The science dealing with objects and phenomena located beyond Earth. In particular, this applies to observations and data. At its core, astronomy is the physically informed cataloging and classifying of the contents of the universe in order to better understand what is out there.

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18
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6answers
3k views

Why are the orbits of planets in the Solar System nearly circular?

Except for Mercury, the planets in the Solar System have very small eccentricities. Is this property special to the Solar System? Wikipedia states: Most exoplanets with orbital periods of 20 ...
6
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1answer
117 views

What prevents stars in globular clusters from merging over time to form a black hole?

Globular clusters are apparently very very old, and the density of these clusters appears to increase as one approaches the center of a cluster. Orbits are bound to be chaotic, since there is no ...
8
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6answers
2k views

What methods can astronomers use to find a black hole?

How can astronomers say, we know there are black holes at the centre of each galaxy? What methods of indirect detection are there to know where and how big a black hole is?
0
votes
2answers
73 views

Sun's Right Ascension appears to be wrong in my Stellarium

The Stellarium version 0.12.4 I have shows the RA of the sun at 22.5 Hour while Google says it is 19h 4m 31s. I am confused. Please help.
11
votes
1answer
762 views

Why don't the black holes appear black in color in images of galaxies taken from HST?

According to NASA a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area According to the documentary Space Unraveling The Cosmos about ...
2
votes
2answers
63 views

Could two identical stars revolve around each other in a common orbit if we only account for Newtonian physics?

Both a parent star and its planet revolve around the center of mass of the system, the reason we see stellar wobble. But if we take this to be true, which it is, there can be a configuration in which ...
0
votes
3answers
480 views

Why wasn't the moon visible during the day a few decades ago?

I was born in 1949. When I was young we played outside and watched the clouds and the sky a lot, and I don't remember ever seeing the moon during the day. Is the sun closer to us now so we see it more ...
5
votes
3answers
89 views

How to measure the effects of the rotation around the Sun?

It is well know that we can measure the spinning of the Earth with a Foucault pendulum. But, is there a similar experiment for the rotation of the Earth around the Sun? I would like to know if we ...
3
votes
1answer
62 views

What is the origin and nature of the “hydrogen envelope” located on the sun side of a comet coma?

It is tempting to assume the origin of the hydrogen is water vapor from the comet, in which case, what process "breaks up" the water molecule (or perhaps ammonia molecule?)? Is the hydrogen actually ...
4
votes
0answers
67 views

Calculus used by Gauss to find Ceres' position?

With very little data, the great mathematician Gauss was able to predict the exact location of where to find the asteroid Ceres. If possible I would like a reconstruction of his work, from the data ...
11
votes
1answer
224 views

What are the alternative theories of dark energy? ($w \neq -1$)

There has been a lot of related questions about dark energy around here but these are usually 2-4 years old and the closest question to mine hasn't really been answered, so I am going to proceed. ...
5
votes
6answers
250 views

Are there free data available online from cosmology (or astrophysics) experiments that anyone can analyse?

One can understand a subject better in physics by trying to solve as many problems as one can from a textbook say. When it comes to experimental physics and data analysis, no book on experimental ...
2
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1answer
156 views

Analyse astronomical data

Recently I was told of a job offer to analyse astronomical data. The job offer states that they want somebody with knowledge of astronomical data analysis software and it will be a plus somebody who ...
0
votes
2answers
34 views

For a planet which has a temperature gradient, hot in the center and cooler on the surface, why do we get absorption lines?

For a planet which has a temperature gradient, hot in the center and cooler on the surface, why do we see absorption lines? Similarly, why do we see emission lines if the planet is hot on the ...
1
vote
4answers
218 views

Astronomy: Are any vanished objects visible to the naked eye?

Are there any objects out there that have since vanished, but because of their distance and the travel time for light, can be seen with the naked eye? Or are those things only visible with ...
0
votes
2answers
35 views

Possible intergalactic celestial objects

We know there are rouge stars floating in intergalactic space, thought to be caused by galactic collision. What other other classes of celestial object could be found floating around in intergalactic ...
0
votes
1answer
44 views

Can life exist in intergalactic space?

A terrifying idea I think, to be so utterly alone. Do you recon life could exist in intergalactic space? A lot of cosmic radiation is shielded from us by the Milky Way's magnetic field, but the ...
3
votes
2answers
55 views

How are the distances to the most distant $\gamma$-ray bursts measured?

This question came to my mind when I read on NASA's website that an explosion (gamma ray burst, GRB 080913) took place 12.8 billion light years away from us. How do they measure such large distances?
-1
votes
0answers
21 views

Classification of a Comet

Google's definition of a comet is: a celestial object consisting of a nucleus of ice and dust and, when near the sun, a ‘tail’ of gas and dust particles pointing away from the sun. This ...
3
votes
0answers
28 views

Classification of small extra-solar system bodies

A Small Solar System Body (SSSB) is an object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite. This encompasses all comets and all minor planets, as well as all ...
2
votes
1answer
42 views

how to measure the age of light?

We measure distances in universe by the units of light year/s or parsec. Which means distance traveled by light in one year equals one light year. Thus the lights we receive from the distant stars or ...
1
vote
2answers
106 views

Is redshift a reliable means to know how fast an object is moving away or towards an observer?

Does wavelength change with distance? How do they know that the change in wavelength can only be caused by the change of speed of the object to the observer? What about noises, distortions, and ...
2
votes
1answer
403 views

Ephemeris calculations: Light time correction of the moon

I am currently trying to calculate apparent positions from raw JPL data. I've got it pretty much figured out, but there is one thing that's bugging me: Has the light time correction of the moon to be ...
1
vote
0answers
13 views

What's the evidence, if any, for some local concentration(s) of dark matter in some region(s) smaller than a galaxy?

What's the evidence, if any, for some local concentration(s) of dark matter in some region(s) smaller than a galaxy? Galactic-sized--or larger--gravitating halos seem to get all the attention. I'm ...
0
votes
1answer
27 views

Prior binary star system as possible explanation of high-velocity Type II Supernovae remnants

From wikipedia(http://en.wikipedia.org/wiki/Supernova) :- "A long-standing puzzle surrounding Type II supernovae is why the compact object remaining after the explosion is given a large velocity away ...
6
votes
1answer
360 views

BBC radiation: What is it?

I'm reading articles about blazars, and in a discussion about models concerning Blazar SEDs (Spectral Energy Densities) there are mentions of different processes: Sychrotron scattering, inverse ...
1
vote
2answers
88 views

Day/night cycle in Greenland

I have heard that in Greenland there is day for 6 months and night for 6 months. Is this true? If so, how does it happen?
3
votes
1answer
52 views

Direct rocky planet formation without protostellar disk?

Inspired by this question, which sort of asked what I'm asking, but emphasized a different point (and the answers accordingly addressed that). Can a rocky planet form directly from the collapse of ...
2
votes
3answers
80 views

Drake's equation

Some years ago i read about this equation that helps us estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. Now, i am wondering if there are results ...
4
votes
2answers
59 views

Optically thick media

What does "optically thick media" mean? Does this property depend on the length of an enclosure? Why is the diffusion approximation for the radiative transfer equation applied to optically thick ...
4
votes
1answer
32 views

Absorption lines in the context of identifying elements in far away celestial objects

I understand that absorption lines are used to identify elements but how are individual absorption spectrums identified in the light that is received by a telescope?
0
votes
0answers
24 views

Mathematical calculation of probability of existence of planet similar to earth [duplicate]

Having a layman level of logic about probability ( read about it 10 years ago, so pardon if it's still not right completely) what i know that the calculation requires some knowledge about the number ...
1
vote
2answers
107 views

How does the evolution of a solar system not break the second law of thermodynamics?

Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck. As I understand it, the solar system evolved from a ...
1
vote
3answers
56 views

astronomy & light pollution

If you were staring to the sky in a big city and electricity is turned off in a big area around you, would you be seeing the sky with little light pollution instantly?
1
vote
1answer
30 views

Factors in predictions for Tolman's Surface Brightness Test?

http://en.wikipedia.org/wiki/Tolman_surface_brightness_test It says: In a simple (static and flat) universe, the light received from an object drops inversely with the square of its distance, but ...
1
vote
1answer
38 views

Import of Celestial Effects on Satellite Radio Interference

Some internet (among other) infrastructure comprises satellites, which beam communications in radio frequencies. These satellites, to ground observers, appear as very small solid angles in the sky. ...
2
votes
2answers
79 views

Would it be possible to detect nuclear explosion on exoplanet?

How strong would have to be nuclear explosion on exo-planet that orbits some other star for it to be detectable outside of that system. Or it would be impossible due to amount of radiation coming from ...
5
votes
1answer
214 views

Red Giant branch and Asymptotic Giant branch

What's the difference between the RGB and the AGB? I can't seem to find an clear distinction anywhere. Thanks.
6
votes
3answers
4k views

What are good books for graduates/undergraduates in Astrophysics?

There are no book recommendations for Astrophysics here. I will write my own answer, but I am also interested in what are others' views on the question (I will NOT mark my own answer as the best one). ...
2
votes
2answers
110 views

Can a point source be located more accurately out-of-focus or in-focus?

Let's say I am taking a picture, and I know a priori that the image is of a single ideal point source of light at infinity. With a perfect imaging system in focus, the image shows an Airy disk. I ...
0
votes
0answers
18 views

Recommendations regarding high-energy astrophysical objects (AGNs, jets, etc.)

I know basic astronomy and I know a few good textbooks for general astrophysics (eg. "Introduction to Astrophysics" by Carroll and Ostlie). I also have a strong background in theoretical physics and ...
2
votes
1answer
45 views

Why 21cm line observations is more helpful compared to other radiation?

Why 21cm line observations is more helpful compared to other radiation? Is it only because of its abundance in whole universe or anything else?
1
vote
1answer
37 views

How stellar aberration is measured?

A simple calculation shows that stellar aberration due orbital motion of earth is roughly 20 arcseconds. My questions are: Practically how this small value is measured? Does this value is in the ...
1
vote
1answer
36 views

Are galaxies “disk” shaped?

When you look a sphere from a fixed observation point, you can easily mistake it for a circle, so I was wondering: are galaxies really "disk" shaped or we just don't have the means to detect the ...
1
vote
2answers
41 views

Redshift 1+z - CMB Temperature lower?

I know that $\frac{\lambda_2}{\lambda_1} = 1 + z$ Suppose a galaxy had redshfit $z=3$. Does this mean that the wavelength becomes $4\lambda$? Then by wien's law where $\lambda \propto \frac{1}{T}$, ...
13
votes
4answers
616 views

Is there a limit to the resolving power of a mirror telescope?

Like, if you hammered out the asteroid 16 Psyche into a 1 mm thick iron foil disc telescope mirror with 2.4x the radius of the Sun, could you resolve details on the surface of an exoplanet? At what ...
3
votes
4answers
4k views

Is it possible for the planets to align?

We've all heard the statement that on the 21st of December, the planets in the solar system will "align" from the point of view of the Earth. I assume this means that they would all be in the same ...
5
votes
2answers
69 views

Abundances of the light element of the Big Bang Nucleosynthesis

This question is related to the Big Bang Nucleosynthesis of light elements, more concretely I do not understand some features of the graph Why do the $^3$He and D abundances go down with ...
6
votes
2answers
437 views

What is the frequency of occurrence of stellar classifications off the HR main-sequence?

An alternative version of this question would be: "if was to pick a star from the $10^{11}$ or so in our galaxy at random, what are the probabilities of it being various kinds of star?" (and I do mean ...
1
vote
0answers
37 views

Any cheap (less than 100 euro) telescope for observing Jupiter? [closed]

The moon is not challenging. I would like to observe the big planets and their rings.