Look into the night sky and you will catch a glimpse of the stars from hundreds of billions of galaxies. Some galaxies swirl blue disks like our own Milky Way, others are red spheres or malformed, lumpy root or something in between. Why the different configurations? It turns out that the shape of a galaxy tells us something about the events of the galaxy’s ultra-long life.
At the very 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 egg yolk, surrounded by a disk of gas and stars ̵
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In theory, disk galaxies are originally formed from clouds of hydrogen. Gravity pulls the gas particles together. Like that hydrogen atoms approach, the cloud begins to rotate, and their collective mass increases, causing their gravity to rise. Eventually, gravity causes the gas to collapse into a swirling disk. Most of the gas is in the rim where it gives birth to star formation. Edwin Hubble, who only a century ago confirmed the existence of galaxies beyond our own, called disk galaxies late-type galaxies because he suspected that their shape meant they were formed later in the history of the universe, according to NASA.
Alternatively, elliptical galaxies – what Hubble called early galaxies – appear to be older. Instead of rotating, like disk galaxies, stars in elliptical galaxies have more random motion, according to Robert Bassett, an observational astrophysicist studying galaxy evolution at Swinburne University in Melbourne, Australia. Elliptical galaxies are thought to be the product of a galaxy fusion. When two galaxies with the same mass merge, their stars begin to pull together with gravity, disrupting the stars’ rotation and creating a more random orbit, Bassett said.
Not every fusion results in an elliptical galaxy. The Milky Way is actually quite old and large, but maintains its disk shape. It has added its mass by simply pulling dwarf galaxies, which are much smaller than our home galaxy, and collecting free gas from the universe. But Andromeda, our disk-shaped sister galaxy, is actually heading directly toward the Milky Way, Bassett told WordsSideKick.com. So for billions of years, the two spiral galaxies could merge, and each of the duo’s star disks will offset the rotation of the other, creating a more random elliptical galaxy.
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These mergers are far from immediate. They take hundreds of millions, even billions of years. In fact, there are ongoing mergers that are moving so slowly – from our perspective – that they seem static. “They have basically been in the exact same state, unchanged throughout human civilization,” Bassett said. Hubble gave these galaxies their own classification – irregular galaxies. To look at them, “they are usually a multi-component mess,” Hummels said. “Irregular galaxies just look like a large train wreck,” Bassett added.
Finally, a less common form, lenticular galaxies, appears to be a mixture of an elliptical and a disk galaxy. It may be, Bassett said, that when a disk galaxy uses all of its gas and cannot form any new stars, the existing stars begin to interact. Their gravitational pull on each other creates a shape that resembles a lens – kind of elliptical, but still a rotating disk.
What scientists have so far uncovered about galaxies and their 3D shapes has been deduced using thousands of 2D images and by relying on other properties, such as galaxy color and motion, to fill in the blanks, Bassett said.
For example, the younger age of disk galaxies is confirmed by their blue color. Blue stars are generally larger and they burn faster and warmer (blue light has a higher frequency and is thus more energetic than red light). Meanwhile, elliptical galaxies are filled with older stars – called red dwarves – which does not burn as hot or fast.
Despite everything we have learned about the massive celestial structures around us, there is still so much we do not know, Hummels said.
“Galaxy formation and evolution is one of the biggest open questions in astronomy and astrophysics,” Hummels said.
Originally published on WordsSideKick.com.