38
votes
Can lightning occur in stars like the Sun?
Stars are not actually gaseous, they are plasmas, i.e., highly ionized. Hence, the entire star is highly conductive and does not easily develop the voltage difference via friction needed for lightning ...
- 2,802
14
votes
Accepted
Do brown dwarfs have stripes?
A number of brown dwarfs have had 'surface maps' created using the light from those stars.
In 2013, observations of 2MASS J22282889–4310262, a brown dwarf 35 light years away, were published. These ...
- 952
12
votes
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A neutron star without a flat surface and an atmosphere?
There is a number of issues with the question, but let me sketch out some kind of answer, so you get something out of it.
The atmosphere of a neutron star is a topic that's a bit speculative. ...
- 17.5k
12
votes
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How can neutron stars have gaseous atmospheres?
Gravity is only important insofar that it is capable of compressing the material to high densities. Whether that material is capable of solidifying depends on the competition between Coulombic ...
- 114k
12
votes
Accepted
Does the Sun have any atmosphere?
Yes the Sun has an atmosphere.
Disclaimer: I'm not sure if you meant this, but your question implies the Sun is a planet. It, of course, is a star and not a planet. Just wanted to make that clear.
...
- 14.4k
11
votes
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Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?
The Ca triplet in the near infrared are extremely strong resonance absorption lines. They are by far the strongest features in the near infrared spectra of cool G,K,M type dwarfs and giants, which ...
- 114k
9
votes
Accepted
Can lightning occur in stars like the Sun?
An essential feature of the lightning is the electrical breakdown - an insulator (air) becomes a conductor for a while, relatively high current flows in the conducting channel for a short while, then ...
- 1,770
8
votes
What happens to oxygen produced on the Sun (or other stars)?
The answer is yes. Molecule formation is common in the outer photospheres of cool stars, and those molecules frequently contain oxygen. Obvious and common examples are TiO, VO.
This chemistry almost ...
- 114k
8
votes
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What happens to oxygen produced on the Sun (or other stars)?
The Sun is a small main sequence star. It does not produce oxygen via fusion. It can't. The temperature and pressure in the Sun's core are too low. Fusion in the Sun is currently limited to production ...
- 27.1k
8
votes
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What is the air pressure in the heliosphere (Sun's atmosphere)?
I am going to assume that what you mean here is "what is the pressure" in the solar wind? There is no "air"!
The solar wind is pretty complex, consisting of a "fast wind" observed predominantly at ...
- 114k
7
votes
Accepted
At what depth below the Sun's surface does the density reach that of water?
The sun's density is 1 gm/cc at approximately 50% of the way down towards the core. if radius of Sun is R then at R/2 the density will be that of water.
@astrosnapper's comment links to this answer ...
- 4,326
7
votes
Accepted
What does this "web on the surface of the Sun" image reflect? What does 789 nm show us?
Well let me take a stab at it. The line in question is said to be a probe of an Fe XI line, that is iron atoms with 10 electrons removed.
You do not get such ions in the solar photosphere, it is far ...
- 114k
7
votes
Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?
The ESA states it pretty clearly (although their figure of 855.2 nm is incorrect; it should be 866.2 nm):
The RVS wavelength range, 847-874 nm, has been selected to coincide with the energy-...
- 33.7k
7
votes
Accepted
Is the gas in a planet forming disk around a star comparable in density to an atmosphere?
The total mass of all the planets is about $3\cdot10^{27} kg$. If you assume the proto-planetary disk to have a radius of $10^9 km$ (somewhat beyond the orbit of Jupiter) and a thickness of $10^7 km$ (...
- 2,314
6
votes
Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?
According to Cropper and Katz 2011 part 2.2,
the RVS working group considered other bands,
but the ~850 nm band is relatively unaffected by absorption in the Earth's atmosphere,
facilitating ground-...
- 16.5k
6
votes
Accepted
Why are helium resonance lines called "resonance lines"?
It appears to be a conventional label that is applied to transitions between the ground state and another energy level (some definitions specify the first excited level) of an atom and is used in all ...
- 114k
5
votes
Accepted
Is there molecular hydrogen in the Sun's atmosphere? If so, how much, and how was that first determined and measured?
According to Formation of the UV Spectrum of Molecular Hydrogen in the Sun (S. A. Jaeggli et al. 2018 ApJ 855 134, also here) molecular hydrogen in the sun was first spectroscopically discovered in ...
- 2,314
4
votes
Accepted
Wilson effect: How "deep" are sunspots?
Interesting questions! I hope I can shed some sunlight on them.
As stated in the abstract that you quote, understanding and modeling of sunspots is an open question, especially the question how the ...
- 3,809
4
votes
What does this "web on the surface of the Sun" image reflect? What does 789 nm show us?
The Sun is pretty much a blackbody for every purpouse except when looking at it with a rather precise spectrometer.
Then again, it is not a constant temperature blackbody. The brightness of these ...
- 1,770
4
votes
Accepted
What was the "optical illusion" that led to erroneous metal concentrations in stellar atmospheres in the galactic center?
The journal paper is Thorsbro et al. (2018).
The facts are somewhat mundane. The atmospheres of cool M-giants are not well understood in detail. The infrared lines of neutral Scandium that had ...
- 114k
4
votes
A neutron star without a flat surface and an atmosphere?
The scale height of a neutron star atmosphere is indeed of cm scale.
An isothermal atmosphere will decay as
$$ \rho = \rho_0 \exp( -kT/mgh)\ ,$$
where $\rho$ is the density, $\rho_0$ is the density ...
- 114k
3
votes
Accepted
Does the Sun's atmosphere have a scale height?
The exponential decrease in density comes out naturally whenever you have a gas in hydrostatic equilibrium. The scale height $H$ is then given by the balance between the kinetic energy of the ...
- 31.9k
3
votes
Accepted
Why does the adiabatic exponent decrease at the ionization zones?
There are two things going on. (1) When you add heat to a gas that is on the threshold of ionisation or partially ionised, some of that heat goes into ionisation. This means that it takes a much ...
- 114k
2
votes
What does the opacity of a molecular transition mean?
I know what an optically think/thick medium is...
Okay so this isn't much more complicated. A medium or material can be optically dense or opaque at one wavelength, but fairly transparent at a ...
- 31.5k
2
votes
Accepted
What is the density profile within the Sun's photosphere? Which one of these is wrong?
I usually don't answer my own questions, but sometimes when the question itself is called into question I make an exception.
The density of the photosphere at $\tau_{5000}=1$ is predicted to be $3 \...
- 31.5k
2
votes
Accepted
Why do we not find larger quantities of helium in the Sun's atmosphere?
Helium is burnt in Sun's core, and it is heavier (specific weight) than hydrogen.
Hence most of the helium stays in Sun's core.
(More details)
- 11.2k
2
votes
How well can we in principle determine $T_{\textrm{eff}}$ of a star?
The question is compromised by saying that you allow arbitrarily perfect measurements.
If we have a bolometer that can measure the amount of flux from a star, at a distance that is known to arbitrary ...
- 114k
2
votes
At what point does an astronomical body's surface stop being gas giant-like and start being sun-like?
A "star" has significant hydrogen fusion occuring in its core. This energy prevents collapse and allows the star to reach a stable position state.
A Brown dwarf does not have significant ...
- 87.6k
2
votes
The sun "burns" hydrogen and even has "campfires" on it, but has anyone calculated a rate of actual chemical burning on the Sun?
I just became curious enough to search a few keyphrases.
This article talks about hydrogen molecules acting as energy "sink" in the sunspots - much like water phase changes on Earth create ...
- 1,770
2
votes
What effect does stellar granulation to have on a chemical analysis of a star's spectrum?
There is no simple answer to these questions - although I could be brief and say
(i) No it doesn't and (ii) no they won't.
If you make a simple two component atmosphere then the observed spectrum will ...
- 114k
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