credit: modification of work by Mark Krumholz
Flows of Interstellar Gas
The most important thing about the interstellar medium is that it is not static. Interstellar gas orbits the galaxy and can become more or less dense, hotter and colder and change its ionization state. A certain gas spot can at some point be neutral hydrogen, then be near a hot young star and become part of an H II region. The star can explode as a supernova and heat nearby gas to temperatures of millions of degrees. Over millions of years, gas can cool down and become neutral again before it accumulates in a dense region where gravity is gathered in a huge molecular cloud.
At any given-time in Milky Way, most of the interstellar gas in terms of mass and volume is in the form of atomic hydrogen. Much denser molecular clouds only take up a small fraction of the volume of interstellar space, but contribute around 30% to the total mass of the gas b/w stars. In contrast, the hot gas produced by supernova explosions contributes negligible mass, but takes up a significant portion of the volume of interstellar space. While the H II regions are visually spectacular, they only make up a very small fraction of the mass or volume of. interstellar material.
However, the interstellar medium is not a closed system. Gas from intergalactic space is constantly falling on the Milky Way due to its gravity, adding new gas to the interstellar medium. In contrast, the gas can collapse into new stars in huge molecular clouds in which gas collects-together-due to gravity. This process blocks interstellar matter in stars. As stars age, evolve, and eventually die, massive stars lose much of their mass. and low-mass stars lose very little. On average, about a third of the matter contained in stars returns to interstellar space. Supernova explosions are so energetic that they can push interstellar mass out of the galaxy and back into intergalactic space.Thus, the total mass of the interstellar medium is determined by a competition between the mass gain of intergalactic space, the conversion of interstellar mass into stars, and the loss of interstellar mass back into intergalactic space due to supernovae. This whole process is known as the baryon cycle; Baryon comes from the Latin word for “heavy,” and the cycle is named because it is a repetitive process experienced by the heaviest components in the universe, the atoms, undergo.
Cycle Of Dust And Heavy Elements
While much of the mass of the interstellar medium is material that has accumulated in intergalactic space over the past billion years, this does not apply to elements heavier than hydrogen and helium or dust, but rather these components of the interstellar medium have been inside Stars in the Milky Way, which they returned to the interstellar medium at the end of their life. What stars “do for living” is to fuse heavier elements with lighter ones and produce energy in the process. As stars mature, they start to lose some of the newly created elements in the reservoir of interstellar matter.
The same goes for grains of dust. Dust is created when grains can condense in regions where the gas is dense and cold. A place where the right conditions prevail is the winds from lumnous cool stars. The grains can also condense into matter kicked up by a supernova explosion. when the emitted gases cool down.
Grains of dust produced by stars can become even grow more when they spend time in the dense parts of the interstellar medium within molecular clouds. In these environments, the grains can stick together or collect additional atoms from gas a-round them. They also make other compounds easier to make, including some of the more complex molecules we discussed earlier.
The surfaces of the grains of dust (see Cosmic Dust), which would appear very large if you were an atom, provide “nooks and crannies” in which these atoms can dwell long enough to find partners and form molecules. Finally, the dust grains are covered with ice. The presence of dust protects the molecules in the clouds from ultraviolet radiation and the cosmic rays that would break them down.
When stars finally begin to form in the cloud, they heat the grains and evaporate the ice. The gravitational–attraction of newly formed stars also increases the density of the surrounding cloud material. Many other chemical reactions take place on the surfaces of the grains in the surrounding gas. newly formed stars, and it is in these areas that organic molecules form. These molecules can be incorporated into newly formed planetary systems, and the early Earth could have sown in-such a way.
In fact, scientists speculate that some of the Earth’s water may have come from interstellar grains. Latest Observations from space have shown that water is abundant in dense interstellar clouds. Since stars are made from this material, water must be present when solar systems, including our own, are formed. The water in our oceans and lakes may originally come from water trapped in rocky material that built up to form the earth. Alternatively, the water could have been brought to Earth when asteroids and comets (formed from the same cloud that formed the planets) later struck it. Scientists estimate that one comet impact every thousand years for Earth’s first billion years would have been enough to explain the water we see today. Of course, both springs may have contributed to the water we enjoy drinking and swimming today.
Any interstellar grain built into newly formed stars (rather than the cooler planets and smaller bodies around them) is destroyed by their high temperatures, but eventually each new generation of stars evolves into red giants with their own stellar winds. Some of these stars also become supernovae and explode. Therefore, the cosmic material recycling process can begin all over again.
