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jimwaltman
2009-Jun-28, 08:38 PM
Anyone have a breakdown of the percent of each stellar objects in the universe? (brown, red, neutron, etc.)


What % is visible?


Would like to know, on the Main Sequence, does the he produced gravitate to the center? If so why does gravity not contract the core?

Thanks:confused:

TonyE
2009-Jun-29, 05:12 PM
I have never seen any percentage breakdown of stellar object types. But the Initial Mass Function tells us that vastly more low mass stars are created than high mass stars. Also the low mass stars live longer.

About He: Stars similar to the Sun have a radiative core so the He manufactured there stays there and gathers in the centre. Once the H in the centre runs out then an expanding shell of H->He burning builds round the core and the He produced in the shell falls down onto the core. Eventually the He core reaches a mass/density/temperature where He -> C burning starts.

Very low mass stars are convective all the way through so the He and H get continually mixed up.

High mass stars have a convective core so the H & He remain mixed in quite a large central region until all the H is used up. Then they start burning the He.

neilzero
2009-Jun-29, 10:39 PM
I saw percentages (many decades ago) something like 95% of the mass is class m stars, 1/2% each of class k, class g, white dwarfs, neutron stars and black holes. The remaining 1.5% is sub atomic particles, dust, gas, F, A, B, O stars. red giant stars, protostars, comets, asteroids, planets and brown dwarfs. I'd guess a modern accounting would increase the percentage of sub-atomic particles, compact stars, dust, gas and brown dwarfs, reducing the class m to perhaps 20% of the total mass. All are more or less visable, if you are close enough. The accretion disk of black holes produces a small amount of visable light. From Earth, most of the naked eye stars are class g, f and red giant. Neil

GalacticBeatDown
2009-Jun-29, 10:40 PM
Hey Jimwaltman,

To answer your question I have heard that you can see 8000 stars in the sky. 4000 on the Northern hemisphere and 4000 on the Southern hemisphere.

For the breakdown of stars this might help.
Of all the stars in the Milky Way a census of them might resemble this:

One in one hundred is a big, giant star. Now you have the breakdown for the other 99 in 100. Approximately one in ten is about the size of the sun. And the other nine out of ten are dwarf stars.

adham
2009-Jul-01, 04:56 PM
Hi guys..
I wonder how much will the apparent magnitude of the star Sirius be, if one was looking at it from a distance equal to 1 AU, or simply if one standing on Earth, and Sirius is in the place of our sun?
Thanks

Nick Theodorakis
2009-Jul-01, 05:14 PM
Hi guys..
I wonder how much will the apparent magnitude of the star Sirius be, if one was looking at it from a distance equal to 1 AU, or simply if one standing on Earth, and Sirius is in the place of our sun?
Thanks

The numbers you want to use are the absolute magnitude (http://en.wikipedia.org/wiki/Absolute_magnitude), which is what the apparent magnitude a star woudl have if it was 10 parsecs from us, or the solar luminosity (http://en.wikipedia.org/wiki/Solar_luminosity), which is how bright a star is compared to the sun. Sirius (http://en.wikipedia.org/wiki/Sirius) A has an absolute magnitude of 1.42 and a solar luminosity of 25.4. For comparison, the sun has an absolute magnitude of 4.83 (and, of course, a solar luminosity of 1, by definition).

Nick

thoth II
2009-Jul-01, 05:46 PM
The formula is:

m = M - 31.5

m = apparent magnitude of star at 1 AU
M = absolute magnitude of star

example: m (sun) = 4.8 - 31.5 = -27
m(Sirius) = 1.4 - 31.5 = -30

If you generalize to any distance :

d = distance to star in parsecs :

m = M + 5 log ( d/10)

d = in AU:

m = M + 5 log d - 31.5