# 70 sextillion stars

#### Phobos

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http://www.cnn.com/2003/TECH/space/07/22/stars.survey/index.html

Ever wanted to wish upon a star? Well, you have 70,000 million million million to choose from. That's the total number of stars in the known universe, according to a study by Australian astronomers. It's also about 10 times as many stars as grains of sand on all the world's beaches and deserts. The figure -- 7 followed by 22 zeros....

Related Astronomy and Astrophysics News on Phys.org

#### stuffy

I wonder who gets to name them all.

#### ranyart

How about the bad news?

Out of all the stars in the universe, how many will go on to make Blackholes?

Lets start with 70,000 million million million Stars, if a certain % go on to make Blackholes after going supernova, so how do we calculate the probable Blackhole count?

Ok, lets say there are ten percent of stars that go on to make BHs.
Then we can say that there is also one BH for every Galaxy in the Universe(presumed to be at Galactic core)someone has to estimate the Galaxy numbers.

The we have to estimate the lifetime of a BH, the average star exists over many million years, so what is the BH age compared to the lifetime of Stars, is it equivelent, millions of years?

A BH does not emmit radiation over millions of years, in fact it dont emit anything detectable, but lets say that a BH exists for the shortest possible moment at the end of a Stars life, the transitional moment from supernova to no-nova( moment a BH exists all light in vicinty ceases).

Anybody know what the total Blackhole count is for our Universe? a rough figure would suffice.

#### ranyart

Also, if I blink during a supernova, can this stop a Blackhole appearing?

#### Janus

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Originally posted by ranyart

Ok, lets say there are ten percent of stars that go on to make BHs.
Way too high of a percent. Only massive stars can become black holes (3.5 solar masses and above.)

The vast majority of stars are small and cool. (95% are smaller than our own sun.)

If you take the region around our sun as representative, There are over 150 stars within 30 ly, and not one candidate to become a black hole among them.

#### drag

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Are we talking about the estimated amount of stars in the
observable part of the Universe ?

Live long and prosper.

#### Ivan Seeking

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Shouldn't this be reported as the number of stars that have existed over some interval in time from our perspective...I guess 12 billion years? We don't really know how many are still out there do we? Or is this accounted for in the estimate; the number of starts that no longer exist but that can be seen?

#### Phobos

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Originally posted by drag
Are we talking about the estimated amount of stars in the
observable part of the Universe ?
That's right. The article says the actual number could be infinite.

#### Phobos

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Originally posted by ranyart
How about the bad news?
Why is a black hole bad news? (unless you're falling into one, of course)

The we have to estimate the lifetime of a BH, the average star exists over many million years, so what is the BH age compared to the lifetime of Stars, is it equivelent, millions of years?
The main sequence of a star can last millions/billions of years. The time it takes for a black hole to die (evaporate) depends on its size...but it's an incredibly long time....
http://itss.raytheon.com/cafe/qadir/q1025.html

but lets say that a BH exists for the shortest possible moment at the end of a Stars life, the transitional moment from supernova to no-nova( moment a BH exists all light in vicinty ceases).
[?]
Why would a black hole (from a stellar remnant) only exist for an instant?
What do you mean by "no-nova"?

Very large stars go supernova...that ejects lots of matter to space and then what's left of the core collapses into a black hole which then persists for what might as well be considered forever (see link above)

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