Talk:Stellar evolution
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Random Question
Random question from a reader, regarding the second paragraph of "Birth" talking about Giant Molecular Clouds, which reads: "The cloud is stable as its constituent molecules are too widely spaced apart for gravity to draw them closer." This doesn't really make sense, since gravity doesn't "cut off" at any distance. In contrast, the page actually *about* Giant Molecular Clouds, (at the bottom of the third paragraph in "General Information"), gives a different explanation: "The clouds have an internal magnetic field that provides support against their own gravity."
Of course that explanation doesn't cover (a) what would cause such a magnetic field and (b) how a magnetic field manages to keep a molecular cloud from collapsing since electrically neutral particles wouldn't be affected by magnetism -- but at any rate we have two pages linked to each other offering different explanations about what holds a molecular cloud apart. Anyone who actually understands astrophysics want to correct one or the other?
(Jan 8, 2005 -- It appears the "Birth" section has radically changed since this comment was posted and no longer contains the statement about particles being too far apart. Hence this comment is obsolete.)
White dwarfs
Updated first location where stability mechanism for white dwarfs is mentioned to link to degeneracy pressure. Although this entry does not yet exist a number of other astronomical entries link to it. It seems to have fallen down the cracks between the contributing astrophysicists and contributing quantum mechanics.
- Alan Peakall
Removed
Removed:
The ball, now a star, begins to shine.
The ball was already a star fusing hydrogen. Rednblu 11:02 20 Jul 2003 (UTC)
This text was replaced.
fusing iron does not liberate energy - because of the vast pressure and temperature, iron is actually forced to fuse. The supernova explosion is less than a fraction of a second away. Iron takes in energy when it fuses, it also takes in electrons. This energy and those electrons had been helping to support the star against it's own gravity. Now, with the support gone, the envelope of the star comes crashing down onto the core at a fine fraction of the speed of light. The implosion rebounds into a shock wave going through the star.
As the shock encounters material in the star's outer layers, the material is heated to billions of degrees, fusing to form the heavier elements. Indeed, all elements heavier than iron-56 are formed in supernovae explosions. In one of the most spectacular events in the Universe, the shock propels the material away from the star in a tremendous explosion called a supernova. The material spews off into interstellar space -- perhaps to collide with other cosmic debris and form new stars, perhaps to form planets and moons, perhaps to act as the seeds for an infinite variety of living things. Rednblu 11:57 20 Jul 2003 (UTC)
1st and 2nd Generation Stars
looking for more info about 1st generation and 2nd generation stars, and what times these have existed. particularly, our sun is 2nd generation; how does its age compare with other 2nd generation stars? is it one of the older? or one of the younger of such stars?
I woudn't call our Sun second generation, but it's recycled material. The life time of stars strongly depends on their age. The more massive ones burn out rapidly, the lighter ones have very long life times (much longer than the age of the universe). There is an entire population of old stars in the globular clusters, which form a spherical halo around what one usually thinks of as the galaxy, namely the central bulge and the surrounding disc. These could be called first generation stars, I guess. Stars form permanently out of the galactic gas, which is (also permanently) being replenished by supernova explosions. The latter occur when massive (and thus short-lived) stars reach the end of their life time, or in binary stars with mass flow between the components. The Sun's age is about 4.5 billion years, which is half its expected life span and one third the age of the universe. Gas from supernova explosions contains more heavy elements (like iron) than the galactic gas contains on average (because supernovae is where those elements are created). Therefore, the gas out of which the stars form becomes more and more enriched with heavy elemnents as the galaxy ages. The stars that form today contain more heavy elements than our Sun, which, in turn, is more metal-rich than the stars in the globular clusters.
Removed
Removed statement about astronomers objecting to the name evolution. I've never heard any astronomer object, and if there are astronomers who do, they are small enough in number so that statement doesn't belong in the first paragraph. Roadrunner 18:02, 4 Mar 2004 (UTC)
Universe's Age
The article makes references to the age of the universe as though we are certain of it, but shouldn't it say that "it is believed that the universe is about 13 billion years old" and "The universe is not believed old enough for black dwarves to exist"?
- We may not know the age of the universe for certain, but it's certain for all practical purposes (as certain as we are about anything else) that it's not old enough for black dwarves to exist. Black dwarves take hundreds to thousands of billions of years to come into existence.
Helium-Carbon Fusion
Edited the statement about helium->carbon fusion lasting only "a few minutes" for a solar-mass star to the more generally accepted 1 billion years.
Heavier than iron
I'm confused with how an iron atom can be fused when being struck by a neutrino in supernova, I mean, wouldn't it break the nuclei apart? How does the additional protons and neutron fuse with the iron nuclei if just a neutrino made impact with it? (Sorry for asking easy questions I haven't studied in college yet)
- Protecter
I'm assuming its something related to neutron capture, just on a more fundamental scale. I mean, neutrons don't break the nucleus either, right? (Plus, they have more mass than a neutrino!) Perhaps the neutrino helps to form bonds or something. -- Natalinasmpf 14:42, 16 Apr 2005 (UTC)
Life Cycle
I assume stars have a life cycle, yes? This article seems to cover mainly just the evolution of a star, classifying them on mass, reporting on its death, then touching lightly on that it can later after its death (or becomes a black hole) be fed into new stars sometime later. This is ambiguous though, but I assume there should be some sort of life cycle or we would have run out of stars at the Universe's current age? Also, about blackholes trigerring nebulae as well? -- Natalinasmpf 14:37, 16 Apr 2005 (UTC)