The Milky Way Galaxy is a barred spiral galaxy located in the Local Group of galaxies, which includes over 50 galaxies. It is estimated to be around 120,000 light-years in diameter and contains between 100 and 400 billion stars, with estimates currently leaning toward the higher end of that range. The galaxy is named after the milky appearance of the band of stars visible in the night sky, which is caused by the light from its stars and gas clouds.
The estimated number of galaxies in the observable universe is currently around 2 trillion (2 x 1012) galaxies, based on the latest observations and models. However, this number is constantly being refined and updated as new data becomes available and our understanding of the universe improves.
Every star we see with our naked eyes is in the Milky Way galaxy. The only object you can see in the sky outside of the Milky Way with your naked eyes is the Andromeda Galaxy.
The galactic center of the Milky Way is about 25,000 light-years away from our Sun.
It’s important to note that the observable universe is just a portion of the entire universe, and there may be many more galaxies beyond our current detection limits. Additionally, the concept of a “galaxy” is not always well-defined and can depend on various factors such as size, shape, and level of activity.
Earth is located roughly halfway to the edge of the Milky Way, at a distance of about 26,000 light years from the center. We reside in a feature known as the Orion Spur (sometimes also called the Orion Arm), which is an offshoot between the larger Sagittarius and Perseus Arms that lie inwards and outwards of our location.
Inside those arms, stars, dust, and gas are more tightly packed than in the more loosely filled areas of the galactic disk, and this increased density triggers more intense star formation. As a result, stars in the galactic disk tend to be much younger than those in the bulge. “Spiral arms are like traffic jams in that the gas and stars crowd together and move more slowly in the arms. As material passes through the dense spiral arms, it is compressed and this triggers more star formation,” Denilso Camargo, of the Federal University of Rio Grande, do Sul in Brazil, said in a statement. The Milky Way currently has four spiral arms, according to the National Science Foundation (NSF). There are two main arms — Perseus and Scutum-Centaurus — and the Sagittarius and Local Arm, which are less pronounced. Scientists still discuss the exact position and shape of these arms using Gaia data.
The biggest known planet in the Milky Way may be HD 100546 b, which is a very large gas giant in the process of forming with a diameter roughly 6.9 times that of Jupiter, or 77 times that of Earth. The radius measurement is highly uncertain, as some of the material surrounding the planet may be masquerading as being part of the planet itself.
The largest planets whose sizes are known for certain are HAT-P-67 b and XO-6b, both with diameters around 2.1 times that of Jupiter. Both of these planets have had their diameters measured directly as they transit their parent star.
In the past, astronomers believed that all of the stars in the universe were contained inside the Milky Way. Following the Great Debate, observations by Edwin Hubble proved that the Milky Way is in fact just one of the billions of galaxies in the universe.
The Milky Way as a whole is moving through space at a rate of approximately 600 kilometers (373 miles) per second. It will collide with the Andromeda Galaxy in around 3.75 billion years.
The Milky Way and the Andromeda Galaxy are part of the Local Group. These two are the biggest members of this group. There are more than 30 other close-bounded galaxy members aside from them. Inside the Milky Way are at least 100 billion planets and anywhere from 200 to 400 billion stars.
Our planetary system is the only one officially called the “solar system,” but astronomers have discovered more than 3,200 other stars with planets orbiting them in our galaxy. Our galaxy is, on average, a hundred thousand light-years across but only a thousand light-years thick. Within this flattened (though somewhat warped) disc, the sun, and its planets are embedded in a curving arm of gas and dust, putting the solar system about 26,000 light-years away from the galaxy’s turbulent core.
A bulge of dust and stars swaddles the galactic center, looking like a dollop of whipped cream plopped on both sides of a pancake. The solar system is zooming through interstellar space at around 500,000 miles an hour. Even at that rate, it takes about 250 million years to travel once around the Milky Way. The last time our 4.5-billion-year-old planet was in this same spot, continents fit together differently, dinosaurs were just emerging, mammals had yet to evolve, and the most profound mass extinction in the planet’s history—an event called the Great Dying—was in progress.
Called Sagittarius A*, the supermassive black hole weighs in at more than four million times the mass of the sun. We’ve never seen this object directly—it’s hidden behind thick clouds of dust and gas. But astronomers have been able to follow the orbits of stars and gas clouds near the galactic center, which allowed them to infer the mass of the cosmic heavyweight hiding behind the curtain.
It’s thought that supermassive black holes are parked in the cores of most galaxies, and some are feeding on nearby matter so greedily that they shoot out jets of powerful radiation visible from millions of light-years away. The Milky Way is embedded in a clump of dark matter that is far larger and more massive than the galaxy itself. In the late 1960s, astronomer Vera Rubin inferred the presence of these invisible halos around galaxies when she observed that stars near the edge of Andromeda were whipping around the galaxy’s center at speeds that should send them flying off into space.
And yet, they weren’t, meaning that some sort of cosmic glue held everything together. That glue, we now know, is dark matter. The study of the Milky Way and its properties is an active area of research in astrophysics, with astronomers using a variety of observational and theoretical techniques to learn more about the galaxy’s structure, formation, and evolution.
Further Reading
Sources
- “10 Facts about the Milky Way” The University of Maine
- “Milky Way galaxy: Everything you need to know about our cosmic neighborhood” Space
- “10 Interesting Facts About the Milky Way” Universe Today
- “40 Interesting Facts About the Milky Way Galaxy” Earth Eclipse
- “Facts about the Milky Way” The Planets
- “The Milky Way Facts” The Nine Planets
- “10 Weird Things You (Probably) Didn’t Know About the Milky Way” National Geographic
