What is the interstellar medium?
Outer space is not empty space. The interstellar medium (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 density of the ISM ranges from 0.003 molecules per cubic centimeter in regions of hot ionized gases, or plasma, 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.
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 supernova. 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.
