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Interstellar Medium

April 9, 2015

The weather can be a bit blustery at this time of year, with strong winds stirring dust into the sky. The dust can make the setting Sun look especially orange or red.

Astronomers see that same “redness” when they look at the stars beyond the Sun, and for the same reason: clouds of dust. In fact, the space between the stars is far from empty. It’s filled with gas and dust that form the interstellar medium. This material accounts for about 15 percent of all the “normal” matter in our home galaxy, the Milky Way.

The interstellar medium is an extreme vacuum by any standard — it’s far emptier than any vacuum created in the laboratory. But there’s a lot of space between the stars, so the gas and dust adds up.

Most of the interstellar medium consists of hydrogen — atoms that were created in the Big Bang. But there’s also a smattering of other atoms and molecules, plus the tiny solid particles known as dust.

Many of the dust grains are about the size of the particles in cigarette smoke. That’s the right size to scatter blue wavelengths of light. In fact, that’s similar to what makes the sky blue, and what makes the Sun look redder when it’s shining through a thick layer of air.

Interstellar dust makes distant stars look redder than they really are, too. So to fully understand the stars, astronomers need to know how much dust they’re looking through — dust that reddens the view of the distant stars.

More tomorrow.


Script by Damond Benningfield, Copyright 2015

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