Lesson Objectives:- Galactic recycling
- Star-gas-star cycle
- Atomic hydrogen gas
- Where do stars tend to form in our galaxy?
As we learned previously, the disk of the galaxy contains the interstellar medium, clouds of interstellar gas and dust from which new generations of stars will be born.
In the early universe, only the two lightest elements, hydrogen and helium, existed. Over billions of years, stars have produced heavier elements and have died, dispersing those heavy elements into the existing interstellar gas. Today, about 2% of the galaxy's gas is made up of these heavier elements while the remaining 98% still consists of hydrogen (about 71%) and helium (about 27%).
Galactic recycling happens through a series of steps called the star-gas-star cycle.
We know that stars are born when gravity causes clumps of gas to collapse in molecular clouds. These stars then shine for millions or billions of years through nuclear fusion, gradually returning material to the interstellar medium through the stellar winds they produce. Eventually, in their deaths, they release a much greater amount of material. For high-mass stars, this material is released through a supernova, and in the case of low-mass stars, they shed their outer layers in a planetary nebula.
For massive stars, the supernovae (super-no-vee) create hot bubbles in the interstellar medium as they blow outwards like a storm front. Eventually, the expanding hot gas bubble will cool, slow down, and merge with the surrounding interstellar medium.
The gas blown out by supernovae is a small fraction of all of the gas in the galaxy. Most of the gas in the galaxy is cool enough that the hydrogen in it is neutral rather than ionized. As a result, the most common form of interstellar gas is referred to as atomic hydrogen gas. It contains mostly hydrogen atoms along with some helium and 2% heavier elements. When the gas from hot bubbles cools, it becomes part of the atomic hydrogen gas in the galaxy.
The atomic hydrogen stage of the star-gas-star cycle lasts for millions of years until the gas cools enough for the hydrogen to clump into molecules, including H2, or molecular hydrogen.
Molecular clouds are heavy and dense compared to other interstellar gas, so they settle toward the central layers of the disk. This is why you see the dark bands running through the middle of the Milky Way.
Inside of these molecular clouds, new stars can form, beginning the whole cycle anew.
Since molecular clouds are dark and hard to see, astronomers look for hot, massive stars and ionization nebulae (neh-byuh-lee) to give them an idea of where stars are actively forming.
Ionization nebulae are colorful blobs of glowing gas. They glow because their atoms are ionized as they absorb ultraviolet light from nearby stars. The colors in the gas are based on which elements are being ionized.
Hot, massive stars and ionization nebulae are mostly found in the spiral arms of the galaxy, since that is where disturbances called spiral density waves have packed gas clouds more densely together, making star formation more likely.