Scientists believe that they may have finally found the answer as to what causes galaxies to die. In a new study, researchers have revealed that dead galaxies, or those that no longer produce stars, are likely killed because they have been strangled to death.

Astronomers know that there are two main categories of galaxies. Those that produce new stars such as our very own Milky Way and those that do not. Until now, however, scientists are not certain what causes galaxies to stop forming new stars.

One of the theories as to what switches off star formation in galaxies is the process known as strangulation, through which the cold gas necessary for star formation is gradually choked off. The other theory posits that gas is suddenly removed from a galaxy, possibly pulled by another galaxy's gravitational  pull.

In a new study published in the journal Nature on May 14, University of Cambridge astronomer Yingjie Peng and colleagues compared 3,905 star-forming galaxies with 22,618 galaxies that no longer produce stars and found evidence that the cause of most of the deaths of the galaxies was strangulation.

Peng, however, said that although they have found that strangulation is the cause of most galaxy' death, there is still a need to understand the mechanism behind strangulation. It is possible that nearby galaxies contribute to the depletion of a galaxy's gas supply essential for star formation.

Stars are mostly made up of helium and hydrogen. The researchers looked at the concentration of metals that are heavier than helium and hydrogen, which form when stars fuse the two elements into heavier elements. They found that dead galaxies contain higher amounts of metals.

When a galaxy's supply of gas is choked off, there are still some gas left inside and this can be used in star formation. The stars can then form elements that are heavier than helium and hydrogen.

The researchers revealed that strangulation takes about 4 billion years to stump a galaxy's ability to form new stars, which is consistent with the age difference between live and dead galaxies.

"Here we report an analysis of the stellar metallicity (the fraction of elements heavier than helium in stellar atmospheres) in local galaxies, from 26,000 spectra, that clearly reveals that strangulation is the primary mechanism responsible for quenching star formation, with a typical timescale of four billion years, at least for local galaxies with a stellar mass less than 1011 solar masses," the researchers wrote in their study. 

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