They have a distinctive shape with spiral arms in a relatively flat disk and a central "bulge". The bulge has a large concentration of stars. The arms and bulge are surrounded by a faint halo of stars. The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. Our Milky Way Galaxy is a spiral galaxy. In barred spirals, the spiral arms of the galaxy appear to spring out of the ends of the bar. Examples of spiral galaxies imaged by the Hubble Space Telescope.
As their name suggests, elliptical galaxies are round or oval, with stars distributed fairly uniformly throughout. They have a bulge and halo, like spiral galaxies, but don't have the flat disk of stars.
The stars in ellipticals tend to be older. Irregular galaxies have no identifiable shape or structure to them. They are often chaotic in appearance, without a bulge or any trace of spiral arms. Also, not all mergers lead to elliptical galaxies- the Milky Way, for example, has merged with multiple dwarf galaxies yet still maintained its disk shape. The last type of galaxy- the lenticular variety- is a cross between an elliptical and disk galaxy.
So far, theorists suggest that when disk galaxies use up all their gas and can't form any new stars, their existing ones start to interact. As such, they tug on each other to create a shape that looks like a lentil- an elliptical but rotating disk. Our understanding of galaxies so far has come from thousands of 2D images and other properties, including galaxy color and motion. As such, there are still sizable gaps in our knowledge of how they form and evolve, making them one of the biggest open questions in the field of astronomy and astrophysics.
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Elliptical galaxies are thought to be a product of a galaxy merger. When two galaxies of equal mass merge, their stars start to tug on one another with gravity, disrupting the stars' rotation and creating a more random orbit, Bassett said. Not every merger results in an elliptical galaxy. The Milky Way is actually quite old and large, but maintains its disk shape.
It's been adding to its mass by simply drawing in dwarf galaxies, which are much smaller than our home galaxy, and collecting free gas from the universe. However, Andromeda, our disk-shaped sister galaxy, is actually headed straight for the Milky Way, Bassett told Live Science. So billions of years from now, the two spiraling galaxies could merge and each of the duo's starry disks will offset the other's rotation, creating a more random elliptical galaxy.
Related: The 15 weirdest galaxies in our universe. These mergers are far from instantaneous. 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 appear static. Hubble gave these galaxies their own classification — irregular galaxies. To look at them, "they are usually a mess with multiple components," Hummels said. Finally, a less common shape, lenticular galaxies seem to be a mix between an elliptical and a disk galaxy.
It may be, Bassett said, that when a disk galaxy uses up all its gas and can't form any new stars the existing stars begin to interact.
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