Look at the night sky and you will see stars from hundreds of billions of galaxies. Some galaxies rotate blue disks like our own Milky Way, others are red spheres or misshapen, clumpy messes or something in between. Why the different configurations? It turns out that the shape of a galaxy tells us about events in the very long life of that galaxy.
At the basic level there are two classifications for galaxy shapes: disk and elliptical. A disk galaxy, also called a spiral galaxy, is shaped like a fried egg, said Cameron Hummels, theoretical astrophysicist at Caltech. These galaxies have a more spherical center, like the yolk, surrounded by a gas disk and stars – egg white. The Milky Way and our closest galaxy neighbor Andromeda fall into this category.
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In theory, disk galaxies are first formed from hydrogen clouds. Gravity drawn gas particles together. Just like hydrogen the atoms approached, the clouds began to spin and their evil increased, causing their gravitational force to rise as well. Eventually, gravity causes the gas to drop on a swirling disk. Most of the gas is on the side, where it feeds star formation. Edwin Hubble, who confirmed the existence of galaxies beyond us just a century ago, called disk galaxies the types of galaxies that were late because he suspected their shape meant they were formed later in the history of the universe , according to NASA.
Alternatively, elliptical galaxies – which Hubble calls the earliest type of galaxies – appear older. Instead of rotating, like disk galaxies, the stars in elliptical galaxies have a much more random motion, according to Robert Bassett, an astrophysic observatory studying the evolution of the galaxy at Swinburne University in Melbourne, Australia. Eliptical galaxies are thought to be a product of a galaxy integration. When two galaxies are evenly spaced, their stars begin to tug at each other with gravity, disrupting the rotation of the stars and creating a more random orbit, Bassett said. .
Not all merging results in an elliptical galaxy. The Milky Way is really old and big, but retains its disk shape. It adds to its mass simply by drawing on dwarf galaxies, which are smaller than our home galaxy, and collecting free gas from the universe. Andromeda, our disk-shaped sibling galaxy, is actually heading straight for the Milky Way, Bassett told LiveScience. So billions of years from now, the two rotating galaxies can merge and each of the duo’s star disks will alternate rotating the other, creating a more random elliptical galaxy.
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These mergers are far from immediate. They last for hundreds of millions, even billions of years. In fact, there are ongoing associations that are slowly moving – from our point of view – that they appear static. “They are usually in the exact state, unchanged for all human civilization,” Bassett said. Hubble gave these galaxies their own classification – irregular galaxies. To look at them, “they are usually a mess with a lot of parts,” Hummels said. “Irregular galaxies are like a huge train wreck,” Bassett added.
Finally, a less common shape, lenticular galaxies seem to be a mixture between an elliptical and a disk galaxy. It may be, Bassett said, that when a disk galaxy uses all the gas and cannot produce any new stars the existing stars begin to interact. Their gravitational tug on each other creates a shape that looks like a lentil – a kind of elliptical but circular disk.
What scientists have discovered so far about galaxies and their 3D shapes has been discovered using thousands of 2D images and by relying on other features, such as galaxy color and motion , to fill in the blanks, Bassett said.
For example, the younger age of galaxies disks is evidenced by their blue color. Blue stars are generally larger, and they are faster and warmer (blue light has a higher frequency and is thus more vibrant than red light). Meanwhile, elliptical galaxies are full of older stars – called red dwarfs – That does not burn as hot or fast.
However, despite all we have learned about the massive celestial structures around us, there is still much we do not know, Hummels said.
“The formation of the Galaxy and evolution is one of the biggest open questions in the field of astronomy and astrophysics,” Hummels said.
Originally published in Live Science.