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Thanks, Glitch. How long does it take for a star to form from a cloud of gas from the time that cloud gets disturbed?
That depends on the size of the star.
For a star like our sun, it takes about 10,000,000 years for the elements (mostly hydrogen and helium) in the nebula to coalesce, collapse, condense, and start the fusion process of a star.
Stars much bigger than our sun (such as a blue supergiant) can take as little as 100,000 years.
Thanks, Glitch. How long does it take for a star to form from a cloud of gas from the time that cloud gets disturbed?
That depends on the mass, temperature, and density of the solar nebula. Our understanding of the Solar Nebular Disk Model (SNDM) only applies to low mass stars (less than ~8 M☉). Our understanding of larger mass star formation is not as good.
Some people prefer to use Jeans Mass as the critical point at which the nebula begins the process of runaway contraction. However, there is a flaw in his initial analysis which allows for a collapse that is slower than that predicted by Jeans' original analysis. This flaw is also known as "Jeans swindle." For this reason I prefer to use the Bonner-Ebert Mass. The Bonner-Ebert Mass is the largest a spherical gas cloud can be before it begins to collapse under its own gravity.
Once the gas cloud exceeds the Bonner-Ebert Mass and begins to collapse, the rate of its collapse will increase, as will its temperature and density, as the radius of gas cloud decreases. In order to create a star similar to Sol in mass takes ~100 million years.
More massive stars will form quicker, and less massive stars will take longer to form. Once the star is born (meaning it has a core temperature greater than 15 million degrees Kelvin so hydrogen fusion can begin), the solar winds will begin and clear out the remaining gas in the solar system. The formation of the planets is relatively quick after that point, taking only about 300,000 to 500,000 years.
Good question. First of all, we can only "see" optically ~8 billion light years, not using gravitational lensing. The only thing that comes close to 13.7 billion light years away was WMAP which occurs at a redshift z = 1089, or ~380,000 years after the Big Bang.
Optically, we can never see what happened before reionization.
Shouldn't we be able to see the reionization period as some kind of impenetrable fog with a powerful enough telescope?
How can there be different size black holes when a singularity is by definition an infinitely dense, infinitely small point?
Black Holes get larger as they accumulate more material. But here's the trick. The area of the event horizon increases proportionate to the matter/energy it accumulates.
They aren't considered an "infinitely dense point" any longer. Black holes being considered an infinitely dense point is not consistent with current theory.
That depends on whose theories you are using at the time. You are right, according to Einstein gravity is an effect, caused by mass warping space/time. However, under Newtonian mechanics, to answer why an apple falls from a tree, gravity is most certainly a force.
Black Holes get larger as they accumulate more material. But here's the trick. The area of the event horizon increases proportionate to the matter/energy it accumulates.
They aren't considered an "infinitely dense point" any longer. Black holes being considered an infinitely dense point is not consistent with current theory.
Is the event horizon like a spinning CD around a black hole or is it a sphere?
Someone correct me if I'm wrong but I think gravitons have fallen out of favor as the preferred hypothesis for what gravity really is.
I believe gravity is really just a distortion in space-time. Putting a large body in space "compresses" the space around it thus creating what we experience as gravity.
Is the event horizon like a spinning CD around a black hole or is it a sphere?
It is a sphere however, faster rotating black holes can have bulges near the equatorial line. The accretion disc around the event horizon is disc shaped as it's name suggests.
Last edited by Adric; 08-06-2014 at 12:22 AM..
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