The Interstellar Medium—What is it?

Most of us, at one time or another, have learned that space is a perfectly empty vacuum.  But the vast space between the stars is really not empty,  it  is sparsely sprinkled with gas and dust.  This ‘stuff’ in the vast spaces between the stars is known as the Interstellar Medium (or ISM). 

About 99% of the ISM is gas (hydrogen and helium),  the remaining one percent consist of  heavier elements and dust.  The gas is extremely dilute, with an average density of about 1 atom per cubic centimeter.   The air we breathe is approximately 30 quintillion (30,000,000,000,000,000,000,) times more dense than the ISM.  Picture this:  an "empty" coffee mug in the ISM  would contain about 500 hydrogen molecules.  The same "empty" coffee mug sitting on your desk contains about 1500 quintillion gas molecules - mostly nitrogen, hydrogen and oxygen.  

The dust in the ISM is made of tiny, irregularly shaped particles of silicates, carbon, ice and iron.    In areas where the dust is thick, the light from nearby stars can be completely blocked - similar to the way dark clouds block light from the sun. 

The picture at 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.  This is know as extinction.  The interstellar dust scatters blue light more effectively than red light - which means that most of the light that reaches us through the interstellar dust is reddish.  This is known as interstellar reddening.  A similar process happens here on earth at sunset - which is why sunsets often appear red.

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 gas and dust  illuminated by  and reflected light from the star it surrounds.  The picture  was taken by the Hubble Space Telescope, and is of NGC 1999, a nebula in the constellation Orion. 

Unlike the dust in the interstellar medium, which can only reflect or block light, the gas in the interstellar medium glows in visible and many other wavelengths.  In the region of hot, newly-formed stars, clouds of  hydrogen gas are ionized by the ultraviolet radiation emitted from the stars.  When free electrons recombine with the ionized hydrogen, visible red light is emitted from the hydrogen gas.  This accounts for the red colors in photographs of emission nebulae, such as the Trifid and Orion Nebulae.

The hydrogen gas in the ISM can also form relatively cool clouds that emit light in the radio band of the electromagnetic spectrum.  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 M33, about 2.7 million light years away in the constellation Triangulum. 

(If you are interested in learning more about the ISM - check out this collection of links to various sites with more information.)