It must be one of the oldest questions. When you gaze at
the sky, you marvel at its immensity. Have you ever, at
some stage of your life, looked up into the night sky
and wondered just how many stars there are in space?
The question has fascinated scientists as well as
philosophers, musicians, and dreamers through 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? How easy it is
to ask this and how difficult it is for scientists to
give a fair answer!

ESA’s Hipparcos mission and its successor, Gaia, are
star mappers and therefore obvious starting points to
derive information. Between 1989 and 1993, Hipparcos
mapped over two and a half million stars within our
galaxy. Due for launch around 2012, Gaia will extend
this work to about a thousand million stars. However,
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 thousands of millions upon millions of other
galaxies also! The mathematics begins to get vaguer and
larger.

Telescopes cannot yet see individual stars in distant
galaxies. Astronomers are therefore a long way from
counting each star. Even the James Webb Space Telescope,
the NASA/ESA successor to the Hubble Space Telescope,
due for launch around 2010, will be unable to do that.
Even if it could, counting the stars in the Universe
would be like trying to count the number of sand
grains on all the beaches that are on Earth. However,
astronomers want surer and smarter ways to arrive at
reliable numbers.

Knowing how fast stars form can bring more certainty to
calculations. Among other things, ESA’s infrared space
observatory, Herschel, launching around 2007, will chart
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.

In 1995, an image from the Hubble Space Telescope
suggested that star formation had reached a crescendo
at roughly seven thousand million years ago. Recently,
however, astronomers have thought again. Göran
Pilbratt, project scientist for Herschel, explains,
“The Hubble Deep Field image was taken at optical
wavelengths and there is now some evidence that a
lot of early star formation was hidden by thick dust
clouds.” Dust clouds block the stars from view and
convert their light into infrared radiation, rendering
them invisible to the HST. “Herschel is designed to
view exactly the time in the evolution of the Universe,
at the right wavelengths where we think the majority of
the obscured star formation can be seen,” says Pilbratt.

So with Herschel, astronomers will see many more stars
than previously. We will be one step closer to provide
a more reliable estimate to that question asked so
often in the past — “how many stars are there in the
Universe?”.

USEFUL LINKS FOR THIS STORY

* More about star types
http://spdext.estec.esa.nl/content/doc/e2/31458_.htm
* More about Herschel
http://sci.esa.int/herschel
* More about Hipparcos
http://spdext.estec.esa.nl/home/hipparcos/
* More about Hubble
http://sci.esa.int/hubble/
* More about Gaia
http://sci.esa.int/gaia/

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=16&cid=12&oid=31456&ooid=31457]
Hubble’s Deep Field Image provided the first clues about
numbers of stars.

The famous 1995 image of the Hubble Deep Field. By
analysing the galaxies revealed here, astronomers made
their first estimates of the history of star formation
in the Universe. However, there are many stars that
cannot be seen on this image because dust clouds hide
them from view. ESA’s Herschel mission will see them,
providing a more accurate determination of the amounts
of stars that have formed in the Universe.

Image by R. Williams (STScI), the Hubble Deep Field Team
and NASA and ESA.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=16&cid=12&oid=31456&ooid=14060]
Hipparcos mapped millions of stars in our Galaxy, but how
many more are there?

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=16&cid=12&oid=31456&ooid=31323]
Herschel’s telescope will collect infrared radiation from
distant stars. Image shows telescope, vessel containing
liquid helium cryostat (narrow, middle part), and service
module at the bottom.