Lesson 51: Galactic Recycling
In this lesson,
we will look at how galactic recycling occurs through the star-gas-star cycle.
We will also see where in the galaxy most star-forming activity tends to occur.
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.