Is there a significant population of neutron stars outside the galactic plane? Pulsar kicks originating from supernovae can impart neutron stars with speeds of 100-500 km/s, often at or close to the escape velocity of the Milky Way. Even if a pulsar fails to escape the galaxy, it can be launched into unusual orbits. Assuming that the velocity distribution is isotropic, a decent fraction of such pulsars should have large orbital inclinations after the kicks, such that they rise above the galactic plane.

I have two questions regarding this:

Is there a large - i.e. noticeable - population of such neutron stars, distinguishable by modern telescopes, or is the fraction of total neutron stars relatively small?
Does observing a pulsar - at any wavelength - in such an orbit have any advantages over observing an identical pulsar firmly in the galactic disk? I would assume that some kicks could propel them high enough that scattering along the line of sight could be lessened, leading to a smaller dispersion measure smearing, and could also isolate the pulsars from ambient radio sources in the disk, such as a the galactic center.

I understand that "noticeable", at the least, is a bit subjective, but I can't come up with any non-arbitrary criteria to change that.

I'll partly answer your question with the following plot from the ATNF pulsar database (which is easy to play around with). The plot shows the Galactic z coordinate of the pulsars (distance from the Galactic plane in kpc) versus their estimated age (from their spin down rates, in years).

The basic picture is that young pulsars are all near the Galactic plane, because they are born from high mass stars that are born, live and die near the plane and even if the neutron star got a kick, there has been insufficient time to move far from the plane. A typical kick might be 500-1000 km/s, roughly 0.5-1 kpc in a million years.

Once pulsars reach a million years old, then indeed some reach a kpc above/below the Galactic plane and then a few even older examples reach 10 kpc.
Most normal pulsars only show pulsar activity for 10-100 million years (the older ones on the plot are probably a different population of pulsars in binaries), so they aren't observed at much greater distances (though there will be old neutron stars out there).


Why is Mars' atmosphere so thin? Is it because there isn't life to exhale enough oxygen and carbon dioxide? The size of Mars is about the same as Earth, so isn't there enough gravity to hold enough air?

The escape velocity of Mars is about $5.03 \mbox{ km}/\mbox{s}$ , that of Earth about $11.2 \mbox{ km}/\mbox{s}$ , that of Moon about $2.38 \mbox{ km}/\mbox{s}.$

The escape velocity is an important ingredient for the stability of a planetary atmosphere.
From this it's not surprising, that the density of the Martian atmosphere is between that of Moon and that of Earth.

The loss of most of the magnetic field of Mars makes its atmosphere less resistant to erosive solar wind.

(Actually things get more complicated, since sequestration and outgassing, meaning an exchange of atmosphere with rock, as well as sublimation, and resublimation of water and carbon dioxide take place. Water vapor is decomposed into hydrogen and oxygen by photolysis, hydrogen escapes to space due to its low molecular weight, and oxygen tends to be bound chemically to iron-bearing rocks. On the other hand solar wind provides a steady supply of hydrogen, which is bound by rocks in the top few micrometers; "ChemCam saw peaks of hydrogen and magnesium during the first shots that we didn’t see in subsequent firings.")

The history of the Marsian atmosphere isn't yet known in full detail. NASA's MAVEN mission is designed to find out more.


Could stellar occultations be used to survey Kuiper belt objects? The first hint we had as to the shape of Ultima Thule was via an occultation study using an array of small telescopes. Has anyone investigated the feasibility and effectiveness of conducting a survey campaign using the same technique?

Given that this was managed for an arbitrarily selected object in a window of few years, it would seem that, if you just wanted to do the same thing for as many Kuiper belt objects as you can manage, that you should be able to get similar data for a significant number of objects a year at relatively low cost.

Edit: the kind of project I'm wondering about would be targeted at getting detailed shape (e.g. what fraction of KBOs are contact binaries?) and size data. For example a line of a few hundred 0.5m class interments at 100m to km spacing.

Edit 2: I guess I kind of didn't actually say what the target of the survey would be. To clarify, the objective would be to collect fine detail on the shape of KBOs. E.g. how common are contact binaries?

Yes this is possible and has been tested by the TAOS (Transneptunian Automated Occultation Survey) group among others. That group is in the process of building the TAOS-II project which will use several telescopes and fast readout CMOS cameras (since the occultations only last a few seconds) to search for occultations caused by unknown Kuiper Belt Objects. Multiple telescopes are very useful to build confidence that you have a real event and not atmospheric fluctuations or other instrumental effect. A detailed workthrough and calculations of how many objects you would see, what is needed to detect them and resolve the Fresnel diffraction and other issues is given in a section by Roques et al. in the Solar System Beyond Neptune book and which is available here.

In general for surveys such as occultations by unknown KBOs, it is etendue (product of telescope collecting area and amount of sky covered in a single exposure) that matters (but it is often more complicated as survey depth doesn't just scale with telescope area) in that higher etendue value equals "better" survey. So for the camera(s) in a survey, it is sensor area that matters, which is a product of pixel size and no. of pixels (and number of sensors and size of gaps between if you are tiling them). The telescope area/diameter tends to be set by how bright your targets of your survey are and practicalities such as cost and manufacturability tend to influence the f/ratio and therefore focal length and pixel scale at your detector. Rarity of your targets and how many you want to find, drive how much area you need to cover. (The telescope area also influences this since in general you get more targets per area if you use a bigger telescope and go fainter). So you end up trading all of these things off to get your best survey for your particular science and type of targets, normally constrained by available or potential budget.

In the case of trying to time-resolve occultations by KBOs, your exposure time is set by the event timescale which depends on the velocity of the KBO across the line of sight and the diameter of the KBO. For typical KBO velocities of 20 km/s, this means that a Pluto (approx 2000km diameter) occultation lasts $2000\,km/20\,  kms^{-1}=100\,s$. If you wanted to find small 20 km KBOs similar to 2014 MU69/Ultima Thule, then the events are $20\,km/20\,  kms^{-1}=1\,s$ long (both of these calculations assume the occultation goes over the largest part of the KBO and isn't grazing).

Given that you will want several points during the occultation, both to convince yourself it's real and to measure diameters etc, you will need at least 5 points across the event. For the 20km KBOs/1s events, this means at least 5 frames per second, ideally more. This means that you need to get enough signal-to-noise on your target stars in a 0.2 or 0.1s exposure.

As discussed in the reply comments to the question author, scientific CMOS sensors such as the Andor Marana (a commercially available closeish match to the custom detectors used by TAOS-II with ~$31k pricetag; Jan 2019) will be better at both the 'signal' part (due to higher quantum efficiency) and the 'noise' parts (larger, less noisy pixels, cooling and temperature stabilization) than a consumer DSLR sensor. Looking at a study of DSLR dark current by Photonics.com, the dark current is ~50 counts/pixel/second (the Photonics.com figure is for 30s exposures and assuming 1500DN) rising to ~100 counts/pixel/second (averaging at ~3000DN for 3 of the pixels) after 100 frames as the sensor heats. For the Marana, the figure is 0.26 counts/pixel/second (taking datasheet value of 0.2 electrons/pix/s and assuming a gain of 1.3 to match the full well capacity of 85k electrons to the 16 bit ADC (65536 levels)), an almost 400x lower value. (The ADC data range is another area of difference; the Marana can do 16bit digitization whereas DSLRs are typically 12 or 14bit, giving only 4096 or 16,384 levels to record all the signal levels in the image)

The other use of occultations is to monitor predicted occultations by known asteroids and Kuiper Belt Objects. This can be used to get a better idea on the size and shape of the object (as you note for Ultima Thule/2014 MU69), search for additional satellites or rings around the object (as has been done for the KBO Haumea (Nat Geo article) and the Centaur Chariklo (Space.com article)) and look for and monitor changes in the atmosphere (as has been done for Pluto since the first occultation in 1988).


How hot can a planet be? Given that some exoplanets, particular "Hot Jupiters", orbit very closely to their parent star, how hot can these planets become?

What is the hottest exoplanet discovered so far?

The temperature of an exoplanet is dependent on a number of factors, including:

The distance between the exoplanet and the parent star.
The type (energy output) of the parent star.
The ability of the exoplanet's atmosphere to distribute the heat energy.

Recent discoveries, such as reported in the article "Sizzling Planet Makes Some Stars Look Cool" (Than, 2007), have found that some exoplanets are hotter than some types of stars.

For example, the exoplanet  HD 149026b  has a star-facing side of over 2000C (3700F), this planet reflects so little light that it would appear like a "glowing piece of charcoal". It is believed (from the article) that the planet is tidally locked and the 'dark side' of the planet is believed to be substantially cooler.


What is a cold white dwarf? Is a cold (3800K, or so) white dwarf still be considered/in degenerate state. Or, how can a gas be degenerate and cold?

I'm not entirely sure what your source of confusion is, so I'll just describe how temperature and degeneracy relate and hope that clears it up.

Fermions

All particles which are fermions are subject to the Pauli Exclusion Principle. This means no two fermions can occupy the same quantum state. They will resist doing so. What that really means, doesn't matter too much so for now, just accept it as an axiom.

Electrons are fermions. A star, before it becomes a white dwarf, is filled with electrons just minding their own business. Then, as the star begins to form itself into a white dwarf, the crushing pressure of gravity squeezes all the electrons into a tight space. Gravity tries to push these electrons into the same quantum state, but the Pauli exclusion principle forbids this.

Fermi Energy

Note: The following is a gross over-simplication, but it gets the idea across without being too terribly wrong. Don't take it as gospel truth.

You can define a set of all the possible quantum states. For simplicity, I'm just going to call them $\mathrm{state}\ 1$, $\mathrm{state}\ 2$, ..., $\mathrm{state}\ 67$, ..., $\mathrm{state}\ 10^{10483}$, etc. (ordered by energy). From above, we know each state can be occupied by only one electron. In general, these electrons will occupy a variety of non-sequential states. Pre-white dwarf, we might order the electrons by state and say

$\mathrm{electron}\ 1$ is in $\mathrm{state}\ 43$,
$\mathrm{electron}\ 2$ is in $\mathrm{state}\ 12084$,
$\mathrm{electron}\ 3$ is in $\mathrm{state}\ 4.187\times10^{78}$
etc.

As the electrons are squeezed in the white dwarf, they'll try to get as close to the same quantum state as possible without violating the Pauli exclusion principle. A fully degenerate star would have electrons in the following states:

$\mathrm{electron}\ 1$ is in $\mathrm{state}\ 1$,
$\mathrm{electron}\ 2$ is in $\mathrm{state}\ 2$,
$\mathrm{electron}\ 3$ is in $\mathrm{state}\ 3$
etc.
$\mathrm{electron}\ n$ is in $\mathrm{state}\ n$

In this completely degenerate state, all electrons have been forced to occupy all the lowest energy states. The Fermi energy is the energy of the last electron in the highest state. If all electrons happen to be squeezed such that they're all below the Fermi energy, then we consider the object degenerate.

Temperature and Degeneracy

Gravity might try to squash all the electrons down into the lowest quantum states, but the ability of gravity to achieve this depends on the temperature. Electrons at a higher temperature have higher energies and it becomes harder for gravity to suppress them. With a certain set of assumptions1, one can say that for a star of a given temperature, $T$, and a given density, $\rho$, it will be degenerate if2:

$$\frac{T}{\rho^{2/3}} < 1261\ \mathrm{K\ m^2\ kg^{-2/3}}$$

From this equation, you can hopefully see that being colder actually helps a white dwarf be degenerate. The two ways to make a star (or anything really) become degenerate are to cool it down and increase the density. For white dwarf formation, the fact that it's degenerate is more a function of the high density than it is the cool temperature.

1Assumptions and preceding equations are in An Introduction to Modern Astrophysics, 2nd ed., pg. 566.

2As a preemptive measure, this equation does not apply universally. You cannot apply it to yourself and declare yourself degenerate!


Impact verses vacuum How do asteroids, comets and other stuff stay together when colliding in the empty vacuum of space, when they are made up of non-sticky substances like rock?

My understanding is that the force of gravity from small objects is very weak and when travelling at speed I don’t see how they join together.

All these objects are rocky, also the icy ones given the low temparatures at their usual orbits. Your observation is also correct, that mutual collisions at usual orbital velocities in today's solar system tend to be catastrophic among the remaining small objects (comets, asteroids, KBOs etc); that is even true when you consider that usual collision velocities are smaller than orbital velocities as most objects rotate prograde and only objects in similar orbits collide with approx. differential Keplerian velocities. The objects don't grow anymore today.

The collisions you likely refer to occur during planet formation. Planet formation happens concurrent to star formation inside the star's accretion disk which evolves into a protoplanetary disk around the pre-main sequence star. This protoplanetary disk consists of both, gas and solid particles (mass ratio ~100:1) so that motion of the objects is not in vacuum but influenced by the gas - the smaller the more. The outcome (sticking, bouncing, fragmentation) of mutual collisions depends on what the particles look like (how porous or compact) and what is their relative size which defines for small particles their collision speed. You can extend these physics to larger particles until they reach a size at which their motion decouples from the gas. Then likely processes kick-in which combine the motion of gas and interaction with collective behaviour of these 'pebbles' (like disk turbulence, streaming instability which forms zones or vortices in the protoplanetary disk in which the small bodies can amass and gently collide to form big bodies, etc), so that these pebbles agglomerate to larger bodies, the so-called planetesimals which are of a few km-size like today's comets.

These bodies are big enough that they are only little affected by gas - the bigger the less drag compared to their mass they experience and thus will rotate with Keplarian speed around the being-born star - and catching up with smaller bodies in the same or very similar orbit, so that they gather them to form even bigger bodies. Thus a few 'lucky winners' will experience run-away growth, collecting the smaller bodies in their orbital distance which increases even more the bigger they become as gravity will start to play a role. The star enters main sequence and the emerging solar wind will push out the gas of the disk so that a debris disk, thus big particles, planetesimals and protoplanets remain, entering the phase of late heavy bombardment where many small bodies collide with the bigger bodies, effectively clearing especially the inner part of the solar system from many small bodies (as the collision timescale is particularily short)

Many, many details have been left-out here, of course and in parts it may even be over-simplified.

So in essence: what you see in the videos only works because there is still gas which slows down things - and because stuff is in similar orbits and thus their relative velocity is small so that gravity is NOT overcome by the shattered bodies when they collide.


On the consistency of different well-polished astronomy software I have purchased a custom wedding band from a seller that claims the ring will show the constellations visible at the horizon on a specific date at a certain date and time.

However, I have fired up Stellarium and set it up to look at a specific example shown in the product page - and I cannot find any set of parameters which looks like it: Jupiter's not in the right place, some constellations are under the horizon (Virgo, Libra, Leo), some others very high in the sky (Andromeda, Perseus)... Although it's the first time I use Stellarium, I don't see how I could make a mistake:

Location set to London Bridge (even the preset London to be sure)
Date and time set just like in the example (double-checked 5 times)
Cylindrical projection (tried all of them though), offset -30%
FoV 210° (to have the 360° mapped to the entire screen - I have
absolutely no idea why 180° does not work)
Elevation lines horizontal (although I tried moving around to align everything like in the pictures for every projection method - in vain)

Here is the example:

Am I missing something, or is their sky map wrong? If yes, is it possible that two pieces of well-polished software like Stellarium and this one (looks just as good as Stellarium) have different star locations? How come (isn't there a standard star almanach?)?

If you set the date to 2018-04-06, Stellarium shows the Moon and planets in positions matching the example image.
Any good planetarium software should produce a similar result.

Most likely the vendor cut and pasted two screenshots (note the seam) for April 2018, overlaid "January 1973," and hoped customers would not check.
Perhaps you could ask them to send you an image for approval before printing the ring.


Expansion again: where does the energy come from? Is it sensible to ask for the overall increase in potential energy when looking at the baryonic mass in the universe moving away from each other, that is, moving against the direction of the force towards a higher energy state in each others however weak gravitation field? I can't imagine that nature has the same take on this matter like a numerical simulation (a distance which it deems "large enough to make effects negligible so let's forget about it completely"), i.e. the energy can't be zero... where does it come from?

Conceptually there are several things going on here.

Where does energy conservation come from? In modern understanding, energy is the Noether charge of time translation symmetry, as found by Noether's first theorem.
 But in general relativity, the metric is dynamical, and so in general we simply don't have any time translation symmetry. Static spacetimes do, and there is also a form of energy conservation for spacetimes that regain time translation symmetric far away from the gravitating system (e.g. ADM energy of asymptotically flat spacetimes). But those are the exceptions, not the rule.

In other words, in general relativity we don't have any scalar notion of "energy" that's applicable globally. Vacuously, it is neither conserved nor violated.

But what about locally? In a local inertial frame, energy is exactly conserved, but the gravitational forces exactly vanish.

One thing you can do in the context of cosmology is look at the Friedmann equations as some sort of analogue of energy conservation, by making a balance between the terms describing cosmic expansion and the energy density, pressure, and cosmological constant. The Friedmann equations come from the components of the Einstein field equation connecting the Einstein curvature tensor and the stress-energy tensor: $G_{\mu\nu} = 8\pi T_{\mu\nu}$. According to this interpretation, Einstein curvature always exactly balances the stress-energy of the matter in spacetime. But this is just a restatement of a dynamical law, so it's not really a "conservation" law.

The Einstein field equation itself can be found from the Einstein-Hilbert action, and trying to apply Noether's second theorem simply shows that the covariant derivative of the stress-energy tensor vanishes: $\nabla_\nu T^{\mu\nu} = 0$, which is analogous to $\nabla\cdot\mathbf{B} = 0$ of electromagnetism: "there are no local sources or sinks of [stress-energy/magnetic field] anywhere." This is actually trivial, because the covariant derivative of Einstein curvature always vanishes (a theorem of geometry lacking any physics), so Noether's second theorem didn't tell us much more than we would have known otherwise.

Because the derivative is covariant rather than partial, many people don't consider this to be a true conservation law either. Certainly it gives no information as to "how much" energy there in spacetime--that's still undefined.

So we have the following issues:

There is no global energy conservation is general relativity, except for very special spacetimes, and the FRW family used for Big Bang models don't qualify.
In a local inertial frame, energy is exactly conserved, but there are no gravitational forces. (Local inertial frames only exist as first-order approximations anyway.)
One can interpret the Einstein field equation as Einstein curvature exactly balancing the stress-energy of matter, which is also motivated by interpreting the Friedmann equations of cosmology as a balance between cosmic expansion and local energy, pressure, and cosmological constant. However, this is actually a dynamical law.
The vanishing of the covariant derivative of stress-energy can be construed as an analogue of local energy conservation, although doing so is conceptually misleading.

Addendum: It's notable that there is yet another sense in which the total energy of the a spatially finite universe is exactly zero. Intuitively, one can try to measure the content inside some closed surface, and then expand that surface to try to enclose everything in the universe. However, for a closed universe, that surface will contract to a point, thus enclosing nothing (picture a circle around the north pole of the Earth, and expand it to try to enclose all of Earth's surface--it just contracts to a point at the south pole).

More formally, one can find a sequence of asymptotically flat universes (for which, again, the energy actually is defined) that approximate a spatially finite universe. In the limit in which the approximating universes "pinch off" and separate from the asymptotically flat region (thus becoming actually finite), the ADM energy goes to $0$.


Why can we observe the Cosmic Microwave Background no matter the direction we look? I often read that the CMB was released from everywhere in the Universe, in every direction.

If that statement is true, can someone elaborate what "everywhere in the universe, in every direction" means ? If not, why can we observe it in every direction ?

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and electrons couldn't form neutral hydrogen, since every time it "tried", an energetic photon would knock off the electron.

This gas was everywhere. And photons traveled and scattered in all directions:

Photons (purple) scatter on free electrons (green), and both are mixed with protons (red).

380,000 years after the Big Bang, the temperature had fallen sufficiently that neutral atoms could form (this is called recombination). The radiation, which until now had scattered continously on free electrons, could now stream freely between the atoms (this is called decoupling).

So they did. Still in all directions:

This free streaming is still taking place. Photons travel in all directions, and are everywhere. The photons that you are able to see, are the ones that started out at a particular distance from you, and in a particular direction, but other photons started out at smaller and larger distances, and in other directions. You just don't see them, because you happen to be right here. But a person in another place of the Universe would see the same as you.

The photons that we observe as the CMB come from a region we call the surface of last scattering, because it corresponds to the surface of a shell centered at us. But there is nothing special about this "surface", except that is consists of all points in the Universe that are so far from us, that it takes a photon roughly 13.8 billion years to travel. And because of the expansion, these points are now roughly 47 billion lightyears away from us.

In the figure below, the arrows show CMB photons. All have the same length; they start where they were emitted and end where they are today. What we observe as the CMB are all arrows that end at the Milky Way (in the center). Other arrows may be observed by other observers in other galaxies that have their own surface of last scattering around them.


Recording Spectral Lines at Home I was wondering if it would be possible to record the emission/absorption spectra of stars without expensive spectroscopy equipment. Would it be possible to somehow utilize diffraction grating? I own a Newtonian with a 130mm aperture if that helps.

Here is a link to a diffraction grating that can do what you want. It is mounted in a 1.25" filter ring that attaches to an eyepiece, or to most astro cameras. I believe they also sell adapters for other cameras and software to extract spectra from the photos.

Here is a link to a spectra of Vega that I took with a 120 mm refractor using  the Star Analyser grating.


Why is the universe full of spinning objects? Almost all celestial objects are rotating in some way about their axes. Why is this and should we expect the rotation to slow as the universe ages?

The MinutePhysics channel on YouTube made a wonderful short animation that answers your very question. It explains how a cloud of gas and dust with zero net angular momentum collapses down to form a flat spinning object such as a galaxy or our Solar System.

The answer involves a few things:

The universe conserves angular momentum in every interaction between
physical objects.
Mathematics says a big cloud of gas and dust must have exactly one plane in which the overall angular momentum is zero.
When particles of gas and dust collide, any angular momentum they have outside that plane is cancelled, so over millions of years and (countless numbers of collisions) the cloud ends up flattening.
But as they collide, their angular momenta within that one plane are
conserved, so the flattened system ends up spinning.


What is the accepted theory as to why Uranus' axis is tilted so severely? The planet Uranus is another solar system anomaly, where according to the NASA profile has an axial tilt of 97.8 degrees, also considered to be retrograde.  This NASA summary "Uranus" suggests the current theory of a large planet-sized impact earlier in its history.

Does the planet-impact theory still hold true or have new accepted theories come to light?

Most of all, are there any results from any simulations available?

A note, this is posted as a separate question to my other question "What is the current accepted theory as to why Venus has a slow retrograde rotation?" as the axial tilt is significantly different.

Nasa.gov speculates that
the most likely possibility is that an object with a mass close to that of Earth's collided with Uranus, causing it to rotate on its side from then on.
A recent test by Space.com suggests that "Planet Uranus Got Sideways Tilt From Multiple Impacts". These findings suggest that two or more smaller collisions probably occurred, asserting that the early solar system was probably more turbulent than most would think.


Help in determining the features of an unusual, fictional star system (Hope this is the right Stack-exchange site for this question)
I'm working on a sci-fi RPG campaign, set on a very atypical location. Since this is a work of fiction, there's enough room for speculation and artistic license. But I'd still like to start from a more astronomically sound perspective.

Background
Consider the following system:

A single moon, with properties roughly comparable to Earth's so that it can support human life (1G gravity, Nitrogen-Oxygen-Carbon atmosphere, soil and liquid water etc.). This moon is tidally locked with it's planet.

A single gas-giant planet, with a size roughly comparable to Saturn. Orbiting it's star so that it completes a cycle about every 12 earth years.

A Star, larger than Sol and emitting enough energy to keep the moon in a comfortably warm temperature.

The features above may be altered to ensure possibility of human survival on the moon, and if possible, to keep the moon tidally locked with a large planet.
Now, the complications (inspired by a short story by Larry Niven):

The whole system (star, planet and moon) are traveling at a relativistic speed across the galaxy.
(Based on the theory that some parts of the universe might be made from anti-matter) The whole system is made from anti-matter, but is currently traveling through a normal-matter area of space.
Note: the "human life" on the moon are also based on anti-matter...

Finally, the story will begin with a small community of primitive "anti-matter humans", living on the moon in a place from which the planet fully visible, and at the zenith.
The Questions
I'd appreciate ideas, insights and comments on the following:

From the moon, how will the sky look like?
What will the equivalent of a "day and night cycle" (sun-up to sun-up) look like?
Will the winds on the moon have any "non-earthlike" patterns?
Considering that at relativistic speeds, the odds of the bodies in the system hitting hydrogen particles in the vacuum of interstellar space are significant (is that even correct?), will that create any special aura or energy streams from the sun or planet?

Thank you for your help.

First it will be important to consider the term 'relativistic speed'. If by that you mean something like 0.1c, it will only change the colour of the stars as you mentioned in the bounty description. However, if it means something with higher Lorentz Gammas (like 0.9c or 0.99c), all other relativistic effects come into play. There's relativistic beaming and aberration combined, which means that all the light that you receive from systems with respect to which you're moving at relativistic speeds will only be received in small beams towards the direction you're moving in and those will be strongly blueshifted and intensified. Watch this animated video for an illustrated view of what I mean - https://www.youtube.com/watch?v=JQnHTKZBTI4. (It's not a high quality video, but it is very effective in demonstrating what I just wrote.)

So, in effect, you will possibly see only three major sources of light in the sky:

The parent planet
The parent star
The bright patch of the sky in the direction of motion, highly blueshifted, with brightness dependent on your speed

Their relative brightnesses will depend on:

distance to parent star
distance to parent planet
albedo of the parent planet
speed of motion through the ism

For a day and night cycle:

The parent planet will obviously be at the zenith the whole time (or fixed location in the sky if you're at a different location).
The parent star will appear to move around the sky with the same period as the revolution period of the moon - say a few days at the most (will be fairly small as compared to the 12 year revolution period of the planet, so you can ignore 'synodic' effects to the first order for a single day/night cycle).
The sky will also move with roughly the same period as the sun, ignoring synodic effects.

Winds will be very complexly affected by the three sources (since heating due to radiation affects winds). There will also be tidal effects due to the parent planet which also play a role in the winds (similar to how tides work on the earth). So, you will have to take into account heating from the star, heating from the planet (which is constant due to tidal locking), tidal effects of the planet (again constant) and heating from the sky. Now the radiation from the sky can also act in a weird way, since blueshifting can give rise to strong emissions in UV, X-ray and Gamma ray photons, plus relativistic cosmic rays (everything you encounter will be in the form of cosmic rays due to the relativistic speed of your motion). These can ionize the atmosphere, heat it, cause there to be particle showers and aurorae, which can all affect wind patterns in complex ways, which I cannot exactly say much about, given my limited knowledge in atmospheric sciences.

As I mentioned above, yes, all interactions will be relativistic and will be similar to cosmic ray bombardment onto the moon you live on. Things will get nasty and you'll be having geomagnetic storms all the time. Satellite communications will probably never work. Expect these particles to arrive in the previously mentioned bright region of the sky due to aberration still applying to this case. You will see similar aurora like regions on the parent planet (http://www.spacetelescope.org/static/archives/postcards/screen/hst_postcard_0008.jpg), and the star will probably not be affected that much.

Oh and by the way, due to the antimatter nature of your system and the matter nature of the ism, the atmosphere would thin over time, and later these antimatter cosmic rays will start bombarding the moon. So if you stay in the ism for long enough, living on the moon will be difficult. Also, I am assuming that life evolved on your moon in an antimatter ISM and you have entered the matter ism only recently, as otherwise, the moon won't be stable for long enough for life (intelligent life) to evolve.

I will try to see if I can think of some other effects, but for now, I guess this will do?!


Is there any ephemeris generator that gives you accurate distances between Earth and other bodies? I've tried JPL's HORIZONS ephemeris generator and while it gives accurate celestial coordinates for an object it doesn't give the geocentric distance.
JPL's Solar System Simulator gives the distance to an object at a certain time however only down to a few digits.
Is there an accurate tool that generates exact (meaning down to the meter) geocentric distances between Earth and another Solar System body over increments of time?

JPL HORIZONS gives the distance from the observer to the target as delta.
In Table Settings this option is 20. Observer range & range-rate.
For asteroids and comets, it can also give distance uncertainty (39. Range & range-rate 3-sigmas) as RNG_3sigma.
You can choose units of au or km.
For example, with Table Settings: QUANTITIES=1,20,39; range units=KM, the geocentric ephemeris for asteroid (153201) 2000 WO107 around its 2020-11-29 close approach to Earth looks like:
 Date__(UT)__HR:MN     R.A._____(ICRF)_____DEC            delta      deldot RNG_3sigma RNGRT_3sig

 2020-Nov-29 03:00     08 48 22.83 +14 26 33.8 4.3065970451E+06  -1.1137937   143.5915  0.0000166
 2020-Nov-29 04:00     08 43 44.11 +14 51 56.9 4.3035317776E+06  -0.5890828   144.6349  0.0000171
 2020-Nov-29 05:00     08 39 04.01 +15 17 00.3 4.3023570178E+06  -0.0635364   145.6159  0.0000175
 2020-Nov-29 06:00     08 34 22.78 +15 41 42.1 4.3030745286E+06   0.4621549   146.5331  0.0000180
 2020-Nov-29 07:00     08 29 40.62 +16 06 00.2 4.3056835832E+06   0.9872992   147.3852  0.0000185

In other words, at 05:00 UT, the asteroid is 4,302,357 ±49 km from the center of Earth.
Though its orbit is determined well enough for a permanent number designation, even 1 km precision would be unreliable.
Its perihelion distance q is only known to ±8.7 km and perihelion time tp to ±1.7 s.
New observations may improve this.
Update: new observations of (153201) 2000 WO107 improve its q to ±6.9 km, tp to ±0.3 s, and distance at 2020-11-29 05:00 UT to 4,302,522.2 ±1.1 km.
That precision is largely due to radar data, which are not always available.


22nd is shortest day in some places, but the 21st is shortest in other places. Can this be true? The Winter Solstice in the Northern hemisphere in 2015 is December 22 at 04:49 UTC.  Where I'm located in Nova Scotia, Canada, that's December 22, 00:49 AST.  And so I would say the shortest day of the year is tomorrow, the 22nd.   But for my friends in Ontario, that's December 21, 23:49 EST.  And so, for them, the shortest day is today, the 21st.

Does this make sense?

Sure it can be (and is) true. Everyone is going through the Earth's "zone of lightness" at different times. Presumably whichever of those times is "nearest to" the actual moment of solstice will be the "shortest period between sunrise and sunset" for any given person (longitude) (ignoring natural and time-zone variations as noted in other comments). As you note, for your friends and everyone west of them up to the Date Line, that will likely be 12/21. And for you and everyone east of you around to the Date Line, that will likely be 12/22.

Maybe it helps (or maybe not) when you realize that people very close to the Date Line itself but on opposite sides experience 12/21 and 12/22, resp., as "the same day".


Where can I find source data for the graph of metric expansion over time, as shown in this ESA web page? This web page from ESA/Hubble, shows the size of the universe over time. (I know that "size of the universe" is a gross simplification and open to multiple definitions and ambiguity, but this kind of graph is roughly what I'm after).

But no source data is cited. This is the closest I can find to what I'm after - where can I find source data to match this graph?

I was hoping a physicist might answer this, as I'm just an enthusiastic armchair scientist whose undergrad calculus was 30 years ago. But as no one else has had a go, let me try an answer.

The graph in the web page is a simplistic and somewhat exaggerated representation of the evolution of the cosmic scale factor a(t) over time. You can derive the "source data" yourself using the following metrics for the three distinct periods in the expansion of the Universe, since the only variable is t which in this case is the age of the Universe. The scale factor a(t) is dimensionless.

For the earliest period of the Universe up to about 47,000 years after the Big Bang, the Universe was dominated by radiation. The evolution of the scale factor in the FLRW metric for this period is given by: $$a(t) \propto \sqrt t$$

After that very brief time (in cosmological terms), matter took over as the dominant factor in the Universe for the next 9.8 billion years, and the evolution of the scale factor is given by: $$a(t) \propto t^{2/3}$$

Finally, the Universe in the current period is dominated by dark energy, and the evolution of the scale factor is given by: $$a(t) \propto exp (Ht)$$
where H is the Hubble constant.

In practical terms, if you'd like to generate a graph similar to that in the web page, you can set $t_1$ = 13.79 Gyr (i.e. the current age of the Universe). I found that the first era of radiation domination is so short that it's not worth including in the graph.

Here's my simplistic rendition:


How can astrophysicists discriminate between pp-chain solar neutrinos and CNO-cycle ones? Astrophysicists at the Borexino experiment in Italy have recently claimed that they have detected CNO-cycle neutrinos coming from the Sun.
It was the Cover story for the November 26 issue of Nature.
I read the whole Paper, and accompanying editorial, yet unless I missed something, they do not say what makes a CNO-neutrino different from an ordinary pp-neutrino.
They go to great lengths to describe how they excluded all types of background noise, but did not say whether CNO-neutrinos are higher or lower in energy than 'proton-proton chain' neutrinos....
Presumably, CNO neutrinos are higher in energy than pp ones?  How many eV or keV?

Nuclear reactions can have particular energy spectra, which arise based on how energy and momentum are distributed among the product particles. This holds true for neutrinos produced in the many different stages of the CNO cycle, as well as the pp chain. The authors began the paper with a figure showing the theoretical neutrino fluxes for the various components of the CNO cycle:

The dominant CNO neutrino producers are the steps involving $^{13}\mathrm{N}$ and $^{15}\mathrm{O}$. The peak of the nitrogen spectrum lies in (or at the edge of) a region where the ratio of CNO neutrinos to background is the highest:

Fitting the observed spectra with models of CNO neutrino emission indicated that such neutrinos were indeed present. I'd draw an analogy with making electromagnetic observations: if your statistics are significantly better when, say, you add a black body component to a non-thermal power law fit, then you have evidence of black body emission. It's basically the same thing here - adding another component to a spectral fit.


Details on the telescope(s) on the Chinese Space Station 天和 The ISS has a telescope on board. Does the Chinese Space Station have one on board as well?
I heard about Xuntian aka Chinese Space Station Telescope (CSST), but not too many details. Would that be the only telescope? What would be the primary scientific goals?

The Chinese Space Station (CSS) has many instruments. It is planned to conduct a spectroscopic survey of hundreds of millions of galaxies. The Chinese Space Station Optical Survey (CSS-OS) is a planned full sky survey with applications to, e.g., cosmology and photometry. It will "probe weak and strong gravitational lensing, galaxy clustering, individual galaxies and galaxy clusters, active galactic nuclei..." This effort has been in the works for over a decade, e.g., here. There are also many laboratory experiments planned.
The CSS is the culmination of the Tiangong program, which began in 1992, and this is why it's known as the Tiangong Space Station. The word "tiangong" means "heavenly palace."
Xuntian is a planned telescope that will co-orbit the CSS for periodic docking. It is expected to have a much larger field of view than the Hubble Space Telescope, and may rival the James Webb Telescope.
Keep in mind that the efficacy of Chinese cooperation with international science programs is controversial.


Calculating the Solar Day of an Exoplanet? I am currently in the process of compiling a list of stars and their planets, given the information in the European Star Catalogue found here, but I'm struggling to find out how to calculate a planets solar day. To be exact; How long, in earth hours, is the solar day of a planet found in this catalogue?
I know the catalogue is incomplete and that it's likely impossible to calculate the solar day for some of these planets due to this, but that's okay. I could also accept the sidereal day rather than the solar day.
I tried to use the formulas found here but I have a hard time finding out what numbers to plot into the equations, given the information available in the exoplanet catalogue. I've been doing a lot of googling and I'm not an astronomer, so it has been a bit of challenge!
I hope some of you can help me here.

This source doesn't give sufficient information to get the length of either a stellar or sidereal day.  The rotation rate of a body is not governed by the orbital parameters.  The Earth, for example, has a slowing spin rate as the Moon retreats due to tidal interactions.  This doesn't affect the Earth's orbital parameters.
The fields given are:
Name
Planet Status
Discovered in
Mass
Mass*sin(i)
Semi-Major Axis
Orbital Period
Eccentricity
ω
Tperi
Radius
Inclination
Update
Detection Method
Mass Detection Method
Radius Detection Method
Primary transit
Secondary transit
λ
Impact Parameter b
Time Vr=0
Velocity Semiamplitude K
Calculated temperature
Measured temperature
Hottest point longitude
Geometric albedo
Surface gravity log(g/gH)
Alternate Names


What does Eddington Limit (Eddington Luminosity) Show? We know Eddington Limit (Eddington Luminosity) is calculated with this formula:

$$L_{Ed} \approx 3 * 10^4 * L_{sun} * \frac{M}{M_{sun}}$$

Example: For a black hole with $10^4 M_{sun}$, we get
$$L_{Ed} = 1.16 * 10^{35} W$$

My question is what does it really show? What do we get by calculating this Number? Is this formula really used to calculate the luminosity or it is used for calculating mass?

The Eddington limit represents the maximum luminosity that can be achieved by a body (such as the star) when there is hydrostatic equilibrium (http://astronomy.swin.edu.au/cosmos/H/Hydrostatic+Equilibrium). For luminosity greater than Eddington limit, the radiative force of the luminosity on matter exceeds the gravitational force on the matter. If the luminosity radiated by an accretion disk exceeds the Eddington limit, the matter falling towards the supermassive black hole can be blown away.


What are the current observational constraints on the existence of Nemesis? Nemesis is a hypothetical companion to the Sun on a very eccentric, long-period orbit. The star supposedly returns every few tens of millions years, driving comets into the inner solar system and causing extinction events. Given our very stringent observational limits from infrared surveys (such as WISE), is its existence definitively ruled out?

Few important points about WISE:

it was able to detect anything with a temperature above 70-100 K, whereas the coolest brown dwarfs are in the 500-600 K range (the coolest was discovered by WISE itself, see Mainzner et al., 2011);
it was able to detect objects larger than 1km up to 3 AU from the Sun, or objects of 2-3 Jupiter masses in a distance up to 7-10 light-years (see here and here);
the closest brown dwarf detected by WISE is a brown dwarfs binary system, located at 6.6 light-years from the Sun.

So I would say we can be pretty confident that Nemesis existence has been ruled out.


Question from Introduction to Modern Cosmology by Andrew Liddle The exact question goes like this:
In the real Universe the expansion is not completely uniform. Rather, galaxies exhibit
some random motion relative to the overall Hubble expansion, known as their
peculiar velocity and caused by the gravitational pull of their near neighbours. Supposing
that a typical (e.g. root mean square) galaxy peculiar velocity is 600 km s-1,
how far away would a galaxy have to be before it could be used to determine the
Hubble constant to ten per cent accuracy, supposing

(a) The true value of the Hubble constant is 100 km s-1 Mpc-1?

(b) The true value of the Hubble constant is 50 km s-1 Mpc-1

Assume in your calculation that the galaxy distance and redshift could be measured
exactly. Unfortunately, that is not true of real observations.

I started off by writing the hubbles equation and as they mentioned it should be accurate to 10% so its peculiar velocity (given as about 600 km/s) is less than or equal to 10% of its expansion velocity. But this gives me different answer.

The actual answer to the problem as stated in the book is
Slightly different answers are possible depending how you deal with the rms velocity.
You should get something like r > 35 Mpc for H0 = 100km s-1 and r > 70Mpc
for Ho = 50kms-1

What am I missing here? Has it got to do with the RMS velocity as mentioned? Please explain.

From the definition of the rms (e.g. here),
$$
\mathrm{rms}^2(x) = \langle x \rangle^2 + \sigma_x^2,
$$
where $\langle x \rangle$ is the mean value, and $\sigma_x$ is the dispersion.

For galaxies with random velocities, the mean velocity should be $\langle x \rangle = 0$, unless they drift in some direction. Hence, the velocity dispersion should be $600\,\mathrm{km}\,\mathrm{s}^{-1}$.

For randomly distributed velocities, the speed $V_{||}$ you measure along your line of sight (LOS) is a factor $\sqrt{3}$ smaller, i.e. $\sim350\,\mathrm{km}\,\mathrm{s}^{-1}$.

Thus, to get a 10% accuracy, the mean value should be 10 times larger, i.e. $\sim 3500\,\mathrm{km}\,\mathrm{s}^{-1}$.

For a Hubble constant of $H_0 = 100\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$, that corresponds to a distance of
$$
d = \frac{v_{||}}{H_0} \simeq 35\,\mathrm{Mpc},
$$
while for $50 \,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ you get $70\,\mathrm{Mpc}$.


Finding the diameter of a molecular cloud using period of intensity variation of hydroxyl maser So I have this problem (which I  translated to English as accurately as could):

  When studying a compact molecular cloud with radiotelescope, at a wavelength of 18 cm, hydroxyl (OH) maser(ic) radiation was registered. Radiation intensity changes at about 5 min cycle. Evaluate the maximum diameter of the cloud.

Actually we learned very little about molecular clouds at high school and I have no idea how to solve this problem.

If the intensity change cycle is wave period, I can calculate wave velocity, which would be very low (as well as the frequency). I'm not asking for the answer, some hints would be enough. Thanks.

The principle that you apply is that the properties of a body cannot change faster than it the time it takes for light to cross the body.  So pulsars can be very small, which we know from the high frequency of their variation.

A body which varies with a period of 5 minutes can't be larger than about 5 light-minutes across.


Is the point of this paper to say that Pluto's status as a planet should have been handled scientifically, rather than by a vote? The abstract of a paper currently in-press in Icarus: The Reclassification of Asteroids from Planets to Non-Planets (Metzger et al. 2018, https://doi.org/10.1016/j.icarus.2018.08.026) says:

It is often claimed that asteroids‘ sharing of orbits is the reason they were re-classified from planets to non-planets. A critical review of the literature from the 19th Century to the present shows this is factually incorrect. The literature shows the term asteroid was broadly recognized as a subset of planet for 150 years. On-going discovery of asteroids resulted in a de facto stretching of the concept of planet to include the ever-smaller bodies. Scientists found utility in this taxonomic identification as it provided categories needed to argue for the leading hypothesis of planet formation, Laplace‘s nebular hypothesis. In the 1950s, developments in planet formation theory found it no longer useful to maintain taxonomic identification between asteroids and planets, Ceres being the primary exception. At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets. This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage where it had hovered during the period 1801 – 1957 to a low level that held constant thereafter. This marks the point where the community effectively formed consensus that asteroids should be taxonomically distinct from planets. The evidence demonstrates this consensus formed on the basis of geophysical differences between asteroids and planets, not the sharing of orbits. We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse, by which perspectives evolve with additional observations and information, just as they did in the case of asteroids. (emphasis added)

On the website there is also:

Highlights

The literature on asteroid classification is reviewed from 1801 to the present.
Scientists considered asteroids to be planets until the 1950s.
Thousands of asteroids sharing orbits was not debarring from planet status.
They were reclassified as non-planets on the basis of geophysical characteristics.
Taxonomical terms like planet are determined by the scientific process, not voting. (emphasis added)

It sounds like they are saying that Pluto's status as a planet was removed by a vote, more than by scientific consensus, and that while the title suggests that the paper is about "Asteroids" it seems the scenario might actually be proxy for Pluto's demotion.
Question: Am I misunderstanding, or is that really the main point of the last few sentences of the abstract, if not the entire paper?
Here's an example of another situation of classification being in flux: Do astronomers generally agree that the distinction between comets and astroids is not so clear?

Part of the problem here is semantics.  The meaning of "planet" was really poorly defined prior to 2006 (and some argue that it still is).  In the early 2000's, this started to become an issue. You can blame Mike Brown.

In January 2005, Mike Brown's team at CalTech in Pasadena, California, discovered an object we now know as Eris (they nicknamed it Xena early on).  The early observations of Eris suggested it was larger than Pluto (which was later conclusively proven incorrect with the New Horizons flyby of Pluto in 2015). But larger or not, it was close in size to Pluto.  Did this make it the tenth planet?  If so, then what about the other bodies like Sedna? Haumea? MakeMake? For that mater, what about Ceres, or Chiron?  By some counts, you could say we had upwards of 23 planets.  Or do we only have 9? Or 8?

For that matter, what makes a planet a planet?

The word planet comes to us from the ancient Greek "planetes asteres" which meant "wandering stars."  To the ancients, even well before the Greeks, a few of the lights in the night sky were definitely different from the others.  The stars remained in fixed patterns.  Sure, they rose and set, and they rose and set at different times through the year, but their relative positions were always the same - as far as the ancients could tell.  But five of them were different.  Their positions changed.  Two of them would rise before the sun in the morning for a while, then disappear and set after the sun in the evening for a while.  These are Mercury and Venus.  The other three, Mars, Jupiter, and Saturn, all seemed to move in the same direction, interestingly through the constellations they knew as the zodiac, but now and then they'd back-track a little bit, then move forward again.  Very odd.

The ancients had no idea that they were, in fact, other worlds, completely different in nature from the burning stars.  Of course, for that matter, they had no idea what the stars really were either.  They just knew the planets, though they looked pretty much the same as stars, moved differently. They were wandering stars.  It wasn't until Galileo turned his scope on the heavens that we learned that these aren't just bright points of light like the other stars - but whole different worlds.  The ancients barely could see Uranus, and gave it little heed, and couldn't see Neptune at all.  Other, smaller bodies like Pluto, Ceres, and others were far to small to see without a telescope.

When Herschel discovered Uranus in 1781, he first thought it was a comet.  In 1783, observations and calculations had lead to the discovery that Uranus' orbit was nearly circular, and it behaved like the other planets, and so it was accepted as one (and Herschel wanted to name it George).  When the predicted movements of Uranus didn't match the observed motions, astronomers and mathematicians suggested the reason was due to another planet, further out, pulling on Uranus and affecting its orbit.  In 1846 it was finally discovered, largely based on mathematical predictions.  At the time, no telescope could show either Uranus or Neptune as more than a very small disk.  But they were in nearly circular orbits, and that seemed to be good enough.

Ceres was discovered in 1801, and for several years was considered a planet. German Astronomer Johann Elerte Bode had previously observed what he thought was a mathematical relationship between the planetary orbital radii.  According to his calculations, there should be a planet between Jupiter and Mars, right where Ceres was discovered by Guiseppe Piazzi.  Bode's observation was actually erroneous - there was no such mathematical realationship, but it was convenient that an object was found there.

As it turned out, Neptune's orbit didn't quite fit the mathematical predictions either.  This lead to a belief that there might be a tenth planet causing the orbital discrepancies.  In the late 1800's and into the 1900's, the hunt was on.  Wealthy eccentric and astronomer Percival Lowell founded the Lowell Observatory in Fagstaff, Arizona, and set out to find "Planet X."  In 1929, the director of the Lowell Observatory hired a 23 year old farmer from Kansas who'd been constructing his own telescopes, mostly out of salvaged farm equipment, and doing his own observations. Clyde Tombaugh was brought in to work on the hunt for Planet X, a rather arduous task involving taking pictures of the same patch of sky a few days apart and then comparing them using a device called a "blink comparator" and looking or something different between the two images.  On February 18, 1930, Tombaugh noticed a tiny dot moving between images taken a few weeks before. Looking back at a previous image a few days before, he added to the evidence.  After capturing a few more images still and comparing, he finally reported the discovery on March 13, 1930.  He had discovered Pluto - the ninth planet!

Well, at least, that's what the hype said.  It didn't take too long to notice that Pluto was a bit different.  While no two planets orbit in the same precise plane, the previously-discovered eight are very close to co-planar.  They also have orbits that are very close to circular.  Pluto's orbit is tilted dramatically with respect to the other planets, and its orbit is significantly more elliptical.  In fact - Pluto's orbit crosses inside that of Neptune (though they'll never hit each other).

Pluto was also tiny. Our own moon is larger, as are several other moons of other planets. In 1978, Pluto's moon Charon was discovered. With further research, it was found that Charon was so large, that the point of orbit between them, what is known as the barycenter of the orbit, is actually well outside Pluto's radius.  Many astronomers were starting to talk of Pluto-Charon as a "co-planet."

Then, in the late 1990's and into the 2000's, a bunch of other objects started turning up out past Pluto.  Some of them were spherical, like a planet.  So, were they planets?

If you take a look back at Pluto's discovery, you can make an argument that the only reason Pluto was called a planet from the start was because Tombaugh was trying to find a planet.  He found something, there's no doubt there.  But would it have been called a planet if they weren't specifically looking for a planet?  As it turns out, Pluto's mass (including the whole Pluto/Charon system which includes four other "moons") isn't nearly enough to explain the discrepancies between the observed and calculated positions of Neptune (which, as it happens, were later explained away by calculations based on Einstein's discovery of special relativity).

When Mike Brown discovered something that was potentially larger than Pluto, but much further out, and just as weird, people started questioning what it meant.  If Eris was a planet, then what made it a planet and not Sedna, or Orcus, Or MakeMake, or the others.  And then, if those were planets, why not Ceres, which had been downgraded to an asteroid.  What does "planet" mean?

So here come the semantic police. The problem is, who decides what a word means.  We run into this in language all the time.  But for science, we need precise definitions so we can make and categorize observations.  If I simply define a star as a bright light in the sky, that includes planets, the moon, and even airplanes.  For science to work, we have to have agreed upon definitions - operational definitions.

Of course, in many cases, there's no single body to define things.  There is no "official" body to make these decisions.  The purpose of the International Astronomical Union is to provide an agreed-upon group to set standards like this.  When they met in 2006, they had a working group to formulate a definition of what should define a planet and differentiate it from another body. To some greater or lesser extent, it IS arbitrary - as is all language.

When I say fruit, you might think "banana" or "apple." But not everyone would think of "tomato" as a fruit, even though it is, by definition.  How about a cucumber?  Both a tomato and a cucumber are the fruiting bodies of their plants.  They are, by definition, fruits.  Yet how many people do you know that talk of a cucumber as a fruit?  Most people consider fruits to be sweet while vegetables are savory.  So, to an extent, it will depend on your point of view.

But for science, we need a standardized set of definitions.  And this is what the IAU did in 2006.  Their working group formulated a set of criteria to define a planet, and put it to a vote of the general assembly, which passed it.

The authors of the cited article state that "taxonomical terms like planet are determined by the scientific process, not voting."  However, when you are arguing semantics, then you are talking about consensus.  Looking at another taxonomic system, scientists agree that a "cat" is a cat because it has certain characteristics.  More recently, those characteristics include genetics and assumed evolution, but historically taxonomy was more based on observed characteristics.  In defining a planet, we have the same process at work - only the defining characteristics were put to a vote.

Regardless of what we call Pluto, no one argues that it is not a definite body of some substantial size orbiting the star Sol.  The argument does not deny Pluto's existence, only its classification.  Pluto, along with the eight other fully-accepted planets, along with the thousands of asteroids, along with the myriad other trans-Neptunian objects, Plutinos, Kuiper Belt objects, and everything else orbiting the sun, are all planet-like in some ways, and not in others.  At some point, a definition must be agreed upon, and this is exactly what the IAU did in 2006.  Not everyone agreed, but that wasn't necessary.  We weren't stating a scientific fact, we were assigning a label.  It's not a FACT that anything is a planet, it is simply a label of convenience.  In fact, if you wanted to be nit-picky, you could argue that Jupiter and Earth are not at all the same type of object, and that they shouldn't both be called planets but should have different classifications.

By reclassifying Pluto, the IAU basically played a game of "one of these things is not like the others" and drew a line.  And whether or not you agree with that line, the majority of the assembled members of the convention did, and adopted the definition.  Sure, it could be changed, but why bother? It really makes no difference other than in the minds of people who don't understand otherwise.  It's PR, and nothing more.


An influx of water on Venus What if a moon like triton or a frozen extraterrestrial body containing lots of frozen ice were set on a collision course with Venus.  Would an inlfux of huge amounts of H2O precipitate a possible terraforming of venus in such a case?

This is a tricky question because a lot of pros and cons come into play. Terraforming isn't a "yes or no" answer as much as it's a pros vs cons answer and as you'll see.  This answer might be too broad for this site, but that's for the powers that be to decide.
The short answer is mostly no.   There's several reasons why a large impact wouldn't be ideal and probably, adding water to Venus hurts more than it helps because water is a strong greenhouse gas.
The longer answer:
Any collision of that size is problematic.   It doesn't take that large an object to kill nearly all life on Earth or set the oceans boiling.   Just 20 or 30 miles across might be big enough for that.   A moon over a thousand miles across would vaporize the surface of Venus and create enormous blowback, losing a lot of the material into space that you wanted to add.   An impact that large might create a moon around Venus - and that in and of itself might not be so bad, unless the moon's orbit was unstable and then you could have a inner solar-system rogue moon that might destroy Earth one day.
An impact of the size of Triton on Venus could also create a lot of debris that might threaten Earth with multiple dangerous near Earth objects that would need to be tracked and perhaps deflected.   So, it's not a good idea to hit Venus with something extreemly massive.
Moving Moons is also hugely difficult.   Moons with the amount of water you're considering are enormously massive and energy intensive to move and worse, Moons are in their planet's gravity wells, so you have two gravity wells to work against.   You know the old joke, I used to walk 10 miles to school, uphill both ways.  Gravity wells actually are like that - uphill in both the towards the sun and away from the sun.   Sending something into Venus requires a lot of acceleration.
A quirk about gravity wells is that, far away is much easier.   It's much easier to push a distant Kuiper belt object with a highly eccentric orbit into Venus than it is to push a much closer asteroid belt object or gas giant planet's Moon into Venus.  Probably orders of magnitude easier.
It's still an enormous amount of energy to move anything that massive, but a distant, eccentric orbit Kuiper belt object would be the place to start.
Also, many small impacts is almost certainly better than 1 large impact in terms of retaining the materials.  Unless your goal is to blow away some of Venus' atmosphere, which might not be a bad idea, but lets save that for another question.
Part II.  Just add Water.
Venus needs water, but it probably needs other things first.   It's too hot and water on Venus would evaporate and become a greenhouse gas.   Adding that much water to Venus would only increase it's runaway greenhouse effect and that's the opposite of what you want.
An additional problem is that the icy bodies you mention also have CO2 and adding more CO2 to Venus would only increase it's problem of too much CO2.
Venus needs to be colder.  That's step one.   It's vast amount of atmospheric CO2 needs to be reduced.  That's also step one.   While water is necessary for photosynthesis, there's no reliable way to make photosynthesis happen on Venus unless it gets colder first, or the photosynthesis is done in air-ship but that's probably too little, too slow and too expensive.
Venus is also quite dark on it's surface due to constant cloud cover so artificial light would probably be required too.  If you could get photosynthesis going on Venus on a large scale, then adding lots of water would help, but that's a pretty big if.  Water in tandem with photosynthesis would be a nice first step.  Water by itself would do very little and probably make things worse.
Another problem, though it might be slow enough not to matter when you're talking oceans of water (and Triton has perhaps 5-10 times as much water as all the oceans on Earth), so this might not be an issue at all in your scenario, but Venus, like Mars, loses water into space.  It's gravity is too weak for it's temperature and lighter molecules like Water, Methane and Ammonia escape from Venus into space and that would be true even if you gave Venus a strong magnetic field.  It's a product of temperature and escape velocity, called Jeans escape.
There are other theoretical methods.
Deliver a hundred million billion tons of calcium to Venus' surface or mine it from Venus' crust might work and let the calcium absorb the CO2.  Delivering that much calcium would be next to impossible and mining on Venus would be enormously difficult due to it's very high temperature, so neither of those are easy.
Most rocky planets have plenty of Oxygen stored in the rocks that make up their crust and Venus already has plenty of Nitrogen. What Venus really lacks is hydrogen, so any hydrogen rich elements might be useful in the long run, but the first step is some kind of process to reduce the CO2 and get the planet to begin to cool down.  Maybe a sun-shield but one that size would be a very expensive project and very far outside of our ability right now.
Removing what's basically an ocean's mass of CO2 is very difficult.  CO2 is a very stable molecule and not easily removed from an atmosphere.


For what parameter does the abbreviation MA stand for? In the sentence:
"We added a Keplerian model with P = 120.713 days, m = 14.629M⊕, e = 0.0448, ω = 251.36°, and MA=216.88° plus Gaussian noise with rms=5.9696 ms-1."

P is the orbital period, e the eccentricity,... But what is MA? I'm guessing its the mean anomaly, but the mean anomaly is a time-dependent variable which (for my understanding) does not make sense in this context...
Quote is from https://iopscience.iop.org/article/10.1088/0004-637X/719/1/890 (page 24).

It stands for mean anomaly. The start of section 3 gives the epoch.

The epoch for all the fits in this work is
JD 2450602.093.


Could there be a planet at the barycenter between two or more stars revolving around each other? In binary star systems, could there be a planet the stars revolve around, with eternal day on all sides?
1st scenario: Imagine a binary system consisting of two Sunlike G5V stars of 1 solar mass each, orbiting each other, and at the barycenter between them is a planet (which would make the planet itself revolve at the same horizontal speed around its axis, making each sun appear always above the same location on the planet). If the stars revolved around the planet both sides would be illuminated similarly. I don't see why it should be impossible. A planet that once was the outermost one in orbit around one of the stars got ejected from its orbit by the other star's gravity, migrating into the barycenter between them.
2nd scenario: Imagine there's a planet at the Lagrangian point between Alpha Centauri A and B. If the stars revolved around the planet both sides would be illuminated similarly. Would that be possible?
Do situations like those above occur or was such even observed?

No.  Such an arrangement is at best "metastable".  That is, although there are periodic solutions to the three body problem (stable orbits) an infintesimal perturbation (eg the proverbial butterfly flapping its wings) will push the system off the stable orbit and into chaos.  Getting a planet to remain at the barycentre is like trying to balance a pencil on its sharpened point.
With two bodies, each orbits around the barycentre.  But with three bodies, the bodies don't orbit around the three-way barycentre. And a planet placed near the barycentre of two stars will not tend to remain in orbit around that point.
The Lagrangian point L1 is also at best metastable. Satellites that orbit the sun at the Earth-Sun Lagrangian point need to fire their engines and do regular "station keeping" to keep them from drifting away.
The L4 and L5 points can be stable. Bodies at the L4 and 5 points are called "Trojans".  However no Trojan exoplanets are known.  A Trojan planet would see the two stars separated by (a variable amount averaging at) 60 degrees


Likelihood of extra planets in systems identified by Kepler? Kepler has found some systems with multiple planets and shown that it's likely that 7% of sun-like stars have Earth-like planets (video).  I don't think we know enough about extra solar planets to know how planetary systems are organized on average, but the multiple-planet systems are few and would be ones where the orbits of the planets are close to the star and/or flat.  In our own solar system for instance in the small strip of the sky where an observer could see the Earth transit, there are only two small sections where they could also see Venus transit, and if they did happen to see both the Earth and Venus, they would not be able to see any of our other planets transit (up to Saturn at least).

I wrote a small program to show roughly the visible area of sky where each planet could be seen to transit.  Shown is 4 degrees above or below the ecliptic:

Is there just not enough known about the variability of inclinations in orbits of planets in general to speculate about what percent of confirmed candidate planets might have siblings with different inclinations that we cannot see?  If other planetary systems had varying inclinations in their orbits like ours, it looks like for every 'Earth' they found, any other planets would only have about a 7% chance of also being visible.  Is there any reason to think that other systems with multiple planets might have less or more variability in the inclinations of the orbits of their planets?

There are many planets that Kepler couldn't detect because the ecliptic is inclined so no transit occurs from the perspective of Earth. We are much more likely to see a transit if the transiting planet is close to the star, and so in Kepler's sample of stars, a disproportionate number of the planets discovered orbit very close to the star. It is probable that most stars have planets, and many of those planets orbit much further out than the average in the Kepler sample. And, moreover, the stars which have been found to have planets are likely to have others, orbiting further from the sun, and inclined, so they are not detectable by transits. Kepler has not detected Earth size planets orbiting in the habitable zone of sun-like stars.

So far we do not have the technology to detect a planetary system that mirrors the Solar System. Our alien friends, if they are very close to the ecliptic may detect transits of Earth but would likely miss the other planets. And there is no reason to suspect that the Solar system is not typical in the range of inclination of its planets.


What are the criteria for the classification of galaxies? There are lots of galaxies, for example, the Milky Way and so on. These galaxies consist of lots of stars. I want to know how galaxies are classified. Is it only by their shape that is caused by gravity? Or are there some other criteria?

There are three main classes of galaxies: Irregulars, Ellipticals, and Spirals. Irregular galaxies, as their name suggests, do not fit into the "normal" classification scheme.

So, how do we distinguish between elliptical and spiral galaxies?

Brighness profile

The radial brightness profile of an elliptical galaxy follows a deVaucouleur law ($r^{1/4}$).

Spiral galaxies have an exponential radial brightness profile, although their central regions ("bulge") also follows a deVaucouleur law.

Star formation

Stars are formed in the spiral arms of spiral galaxies (and can be formed in irregulars), while elliptical galaxies tend to only have old, and consequently low mass, stars.

Components

As far as we can tell, all galaxies consist of a dark matter halo and stars. In addition, spiral galaxies also have clouds of dust and gas. If conditions are right, these can form new stars. (Some ellpticals have a very thin, very hot gas component as well, but there is a lot less of it than in a spiral galaxy).

Kinematics

Spiral galaxies are rotationally supported, while elliptical galaxies are mainly pressure-supported (i.e. they act like an ideal gas, with stars as gas molecules). There are some rotational features present in ellipticals, but they tend to be minor compared to the overall random motion.

A graphical overview of the various galaxy types is usually shown in the Hubble tuning fork diagram. Note that this does not indicate an evolutionary progression from one type to the next.


Surface of the Sun, or Jupiter, etc I keep hearing and reading statements that refer to the "surface of the Sun" (how hot the surface of the sun is) or the "surface of Jupiter" (when the Shoemaker comets hit Jupiter). I find this to be very confusing and unscientific (especially when stated by astronomers).
If the Sun and Jupiter are basically balls of gas they don't have a surface. It is like saying that the surface of the Earth is somewhere in the upper atmosphere.
Can anyone help me understand how a ball of gas can have a surface.

Yes, there is no surface in the sense as we have on the Earth.
However, there is the Barometric High/Pressure formula. It says, that in an ideal gas of atmosphere, the density (and pressure) increases exponentially high the decrease of the height. Its cause is understable even without differentiation: all horizontal layers of the atmosphere holds the mass of all the layers over it.
The result is, that in the atmosphere of the Earth, it is a good estimation of the pressure, that it halves with every $\approx$ 5km of height. Thus, 5km height we have about half of our surface atmosphere density, 10km heigh we have about a fourth, and so on.
In gas planets or stars, where the whole body is gaseous, this barometric formula holds only until the gas loses its ideality, i.e. its density does not grow more linearly with the pressure. In layman terms, it means that its molecules are so close to each other, that doubling the pressure does not halve the volume any more.
This happens typically in very high pressures (some thousand bars). At this point, the gas is already so dense, that we can say, we are already "inside of the body". If the gas is radiating (for example, the thermal radiation due to the $\approx$ 6000K temperature of the Sun), it also means that it is not transparent any more, thus we can not see inside it.
While there is no surface on the Sun (Jupiter, etc), the height difference between the near-total vacuum, and between the point where the atmosphere is not an ideal gas any more, is surprisingly small because of the exponentiality of the barometric formula. For example, the photosphere of the Sun, is only about some hundreds of km high. This is the layer,

Which is not enough dense to be able to fade the region below it;
But it is already enough dense to be its light enough well visible.

While there is no solid surface, this some hundreds of kms can be considered as "the surface of the Sun", particularly if we compare it to its $\approx$ 1.4million km diameter.


Does the universe prefer a velocity? As I understand it, the distribution of matter is roughly homogeneous across space -- anywhere I stand in the universe, there will about the same amount of matter in every direction around me.

Is the distribution of matter across velocities roughly homogeneous?  If I flew away from the center of the Milky Way at .999c, would all directions look roughly the same, or would I notice a wind of galaxies and neutrinos traveling at a preferred velocity?

Tags I cannot add: cosmological principle, homogeneity, isotropy

There is a preferred standard of rest, that of the Cosmic Microwave Background (CMB). You can detect motion with respect to the microwave background. So move fast with respect to the background and the CMB in the direction of motion will be blue-shifted and in the opposite direction will be red-shifted compared to a direction orthogonal to the direction of motion.

The Earth motion has been measured with respect to the CMB, see here

You could/can also detect your motion from the anisotopy of galactic red-shift in the same way.


How can a supermassive black hole cause so much energy to enlighten its matter when its massive gravity prevents light to escape? To cite the German newpaper article Astronomen beobachten erwachendes Schwarzes Loch:

Das Materie-Monster sitzt den Angaben zufolge im Herzen der 42 Millionen Lichtjahre entfernten Polarring-Galaxie NGC 660, deren Aktivität innerhalb weniger Monate Hunderte Male zugenommen hatte.
Erst wenn die Massemonster große Mengen Materie verschlucken, werden sie aktiv. Bei diesem Prozess wird so viel Energie frei, dass die Materie hell aufleuchtet, bevor sie im Schwarzen Loch verschwindet und ein Teil von ihr in Form von Jets weit ins Weltall hinaus geschleudert wird.

This translates roughly to:

According to the data the matter-monster is in the middle of the 42 million light years far away polar ring galaxy NGC 660, whose activity has increased a lot in only a few months.
Only when these Matter-monsters swallow big amounts of matter, they become active. This process releases so much energy, that it brightly enlightens the matter, before it disappears inside the black hole. A part of the matter is flung out in the universe in the form of jets.

My physics teacher told me once, that a black hole is just a very small-sized and heavy object which has so much gravity that nothing at all, not even light can't escape of its gravity. This explanation is also supported by this SE.astronomy - If nothing travels at the speed of light, except light, how can a black hole also pull light into itself? question.

If a "normal" (not supermassive) black hole can already prevent light to escape, how can matter which is pulled into the black hole can produce energy/light which can't escape the gravity of the black hole?

How can a supermassive black hole be able to pull matter, but not the light photons of the energy?

Additionally: Why is part of the matter which is pulled into the black hole flung (i.e. accelerated) into the universe? I understand why this matter is maybe divided - the acceleration increases as far as i know quadratic, i.e. the differences in terms of the actual acceleration, in dependence of the location may be so huge that the matter can't be held together. But i don't understand why a part of the matter is accelerated in the exact opposite direction, as a force bigger as the gravity of the supermassive black hole would be needed. Therefore: Why is a part of the matter accelerated in the opposite direction (i.e. out of the black hole's gravity) than the other part?

Note: My physics education is rather limited. I know a bit about the Newtonian gravity and a bit about the theory of Conservation of energy. But that's all I know about physics.

It is quite correct that a black hole has so much mass that light cannot escape from a region around the black hole. The edge of this region is called the event horizon. If you cross an event horizon you are never coming back. That applies equally to light, and matter.

Around the black hole there may be matter in orbit. Since the Black hole has such strong gravity, the speed of the orbiting matter will be very fast. In fact it will be close to the speed of light. This high speed gives it lots of energy. The matter will form a disc, called an accretion disc, around the black hole, and collisions in this disc will cause the matter to heat up, to millions of degrees. At these temperatures, the disc will glow with X-rays.

On the part of the disc closest to the black hole, matter will be falling in from the disc, but before it reaches the black hole it can get enough energy to be ejected out, at very high speed, close to the speed of light. It gets ejected at right angles to the disc, at the poles of the black hole. These are the "jets". Intense radiation is produced along these jets. Blazars are distant supermassive black holes with jets that are pointed right at us.

So the black hole itself is "black", but the matter orbiting around it may be very bright.


How big would the asteroid belt planet be? As I understand it, the asteroid belt exists because the gravitational force of Jupiter prevents the asteroids from accreting (is that a word?) into a planet.

If, however, Jupiter didn't exist and they did create a planet, how big would that planet be?

The mass of the asteroid main belt is estimated at 4% the mass of our moon according to Wikipedia so any object formed from the aggregation of that mass would not be a planet.

It would be the size of a very small moon.

Even if all the asteroids in the solar system were combined, the total mass would be below a third of the moon's mass.


What makes an exoplanet to orbit around its star backwards?And why did we consider our orbital movement to be forward? I read somewhere that an exoplanet in the name of HAT-P-7B orbits its star backwards.Why?

And why did we consider our orbital movement to be forward?Is there some scientific reason to consider the movement as forward?

All the planets in the solar system orbit the same way. This is not a coincidence. Planets all form from the same protoplanetary disc that surrounded the young sun. The particles in the disc orbited the same way as the sun rotates, and the planets continue to orbit in the same direction.

In this context forward means orbiting in the same direction as the star rotates. We would expect most planets to orbit "forwards" for the same reason that all the planets in the solar system orbit forwards.

What probably happened with HAT-P-7b is that it was thrown into an odd orbit following a close encounter with another planet, or perhaps another star. It is also conceivable that it is a captured rogue planet. See a New Scientist article about the planet


At what latitude could Orion's belt and Aldebaran be ~10 degrees above the horizon at the same time? In his appearance on television contained in the video The Mystery That Keeps Neil deGrasse Tyson Up At Night, in the backdrop behind astronomer and educator Neil deGrasse Tyson and the talk-show host, what latitude on Earth would have Orion's belt and Aldebaran at equal elevation (about 10 degrees) above the horizon? Could that be Sunrise/Sunset, or are those just city lights?

He's known to be a stickler about such things (the story about the stars in the movie Titanic for example). This background must be calculated from a particular latitude at least (if not longitude and/or season).

This sky configuration is consistent with 40 degrees N, at about 6.30am (local time) at the end of November.  If you went to mid November, this would look the same at about 7.30am (ie 1 hour per half month).

The latitude of New York is about 40N, which makes sense for a this particular US TV show,  which is recorded live in New York City, "just down the street" from where Dr. Tyson works as directory of the Hayden Planetarium.

The way I guessed was from a combination of things. From where I live I'm familiar with the eastern sky, so it was obvious this wasn't looking east. So I played around with different times of day and directions, which confirmed this was a view from looking west, and south of where I live (50N).

This can be quickly confirmed with any on-line "planetarium" or sky viewer. For example here's a shot from in-the-sky.org in planetarium mode, set to New York City, 6:15 AM November 20. It puts Aldebaran and Orion's belt both at about 10 degrees above the Western horizon, with only the beginning of twilight. Obviously other combinations of nearby dates and times can work as well.

The point is that the light in the image in the question is not the sunset or sunrise, so it must be a distant city in the West.


What's fixed in Ariel that is moving in other planet-hunting space telescopes? The BBC News article UK will lead European exoplanet mission says:

Steady platform
And one aspect that would work in Ariel's favour is the absence of any moving parts in its build, commented Plato team-member Dr Don Pollaco from Warwick University, UK.
"The issue with all of these planet experiments is that the signals you are looking for are so incredibly small that any systematics in the instrument itself will dominate the signal," he explained.
"And the systematics are often associated with bits that move. So the great thing about Ariel is that it is fixed-format - nothing changes," he told BBC News.

Question: What's fixed in Ariel that is moving in other planet-hunting space telescopes? Is it only aspects of the satellite's telescope and payload module, or does this difference include aspects of the satellite's attitude control system and other things in the service module as well? See ESA's Ariel.

I think this is a case of selective quoting/editing in the media reporting otherwise something major has changed in the design and plan for the mission that hasn't been released or published officially. The large and comprehensive ARIEL assessment report which was published less than 2 years ago discusses mounting all of the optical components on a large, thermally isolating optical bench to prevent movements of the optics introducing noise into the transit signals. However it also says (page 66) that secondary mirror (M2) will have the normal 3 axis position control to provide focus and tip-tilt. This is also mentioned in a Phase A study report published in 2018. The stability will be improved by the launch of Ariel into the Earth-Sun L2 Lagrange point which is more thermally stable and doesn't suffer from Earth eclipses compared to Kepler or TESS which are closer to the Earth.

This is the normal and sensible accommodation incorporated into satellite optical systems in order to compensate for anything moving slightly during the pretty violent shaking that occurs during launch on a rocket. Locking everything down pre-launch with no adjustment would be a very risky undertaking.

There are no more recent publications listed on the Ariel mission website that would suggest that the optical system has been redesigned to eliminate all moving parts although it is possible a study has been done and the result have not yet been communicated publicly.


Is CMB Polarization simply the temperature gradient of the CMB? Firstly, I have to admit that I don't fully comprehend the concept of "CMB Polarization". When I think of "Polarization", I normally think of the optical definition of polarization, as defined by the electric field. What I currently think is that CMB Polarization is simply the temperature gradient of the CMB. The Wikipedia page also said that:

  The cosmic microwave background is polarized at the level of a few microkelvin.

Since microkelvin is a unit of temperature, CMB Polarization must be somehow related to temperature. Is CMB Polarization simply the temperature gradient of the CMB?

Polarization of an electromagnetic wave refers to the extent to which the electric field (which is always perpendicular to the direction of wave motion) oscillates in one particular direction, rather than randomly in the plane perpendicular to the wave motion.

Complete linear polarization means the electric field oscillates to and fro along one particular axis.

Polarization of the CMB is produced by Thomson scattering from free electrons just before they combine with protons and the Universe becomes transparent to the CMB radiation.
The polarization occurs because the radiation field seen by the electrons is not uniform in space (not isotropic), it varies according to which direction the radiation is coming from. These anisotropies are the fingerprints of what went on in the early Universe and what the values of the cosmological parameters are.

There is an excellent primer on this topic - http://background.uchicago.edu/~whu/intermediate/polar.html

The CMB is a blackbody spectrum at a temperature of 2.7K. Differences in this temperature are seen on a variety of scales at levels of 30-60 microKelvin ($\mu K$). These are the "ripples" first seen by COBE, then WMAP and Planck with increasing precision and sensitivity to smaller angular scales.

Whereas expressing these spatial variations as differences in temperature seems straightforward, it is not so obvious why you would express polarization signals in the same terms.

In brief, what is going on here is that when a polarization signal is measured it is actually finding the difference between two perpendicular polarization intensities. Because these intensities are measured as changes of temperature in the bolometer arrays used as detectors, this is how the results of the polarization are expressed.

The polarization anisotropies are expected to be an order of magnitude smaller than the CMB temperature variations. That's because repeated scattering from free electrons can dilute the signal, so it is only light that has been scattered once that contributes.


What is the formula to figure out how far away a star is? I have tried to search for this but it won't show me the formula or "how to" figure out the formula. So what is the formula, and how do you figure it out?

(The formula for stars within 400 light years.)

There is no single formula that tells you how far away a star is. In general, it is difficult to find the exact distance to a star just using its brightness since the brightness of stars vary.

For close stars parallax is used to accurately tell the distance to the star. For stars farther away more complex ways of finding the distance to the star must be used based on how far away the star is, these methods make up the cosmic distance ladder.


How was an infrared picture of the entire universe taken? (click to enlarge)

How was this picture of the supposed entire universe taken in infrared? Also, why does it seem as though almost all of the matter is in a line formation over the center? It makes sense why most matter would be concentrated in the center, but why is it in a line?

This picture was taken by the WISE (Wide-field Infrared Survey Explorer) space telescope. It is in deed an infrared picture of the entire universe but what you see in this line is our home galaxy, the Milky Way.

This telescope has taken infrared pictures of the entire Universe (as we see from the Earth) and joined them in this full-sky view.

More info: http://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer


Does earth have retrograde motion? I am a beginner.  Please let me know whether earth has retrograde motion

If it does not, let me know the reason

It does not have a retrograde motion. It has a prograde rotation, and a prograde orbit of the Sun.

If you were to look down on the solar system from far above the north pole you would see the planets orbiting the sun counter-clockwise. And the planets, including the earth also spinning counter clockwise. The Moon also orbits the Earth counter-clockwise.  All this motion is in the same direction, and called prograde.

There are a couple of exceptions: Venus spins clockwise. Venus spins in a retrograde motion. There is a moon of Neptune, Triton, that orbits retrograde. Uranus spins on its side.

The reason is that most objects rotate the same way is that the planets and the sun formed out of the same cloud of gas and so have the same direction of spin.

There is another sense of "retrograde". As the Earth overtakes the other planets they seem to move backwards in the sky for a while.

From the inner planets, Earth would appear to move rather like mars, with periods of retrograde motion. From the outer planets Earth would move like Venus, moving from being a "morning star" to an "Evening star".


Does the universe expand at the same rate everywhere in the universe? Specifically, I am wondering if some areas of the universe expand faster than other areas and whether the faster expanding areas diffuse the expansion through the slower expanding areas or does the expansion occur at a uniform rate throughout the entire universe.

What's outside the observable Universe, we can't say anything about, but averaged over large enough scales ($\gtrsim$ a billion lightyears), it does indeed seem to be expanding uniformly.

However, the presence of mass, or more generally energy, retards the expansion. This means that on the scale of clusters of galaxies, the Universe expands more slowly, and on the scale of galaxy groups, the galaxies' mutual gravitational attraction will prevent them from receding from each other. This is also why our galaxy, Solar system, planet, and bicycles will never get torn apart (unless the cosmological constant is not a constant).

Conversely, in mass underdensities, i.e. the huge voids between clusters and filaments of gas and galaxies, expansion is increased (relative to denser regions). In fact, it has been hypothesized that that the observed accelerated expansion of the Universe is not due to dark energy, but could be an "illusion" from accidentally living in the center of a huge underdensity (e.g. Zibin et al. 2008). More recent observations seem to rule out this possibility, though