Hubble's Deep Field Image provided the first clues about numbers of stars. Credit: R. Williams (STScI), the Hubble Deep Field Team and NASA and ESA

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 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. Goran 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?".