Wikipedia:Reference desk/Archives/Science/2015 July 5

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July 5

Stars

The following is the statement of confusion: In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are called molecular clouds.

Q:

1. Does this mean population I stars are burning H2 and helium in its core? Done
2. Which stars turned H into H2? Done
3. ‘A still gaseous body before any star formation has taken place’ & ‘over-dense region of dark matter in the very early universe’ – Does this mean ‘the foggy universe’ time?

Space Ghost (talk) 21:06, 5 July 2015 (UTC)

H₂ is the normal form of Hydrogen under reasonably cool conditions - e.g. on Earth. In this form, two hydrogen atoms share their electrons in a covalent bond. Under most reasonable conditions, H₂ will form spontaneously. However, the H-H covalent bond is a chemical bond. At the temperature of stars, all chemical bonds break down, and indeed, even the bond between the H nucleus and its electron breaks down. Matter in a star is, nearly without exception, in the form of a plasma. The nuclear reactions that "burn" Hydrogen affect only the nuclei, not the electron shell, of hydrogen. Main sequence stars generally burn Hydrogen, but there are population I stars that have evolved far enough to burn Helium (but not H₂) and have turned into red giants. --Stephan Schulz (talk) 22:08, 5 July 2015 (UTC)

Thanks -- Space Ghost (talk) 19:18, 6 July 2015 (UTC)

Star formation, Evolution & Mass

1. Does it occur only in a giant molecular cloud? Doesn't it occur in a small/medium molecular cloud? Is it called the cold molecular cloud?
2. Does a star's mass increase and decrease with its age, like 'temperature' and 'luminosity'? If so, how? - is it from accretion during its complete life time? How does the mass work out anyway? Does it depend on the chemical element?

Space Ghost (talk) 21:06, 5 July 2015 (UTC)

2. A stars mass will generally decrease with age. Solar wind gradually bleeds off plasma, and mass. This solar wind is driven by the nuclear fusion of the sun, which releases lots of EM radiation. When two nuclei combine to form a third nucleus, that third nucleus will have a mass of less than the sum of the two original nuclei. The difference in mass will result in photons (electromagnetic radiation, light, IR etc.) being given off. The photons leaving the star will also reduce the mass. The only chance of it increasing in mass is colliding with another star, or swallowing a large planet. Big stars burn faster than small stars, e.g. Betelgeuse is a very good example. Early stars with low metallicity burn slower than later starts. Martin451 23:48, 5 July 2015 (UTC)

Is the reason a star moving through a cloud couldn't pick up mass, that the solar wind would blow the cloud out of the way ? StuRat (talk) 00:49, 6 July 2015 (UTC)

I would think that wouldn't be significant. The amount of mass lost to the solar wind is tiny/insignificant. If the cloud were much denser than the solar wind, the volume of the solar wind pressure would decrease as the solar wind heats it. I'm not sure what density it would take to move that pressure volume boundary to within the star itself. I would tend to think there simply aren't enough conic solutions that result in the collision with the star. How much mass does the earth gain from the solar wind vs. loss from energetic escapes? I think the loss of solar mass to both fusion and solar wind over the entire lifetime of the sun is about 0.1% or so IIRC (before Red Giant phase). The solar wind doesn't appreciably affect the orbits of planets. --DHeyward (talk) 08:45, 6 July 2015 (UTC)

Strange things can happen in binary star systems - see cataclysmic variable star. Gandalf61 (talk) 10:13, 6 July 2015 (UTC)

[[File:|25px|link=]] 'Zero age main sequence' (ZAMS)! I forgot, sorry. But I'm still not clear of the issue.

Lets say (for example) when the cloud is fragmenting, its accreting material creating a round object, as its becoming heavy, it went up to 5 solar mass and turned into ZAMS. As it decreases its mass, at what mass does it become a red giant, white dwarf, black dwarf? - (the mass is just an example btw) - I understand that once it fuses all of its hydrogen but at what mass it burns all its hydrogen then goes into helium burning phase?

Mass is not the determining factor, as stars with greatly different masses could all be at the same stage (e.g. Main Sequence G2, like our Sun) at the same time. However they would have started at different times as the mass of a star does greatly influence how fast they progress through the various stages of Stellar Evolution, an article which you should read thoroughly as it should clear up many of your misconceptions and answer most of your questions. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 20:53, 6 July 2015 (UTC)

I read it, it uses the word 'mass' mainly to differentiate the evolution of 'cold molecular cloud' & 'GMC' formations, and does not clarify at what mass it changes its phases after the ZAMS. If I knew, say for example, a protosar goes to main sequence after it becomes 5 solar mass, then it goes to orange after losing 2 solar mass, while it is on 3 solar mass and being orange, it goes to the red phase after losing 1 solar mass resulting in with 2 total solar mass leftover, and so on. - If you know what I mean - I need a rough/original figures in order to understand how much mass it decreases in each phase after ZAMS, for low, medium, massive stars, in order to go to the next phase... -- Space Ghost (talk) 19:07, 7 July 2015 (UTC)

Also, I don't understand about the 'early stars with low metallicity burn slower than later starts', I thought the early 'quasars' (not known of its mass) and 'III' stars burnt faster than expected because they had over 100 mass. No information found on 'II' stars mass even though its visible till today. -- Space Ghost (talk) 19:28, 6 July 2015 (UTC)

Two stars of the same mass, the one with low metallicity will burn slower than the one with higher metallicity. The Sun is expected to have a life of 10 billion years. Stars like SM0313 are already 13 billion years old, and likely to burn for a long time. Stars with larger masses burn faster, Betelgeuse mentioned above has a mass of 8-20 solar masses, and is currently 8-10 million years old and expected to go nova in the next million years. Its life will be one thousandth of that of the sun. The early quasars were even bigger than Betelgeuse. Martin451 00:42, 7 July 2015 (UTC)

I'm taking your words for it... Thanks -- Space Ghost (talk) 19:08, 7 July 2015 (UTC)

---

Clarification required:

1. I understand that a Star can accrete, what I forgot i.e. up to what age/time? I assume, even up to the black hole time? Am I right?
2. When does it become a neutron star, after a white dwarf or it depends on its mass/accreation? I believe neutron star do not accreate as it spins extremely fast...
3. Is after a 'supernova' where you don't actually see nothing left at all of a star? I mean 'no round ball'?

Space Ghost (talk) 19:20, 6 July 2015 (UTC)

1) A star could continue to accrete material, if it's surrounded by a dense enough cloud that the solar wind can't push it away. I'm not sure of how dense it would have to be, though. Also, if the cloud was rapidly heading towards the star that would change the calculations. At some mass and/or time a nova or supernova would occur, before it got to a black hole.

2) See neutron star. They can accrete material from a binary system twin.

3) Supernova often do leave a core at the center, perhaps a neutron star, perhaps a black hole, depending on mass, etc. See the chart in supernova#Core_collapse. StuRat (talk) 20:16, 6 July 2015 (UTC)

Okay, thanks -- Space Ghost (talk) 19:08, 7 July 2015 (UTC)