The interstellar medium is a mixture of gas and dust found between stars in a galaxy, with 99% gas and 1% dust. Astrophysicists study it as it influences the star formation process. Francis Bacon first acknowledged its existence in 1626. It is cold and blocks visible light, but infrared rays can get through.
The interstellar medium is the name scientists give to the highly dispersed gas and dust found between stars in a galaxy. Of the mixture, 99% is gas and 1% is dust. Of the gas, 90% is hydrogen and 10% helium. The density of the interstellar medium varies depending on where you are and in which galaxy, but ranges from a few thousand to a few hundred million particles per cubic meter, with the Milky Way averaging about a million particles per cubic meter. Contrast this with the intergalactic medium, which has only 100-1000 particles per cubic meter, or the average density of the universe, which contains numerous massive voids, bringing it down to just one particle per cubic meter.
Astrophysicists are very interested in the precise qualities of the interstellar medium because it strongly influences the star formation process. Dense regions are more likely to condense into stars. Very old galaxies, such as those that feed quasars, are thought to have higher average densities than present-day galaxies, which aggregated into stars and planets. Aggregation continues all the time on all scales of the interstellar medium, as new stars are born and die in clouds called nebulae. The launch of the Hubble Space Telescope has greatly improved our understanding of these bodies and how they move and interact with the interstellar medium.
The first philosopher to acknowledge the existence of an interstellar medium was Francis Bacon, who wrote about it in 1626. Francis Bacon also originated the scientific method. He assumed that the interstellar medium moved with the stars, and he was right. The scattered particles of the interstellar medium move nearly a million miles per hour around the galactic center. Depending on how close the particles are to the galactic center, they take anywhere from a few million years to a couple of hundred million years to complete a full rotation around the galaxy.
The interstellar medium is cold and does a good job of blocking visible light where it is dense. We have trouble seeing our galactic center because the dust makes it a trillion times dimmer than it would otherwise be, at certain wavelengths. In the infrared portion of the spectrum, rays get through, so observers looking at the galactic center have to rely on the infrared.
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