Is our universe dying? It might be, because something is stopping galaxies from producing new stars.
Starbirth region in the Carina Nebula [Image by the Hubble Telescope]
Consider this to be a cosmic murder mystery, with astronomers as some kind of galactic sleuths. More and more galaxies are now seemingly losing the ability to form new stars, which is certainly something of a puzzle. Just 10 billion years ago, galaxies were birthing new stars left and right. There must be something that's slowly but steadily sapping these galaxies of their ability to form stars—and astronomers have found what the culprit is.
Astronomers are researching a galaxy about 310 million light years away, and it's serving as a kind of star witness as to what is killing these galaxies. The culprit, apparently, is something that removes gases from galaxies, thus sapping it of its ability to create stars.
So whodunnit? Astronomers point the finger at a process called ram-pressure stripping, identified to be a leading cause of death among galaxies.
A computer simulated view of a galaxy cluster [Image by NASA, ACS Team, Rychard Bouwens (UCO/Lick Obs.)]
When we look up at the stars in the sky, we don't really notice any changes. Of course, this isn't representative of the truth. Stars and galaxies do in fact undergo several different changes during their lifetimes. One of these changes, apparently, is losing their ability to create stars.
So now that we know what's taking this ability away from galaxies, what exactly is ram-pressure stripping, anyway? First, let's look at the positioning of galaxies in their places in the universe. Usually, the gravitational attraction between galaxies pulls them together to form a cluster. However, there are still spaces in between these galaxies, which are filled with hot gas.
Galaxies don't stay still, hanging motionless in space. They hurtle through the universe, though it doesn't feel that way to us. Thus, because these galaxies continue moving, the hot gas in the spaces between them act like a headwind. This headwind is then able to strip galaxies of their gas, thus taking away their ability to form new stars. Dense interstellar gas and dust, which form molecular clouds, are the stuff of starbirth. If the interstellar gas isn't dense enough for viable molecular clouds, no new stars will form.
Will our own galaxy lose the ability to form new stars as well?
Another thing that astronomers have dubbed “strangulation” can also keep new stars from forming. Think of a galaxy that is entering a new cluster for the first time. The cluster's dark matter halo tugs on the new galaxy's gravity, thus pulling gas from the galaxy itself. This leaves the galaxy without the ability to form new stars of its own.
Astronomers have a name for these infertile galaxies—”red and dead”. Red, because all the stars in that galaxy are already old red stars. There are a number of other processes that can strip star-forming gas from galaxies, so it's not such a surprise after all that so many are “red and dead”.
All we can do from where we stand, however, is observe as these galaxies lose their interstellar gas. Fortunately, these findings give us an insight into the evolution of galaxies as they move across the universe.
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