# Tag Info

40

No. Besides the 13 Jupiter-masses required to ignite deuterium burning, and make Jupiter into a Brown Dwarf, there is a clear difference between the formation pathways of Brown Dwarves and Gas Giants. Gas Giants are planets, that form via processes in their parent protoplanetary disc. Contrasting this, Brown Dwarves form via direct fragmentaion of the ...

33

Human color vision is based on three types of "cones" in the eye that respond differently to different wavelengths of light. Thus, not counting overall brightness, the human color space has two degrees of freedom. In contrast, the spectra of stars are very close to a black body, which depends only on effective temperature. As one varies the temperature, the ...

29

Most of the galaxy's gas is not incorporated into stars and remains as gas and dust. This is not really my area of expertise, but papers such as Evans et al. 2008 and Matthews et al. 2018 seem to suggest that in the Giant Molecular Clouds where most stars in the Milky Way Galaxy form, the star formation efficiency is about 3-6%. So the vast majority of the ...

21

I think you've answered your own question. if 1st and 2nd stars generation burned hydrogen to helium and more heavier elements, then should it be like 90% of all universe hydrogen already converted to helium and something else? If yes, then there should not be enough hydrogen to make the Sun. Clearly the Sun does have enough Hydrogen to form and the ...

21

There is very little mixing in the core of the Sun, where the stratification is fixed by radiative (rather than convective) heat transfer. The heavier helium does "fill the core", but takes about 12 billion years to do so, during which time, the concentration of helium gradually increases. During its main sequence lifetime, most of the energy generation ...

18

Good question - the answer is that stars are not in general well mixed - or rather, the nuclear-burning core is not well-mixed with the rest of the star. That means that a star will finish hydrogen burning even when about 80% of the hydrogen in the star is still available, but is situated outside the core and cannot be mixed into the core (or at least not ...

15

Short answer: no It all of course depends on how you define the term failed star. In general, a star should be able to generate heat by fusing atoms together, and it requires about 13 times the mass of Jupiter for conditions to be adequate for sustained deuterium fusion, and about 63 times the mass of Jupiter for fusion of lithium to take place. All other ...

14

Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, yes, ${\rm H}_{2}$ does get dissociated by high-energy photons -- and also by cosmic rays, which can penetrate dense, dusty clouds that block most of the high-...

10

Where did the hydrogen come from if that is the first element used up in the star's life cycle? Hydrogen is indeed the first element that is fused into helium in the core of a star -- but not necessarily the entire star. For a star to consume all of its hydrogen would require the star to be fully convective. Only the smallest of stars are fully convective. ...

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 $... 8 Stars are responsible. HII regions$^\dagger$can refer to several things, but usually I guess one thinks of the volumes around star-forming regions. The more massive a star is, the faster it burns its fuel, and at a higher temperature, meaning that the peak of their spectra are more toward the high frequencies. The most massive stars of a stellar population ... 8 If we don't wait for too long, where we don't know for sure, whether protons or atomic nuclei stay stable forever, there won't be just one single element. The most abundant element in the universe will probably stay hydrogen, since it will stay in the intergalactic medium, and thin out by accelerating cosmic expansion. The second-most abundant element will ... 8 The mass of an average galaxy appears to be totally dominated by dark matter, so your calculation would not give the galaxy mass. Even if all you wanted was the baryonic (non dark matter) mass then what you suggest will be very much a lower limit. For example you can look at this paper by Chabrier (2001), who estimates that gas forms less than half the ... 7 The idea of removing material from the sun for either fuel or to lengthen its lifespan is called "star lifting". Various hypothetical methods have been suggested, including heating parts of the atmosphere or squeezing it using magnetic fields. My own tentative suggestion is to combine these with chaos control to manipulate turbulence into making coronal mass ... 7 Stan has essentially answered this in his comment, which I will attempt to spell out a little more laboriously. The significant majority of our Sun's energy output comes from the proton-proton chain. This was advocated by Eddington back in the 1920's, but at that time your basic concern was a very real and major problem. Objects with like electrical ... 7 The slowest reaction rate in the pp chain determines how quickly hydrogen can "burn" in the core of a sun-like star. That rate-determining step is actually the fusion of two protons to form deuterium via the diproton and a weak interaction decay. The fusion of lithium, whereby it fuses with a proton and then splits into two Helium nuclei is actually part of ... 7 Hydrogen was not "created at the moment of the big bang". Particles (leptons and quarks) can attain mass via the Higgs field after the epoch of electroweak symmetry breaking, that occurred about a picosecond$(10^{-12}$s) after the big bang. Only after this was it possible to form the building blocks of hydrogen. In fact the formation of stable protons ... 7 If we only consider gravity, one answer may be found using the Hill sphere. This is the distance the gravity of a body dominates over the sun: $$r_H \approx a \left(\frac{m}{3M_\odot}\right)^{1/3}$$ where$a$is the semi-major axis,$m$the mass and$M_\odot$the sun's mass. Now, an actual body has some nonzero density$\rho$and$m=(4\pi/3)\rho r^3$. If the ... 6 About 70% of the baryonic matter in the universe is hydrogen, with a mean density of about$4\times 10^{-29}$kg/m$^3$. Most of the stars that have ever been born are still alive, since an average star is only about$0.25 M_{\odot}$and has a lifetime much longer than the age of the universe (so very little material has actually been recycled). If we ... 6 The question is if 1st and 2nd stars generation burned hydrogen to helium and more heavier elements, then should it be like 90% of all universe hydrogen already converted to helium and something else? Only a tiny portion of the primordial hydrogen has been converted to helium or something else. The explanation is fourfold. Most of the universe's primordial ... 6 The role of H$_2$is to allow primordial gas to cool down sufficiently to allow the collapse to start and then to hold the gas as a relatively low temperature as it gets much more dense. The formation of H$_2$is essential because atomic hydrogen simply has no way of cooling itself below temperatures that are capable of exciting the$n=2\$ level and then ...

5

Different sources give significantly different values for the conditions under which hydrogen shifts from molecular to metallic. See the discussion in the comments associated with this question. Pressures from about 20 to 300 GPa and temperatures very broadly around 10 000 K seem to be the most common. Experiments can only just reach these pressures, at ...

5

Hydrogen and oxygen only react when there is sufficient energy. For instance, the autoignition temperature of hydrogen at 1 atmosphere is 536 °C. This is why you can do that experiment with mixed hydrogen and oxygen in a balloon, that only explodes when you touch the balloon with a lit taper. Space is cold. Molecular clouds have temperatures in the tens of ...

5

There are several misconceptions in your question. First, a star does not vacuum everything in its vicinity. Rather it forms from a condensation in a gas cloud, which in turn collapses to a proto-star surrounded by a gas disc, which can contribute further material. Once formed in this way, a star typically does not acquire more gas (exceptions are symbiotic ...

5

Protium is a proton + an electron. Under enormously high pressure, it's energetically favorable for electrons to merge with protons and become neutrons - see here. are stars mostly protons By mass, yes, at least before they get too old. The mass of the universe is more complicated, but anything solid that we think of as matter is made of atoms, ...

5

This is one of those questions that is easy to state but complicated to answer - and this won’t at all be a complete answer, but mostly a quick outline of some important factors to consider and terms you might search for in order to learn more. The question of why the interstellar medium (ISM) has the structure it does is a long-standing one, and one that a ...

4

Yes, as mentioned elsewhere, it is possibly possible. Dark matter particles may be intrinsically unstable (though having long lifetimes, which are at least significantly longer than Hubble time). Check for more info here: http://arxiv.org/abs/1307.6434

4

The lines that appear in a stars spectrum mainly reflects its temperature not its composition, see here O-type stars start out with the same sort of composition as other stars, that is they are mainly H and He (approximately 75% and 25% by mass) with traces heavier elements.

4

Metallic hydrogen is an odd substance. When you push hydrogen atoms very close together, their electrons can come free, and move around, instead of being tightly bound to the atomic nuclei. As this form of hydrogen would conduct electricity, it behaves like a metal. At least this is the theory. Nobody has been able to produce enough pressure to actually make ...

4

It is difficult to work out an actual numerical answer, but let me point out some things that I think would determine the lower bound. If two objects are close enough, they are attracted by Van der Waals forces. That only operates over a very close range, but it sets a minimum distance before something besides gravity dominates. That is relevant because low-...

Only top voted, non community-wiki answers of a minimum length are eligible