Galaxies: Cities of Stars
Galaxies are like cities made of oodles of stars, gas, and dust bound together by gravity. These beautiful cosmic structures come in many shapes and sizes. Though there are a slew of galaxies in the universe, there are only a few we can see with the unaided eye or backyard telescope.
How many types are out there, how’d so many of them wind up with weird names, and how many stars live inside them? Hold tight while we explore these cosmic metropolises.
Galaxies come in lots of different shapes, sizes, and colors. But astronomers have noticed that there are mainly three types: spiral, elliptical, and irregular.
Spiral galaxies, like our very own Milky Way, look similar to pinwheels! These galaxies tend to have a bulging center heavily populated by stars, with elongated, sparser arms of dust and stars that wrap around it. Usually, there’s a huge black hole hiding at the center, like the Milky Way’s Sagittarius A* (pronounced A-star). Our galactic neighbor, Andromeda (also known as Messier 31 or M31), is also a spiral galaxy!
Elliptical galaxies tend to be smooth spheres of gas, dust, and stars. Like spiral galaxies, their centers are typically bulges surrounded by a halo of stars (but minus the epic spiral arms). The stars in these galaxies tend to be spread out neatly throughout the galaxies and are some of the oldest stars in the universe! Messier 87 (M87) is one example of an elliptical galaxy. The supermassive black hole at its center was recently imaged by the Event Horizon Telescope.
Irregular galaxies are, well … a bit strange. They have one-of-a-kind shapes, and many just look like messy blobs. Astronomers think that irregular galaxies’ uniqueness is a result of interactions with other galaxies, like collisions! Galaxies are so big, with so much distance between their stars, that even when they collide, their stars usually do not. Galaxy collisions have been important to the formation of our Milky Way and others. When two galaxies collide, clouds of gas, dust, and stars are violently thrown around, forming an entirely new, larger one! This could be the cause of some irregular galaxies seen today.
Now that we know the different types of galaxies, what about how many stars they contain? Galaxies can come in lots of different sizes, even among each type. Dwarf galaxies, the smallest version of spiral, elliptical, and irregular galaxies, are usually made up of 1,000 to billions of stars. Compared to our Milky Way’s 200 to 400 billion stars, the dwarf galaxy known as the Small Magellanic Cloud is tiny, with just a few hundred million stars! IC 1101, on the other hand, is one of the largest elliptical galaxies found so far, containing almost 100 trillion stars.
Ever wondered how galaxies get their names? Astronomers have a number of ways to name galaxies, like the constellations we see them in or what we think they resemble. Some even have multiple names!
A more formal way astronomers name galaxies is with two-part designations based on astronomical catalogs, published collections of astronomical objects observed by specific astronomers, observatories, or spacecraft. These give us cryptic names like M51 or Swift J0241.3-0816. Catalog names usually have two parts:
A letter, word, or short acronym that identifies a specific astronomical catalog.
A sequence of numbers and/or letters that uniquely identify the galaxy within that catalog.
For M51, the “M” comes from the Messier catalog, which Charles Messier started compiling in 1771, and the “51” is because it’s the 51st entry in that catalog. Swift J0241.3-0816 is a galaxy observed by the Swift satellite, and the numbers refer to its location in the sky, similar to latitude and longitude on Earth.
There’s your quick intro to galaxies, but there’s much more to learn about them. Keep up with NASA Universe on Facebook and Twitter where we post regularly about galaxies.
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