h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Traveller in Time not entirely serious
"This was a question 'Waterrat' (7) asked me last night just before bedtime.
My answer in such cases is: 'twice the half', we have not really counted them, are fairly sure there is not an infinite number, but certainly more then 10^9.

Anyone a more conclusive answer ?"

.
6.24x10^24

When I wrote my 'Galaxy Song' Entry (A2163133), this was the one point I had the most difficulty finding a source for. As I put in the Entry:

"It is very difficult to work out how many stars there are in the Milky Way. Estimates vary from a hundred-billion - as Eric quotes - up to a few trillions."

And that's just one galaxy..!

http://www.space.com/scienceastronomy/star_count_030722.html
Imperfect Estimate Claims Universe Has 70 Sextillion Stars


... stars are forming all the time, so the number is never fixed

Also, when telescopes are pointed at a galaxy that is, say, 10 billion light-years away, the light it detects left that galaxy 10 billion years ago. The galaxy has changed greatly since then, and astronomers can only guess at how it evolved -- and how many stars it might now contain -- by studying more mature galaxies closer at hand.

Finally, astronomers do not know if our apparent universe is the Universe. There may be much, much more out there that is not observable and never will be known.

If these are not enough problems, scientists even have difficulty counting the stars in our own Milky Way Galaxy. The figure is typically put at between 100 billion and 300 billion.

One thing is certain: We cannot see most of the stars that are out there.

There's only a handful of true stars.

Sean Connery, Barry Manilow, Elton John, Maggie Smith, Michael Caine, Lauren Bacall...

How many stars can you see in the sky? About 5,000.

How many in our galaxy? About 100 billion.

How many galaxies are there? Well we can see more than 100 billion. And there are others that we can't see because they are too far away.

So we have any estimates of how many stars are in adjacent galaxies?

Our galaxy has two irregular satellite galaxies, which are rather small, but the nearest proper galaxy to us is M31, which is slightly bigger than ours. Most galaxies are spiral, and most spirals have about the same number of stars as ours, that is, about 100 billion. There are a fair number of elliptical galaxies as well, with less stars, but you won't be too far out if you just assume 1x10^11 stars per galaxy and 1x10^11 visible galaxies, which gives 1x10^22 stars.

Thanks Gnomon.

'...which gives 1x10^22 stars'. This seems to be a reasonable number./ I wrote down in my teaching file that 'there are more stars in the Universe than all the grains of sand on every beach and in every desert on Earth (70 thousand, million, million, million of them = 70 x 10^21 (70 sextillion). I didn't note where I got this info from but I noted also 'see Genesis 22:17'.

n.b. compare this with Avogadro's Constant (No of atoms in 12g of carbon-12 (6.023 x 10^23)

I don't know, but it's a truly astronomical number.