Some galaxies are similar to the Milky Way, but some are quite different.
MESSIER 81 GALAXY
A composite image of the Messier 81 (M81) galaxy shows what astronomers call a "grand design" spiral galaxy, where each of its arms curls all the way down into its center. Located about 12 million light-years away in the Ursa Major constellation, M81 is among the brightest of the galaxies visible by telescope from Earth.
Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also contain supermassive black holes at their centers. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion stars that spin around Sagittarius A*, a supermassive black hole that contains as much mass as four million suns.
The deeper we look into the cosmos, the more galaxies we see. One 2016 study estimated that the observable universe contains two trillion—or two million million—galaxies. Some of those distant systems are similar to our own Milky Way galaxy, while others are quite different.
Types of galaxies
types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.
Before the 20th century, we didn't know that galaxies other than the Milky Way existed; earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere.
SPIRAL GALAXY
A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. That spinning motion, at speeds of hundreds of kilometers a second, may cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel. Our Milky Way, like other spiral galaxies, has a linear, starry bar at its center.
LENTICULAR GALAXY
Lenticular galaxies, such as the iconic sombrero galaxy, sit between elliptical and spiral galaxies. They're called “lenticular” because they resemble lenses: Like spiral galaxies, they have a thin, rotating disk of stars and a central bulge, but they don't have spiral arms. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space.
ELLIPTICAL GALAXY
Elliptical galaxies are shaped as their name suggests: They are generally round but can stretch longer along one axis than along the other, so much so that some take on a cigar-like appearance. The universe's largest-known galaxies—giant elliptical galaxies—can contain up to a trillion stars and span two million light-years across. Elliptical galaxies may also be small, in which case they are called dwarf elliptical galaxies.
IRREGULAR GALAXY
Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by. They are full of gas and dust, which makes them great nurseries for forming new stars.
Galaxy origins
The universe's first stars ignited some 180 million years after the big bang, the explosive moment 13.8 billion years ago that marks the origins of the universe as we know it. Gravity had sculpted the first galaxies into shape by the time the universe turned 400 millions years old, or less than 3 percent of its current age.
Astronomers now think that nearly all galaxies—with possible exception—are embedded in huge haloes of dark matter. Theoretical models also suggest that in the early universe, vast tendrils of dark matter provided normal matter the gravitational scaffold it needed to coalesce into the first galaxies.