18 votes
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Metallicity in gravitational wave astronomy

As you say, metallicity in this context refers to the proportion of the interstellar medium a star forms from that consists of elements heavier than helium. It can be expressed as a (mass) fraction (...
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17 votes
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Is oxygen an alpha element?

Two things. The abundance of oxygen is a difficult thing to measure in optical spectra - much harder than Mg, Ca, and Si. So these latter are usually used to represent "the alpha elements". ...
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14 votes
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Define stellar metallicity

[Fe/H] is the logarithmic ratio of the number of iron nuclei per proton in the star to that in the Sun, $${\rm [Fe/H]} = \log_{10} \left(\frac{n({\rm Fe}_*)/n({\rm H})_*}{n({\rm Fe}_\odot) / n({\rm H}...
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Why do we talk about metallicity in stars instead of more specific elements?

The term "metal" arises in astronomy since almost all the prominent lines in a stellar spectrum arise from metals like Na, Mg, Ca, Fe. In fact, when one looks in enough detail one can also ...
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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 ...
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9 votes

Why does metallicity increase the opacity inside stars?

Both free-free and bound-free absorption are strongly dependent on the atomic number $Z$ of nuclei in the gas. For free-free absorption it is simply that the emissivity per unit volume of electrons ...
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Why do type Ia supernovas produce more iron than type II

Context Iron has the highest nuclear binding energy per nucleon of all the elements (not completely true, but sufficiently accurate in an astronomical context). So, fusion of light elements into iron ...
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Why do certain massive stars leave no remnants?

The gap appears because of pair instability supernovae. In short, as one looks at such massive stellar cores at increasing temperatures, an ever-larger fraction of the photons are sufficiently ...
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7 votes
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Why is the metallicity of elliptical galaxies low?

It isn't true that elliptical (or early-type) galaxies are metal-poor. This sounds like a pop-sci myth being propagated. The first line of the abstract of Pipino & Matteucci (2006) states that "...
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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-...
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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-...
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Metals and dust locked into planets

The amount of metals locked up in planets is completely negligible compared to that of the interstellar medium (ISM). First, in a typical stellar system, the mass fraction of planets is less than 1%, ...
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5 votes
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How could lithium burning take place in a quasi-star?

Lithium, along with Hydrogen and Helium, was one of the 3 elements created in the Big Bang. Thus, it should exist to some part in any star that hasn't burnt all of it out, and as mentioned, it's not ...
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Reason for a correlation between Hot Jupiters and higher metallicity in Kepler data

This is a well known, well researched phenomena. Yes, there certainly is a correlation between metallicity and the likelihood of observing a hot Jupiter. There are two classes of explanation. (1) ...
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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 ...
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4 votes

How to calculate metallicity of a star that is made of iron of 20kg and Hydrogen 1000 kg?

You cannot calculate the solar ration of $\mathrm{Fe/H}$ nor can you calculate the logarithmic ratio of iron to hydrogen for a star - both are entities you have to measure. So in order to calculate ...
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Have any planetary systems been found around Population II stars?

Kapteyn's star has 14% the metallicity of the Sun and is about 11 billion years old and is in the galactic halo. It has two reported planets. I don't know if 14% is low enough to count as Population ...
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Why is the carbon/oxygen ratio at low metallicities important to study?

I think I can in part answer your questions. The [CII] ($\lambda=158\,\mu m$) and [OIII] ($\lambda=88\,\mu m$) are the most brightest IR emission lines in the local Universe Stacey et al. (1991). The ...
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Why could Quasi-stars ("black hole stars") have only existed when everything was hydrogen and helium? (no metal "contamination")

The presence of heavier elements makes the medium absorb more radiation. This means a nascent star that would have been able to collapse in the absence of metals, will lose its outer layers to ...
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How to evaluate the fit of an isochrone to a stellar population?

Finding the best-fitting isochrone, a.k.a. isochrone fitting, is a standard approach to determine the age of globular clusters. This problem can be solved with a least-square method, where the data to ...
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Relationship between metallicity and color? Should Pop. I stars be blue?

I think the misunderstanding (which I've been asked about before) is because Population II stars are redder as a population, even though for a given mass, a main-sequence Pop II (i.e. metal-poor) star ...
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3 votes
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Why the forbidden lines of [OIII],[NII] and [SII],[OII] are sensitive to electron temperature and electron density, respectively?

Forbidden lines like these arise in thin gases, where collisional de-excitation is unlikely. Forbidden lines become "quenched" by collisional de-excitation when the densities reach levels that depend ...
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2 votes

Why do certain massive stars leave no remnants?

Electron capture supernovae occur in a narrow regime where electron captures on to products such as Mg and Ne are happening at a rate that removes support of the core against collapse "faster" (or ...
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Metalicity and age of bulge stars vs halo

Your intuition is largely correct: the key is that the proto-bulge region had a deep enough potential well so that the supernovas couldn't expel the remaining gas, and so new stars could form out of ...
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1 vote

Formation of the First Stars

Metallicity impacts star formation and stellar evolution in many different ways. Star formation A dense core of a molecular cloud becomes unstable and starts collapsing under its own gravity. During ...
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Why is the metallicity of elliptical galaxies low?

Elliptical galaxies have no regions of high density, so they are not forming stars any more. So all the stars in an elliptical galaxy are old stars, and as you say, tend to have low metallicity (...
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1 vote
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How can ionized emission line flux decrease as a function of increasing metallicity or abundance?

Metallicity and abundance Metallicity Without specifying a given metal, the term "metallicity" — abbreviated $Z$ — usually refers to the total metallicity of all elements, i.e. the mass fraction of ...
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