Like everything else in existence, galaxies have a lifetime. While theirs stretch billions of years longer than ours, eventually, they too come to an end. The question on many scientists minds has been what causes the lives of galaxies to come to an end.
Astronomers now believe that they have solved this mystery: they believe that galaxies are strangled to death and are unable to make new stars.
Among the ways that astronomers classify galaxies, one way is to divide them between live galaxies which have lots of gas, and dead galaxies which have very little gas. Stars need interstellar gas to form, therefore no star formation occurs in the dead galaxies.
Before now, scientists didn’t know what was halting star formation in galaxies, being described as one of the most challenging questions of astronomy in the past two decades.
There are two theories for what may be stopping stars from forming. One is that gas is suddenly removed from galaxy, potentially being taken by the gravitational pull of another galaxy. Another theory, the strangulation theory, states that the supply of gas for new stars is slowly choked off. After examining thousands of nearby galaxies, researchers have found proof that most galaxies die through strangulation.
Stars are mainly made of hydrogen and helium. Through fusion, hydrogen nuclei combine to form helium and this produces energy. Researchers focused on the concentrations of elements heavier than hydrogen and helium – these elements are formed by further fusion of lighter elements.
It was found that dead galaxies have much more of these heavy metals than live galaxies did. This is consistent with the explanation of how strangulation would cause galaxies to evolve.
After a galaxy’s supply of gases is slowed, there is still some present in order for stars to form. These stars will form the heavy elements. In comparison, when gas is removed from a galaxy, star formation ceases.
According to researchers, the strangulation theory is accurate for any galaxy up to 100 billion times heavier than our Sun, which, accounts for more than 95% of all known galaxies, is a very accurate theory. For larger galaxies, it’s still not known whether strangulation or sudden-removal is responsible for galaxy deaths.
While scientists now know that strangulation is the main cause for how galaxies end, they still need to find the mechanisms behind what causes it to happen. One possibility is that other galaxies may take one galaxy’s gas supply.
The next step in the research is to examine far away galaxies and and see what the universe looked like when it was many years younger (due to the speed of light, when we look at something one hundred light years away, we are seeing what it looked like one hundred years ago). This step will allow us to see more about galactic evolution.
Further advances in technology, such as the Multi-Object Optical and Near-Infrared Spectrograph, and the James Webb Space Telescope, will make this kind of research possible in the upcoming years.