Physics Question #301

Jonas Malmgren, a 20 year old male from the Internet asks on December 12, 1997,

Is the interstellar space between the galaxies completely empty? Not even a single star or a nebula?

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The answer

Donald J. Barry answered on December 12, 1997

Only in the last decade have we started to understand the details of the intergalactic medium. Many questions remain. We know that gas clouds exist between galaxies - we see these because they remove specific colors of light from more distant objects that we see through them. Since these clouds are at different distances and have different redshifts, hydrogen gas inside them removes light of a different color, depending on the distance. The result is a so-called "Lyman Forest" of features in the spectrum of the star, each absorption feature (like a tree) representing a gas cloud.

We haven't found nebulae inside gas clouds because the ones near us are either too tenuous to hold together or have already long-since collapsed to form galaxies. To look back to the era when many clouds are forming galaxies, we have to look so far away that it's difficult to see individual features like nebulae.

We do expect to find the occasional stars, not in big voids necessarily, but near galaxies because galaxies occasionally shoot out stars at escape velocity from close encounters between stars inside the galaxy. But galaxies themselves are pretty faint, and they have hundreds of billions of stars. Finding a single one is like finding a needle in a haystack. There have been several attempts to find them, none unambiguously successful.

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