Buzz
The Andromeda Galaxy, the closest large galaxy to our Milky Way, is a colossal structure, boasting about three times the number of stars as the Milky Way. It is clearly visible to the naked eye from Earth and resides in the Andromeda constellation, which is where it gets its name.
More than just a stunning object in the sky, the Andromeda Galaxy is a dynamic and active region of space. It is even predicted to collide with the Milky Way in the distant future. If you're intrigued by that, you'll be captivated by these ten fascinating facts about the Andromeda Galaxy.
10. Often Referred to as Messier 31
Our nearest neighboring galaxy is also known as Messier 31 or M31. This name was given by Charles Messier, the French astronomer who cataloged this massive galaxy. Messier cataloged a multitude of objects in the Northern Hemisphere skies, which are collectively referred to as the Messier objects or the Messier Catalog. (It’s worth noting that Messier wasn’t always the first to spot the objects in his catalog.)
In 1757, Messier began his search for Halley’s Comet, but a miscalculation led him to a different part of the sky. There, he discovered a nebula, which later became the first entry in his catalog: M1, also known as the Crab Nebula. In 1764, Messier added M31 to the catalog. By the end of that year, he had documented 38 objects, and by 1781, he had recorded a total of 103 objects, 40 of which he had discovered himself.
9. Named After The Andromeda Constellation
If you gaze into the northern night sky between the “W” shape of Cassiopeia and the Great Square of Pegasus, you’ll find the Andromeda constellation. This star pattern is named after the mythological princess Andromeda, the wife of the Greek hero Perseus. Cataloged by the Greek astronomer Ptolemy in the second century, it is also known as Chained Maiden, Persea, or Cepheis.
The Andromeda constellation hosts a variety of other deep-sky objects. Positioned outside the galactic plane, Andromeda does not feature any clusters or nebulae of the Milky Way. However, it is home to other visible galaxies, with the Andromeda Galaxy being the most famous. This galaxy, which shares its name with the constellation, is one of the most notable objects in the night sky and can be seen with the naked eye.
8. Bigger Than The Milky Way
In astronomy, distances are often measured in light-years, but some astronomers prefer to use parsecs. For larger distances, they turn to kiloparsecs (1,000 parsecs) and megaparsecs (one million parsecs). The Milky Way spans about 100,000 light-years, or 30 kiloparsecs, across. While this might seem vast, it’s small compared to some other galaxies.
The Andromeda Galaxy has an estimated diameter of 220,000 light-years, more than twice that of the Milky Way. It is the largest galaxy in the Local Cluster. If it were bright enough, it would appear larger than the Moon in the sky (as shown above), despite being much farther away. The galaxy is 2.5 million light-years from Earth, while the Moon is only 384,400 kilometers (238,900 miles) away—giving you a clearer picture of just how immense it truly is.
7. One Trillion Stars
While the Milky Way is estimated to contain between 100 billion and 400 billion stars, the Andromeda Galaxy is home to roughly one trillion stars. As part of this vast star population, NASA’s Hubble Space Telescope discovered a unique and rare group of hot, bright stars.
Blue stars are typically young and hot, but the blue stars within the Andromeda Galaxy are older, Sun-like stars that have shed their outer layers, revealing their blue-hot cores. These stars are found throughout the galaxy's center and are most visible in ultraviolet light.
6. It Has A Double Nucleus
An intriguing feature of the Andromeda Galaxy is its double nucleus. Each of the two light peaks holds millions of densely packed stars and they are only five light-years apart. The first nucleus is referred to as P1, while the second is named P2.
Some astronomers have concluded that the nucleus isn’t truly two separate entities, but rather one asymmetric cluster of stars orbiting a supermassive black hole. The stars in P1 follow elliptical orbits around the black hole and mostly shine in the outer regions, causing the area to appear bright. The second nucleus forms because stars group together as they approach the black hole.
5. Will Collide With The Milky Way
A monumental cosmic event is on the horizon, as the Andromeda Galaxy is racing toward the Milky Way at a speed of 400,000 kilometers per hour (250,000 mph), fast enough to circle the Earth in just six minutes. The collision is expected to occur in about 3.75 billion years. What will become of Earth after this collision?
Experts believe that Earth will likely survive the cosmic collision. Since galaxies are mostly empty space, our planet should remain largely unaffected. However, the event will create a spectacular show, visible throughout the sky for many years. The two galaxies will continue to interact until their central black holes draw near enough to merge. Once merged, our solar system will be part of a different type of galaxy—an elliptical galaxy. If it weren't for the Sun swallowing the Earth in about five billion years, the elliptical galaxy's sky would shine brightly at night.
4. Magnitude of 3.4
In astronomy, absolute magnitude refers to the apparent magnitude an object would have if it were located at a standard luminosity distance from Earth. This measure enables comparisons of an object's brightness without factoring in its distance.
The Andromeda Galaxy has an apparent magnitude of 3.4, making it one of the brightest Messier objects. On a clear, moonless night, the Andromeda Galaxy is visible to the naked eye, even in areas with moderate light pollution. When photographed through a large telescope, it may appear more than six times as wide as the Moon, but only the central, brighter region is visible with binoculars or smaller telescopes.
3. Was Once Mistaken for a Nebula
A nebula is a vast cloud of gas, dust, hydrogen, helium, and plasma, often serving as the birthplace of stars. Distant galaxies were frequently mistaken for such expansive clouds. In 1924, astronomer Edwin Hubble revealed that the so-called spiral nebula Andromeda was actually a galaxy, proving that the Milky Way was not the only galaxy in the universe.
Hubble discovered several stars in the Andromeda Galaxy, including Cepheid variable stars, whose brightness fluctuates and can be used to measure distance. By determining the distance to these stars, Hubble calculated that the Andromeda Galaxy was 860,000 light-years away, more than eight times farther than the furthest stars in the Milky Way. This confirmed that Andromeda was indeed a galaxy, not a nebula as initially believed. Hubble later went on to document numerous other galaxies.
2. Home to 450 Globular Clusters
Globular clusters are tightly packed groupings of ancient stars, often containing hundreds of thousands, or even millions, of stars. These clusters are crucial for understanding the universe’s age and can also reveal the location of a galaxy's core. While astronomers have cataloged 200 globular clusters within the Milky Way, about 450 have been observed in the Andromeda Galaxy.
The true number of globular clusters in the Andromeda Galaxy could be even higher, as the galaxy's outermost regions remain underexplored. If the globular clusters in Andromeda follow a similar size distribution to those in the Milky Way, the total count might range from 700 to 2,800.
1. Teeming with Black Holes
The Andromeda Galaxy initially had nine known black holes, but that number soared to 35 by 2013. Astronomers detected 26 new black hole candidates, making it the largest discovery of such objects outside of our own galaxy. Most of these black holes are five to ten times the mass of our Sun, with seven found within 1,000 light-years of the galaxy's center.
Astronomers predict that many more black holes will be discovered in the Andromeda Galaxy in the future. In 2017, two supermassive black holes were found orbiting each other. They hold the record for the closest-orbiting pair ever observed. These two might violently collide and merge into one massive black hole in less than 350 years. Currently, they are just 0.01 light-years apart, only a few hundred times the distance between Earth and the Sun. The sheer number of black holes in the Andromeda Galaxy, along with their incredible movements, is truly astonishing.
