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The universe contains about 100 billion galaxies, and no two are exactly alike. In some, stars whirl around the center in neat, circular orbits, making the whole galaxy seem to spin in unison like a pinwheel. In others, the stars move in their own disorderly routes, sometimes in long, stretched ellipses and sometimes on wildly tilted orbits around the galaxy’s center of gravity.
Disorderliness comes with age, at least for stars
Croom and his colleagues used a telescope at Siding Spring Observatory to survey about 3,000 galaxies. The survey, using an instrument called SAMI (or the Sydney-Australian-Astronomical-Observatory Multi-object Integral-Field Spectrograph), collected data on the galaxies’ age and mass, as well as whether they were part of crowded galaxy clusters or more solitary. SAMI also measured the motion of the stars in each galaxy, so Croom and his colleagues could tell whether the galaxy was a neatly spinning disk, or a messy bunch of stars doing their own things.
The astronomers were interested in what made some galaxies orderly and others more free-spirited: the galaxy’s own mass, interference from neighboring galaxies’ gravity, or something else. Their answer turned out to be age: galaxies with older stars, on average, tended to be more disorderly than galaxies with younger populations of stars.
What’s important here isn’t the age of the galaxy itself, but how long ago it stopped forming new stars. In a galaxy that’s still producing new stars (in stellar nurseries like our nearby Orion Nebula), new stars are born even while other stars are aging and dying, so the average age of the galaxy’s stars stays fairly young. But when a galaxy stops producing new stars — often because it runs out of material — there are no new young stars to replace the aging ones, so the galaxy ends up with a population of older stars.
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Galaxies get more chaotic as their stars age, a new study suggests.
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