expected to look much more like the "modern" Universe.  In fact, star formation in these scenarios is still far from settled.  The main reason is that you need to follow how the gas evolves from the scale of the protogalaxy all the way down to the protostars inside.  This is a range of scales something like $1\,\mathrm{AU}/10,000\,\mathrm{ly}\approx10^{-9}$, which is numerically very difficult.

And, finally, to answer the question directly, in neither of the supermassive star options do the stars really collapse in supermassive black holes.  SMBHs are so big that they can only have grown so large through accretion.  The supermassive stars would collapse into their progenitors (or "seeds"), which would subsequently grow to such large masses.


How to convert apparent visual magnitude to SI units? How do I convert the apparent visual magnitude of astronomical objects to SI units (lumens)?

I found this from Wikipedia:

$$\mathrm{illuminance} = 10^{(-14.18 - M_\mathrm{V})/2.5}~\mathrm{lx}$$

https://en.wikipedia.org/wiki/Illuminance#Astronomy

Looks to be right.  I checked the formula with the luminous flux of Sol given by Wikipedia as 3.75×1028 lumens, with an (extraterrestrial) luminance at 1 AU of
$$\frac{3.75 \times 10^{28}~\mathrm{lm}}{4\pi~\mathrm{au}^2} = 133000~\mathrm{lm/m^2}$$
The formula above gives instead a (terrestrial) illuminance of 105700 lm/m2 for an apparent visual magnitude of -26.74 (which makes sense since there would be less illuminance on the surface beneath an atmosphere).

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


How stable are Lissajous orbits? Now that the Gaia Space Telescope is on it's way to the Sun-Earth L2 Lagrangian point (SEL2), I start wondering about the stability of Gaia's orbit there. The Planck Telescope is already there, as was Wilkinson Microwave Anisotropy Probe (WMAP) and other probes, and from Wikipedia I learned that:

  In practice, any orbit around Lagrangian points L1, L2, or L3 is
  dynamically unstable, meaning small departures from equilibrium grow
  exponentially over time.

Gaia has some kind of Orbital Maneuvering System (to borrow a Space Shuttle term) and some propellant on board, so has Planck, however I wonder how deterministic these orbits are and if both Planck and Gaia have automatic corrections and collision detection in their flight computers; L2 is "only" 1.5 million km (or about 5 light-seconds) away so surely there's time for manual correction.

Does anyone know a source which tells how different Gaia's and Planck's orbits are, if there are intersections between their orbital planes or even how likely the need for an unplanned orbital correction is? I know Lissajou-shapes from maths classes and I know how much the projected trail can differ depending on the precision of data types used in calculations (e.g. float vs. double). How does ESA/NASA handle this, now that SEL2 seems it will become a crowded place?

Gaia's original science and technology report (see page 221, see also the summary) gives an analysis of the Lissajous orbit. From what I understand Gaia will be placed in a small amplitude Lissajous orbit, giving it an orbital radius of $\sim400000$ km away from $\sim100000$ km along the Sun-Earth axis.

In addition to the fact that this orbit is intrinsically unstable, variable radiation pressure from the sun causes stochastic destabilization of the orbit. The prediction is that once in orbit, a small velocity correction of $\sim1$ m/s is needed about once a month in order to maintain the orbit.

From a summary on Planck's orbit it seems that this applies to Planck as well.

That the SEL2 point is being 'crowded' doesn't seem like such a big problem to me. Once the satellites are turned off (both Planck and Herschel are no longer active) their orbit will rapidly destabilize, effectively removing then from the region. But more importantly, the orbital radius is comparable to the distance to the moon, this really is a tremendously large amount of space. Since these satellites are only a few metres across, the probability of a collision is likely negligible.


calculate sunset in flight I found an answer at
https://physics.stackexchange.com/questions/26795/how-to-calculate-the-amount-of-night-time-during-a-flight

but I was stuck at the part:

$\cosθ_1=\sinφ_1\sinφ+\cosφ_1\cosφ\cos(λ−λ_1)$

$\cosθ_2=\sinφ_2\sinφ+\cosφ_2\cosφ\cos(λ−λ_2)$

from which  $ (λ(t),φ(t))$

can be derived (after some tedious calculations).

After choosing an arbitrary time t, you have the two equations you reference and two unknowns: $ λ$ and $φ$. The problem is that the equations cannot be solved directly, so an iterative approach is required. For example,

assume that $(λ−λ_1)=θ_1$. Note that this also lets you calculate $λ$
solve for $φ$ from the first equation
use that value of $φ$ in the second equation to solve for $λ$
If the values of $λ$ from equation 1 and 2 are different, try another value for $λ$ in the first equation.
repeat until the assumed value for $λ$ in equation 1 and the calculated value from equation 2 are equal.

It has been a long time since I did spherical trig, but I think the solution on the Physics SE is inefficient. It looks like they are calculating the sun's altitude at multiple point (multiple times of t) along the path, so the two equations two unknowns needs to be solved multiple times -- that is inefficient. It should be possible to determine the equation of the great circle through the departure and destination point. Essentially you need to calculate the longitude of where it cross the equator and the inclination. That probably needs to be solved iteratively, but it only needs to be solved once. Then for any time t, the position $λ$ and $φ$ can be calculated directly, and the sun's altitude can be calculated directly.


Can Earth have a ring? Why doesn't the earth have a ring?

If solid material such as dust and moonlets, and common components of satellite systems are brought nearer to the Earth, then what will happen? Will it take the form of a ring or fall down on the Earth?

Most planets don't have rings.   The ring region is inside the Roche limit which is quite close to the planet.  A ring system outside the Roche limit needs to be either very faint or, it would over time coalesce into a moon or possibly pair of moons at Trojan points to each other.

There's different Roche limits for rocky bodies vs icy, but an icy body couldn't exist in Earth's orbit for long, so it's not really relevant for the Earth.  Saturn's rings are mostly ice, which extends about twice as far as the rocky body Roche limit.

How long a ring system lasts depends on a few factors.   How big it is, whether it's given new material regularly or not, what it's made of and how vulnerable it is to the sun's rays.   Saturn's rings, for example, are losing material, raining down on Saturn as well as slowly adding material from Enceladus, one of it's small and close moon's that gives up ring material by tidal-volcanic eruptions.

Earth's large moon could (not saying it would but it might) play a role in disrupting and Earth-Ring system more quickly than it would with no moon.   A 2 body system with the 2nd body as large as the moon isn't gravitationally friendly to other orbiting objects.

Many man made satellites orbit well within the Roche limit but they are small enough to be held together and not be affected by tidal forces.   Larger objects, just a few miles across, perhaps less, are more vulnerable to tidal forces and tend to break up inside the Roche Limit.


Does nucleosynthesis happen near a black hole or during its formation? During its lifetime, a star can synthesize elements to iron because of the pressure and temperature. A neutron star is composed of iron nuclei 56 and up to krypton 118.

According to that, a black hole (which concentrates matter to an increasingly tiny space during infall) must also have an unimaginable pressure/temperature during its formation and as additional matter falls into it.

Can it therefore synthesize new elements?

After a google search the answer is yes, in theory. There is nucleosynthesis associated with the Black Hole accretion disks, see here, here and here. These sources look at the nucleosynthesis in the hotter parts of the accretion disks surrounding black holes, and how this is effected by the neutrinos produced.

It is also proposed that its possible for very small mass (10e-9 Mass of sun) black holes to be devoured by a neutron star (see here) and produce many of the heavier elements. This method starts with primordial black holes which can be of the required mass compacted into a region the size of a single atom. This tiny black hole could crash into a neutron star and initiate the r-process for nucleosynthesis.


Does the sun's rising/setting angle change every few months? When I lived in Darwin, Australia, I noticed that the Sun set in a slightly different direction to the West. In the evenings, I used to sit on a couch and watch TV. There was a huge window to my right and the setting sun would always shine bright into my face. So I moved a single vertical blind to block it out just about right. In the months of October/November I started noticing the sun shining into my eyes again. Neither the couch nor the blind had moved. After moving the blind again to block the sun, in a few months I would have to move the blind again. I was living alone and there is no way the blind or the couch would move.
When I lived in Canberra, Australia, I had the habit of stepping out of my house early in the morning. I would face the sun as soon as I stepped outside because it was too cold and I wanted the warmth of the sun on my eyes/face. I noticed again in October/November that my usual position of facing the sun was wrong. The sun would rise from a slightly different angle. So if earlier the sun was rising at 130 degrees from my door, it would now be rising at 110 degrees. The door faced the southeast, and because of the very narrow path and vegetation from my door I am very certain of my positioning.
I really don't know what this phenomenon is called. When I lived for so many years in other countries, I never noticed this. Am I just being silly or does the Sun rise/set differently every so many months?

The Sun rises and sets at a different point on the horizon every day. The change is small, so without careful observations, it may take several days or weeks to be fully aware of the change. Mathematically, the position of rising/setting can be found from the following formula:
$$\cos(\theta) = -\frac{\sin(declination)}{\cos(latitude)}$$
where

$\theta$ is the angle measured from due south to the point on the horizon where the object rises. (The angle is the same for rising and setting, if you ignore the change in the Sun's declination during the day.)
declination ranges between approximately +23.5 and -23.5 degrees for the Sun during 6 months (southern hemisphere winter to summer) and then from -23.5 to +23.5 during the next 6 months.
latitude is the observer's latitude.

From 35° S latitude, the winter Sun would rise at $\theta=119$ from due south (or 61° from due north, approximately ENE). The summer Sun would rise at $\theta=61$ from due south (or 119° from due north, approximately ESE). The range is 119-61=58° along the horizon.
At latitudes closer to the equator (0° latitude), the difference is smaller. The range is approximately 47° along the horizon when at the equator. At latitudes closer to the poles, the range becomes larger. At the Arctic and Antarctic Circles, the range is the largest that it can be: 180° (at least mathematically). At these high latitudes, the diameter of the Sun and refraction become important to calculate the precise location of rising.


Are warm circumstellar debris disks still able to be detected at sub-millimeter wavelengths? At these wavelengths a survey is more sensitive to detecting relatively cool disks (T<100k), but would a telescope operating in the sub-millimeter be less sensitive to warm debris disk than say, a mid IR telescope?

Such surveys are designed to detect an excess of emission over that expected from a stellar photosphere.

The emission from a circumstellar disk is often modelled as a blackbody at lower temperature than the stellar photosphere. A hotter blackbody with the same emitting area emits more flux than a cooler blackbody at all wavelengths.

Therefore, if you had a disk of fixed emitting area, it would be easier to detect a hotter disk at sub-mm wavelengths than a cooler disk.

However, if you have a lot of cold disk material and no warmer regions, then because the Wien tail of the blackbody distribution is so steep, it emits almost no radiation at near- and mid-infrared wavelengths, and sub-mm detection is the best way to detect it.


Where can I find Helium 3 in our solar system? What are the best locations within the solar system to find Helium 3 excluding the Sun? Where is Helium 3 most abundant after the Sun?

You wouldn't need to travel too far, the moon has been bombarded with large amounts of Helium3.

https://www.esa.int/Enabling_Support/Preparing_for_the_Future/Space_for_Earth/Energy/Helium-3_mining_on_the_lunar_surface


Counter clockwise rotation of storms at Jupiter's north pole. What explanations have been proposed? On Earth, high pressures and low pressures occur interchangeably. So that where two pressure systems intersect, they move the air in the same direction. But as Juno revealed the first observations of Jupiter's north pole, that doesn't seem to be the case over there. There are eight gigantic storms surrounding a ninth one, and they all move counter clockwise. With two smaller ones in between on one side that do move clockwise.
How can this be explained? How is this possible?

JPL Juno IR image of Jupiter's north pole

Let my try to stretch the analogy to anticyclonic tornados: The vast majority of all tornados move cyclonic, i.e. counterclockwise on the Northern Hemnisphere of our blue planet. There is e.g. a publication on anticyclonic tornados by Howard Bluestein et. al. which says:

It is also possible that the source of vorticity in the anticyclonic tornadoes is not from a parent mesoanticyclone (produced via tilting), but rather from (hypothesis 2) preexisting anticyclonic shear vorticity on the anticyclonic-shear side of the low-level jet associated with the rear-flank downdraft [...]

In simple words: It is not fully clear yet, how exactly these atypical atmospheric vortices form, but there are different possible explanations why tornados exist which rotate against the direction supported by Coriolis force. This reminds me somehow of the Coriolis Force Effect on Drains:

The notion that the Coriolis force determines which direction water spirals down drains is one of the most prominent scientific myths.

Some educational German television show has been able to actively influence the rotation of water spiraling down a drain with appropriate boundary conditions like little wings in the drain (but the clip is not online). In the metereology of planets with solid surface, the effect of the ground on the atmospheric motion is usually termed orographic effect. I suppose the different rotations of the Juno IR image of Jupiter's north pole have similiar "guide wings" in the lower atmosphere, most probably density fluctuations.
Another important factor is that eddies in fluids live really long, plus these eddies can interact with each other. I remember listening to a scientific talk on some kind of "algebra of eddies", but I did not manage to figure out the reference. However, a similar idea can be found within the references of a preprint on the network of solar flares. The idea of this kind of meta-modelling of Navier-Stokes equation is to save computer-time and nevertheless predict eddy dynamics. What that means for your questions: It is not too unprobable that a single anticyclonic eddy formed, and due its interactions with the other eddies in the storm system, other anticyclonic eddies could have been branched of this one. Let me conclude with the words of @AtmosphericPrisonEscape

One can speculate much and say it should have been A or it should have been B, but unless those scenarios are tested in a 3D global circulation model, nobody will know for sure.

Related

Simulating oceans and ice sheets on other planets or moons?


Astrophysics of Supernovae - Energy over Distance? I need this for worldbuilding, but it is a physics question. I want to be able calculate the energy over distance from a hypernova of a variable star like Eta Carinae. Understanding what energy levels would be dangerous to a planet, and dangerous on what level, would also be necessary.

Scientists have numbers saying that a normal supernova would damage Earth from within 50 to 100 light years. I'm more interested in hypernovae, because Eta Carinae is 7500 light years from Earth, and some scientists are saying it could strip our ozone layer literally any minute, day, year, or century now. I have to wonder: if it might be that bad here, how bad is it 5000 light years or 1000 light years away?https://en.wikipedia.org/wiki/Eta_Carinae#Possible_effects_on_Earth

I know we have models to predict these, but after repeated attempts at searching I cannot find them, and maybe I just don't know the correct terms to search. I like this answer to a similar question: https://worldbuilding.stackexchange.com/a/19002 but it appears to be about planets orbiting the dying star having a normal supernova.

I'd like to be able to take the characteristics of the star and create a distance spectrum that goes from planetary evaporation to an aurora light show for a wide range of planets: rocky, Neptunian, or Jovian . Being able to estimate that of any star would be a bonus, but I mostly just want to be able to do massive variable stars.

In short, there are no nice standard formulas for this. One can make some order-of-magnitude calculations, though.

The key formula you need is the inverse-square law: the intensity of a spherical energy source falls off with the inverse square of distance. $$I(r)=\frac{I}{r^2}.$$ The useful thing is that if you know that some source with intensity $I_1$ has certain effects at distance $r_1$ (say, a supernova being dangerous at 50 light years) then you get the same effects from a source with intensity $I_2$ at a distance $$r_2=\sqrt{\frac{I_2}{I_1}}r_1.$$ So a hypernova 10 times more luminious than a supernova would be dangerous out to about 158 lightyears (assuming the initial 50 lightyear range - this is where different papers and models might quibble a fair bit). Notice that you need a fairly large luminosity to get a vast range in this case.

Hypernovas are likely progenitors of gamma-ray bursts that release most of the energy along fairly narrow jets, 2-20 degrees across. That would amp up the range significantly. If the jet covers a fraction $f$ of the sky, that means the range is now $$r_{2 GRB}=\sqrt{\frac{I_2}{f I_1}}r_1$$ where the intensities are the true energy releases. For an opening-half angle $\theta$, $f=1-\cos(\theta)$ so we should expect $f$ to be between 0.00015 and 0.015, basically increasing the range by a factor 81 to 8.1 if one is unlucky enough to be in the beam.

The exact effect on different planets depends on many complex factors. Planetary evaporation is not a simple process unless the star delivers far more energy than the gravitational binding energy. The absorbed energy $\pi \epsilon R^2 I(r)$ where $0<\epsilon<1$ is the absorption efficiency must be much larger than $3GM^2/5R$, or $$ I(r) \gg \frac{3GM^2}{5\pi\epsilon R^3} $$ This scales proportional to mass and density, so to evaporate a ten times more massive planet you need ten times more radiation, assuming the same density.

Merely melting the surface or blowing off the atmosphere would depend on a lot of geophysical details. $\epsilon$ depends a lot on wavelengths, whether there is a plasma layer forming and similar messy issues. The light curve timecourse also matters, since some of the processes will be hydrodynamic flows rather than instant shockwaves or long-term heating.

A crude model might be that the absorbed energy heats up the atmosphere initially as $$\Delta T = \frac{\epsilon I(r)}{ C_P M_a}$$ where $C_P$ is the specific heat and $M_a$ the mass of a one-square meter column of atmosphere. For Earth, $M_a=10300$ kg and $C_P=1.00$ kJ/kg.K. A $10^{47}$ J hypernova at a distance of 50 ly radiating evenly would deposit $10^{47}/4\pi (50 \text{ly})^2\approx 35\cdot 10^9$ W/m$^2$, heating atmosphere if we assume $\epsilon=0.1$ by 345 K - enough to broil us, but not enough to evaporate the oceans (there $M$ is 1000 kg per meter depth, and $C_P=4.2$ kJ/kg.K). Were it just a supernova 100 times less intensive the effect is a balmy 3.45 K. On the other hand, with a focused GRB beam we can get a hundred to 10,000 times more heating (at which point the above formula stops being valid since the air ionises and becomes plasma).


1-parameter NFW dark matter profile This should be a straightforward exercise, but I'm struggling. Help is much appreciated

I would like to parameterize a Navarro-Frenk-White profile with one parameter, using the correlation between total mass and the concentration.

As a starting point, I have the density in terms of two parameters: the total dark matter mass inside the virial radius $r_{vir}$, and the concentration parameter $c=r_{vir}/r_{s}$:

$\rho_{DM}(r) = \frac{M_{DM}}{4\pi A(c)}\frac{1}{r(r_{s}+r)^{2}}$

where the function $A(c)$ is given by

$A(c) = \ln{(1+c)} - \frac{c}{1+c} $

Using the correlation between $M_{DM}$ and $c$ (from e.g. Napolitano et al. 2005)

$c(M_{DM}) \approx 9.195 \left(\frac{M_{DM}}{h^{-1}10^{12} M_{\odot}} \right)^{-0.094}$

it should then in principle be possible to parameterize $\rho_{DM}$ as a function of $M_{DM}$ only. This is how they do it in e.g. Williams et al. 2009.

However, I'm not managing to reproduce that. Sure, given the correlation above, $M_{DM}$ determines $A(c)$, but won't $\rho_{DM}$ still depend on $r_{s}$?

You are missing one last ingredient: the correlation between the virial radius $r_{\mathrm{vir}}$ and the virial mass $M_{\mathrm{vir}}$. The idea is that the orbital time scale of a particle orbiting in the halo becomes roughly equal to the age of the universe at $r_{\mathrm{vir}}$. This implies a relation between the (mean) density of the halo and of the universe:
$$
    \bar{\rho}_{\mathrm{vir}} \equiv \frac{3M_{\mathrm{vir}}}{4\pi r_{\mathrm{vir}}^3} = \Delta \rho_{\mathrm{crit}}(z)
$$
where $\bar{\rho}_{\mathrm{vir}}$ is the halo's mean density interior of $r_{\mathrm{vir}}$, $\rho_{\mathrm{crit}}$ is the critical density of the universe (as function of redshift $z$), and $\Delta$ a parameter that depends on the cosmology. Often $\Delta=200$ is used and the corresponding quantities labeled $r_{200}$ and $M_{200}$ instead. See also this Wikipedia page.

So the way to construct a halo is as follows:

Pick $M_{\mathrm{vir}}$ (or $M_{200}$) and redshift $z$ (and your favorite cosmology)
Use above relation to find $r_{\mathrm{vir}}$ (or $r_{200}$)
Use the relation from Napolitano et al. (2005), or equivalent, to find $c$ and hence $r_s$
Obtain $\rho_{\mathrm{DM}}$ (if you still need it)


Do asteroids originally come from debris of star remnants? As we know, asteroids can be found in the Kuiper belt and Oort cloud, but surely the Sun only captures them, not produces them. I know that supernovae may produce heavy elements such as oxygen, silicon and iron, which is the common composition of asteroids, so is it true that asteroids are come from star remnants?

The asteroids (which are mostly found in the asteroid belt between Mars and Jupiter) and more distant, icy bodies found in the Kuiper belt and the Oort cloud were formed at the same time as the rest of the solar system.

The sun and the planets formed from a molecular cloud, containing mostly hydrogen and helium, but also some other elements that came from the remains of stars, including supernovae. This gas cloud collapsed under gravitation to form a star, surrounded by a disk of dust and gas. The matter in the disc formed the planets, and the Kuiper belt. More distant matter in an extended halo around the star, formed the the Oort cloud.

Thus the sun neither captures nor produces asteroids and other solar system bodies. They were formed at the same time as the Sun.

Note "Kuiper" not "Kepler" belt, and the Oort cloud is the source of comets, not asteroids. Asteroids are rocky; Oort cloud bodies are made of ices.


Why is SWAN dimming? The SWAN C/2020 F8 comet is passing near the Earth, but from Wikipedia

The comet has dimmed a small amount since May 3.
It was expected to possibly reach 3rd magnitude in May, but is now expected to hover closer to magnitude 5.

What is the cause of this dimming?

SWAN seems to be on a parabolic orbit. This means that it is the first time that it is interacting with the Sun, making it a "new" comet. New comets are covered in a layer of very volatile elements that will vaporize pretty fast when the comet is still far away from the Sun. This causes a surge in brightness. But once those very volatile elements are gone, it stops getting brighter and the comet's brightness can even decrease.

In addition to this, it is mostly losing gas, so its tail isn't as visible as if it were a tail of dust.

(From this article, in French)


Which quantum reality will be seen in a future Earth If we orbit near a black hole to travel to future and come back to Earth, which of the quantum realities will be observed in the future Earth?

My question is regarding many worlds interpretation of quantum mechanics according to which multiple alternate futures are possible https://en.m.wikipedia.org/wiki/Many-worlds_interpretation

All of them.

If you sit on Earth then (in the many worlds interpretation) each quantum mechanical observation creates multiple universes, and there is a copy of "you" in each one. So if I were to ask, which universe will I be in tomorrow, the answer is "all of them" (except the ones in which I die)

If I go into space, float around and come back I will come back in all of the universes. Because I have been moving (relative to Earth) there will be a slight clock discrepancy in all the universes.

If I go into space, orbit a black hole, then come back. I will come back in all the universes (except for the ones that I die in). There will be a large discrepancy in clock time but that is irrelevant. I've not done any movie time-travel. It's just that my watch doesn't match the clocks on Earth.

In the Copenhagen interpretation you come back in the one universe (with a certain probability that is close to 1). As the mathematics is identical, there is no experiment that can distinguish between different interpretations of QM.


Determining Horizontal Coordinate System data (azimuth, altitude) for geographic locations I would like to determine, specifically, azimuth and elevation data for any given location on the surface of the earth relative to another location on the surface of the earth (for my purposes, the latter will be London, UK).

Data and APIs are readily available for certain celestial objects, but I haven't had any luck finding data as required.

Can anyone enlighten me as to a method for calculating such data?

Many thanks.

This can be done using the Python package Skyfield fairly easily.

Here is a shell of a script; I used some dictionaries to hold data, you may want to do something else. I've stored a lot of goodies but only printed azimuth, altitude and distance in kilometers.

By default Skyfield's .altaz() method calculates atmospheric refraction for anything higher than -1 degrees (1 degree below the horizon geometrically). You can turn that off by replacing it with .altaz(pressure_mbar=0).

OUTPUT:

 place     altitude  azimuth     distance (km)
Caracas    -33.67   258.26         7075.28
Edinburgh  -2.39    338.98          533.65
Canberra  -76.25     64.93        12377.25

Here's the Python script:

import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import Topos, Loader

load        = Loader('~/Documents/fishing/SkyData')  # avoids multiple copies of large files
data        = load('de421.bsp')
ts          = load.timescale()

Earth       = data['earth']

London      = Earth + Topos(latitude_degrees  = 51.51,
                            longitude_degrees = -0.1275,
                            elevation_m = 11.)

places      = {'Caracas':(10.48, -66.90, 900), 'Edinburgh':(55.95, -3.19, 47.),
               'Canberra':(-35.29, 149.13, 578.)}

now         = ts.now()

answer_dict = dict()

for name, (lat, lon, elev) in places.items():
    place     = Earth + Topos(latitude_degrees  = lat,
                              longitude_degrees = lon,
                              elevation_m       = elev)
    vector_km            = place.at(now).position.km - London.at(now).position.km
    distance_km          = np.sqrt((vector_km**2).sum())
    dic                  = dict()
    answer_dict[name]  = dic
    dic['lat']  = lat
    dic['lon']  = lon
    dic['elev'] = elev
    dic['vector_km'] = vector_km
    dic['vector_distance_km'] = distance_km
    alt, az, d    = London.at(now).observe(place).apparent().altaz()
    dic['altaz']  = alt._degrees, az._degrees
    dic['dist']   = d.km

for name, dic in answer_dict.items():
    print(name, dic['altaz'], dic['dist'])


How to collect photometric data for WW Aur I have a problem with the collection of photometric data for star WW Aur. I can't find any file with HJD and mag column beside this one on Vizier.

I need more than one source of data. I will use the data to create a light curve via Scidavis software.

I did a bibliography search on the object via Simbad and there isn't a lot of data available since the Southworth et al. 2005 article you linked to. Looking at the paper it seems the only other source of photometric data that is mentioned is Kiyokawa and Kitamura, 1975 in Ann. Tokyo Astron. Obs., second series which seems to be a in-house publication of Tokyo Observatory and doesn't seem to be available online. The only recent references are a few times of eclipse, which would help any period change modelling, but not the general modelling of the system.

This situation is not uncommon; there are a lot of binary stars out there and at a period of 2.5 days, this one is quite "expensive" in terms of observing time. This is even more the case if you want to get lightcurves in several filters, to aid the modelling by constraining temperatures etc.


Statistics of elements abundance in exoplanets Recently, I encountered the concept of carbon planets - planets, which would be, unlike the Earth, formed mostly by carbon, instead of oxygen, silicon and magnesium. (I am not counting iron, which is mostly locked in the core). Now, I am interested in the planetary chemical compositions in general. I found interesting articles investigating the topic of variance of the planetary chemical compositions (for example here or here), but it is quite hard for me to make an overall picture of the state in the field.

I would like to ask on these things:

1. What typical compositions of terrestrial planets are expected to occur? (I guess there will be plenty of correlations, some groups of elements occuring together as they are bound, for example to the CNO cycle or other nuclear cycle.)

2. How much different the chemical compositions of terrestrial planets really are? (I.e. are all terrestrial planets mostly similar to each other, or should one rather expect that every planetary system has unique ratios of elements?)

The observational determination of the chemical abundances in exoplanets is in its infancy. In terms of terrestrial type planets, i.e. those of size less than a few Earth radii, the constraints are confined to comparing the measured densities (obtained from the masses and radii of transiting planets found by Kepler and CoRoT) with models of what planets with certain assumed composition would look like. An excellent recent example of this can be found in Dressing et al. (2015). In this paper they make the claim that all of the low-mass
planets are consistent with a single, simple 2-component models (a mixture of 83% MgSiO$_3$ and 17% iron, but that this changes at higher masses, where more volatile elements or significant water are needed to explain their lower densities. The plot below, taken from that paper, illustrates the available data and should be quite up-to-date. Note how all the low-mass planets (and the Earth and Venus) can lie on the same family of models.

I don't think that the authors are claiming that this is exactly what all the planets are made of, but simply illustrating that at present there do not seem to be any large deviations from such a composition (for example, planets that are made solely of iron).

There are relatively few planets on this diagram, because it is difficult to obtain the masses of small transiting planets (it requires detection of the doppler shift caused by the pull of the planet on its host star).

Of course different models yield somewhat different results. For instance, Wagner et al. (2012) used the same data for Kepler-10b and CoRoT-7b and their own detailed models to argue that these planets have an iron core that makes up about 60% of the planet - i.e. a lot more than makes up the Earth.

At the moment the data for the lowest mass planets currently indicate that there could only be a limited amount of diversity. But the information that we are working with, the sample size, and the fact that only masses and radii are
determined, is too sparse to be sure.

From a theoretical point of view there are many ideas. The basic concept about the formation of the terrestrial-type planets is that they form (relatively) close in to the parent star and have compositions that are reflective of what elements and minerals can condense out of the protoplanetary disk at high temperatures. This in turn depends on the balance of elements that are present in the protoplanetary disk, where in the disk the planet forms, the detailed structure of the protoplanetary disk, how it cools and how planets migrate in the disk.  Unsurprisingly, by varying some of these conditions it is possible to create planets with a wide variety of compositions, which as I said above, appears to be mildly contradicted by the available evidence. At the most basic level we understand why the Earth and other terrestrial planets are comparatively lacking in "volatile" elements like hydrogen and helium (and to some extent C, N, O and noble gases), because they (or minerals containing them) could not condense at the high temperatures close to the proto-Sun.

Examples of these theoretical approaches can be found in Moriarty et al. (2014) (of which you are familiar), but also see Carter-Bond et al. (2012) for examples of how chemical diversity might arise. It seems that the Mg/Si and the C/O ratios have the biggest influence on the final compositions of the formed planets. A low C/O ratio favours the formation of silicates and fewer carbon-carrying compounds; but if there is more carbon than oxygen then it becomes more favourable to form carbon and silicon-carbide (I guess this is what you mean by "carbon planets"), but this also depends on the temperature in the region where the planet forms. For reference, the solar C/O ratio is 0.54 and the relative abundance of carbon in the Earth is much lower (than in the Sun) but the C/O ratio measured in other stars can be higher.


Why levels of radio contours maps are given in mJy/beam and what does it mean? I know what a jansky is, but I don't understand what beam (in $\frac{1 Jy}{beam}$) is.
If jansky is a unit of flux density, what kind of unit is $\frac{1 Jy}{beam}$?

Jansky is defined as  $1 Jy= 10^{-23}erg/s/cm^2/Hz$. So all the energy coming from a given solid angle per time, per frequency bin and per detector area.

This is convenient for sources of small angular extent (smaller than observing beam), e.g. for point sources, because the flux remains constant for varying beam sizes. For extended sources, the surface brightness is often described with units of Jy per solid angle. Radio contour maps, e.g. of interstellar clouds, usually show objects that are larger than the beam, so that the observed fluxes are a function of the beam size. To clean the beam influence from the date Jy/beam (beam area in steradian) is used.

So it is a surface brightness. For a flux of 1 Jy measured with a beam of 1 sr solid angle we find:   $1 Jy/beam= 10^{-23} erg/s/cm^2/Hz/sr$.

Note, that 1 sr is already a huge portion of the sky. Beams are much smaller, on the order of arcsec to arcmin diameters.


Multiple Star-system percentages What are the percentages of systems that have x number of stars in them?

What I have found thus far is something like:

Single Star Sytems = 69%
Double Star Systems = ~10%
Triple Star Systems = <20%
Quadruple Star Systems = ???
Quintuple Star Systems = ???
Sextuple Star Systems = ???
Septuple Star Systems = ???

Also... Is there any way to get these percentages for systems that have primaries that are Giants (more than 2 Solar Masses) Major Stars (0.5 to 2 Solar Masses), and Minor Stars (0.5 Solar Masses... red dwarfs)

I have found an article that says...

Giant Stars = 80% have companions
Major Stars = 50% have companions
Minor Stars = 25% have companions

Sources:
http://www.space.com/1995-astronomers-wrong-stars-single.html
http://boards.straightdope.com/sdmb/showthread.php?t=499261
http://www.skyandtelescope.com/astronomy-news/surprise-most-star-systems-are-single/

Your best bet for finding relevant information on this is to look up actual published papers. I'll walk you through my research process to help in the future, as well as provide the results I found.

Step 1: Google Scholar is your friend

I start out using Google Scholar. This is much like Google, but rather than returning any old website, it specifically returns published papers and other scholarly articles. Part of this magic is of course knowing what to search. I went with "multiplicity of stellar systems". Immediately I found two sources which seemed promising.

Stellar Multiplicity and the Initial Mass Function: Most Stars Are Single (Lada 2006)

A Catalogue of Multiplicity Among Bright Stellar Systems (Eggleton & Tokovinin 2008)

I've lucked out because both articles happen to be free so I can see the full contents!

Step 2: NASA ADS

NASA Astrophysics Data System is a very large catalogue of just about every astronomy (and some physics) related paper published in any journal. What's even more amazing, very often the papers here are free, even if they aren't free on the journal's actual web page. If you ever run into a paper from a journal that only gives you an abstract, try looking for the paper here.

It was already free on the journal's webpage, but I was able to look up and find the Lada paper I cited above.

I specifically point you towards ADS for a few reasons.

It has a large collection of free papers.
You may have heard of or even use arxiv, which is a great resource, but very often the papers on arxiv are versions which are pre-publication and not the officially peer-reviewed and published papers. ADS usually has the official published papers (and links to arxiv versions!).
ADS has another hugely important feature. It lists article citations and articles which have cited that article.

Step 3: Backtracking and Forwardtracking

Once you've found a good source (such as the two papers above), you don't want to stop there. There's likely more or better papers and you can use the papers you have to find new ones. What you want to do now is take a look at all the papers your current source has cited, and look at all the papers which cite your current source. ADS very conveniently gives you all this information.

The Lada 2006 source is 10 years old at this point so I wanted to see if there was anything more recent. I was able to click on the "Citations to the Article" link on ADS for that article, and found the list of 217, more recent papers which cited the Lada paper. Scanning through this list I found two promising papers:

Stellar Multiplicity of the Open Cluster ASCC 113 (Guerrero et al. 2014)

An Adaptive Optics Multiplicity Census of Young Stars in Upper Scorpius (Lafrenière et al. 2014)

From here, just rinse and repeat until you've got the information you want.

Results

I found four papers which appear to give some or all of the information you want.

Lada 2006 - This paper is more or less a conglomeration of past research on the fraction of stars which are single star systems. It focused on lower mass stars (G to M) and found that the fraction of single star systems ranged from ~43% for G type stars to ~75% for M type stars (which are the most populous type).
Eggleton & Tokovinin 2008 - I think that this is the best source for your particular question. Within their abstract they state We identify 4559 such bright systems (including the Sun), and the frequencies of multiplicities 1, 2,..., 7 are found to be 2718, 1437, 285, 86, 20, 11 and 2. This implies the fraction of multiplicities is: 59.62%, 31.52%, 6.25%, 1.88%, 0.44%, 0.24%, and 0.04%. However, note they say their measurements have "substantial" uncertainties which they describe.
Guerrero et al. 2014 - This group looked at and around a specific cluster and found a ratio of the number of single to binary stars to be 27:7 within the cluster itself. Including stars around the cluster, the following multiplicities were found (ranging from 1 to 8 companions) 125:27:4:1:0:0:0:1 These two sets of ratios indicate that within the cluster they saw 79% single star systems and 21% binary star systems. Overall they saw multiplicity fractions of 79.1%, 17.1%, 2.5%, 0.6%, 0%, 0%, 0%, and 0.6%, respectively.
Lafrenière et al. 2014 - A study that looked at 91 stars and found 57 single stars (63%), 29 binaries (32%), and 5 triple systems (5.5%).

What this suggests is that the multiplicity of stars is highly variable, still somewhat unknown, and that it depends on the environment you look at (and I found a few papers discussing this very point). The Eggleton paper had the largest sample and so may be the most trustworthy in regards to a true average, but make sure to understand what their uncertainties are.


Black Dwarf Capabilities Can a black dwarf be suitable for manned exploration, once it cools down? And potentially used for space mining?

A 1 solar mass, Earth sized black dwarf would have a surface gravity of about 360 000 g which probably rules out manned exploration by anything we would normally think of as human. For similar reasons, mining would be quite challenging.

Another obstacle is that the Universe is not old enough to have produced any black dwarves yet. The oldest white dwarves still have a surface temperature of thousands of Kelvins.

Indeed, if humanity is still around when they have formed, visiting and mining them will probably be easy.


How long time since a typical asteroid collided with another asteroid? How pristine and ancient are asteroids really? I suppose they melt and reform when they collide with each other. How frequently do they collide with each other?

How ancient they are, I'd have to say as old as the solar system.
Collision frequency:
Collision amongst Asteroid Belts
From ref 1  below: Based on the particle in a box model which assumes a homogeneous distribution in the interactive volume, the intrinsic collision probability is computed to be around $5 \times 10^{-18} km^{-2}yr^{-1}$.
However Wetherill(ref below) have some modifications to Opik's formulation for collision between two elliptical orbits, suggests that the impact is nearly independent of the orbital elements of the test body and is nearly half of the value obatined from PIAB model. It also seems to suggest that intrafamily collisions are higher than collision amongst non family objects.

Collision with the Earth Here it characterized in various classes:

bolides or upper atmospheric explosion,
land impacts leading to local or regional devastation,
ocean impacts leading to regional disasters,
global ecological catastrophe,
mass extinctions

Based on the scale, the impact events are categorized as harmless, Class-1, to devastating, Class-5.

The 440-KT Chelyabinsk event of 2013 belongs to impact hazard class 2, causing regional or local devastation. This was caused by a mere 20 meters wide meteor entering the atmosphere of Earth. For a cumulative number of impacts described for a given reference time interval of $T_{ref}$ = 1Ga ($10^9$ years)
which is long enough to statistically include a meaningful number of impacts from all the classesabove, the class 2 impacts cumulative will be $2\times 10^6$ , with small bodies falling in the size range of 20-100 m. Where the characteristic time scale is of $10^2$ a.

For class 3-5 events leading to regional disasters to global extinction events, the cumulative number of impacts within the reference interval is $10^4$ with characteristic time scale being $10^5$ a. These events can lead to large devastation ranging from an entire country or region (class 3, with an explosive energy of approximately 104 MT TNT), to mass extinctions (class 5 releasing an approximate energy 108 MT of TNT). An example from the history for such event is the classic K-T extinction events 65 Ma ago that wiped out more than 99% of the living species.

References:
1. http://articles.adsabs.harvard.edu//full/1992LPI....23..283D/0000283.000.html
2. http://www.springer.com/fr/book/9780792354666
3. http://onlinelibrary.wiley.com/doi/10.1029/JZ072i009p02429/abstract
(PS: If anyone has a link to the open access for final paper, or a pdf; I would really appreciate a copy.)


Do free protons and neutrons absorb much radiation? To affect astronomers' observations? If so, at what wavelength(s)? In a plasma, or wherever, do the completely ionized nuclei commonly absorb much EM radiation?  Or any free neutrons or protons?  Can astronomers detect this?  Enough so that astronomers take it into account in their observations?
I've read elsewhere that protons, neutrons and nuclei, even when free of any electrons, usually (or only, depending on who you read) aborb and/or emit only hard X-rays and gamma rays...
Is this true?

You cannot have free protons without electrons. Plasmas, in general, are electrically neutral.
It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a photon and conserve energy and momentum) in a plasma at low photon energies. That is basically due to their much lower masses (classically you can think of the electric field of the incoming light accelerating the particles). NB. Electrons in the electric field of ions can emit or absorb photons in a process called (inverse) bremsstrahlung.
High energy (>MeV) photons are capable of causing transitions in the energy levels of nuclei, so can be absorbed. At even higher energies the photons can interact with a nucleus and create particle/anti-particle pairs - pair production.
Very high energy (>100 MeV) photons can scatter from protons and neutrons because these particles have internal quark structure. It is even possible for photons to be absorbed, creating short-lived 3 quark resonances.


Why are O III lines so prominent in the spectra of emission nebulae when the amount of oxygen relative to hydrogen is a million times smaller? Looking at spectra of emission nebulae like the Lagoon Nebula, the $[\text{O III}]$ lines are prominent in intensity. However, the abundance of oxygen is minuscule compared to hydrogen. How then are the $[\text{O III}]$ spectral lines of such high intensity?

This is an excellent question. Think about the way in which emission happens. $\text{H}\alpha$ emission happens when an electron makes a transition from the third energy level to the second, emitting a photon in the process with the energy equivalent to the difference of the energies of the two states, roughly $1.9\text{ eV}$. There's a lot of hydrogen in emission nebulae - and in nebulae in general - and so it plays an important role in the emission spectrum.

Emission from $[\text{O III}]$ comes from a different process: collisions. Free electrons with energies of $\sim1\text{ eV}$ - about the amount of the oxygen's low excitation potential - collide with oxygen atoms. This produces forbidden lines that would otherwise have been unlikely to be created, as the ionizing photons typically would not have the right energies. Therefore, doubly-ionized oxygen (in particular) becomes extremely important in spectra, generally second only in intensity to $\text{H}\alpha$.

Given that $[\text{O III}]$ emission is collision-based, it makes sense that there is a temperature dependence - which there is, as is the case with $[\text{N II}]$ lines and is responsible for their high intensities. Conditions in many hot nebulae (in this case, $>10,000\text{ K}$) are right for such collisions and thus forbidden line emission. Hot host stars can sometimes help achieve high electron temperatures.


How to reduce magnification and increase FOV of a camera attached to a telescope? I am a beginner in astronomy.
I have a Maksutov telescope (90mm/1250mm). Generally I do observation with a 25mm wide angle eyepiece, but sometimes due to light pollution I can't see many stars.
So I modified my webcam and converted to a telescope camera.
I can take detailed pictures of moon craters, but the FOV is very small. Is there any way to increase FOV? Can I set the camera on the eyepiece?
note: I am not expecting high quality DSLR-like images, but expecting more stars due to a longer exposure time.

You are using the webcam as an electronic eyepiece. People have been using various types of cameras as eyepiece for decades.
The reason for the small field of view is the small (1/4") sensor of the camera. Only a part of the actual field of view strikes the sensor. The field of view of your telescope will likely cover a 2" eyepiece. You are putting a 1/4" sensor into the middle of a 2" circle. Tne sensor only sees the center portion of the total field of view.
Most use focal reducers in front of their cameras to increase the field of view of their electronic eyepieces.
I started electronic viewing twenty years ago with webcams and CCTV security cameras. I now use several Mallincam Astro cameras.
Google "electronic eyepiece for telescope" or "electronic assisted astronomy". You will useful information once you get past all the advertising.


What happened before the big bang? Was there space before big bang? Or who knows there might be different heavenly bodies that we might not perceive?

"What happened before the Big Bang" is like asking what Alexander the Great's phone number was. It's a nonsensical question.

Space and time began with the Big Bang. "When" and "where" began with the Big Bang.