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What is the interstellar medium?

Outer space is not empty space. The interstellar mediumAll the gas and dust found between stars. (ISM) is the name for the stuff that is in space between stars in our Milky Way Galaxy. The ISM is mostly made of clouds of hydrogen and helium. The rest of the ISM mostly consists of heavier elements like carbon. About one percent of the ISM is in the form of dust.

In some places in space the ISM is not dense at all, but it is much more dense in other regions. However, even the densest parts of the ISM are 1014 (100,000,000,000,000 or 100 trillion) times less dense than the Earth's atmosphere. The densityThe amount of mass or number of particles per unit volume. In cgs units mass density has units of gm cm-3. Number density has units cm-3 (particles per cubic centimeter). of the ISM ranges from 0.003 molecules per cubic centimeter in regions of hot ionized gases, or plasmaPlasma consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state., to more than 100,000 molecules per cubic centimeter in regions where stars form. On average, there are only 1,000 grains of dust in each cubic kilometer of space! For more information on the composition of the ISM, please visit: cse.ssl.berkeley.edu/chips_epo/EducationBrief/CHIPS-Educational_Brief.htm.

Stars form in regions of the ISM that are dense enough for gravity to pull the gas and dust together to make compact, hot spheres. These protostars eventually become so dense and hot that nuclear fusion begins, and they become stars.

A simulation of the Veil Supernova Explosion. Credit: ESA/Hubble (M. Kornmesser & L. L. Christensen).

Although they are not alive, stars have life-cycles. They are born from the ISM, grow, and die. Some stars die in an explosion called a supernovaA stupendous explosion of a massive star, which increases its brightness hundreds of millions of times in a few days.. After it explodes, a supernova's material is recycled into the ISM.

Exploding stars continually replenish the ISM with their material. In turn, gravity pulls the ISM material together to form more stars.

What is the interstellar medium?

CLICK TO PLAY MOVIE. Clip from IBEX: Search for the Edge of the Solar System. Credit: NASA/Adler Planetarium.

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Last Updated: 22 NOVEMBER 2010
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