# Tag Info

46

The chemical enrichment of the Universe over time is indeed a thing. The plot below (source) shows observational measurements of the cosmic density of ionised Carbon in the Universe against redshift (higher redshift -> further back in time). The abundance of other heavy elements over time shows a similar trend. Stars are explicitly classified based on ...

35

Stars don't "come from" a supernova. Stars come from the interstellar gas in the galaxy, particularly where it is more concentrated into nebulae. This gas is mostly hydrogen and helium, but it is "enriched" with heavier elements from old stars, including from stars that have exploded in supernovae. Over the billions of years since the ...

19

The mean mass of a star in a typical star forming region is about 0.3 solar masses and contains about 1% by mass of elements heavier than helium A typical core-collapse supernova progenitor might have a mass of around 15 solar masses and they will be responsible for dispersing a few (say 3) solar masses of heavy elements into the interstellar medium. i.e. ...

9

One can make a theoretical upper bound by considering the most short-lived star possible $\tau_{short}$, and a large supply of initial hydrogen $M_H$. Then one could calculate the fraction hydrogen that is recycled $r$ after the star ends (with a supernova), and get a total number of generations as $\log (M_{star}/M_H)/\log(r)$. If one uses the solar-mass $... 9 There are two phases to this problem. In order to accrete into stars, a huge amount of angular momentum must be lost to allow so much mass to gather into a small volume. A second problem is how stars like the Sun end up rotating so slowly, when younger versions of stars similar to the Sun rotate much faster. The solution to the first problem may be solved by ... 8 It is actually the other way around: First a massive accretion disc can form, through which material looses angular momentum and accretes onto the star radially, hence being angular momentum poor. However, during the initial free-fall phase, before the disc forms, infalling material can be 'rejected' at the star, either via high pressure gradients or ... 7 In principle, yes. In practice, no. The question has been studied for a long time. The classic treatment is Dyson's papers on "anchor rings" in 1893 (paper I, paper II) but it goes back to the study of "figures of equilibrium" starting with Newton's considerations of the oblateness of the Earth and then continuing with Maclaurin and ... 5 Galaxies consist of dark matter, stars, and gas. While gas is "collisional", i.e. it may interact hydrodynamically and cool, dark matter and, effectively, stars are collisionless. Hence, it is relative easy for an originally more or less spherical, gas-rich galaxy to collapse along the axis of rotation, while centrifugal forces prevents to collapse ... 5 Supernovae are extremely energetic events. The remnants left behind by a supernova are ejected at much greater than escape velocity. Instead of coalescing due to self-gravitation, the remnants disperse and eventually spread throughout vast regions of the galaxy. 4 The size of stars depends on the amount of non-hydrogen elements ("metals") affecting the opacity of the gas that forms them. Metals make it easier to radiate away heat when a gas cloud is collapsing, making it easier for a large amount of gas to accrete (although it may mainly act by increasing the minimum mass of stars that form). There is a fair ... 4 The UV luminosity of a galaxy can be calculated, given a stellar population. This population, in turn, can be calculated given an initial mass function (IMF), i.e. the distribution function of stellar masses. In this case, the UV luminosity should be linearly proportional to the star formation rate (SFR), sometimes written$\Psi$. The UV is primarily emitted ... 4 Black holes do not suck in matter any more than stars and planets do: an object in orbit would remain in stable orbit if nothing perturbed it. However, in the long run ($10^{19}\$ years and more) interactions between stars will perturb their orbits, making many of them end up in the central black hole. An easy way of seeing this is to consider a random close ...

3

The dynamics of the Solar System and the chemistry of the Solar System bodies don't support a hypothesis of a stellar merger later than formation of the protoplanetary disk which would have mixed-up things considerably and heavily disrupt any circumstellar disk. Thus this basically excludes any collision after the time one can start talking about a protostar,...

3

Yes, a blueshift here would mean relative to the systemic velocity of the galaxy - which means that, relative to the galaxy, the material is moving at least partly in the direction of Earth. In many cases the outflow may be a pair of jets being ejected in opposite directions out of the nucleus of the galaxy, but we are less likely to see absorption lines ...

3

This quantity is referred to as star formation efficiency (SFE) by astronomers who study star formation and galaxy evolution. Estimates can vary but typically are around a few percent. In Sec. 4.1 of this paper Inoue et al. review some estimates from the literature. Those numbers are for the central regions of spiral galaxies, and the rate may be lower ...

3

The first stars formed in the universe are likely to have a "top-heavy" mass function. There would be more high-mass stars (as a fraction of all stars) than in the present day and the most massive stars would be more massive than the most massive stars formed in the present day. On the contrary, there would be more low-mass stars formed in the ...

2

Many hot stars are born in multiple star systems because the cores of these stars tend to split (see Jeans instability). With lower mass stars this still can happen. However, there are other ways. For example, in a young star cluster, close encounters with other stars can cause a star to be captured by another one.

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Some stars do form from supernova remnants. But the relative amount of heavey elements is still very small compared to hydrogen and helium. Many 2nd generation stars have more elements than the 1st generation stars. But the percentage of these elements is still very small compared to the hydrogen in the star.

1

Stellar mergers are certainly possible, but also relatively rare. Maybe protostars merging is a bit more common since they have less relative velocity. However, unless the merger is straight it will typically deposit a lot of angular momentum. The sun seems to be a slow rotator for its spectral class. Hence it is not likely it was formed through a stellar ...

1

Yes, although astrophysics does not talk about "purity". The specific term you're looking for is metallicity. In astrophysics, "metals" are all elements heavier than hydrogen and helium, and the metallicity of a star does in fact play a major role in its evolution. It is also on the basis of metallicity that stars are divided into three ...

1

The main reason the late-type galaxies (spirals and irregulars) are blue is that the brightness contribution of the hotter stars (Main sequence O, B, and such) surpasses the contribution of colder, less massive stars (even though, there are more low mass stars than high mass stars). Read more about it here and here. In early-type galaxies (elliptical and ...

1

Based on links on this page, the closest visible star in Orion (that is, the region of the sky called Orion) is Pi3 Orionis (π3 Orionis), a.k.a Tabit, a.k.a al-thābit, a.k.a. Zhāng Qí Liù. This is the point where the arm touches the shield/bow. That page says it is 26.2 light-years away; Wikipedia says its 26.32 ± 0.04 light-years or 8.07 ± 0.01 parsecs away....

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