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 ...


10

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 ...


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 ...


6

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 ...


6

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

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 ...


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" ...


4

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 ...


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

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 ...


4

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 ...


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

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 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 ...


1

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 ...


1

It is quite easy. In fact you do not need a bolometer. You just need to perform Intensity measurements in several parts of the spectrum, and then fit these to a teorethical black body spectrum. Three uses to be enough if it does not happen that you are measuring on a spike or valley in the spectrum caused by an emission or absortion line. The black body ...


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