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ESA Science News
http://sci.esa.int

28 Feb 2003

Who knows how many stars there are?
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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/

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