In areas where the dust is thick,  the light from nearby stars can be completely blocked - similar to the way very dark clouds block light from the sun.  This picture (left) shows the Horsehead Nebula - the dark area in the center of the photograph (shaped like a horse's head) is a thick cloud of interstellar dust blocking the light. Thinner clouds of interstellar dust may dim the light passing through,  without completely blocking it - similar to the way fog can dim light from the sun.  This is known as extinction.

Light from nearby stars can also be reflected from the interstellar dust,  similar to the way light from a car's headlights can reflect off fog.  The picture (right) shows a reflection nebula - a cloud of interstellar dust illuminated by reflected light from the star it surrounds.  This picture of NGC 1999 - a nebula in the constellation Orion - was taken by the Hubble Space Telescope in December, 1999   .

The dust in the interstellar medium can only reflect or block light,  but the gas in the interstellar medium glows with its own light.  Very hot stars can heat up the gas in the ISM until it glows with a red color.  This glowing gas is called an emission nebula.  The Hubble-X nebula,  is an example of an emission nebula.

The hydrogen gas in the ISM can also clump together and form relatively cool clouds.  These cool clouds of hydrogen can collapse - it is here, in these dense, collapsed clouds known as stellar nurseries,  that new stars are born.  This image (left) shows a stellar nursery in the spiral galaxy M-33, about 2.7 million light years away in the constellation Triangulum.

So the ISM contains the seeds for brand new stars!  New stars are formed when hot clouds of hydrogen in the ISM cool and collapse.    In this lesson,  you'll learn a little about how things cool off in general - and you might just be surprised by what you learn!  Maybe you'll have your own ideas about how this strange stuff called the ISM cools off - and about how new stars form.

Finding the Beginnings of Stars

Scientists know a lot about how stars end their long lives - they cool down and shrink,  and may eventually explode and become supernovae.  And they know a lot about how nearby stars and exploding supernovae can heat up the surrounding ISM.  But what they are still puzzling over is how the ISM cools down to form new stars.  This is the mystery that the CHIPS mission is trying to solve.

The next few sections will give you a chance to explore some of the most important science behind the CHIPS mission,  including density, heat,  temperature, and cooling.