38

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 like that in our atmosphere. On the other hand, stellar atmospheres have strong magnetic fields which reconnect, and that causes strong voltage spikes that ...


14

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 were made using the Hubble and Spitzer space telescopes and were able to show changing light patterns and distinct layers of material at different altitudes in ...


12

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. Estimates vary a lot. Regardless, neutron stars can have an atmosphere - sure, gravity is huge, but they are also extremely hot. Some molecules are bound to jump up a ...


12

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. What is an atmosphere? When you ask if the Sun has an atmosphere, you're actually asking a tricky question. What do you mean by atmosphere? How do you define ...


12

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 potential energy and the thermal energy of the particles. The former increases with density, the latter increases with temperature. A dense plasma can still be a gas ...


11

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 will be the majority of the stars observed by the Gaia RVS. The Ca triplet lines are so strong that even in low metallicity halo stars, that have little Ca in ...


9

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 stops. The conducting channel is an insulator again. This requires insulating (dielectric) medium and a means of creating an intense electric fields. Given ...


8

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 of helium. This will remain the case for several billion years. That said, there is oxygen in the Sun, about 1% by mass. This oxygen was produced long ago by ...


8

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 high solar latitudes and a slower more variable wind a low latitudes. Both components essentially consist of an expanding stream of protons, electrons plus a ...


7

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-distribution peaks of G- and K-type stars which are the most abundant RVS targets. For these late-type stars, the RVS wavelength interval displays, besides numerous ...


7

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 entirely happens when temperatures fall below 5000K, because otherwise the molecules are dissociated. Therefore it never occurs in stellar interiors. The Sun ...


7

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 in Physics SE. A reverse image search of that unsourced graphic leads to Thesis: Multi-spacecraft analysisof the solar coronal plasma NASA: The Solar Interior ...


7

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 too cool; the radiation from the photosphere is probably a pseudo-continuum at that wavelength. However, much hotter material in the chromosphere and corona ...


6

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 the physical sciences, not just astrophysics. e.g. He I (58.4 nm) is a transition from $^1$P to the $^1$S ground state. In fact all atomic/ionic transitions can ...


6

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-based preparation and follow-up. In addition to the strong Ca II triplet, this band is rich in lines enabling study of astrophysical quantities other than radial ...


5

Temperature ($T_{eff}$) can be quite tricky to determine accurately as it interrelates to a number of other fundamental measurements. Firstly, remember that the spectrum we observe from stars are pin-point, they give us the entire overall result and not a specific location or part of the star. We need to dissect the various parts to arrive at the fundamental ...


5

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 1977. The model calculation in this reference give a ratio molecular/atomic hydrogen of around $10^{-5}$ at a height of about 650 km (where the $H_2$ emissions ...


4

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 images directly translates to some temperature in the corresponding region of the photosphere. The most bright of them are somewhere 6000K, the darkest pixels are, ...


4

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 stability of sunspots is sustained. There are many models, as your citations indicate. First, some terminological clarification: the "Wilson effect" ...


4

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 at $h=0$, where $h$ is the "height" above this datum point, $g$ is the surface gravity of the neutron star, $m$ is the mass of the particles making up the ...


4

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 previously been used to claim massive overabundances in stars near the Galactic centre are problematic. The paper by Thorsbro et al. shows that the same "anomalies" ...


3

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 larger amount of energy to produce a rise in temperature. (2) However, as the gas becomes ionised the number of particles per unit mass also increases and so the ...


3

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 particles due to thermal motion, $kT$, and the gravitational energy of the particles, $mg$. This is often a good approximation, both in planetary and stellar ...


2

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 $3 \times 10^{-7} \text{g/cm}^3$ in the Holweger-Müller Model Atmosphere7. As pointed out in comments there is a spread in values here. The 1E-06 g/cm^3 density value ...


2

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)


2

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 accuracy, with arbitrarily good spatial resolution, then what we do is measure the bolometric luminosity from a 1 m$^2$ area at the centre of the stellar disk. ...


2

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 climate features. The heat capacity, mass density, adiabatic properties of the molecular hydrogen are pretty much different from the atomic one and these are ...


2

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 be the flux-weighted combination of two spectra. $$ S_{\rm obs} = \frac{A_1 T_1^4 S_1 + A_2 T_2^4 S_2}{A_1 T_1^4 + A_2 T_2^4}\ , $$ where $A_1, T_1, S_1$ are ...


2

This mechanism, being actively emitting in the radio-wavelengths, is certainly negligible for the overall atmospheric energetics at Proxima b. One can conclude this by taking the band luminosities from the cited paper ($\rm 2.51\times10^{20} erg/s$, p.3, first paragraph) and compare them to the solar constant at the planets orbit, which should amount to $\...


2

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 different wavelength. If you look at the dark plastic window on a remote control for a TV or other appliance, you can't see through it. It's optically dense at ...


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