Stars

Have you ever looked up into the night sky and wondered just how many stars there are in
space? This question has fascinated scientists as well as philosophers, musicians and
dreamers throughout the ages.

Look into the sky on a clear night, out of the glare of streetlights, and you will see a
few thousand individual stars with your naked eyes. With even a modest amateur
telescope, millions more will come into view.

So how many stars are there in the Universe? It is easy to ask this question, but
difficult for scientists to give a fair answer!

Stars are not scattered randomly through space, they are gathered together into vast

groups known as galaxies. The Sun belongs to a galaxy called the Milky Way. Astronomers
estimate there are about 100 thousand million stars in the Milky Way alone. Outside
that, there are millions upon millions of other galaxies also!
Hipparcos mapped millions of stars in our galaxy, but how many more are there?
It has been said that counting the stars in the Universe is like trying to count the
number of sand grains on a beach on Earth. We might do that by measuring the surface
area of the beach, and determining the average depth of the sand layer.

If we count the number of grains in a small representative volume of sand, by
multiplication we can estimate the number of grains on the whole beach.

For the Universe, the galaxies are our small representative volumes, and there are
something like 1011 to 1012 stars in our Galaxy, and there are perhaps something like
1011 or 1012 galaxies.

With this simple calculation you get something like 1022 to 1024 stars in the Universe.
This is only a rough number, as obviously not all galaxies are the same, just like on a
beach the depth of sand will not be the same in different places.

No one would try to count stars individually, instead we measure integrated quantities
like the number and luminosity of galaxies. ESA’s infrared space observatory Herschel
has made an important contribution by ‘counting’ galaxies in the infrared, and measuring
their luminosity in this range – something never before attempted.

Knowing how fast stars form can bring more certainty to calculations. Herschel has also
charted the formation rate of stars throughout cosmic history. If you can estimate the
rate at which stars have formed, you will be able to estimate how many stars there are
in the Universe today.