Facts about the Milky Way
We kick off our guide with some cool Milky Way facts before diving deeper into our cosmic neighborhood.
- About 90% of the Milky Way is made up of dark matter. Dark matter is invisible. However, its presence was inferred by scientists because of the gravitational effects on galaxies that seem to be from an unseen source.
- 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 oldest star in the Milky Way galaxy is HD 140283. Also known as the Methuselah star, it is estimated to be at least 13.6 billion years old.
- 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 surface brightness of the Milky Way is relatively low. That means it is difficult to see from any brightly lit urban or suburban location. Still, it is very prominent when the Moon is below the horizon.
- If the Milky Way were reduced in diameter to a width of 100 meters, the solar system would be no more than 1 millimeter in width.
- Inside the Milky Way are at least 100 billion planets and anywhere from 200 to 400 billion stars.
- About 17 billion exoplanets in the Milky Way lie in the habitable zone of their planetary systems. In 2020, it is estimated that the galaxy has more than 300 million habitable planets.
- Surrounding the Galactic disk is a spheroidal halo of old stars and globular clusters. About 90% of which lie within 100,000 light-years of the Galactic Center.
- 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.
- The supermassive black hole in the center of the Milky Way is called Sagittarius A*. It is as massive as 4 million Suns.
The Milky Way
Type: Barred Spiral Galaxy
Size: About 150,000 light-years across
Mass: 1.5 trillion solar masses
Number of Stars: About 100 to 400 billion
The Milky Way is our home galaxy. The Sun is just one of the billions of stars that exist in this galaxy. From here on Earth, the part of it that we see looks like a milky band of light across the sky. We do not know exactly how many stars are in our galaxy. However, it is estimated to be around billions. The number of planets is believed to be just as many too.
The Milky Way is a barred spiral galaxy. Aside from the spiral arms, it has a prominent bar structure in the center. About 90% of it is made up of dark matter, an unknown and invisible component that glues the galaxy together. A very massive black hole lies in its core too.
For a long time, it was thought that the Earth was the center of the solar system. Similarly, it was once believed that the solar system lies near the center of the Milky Way. It turned out, however, that we are located near the edge of the galaxy, in the Orion Arm. The center of the galaxy lies in the Sagittarius constellation.
The Milky Way has 1.5 trillion times the mass of the Sun. Estimates of its size range from 100,000 to more than 200,000 light-years in diameter. It is part of the local neighborhood of galaxies called the Local Group. The smaller members of the Local Group that orbit the Milky Way are called satellite galaxies. Some famous satellites of our galaxy are the Large and Small Magellanic Clouds.
In about 4 billion years from now, our Milky Way galaxy will collide with its largest neighbor, the Andromeda galaxy.
The Milky Way Throughout Time
Our view and understanding of the Milky Way have changed over time.
In the early days, Aristotle proposed that the glowing band was part of our planet’s upper atmosphere. The astronomer Avempace believed that it was composed of many stars. Others had the same idea, stating that these stars are so tightly packed that they look like a continuous cloudy patch.
In 1610, through telescope observation, Galileo Galilei proved that the Milky Way is indeed composed of many stars. In 1750, Thomas Wright proposed that some of the nebula-like objects are actually separate bodies that are similar to the Milky Way. Immanuel Kant supported this idea, calling those nebulae “island universes.”
Charles Messier cataloged a list of these bright nebulae. One of these objects, Messier 31 (Andromeda galaxy), had a supernova in 1917. The American astronomer Heber Curtis was able to observe this event. He also noticed other novas that are unlike those in the Milky Way. He thought they were likely independent objects, supporting Kant’s concept of “island universes.”
Another astronomer, Harlow Shapley, had a different view. He believed that the nebulae-like objects are just within the bounds of our Milky Way galaxy.
The Great Debate
In 1920, the Great Debate was held to discuss the differing views of Harlow Shapley and Heber Curtis. A couple of years later, Edwin Hubble was able to resolve parts of some “nebulae” through a telescope. He was able to estimate the distance of the Andromeda galaxy through Cepheid variables. This proved that it is indeed a separate object very far from us.
Hubble’s observations changed our view of the universe. We now know that it is much much bigger and that the Milky Way is just one of the many galaxies out there.
Milky Way Facts – How Big Is It?
Galaxies are made of gases and dust and there is just no way of knowing where their exact boundaries end.
Measurements of the Milky Way have changed over the years. It was first measured by the American astronomer Harlow Shapley around 1917. Using globular clusters, Shapley calculated the galaxy’s diameter to be around 100,000 light-years. He also determined that the Sun is located about 30,000 light-years from the galactic center. This is close to modern measurements of around 28,000 light-years (8.5 kiloparsecs).
Modern estimates show that the Milky Way is around 150,000 light-years across. However, in 2019, astronomers stated that it is around 256,000 light-years in diameter. It is as massive as 1.5 trillion suns. There are about 100 to 400 billion stars in this galaxy. More than half of them are older than our Sun which is 4.5 billion years old.
The Milky Way might seem very large, but in galactic terms, it is only a medium-sized galaxy. Its largest neighbor, the Andromeda galaxy, is more than 220,000 light-years across. The largest galaxies we know of so far are more than millions of light-years in diameter.
The biggest galaxy that we know of is called IC 1101. It is 50 times larger than the Milky Way. This supergiant elliptical galaxy contains more than 100 trillion stars. Because of that, it is also about 2,000 times greater than the mass of the Milky Way.
Though our galaxy is not the largest, it is also not the smallest. It is still much bigger than Segue 2, the smallest known galaxy. Segue 2 is a satellite of the Milky Way and is known to contain only about 1,000 stars. It is believed that dark matter holds this small galaxy together.
It is said that only 10% of our Milky Way galaxy is visible, while the rest is composed of unseen dark matter. The visible portion is comprised of stars, gasses, and interstellar dust. Hydrogen gas makes up 90% of the interstellar gas and the other 10% is helium.
Knowing the exact number of objects in the Milky Way poses some challenges because of distance. Most of the stars in the galaxy are concentrated near the center and decrease further out.
If there is at least one planet for each star, then our galaxy likely has 100–400 billion planets. Some of them orbit a yellow star like the Sun. It is believed that there are also billions of rogue planets in this galaxy.
Star clusters are common in the Milky Way. More than 1,000 open clusters and 150 globular clusters have been found so far. Some of the most prominent globular clusters are M22, Omega Centauri, and M5, to name a few. Aside from stars, the Milky Way likely contains millions of stellar black holes too.
Milky Way Facts – Features and Characteristics
As a whole, the Milky Way has a low surface brightness. Because of that, more than 30% of the Earth’s population cannot see it from their houses because of artificial lights.
Our night sky is filled with stars from the Milky Way. Some of them are yellow stars in the main sequence just like our Sun. However, the most common ones in our galaxy are red dwarfs. They are less massive than the Sun but they have longer lives. The red dwarfs will be the last stars to shine when all other stars have run out of fuel.
Aside from stars, we can also see galaxies in the night sky. With our naked eye, we can see three galaxies outside our own. These are the Small Magellanic Cloud, the Large Magellanic Cloud, and the Andromeda galaxy.
We cannot see the Milky Way from the outside but we get an idea about its shape through observations. The spiral shape is evidenced by the band-like concentration of stars. The presence of ionized hydrogen called HII regions also supports the spiral shape of our galaxy.
The Milky Way is influenced by its neighbors. Its warped shape is caused by the gravitational influence of the Small and Magellanic Clouds. Our galaxy, in turn, is also disrupting these smaller galaxies.
In 2010, NASA’s Fermi Gamma-Ray Space Telescope discovered a curious structure that lies above and below the center of the Milky Way.
Called the Fermi Bubbles, these two blobs are made up of hot gas, dust, and cosmic rays. This structure is around 50,000 light-years long. We cannot see them directly but they emit stronger gamma rays than any other part of the galaxy.
Much is still unknown about the Fermi Bubbles. However, theories suggest that the structure is likely related to the central black hole of the Milky Way. About 6 million years ago, the black hole probably swallowed massive amounts of matter. This led to large energy jets that caused the said gamma rays and the nearby X-ray sources.
Structure and Galaxy Type
The barred-spiral structure of the Milky Way was confirmed in 2005 through the Spitzer Space Telescope. In galaxies of this type, the bar is believed to pull material from the disk to the center. Because of this, the central bar becomes a nursery for star formation.
The Milky Way is a very big structure. We can look at it in six different parts.
We cannot directly observe the center of the Milky Way because it is obscured by interstellar dust. However, this central region is detectable using radio, infrared, and other nonvisual wavelengths.
Our galaxy’s nucleus is fuelled by Sagittarius A*, a supermassive black hole roughly 4,000,000 times more massive than the Sun.
The Milky Way’s nucleus is surrounded by a big bulge of stars. This bulge is mostly populated with old stars of low metallicity. Aside from stars, there are also globular clusters that make up this spherical bulge.
The galaxy disk is a flat structure of stars that extends as far as 75,000 light-years or more. The space between these stars is filled with gas and dust of the interstellar medium.
Different components can be determined within the Milky Way disk: the thin disk and the thick disk. The thin disk is composed of gas, dust, and younger stars. It contributes more than 90% of the stars in the disk. Meanwhile, the thick disk consists of older stars.
The spiral structure of the Milky Way is hidden to us by dust. Another thing that makes its structure uncertain is that galaxy arms generally twist and merge. Though there is no consensus yet on its structure, our galaxy is thought to have at least two or four arms.
The two major arms of the Milky Way are the Perseus Arm and the Scutum–Centaurus Arm. Completing the four-arm structure are the Norma and Sagittarius Arms. Aside from the major ones, the Milky Way also has smaller arms or spurs. Our solar system is located in one of these spurs, in the Orion Arm.
The spherical component is like the extension of the galaxy’s bulge. It is, however, less dense. This region is made up of globular clusters and metal-poor stars.
The galactic disk is enclosed in a spheroidal halo. Much is still known about this structure. It is thought to extend about 100,000 light-years from the galactic center.
The Universe is a vast place and we can only see a portion of it. The part that we can see is called the observable universe.
The Universe as a whole has been expanding even before the Big Bang. The size of the observable universe is estimated to be around 46 billion light-years. This limit will expand over time which will allow us to see more of the Universe in the far future.
Today, there are about two trillion galaxies in the observable universe. But where does our Milky Way sit among all of these? We can use the surrounding deep-sky objects as a reference to our galaxy’s location.
The Milky Way is part of the Local Group, a neighborhood of about 50 galaxies bound together. The Local Group is part of an even bigger set of galaxies called the Virgo Supercluster, also called the Local Supercluster. It is part of the bigger Laniakea Supercluster which contains the Milky and around 100,000 other galaxies.
On a much broader scale, the Laniakea Supercluster is part of the galaxy filament called the Pisces–Cetus Supercluster Complex.
As for our solar system, the Sun is located in the Orion Arm of the Milky Way. It is approximately 27,000 light-years from the galactic center. As observed from Earth, the center of the Milky Way lies in the constellation of Sagittarius.
The Local Group
We can make sense of our location in the vast Universe by using context. The Milky Way belongs in larger clusters but its most immediate group is the Local Group. It is also called the Andromeda group after its largest member.
The Local Group is a collection of nearby galaxies. It spans 10 million light-years in diameter, with around 50 or more member galaxies. The Andromeda galaxy is the largest in this group, followed by the Milky Way, and the third is the Triangulum galaxy.
It is said that the Local Group has a dumbbell shape. This is because most of the mass is concentrated between its two largest members. Each of them has its own set of “satellites” or smaller galaxies orbiting them.
The Milky Way has at least 60 satellite galaxies. The biggest ones are the Magellanic Clouds, named after Ferdinand Magellan who explored the southern hemisphere. The smallest satellites are only around 500 light-years across. Examples are the Carina Dwarf Spheroidal Galaxy and the Leo Dwarf.
Milky Way–Andromeda Galaxy Collision
Everything moves in space. Our galaxy moves and the Local Group where it belongs moves too. As a result, collisions become a common thing. In fact, our Milky Way is already devouring the Canis Major Dwarf Galaxy.
The two biggest galaxies of the Local Group are set to collide 4.5 billion years from now. Scientists have projected this event because the Andromeda galaxy is heading our way at a rate of 110 kilometers per second (68 mi/sec).
Since galaxies are mostly empty space and the stars are spread out, our solar system will likely survive this collision. This event will unfold in a course of about a billion years until a bigger galaxy is formed. This resulting galaxy has already been nicknamed “Milkomeda” or “Milkdromeda.”
Milky Way Facts – Stories and Mythology
Throughout history, people have seen the Milky Way from different parts of the world. Different cultures have different interpretations of it. This celestial band of stars became a part of their mythology, along with interesting tales about its origins.
One of the most famous of tales is from Greek mythology.
The king of the gods, Zeus, had a child with a mortal woman named Alcmene. This baby became known as Heracles. Zeus wanted his son to have god-like qualities so he brought the baby to suckle his wife’s milk.
Since Hera was sleeping at the time, Zeus sneaked the baby onto her. When Hera woke up, she was so shocked that she flung Heracles away, spattering her milk as she did so. In other versions of the story, Hera, persuaded by Athena, agreed to suckle the young Heracles. The baby bit her hard that she also pushed him away.
The queen goddess’s milk then became the Milky Way.
In Chinese mythology, the Milky Way was the Heavenly River that separated Zhinü and Niulang. These two lovers also represent the stars Vega and Altair.
Zhinü was helping her father, the king, in creating the heavens. She was tasked with the laborious job of weaving the clouds. One day, she met a lowly cowherd and they fell in love.
The young woman was so entranced by her romance that she did not weave clouds anymore. Her father was furious so he devised a plan to separate them. He created the Heavenly River to keep the two lovers apart for eternity.
Still, Zhinü and Niulang can see each other once a year, on the seventh day of the seventh month. On that day, magpies flock together to form a bridge, allowing them to meet.
The story of the cowherd and the weaver girl is celebrated annually in China during the Qixi Festival.
Milky Way Facts – Common Questions:
What is the Milky Way?
The Milky Way is our home galaxy. The Sun, planets, and all other objects in the solar system are part of the Milky Way. It is composed of gas, dust, and billions of stars. Each of these stars likely has its own planetary systems just like ours. The closest star to us, Proxima Centauri C, has two confirmed planets.
Aside from the visible components, a big part of the Milky Way is believed to be made up of dark matter. This galaxy is not alone, in fact, there are billions to trillions of galaxies out there.
How old is the Milky Way?
The Milky Way is about 13.6 billion years old. It formed early when the universe was still young. In fact, the universe is only a bit older at 13.8 billion years. Our solar system formed much later, about 4.5 billion years ago.
The youngest galaxies that we have discovered formed more recently, 500 million years ago. An example of this is the I Zwicky 18 galaxy. It is a dwarf galaxy with an estimated diameter of 3,000 light-years. It lies 59 million light-years away from us.
Does the Sun move around the Milky Way?
The Earth orbits the Sun. In turn, the Sun and the entire solar system also move around the center of the Milky Way. It takes the Sun 225 to 250 million years to orbit the galactic center. We call this length of time “cosmic year” or “galactic year.”
In this journey, the Sun moves at a speed of 800,000 kilometers an hour (500,000 miles per hour). Still, it takes hundreds of millions of years to complete a trip. To date, the Sun has orbited the Milky Way 20 times.
Does the Milky Way revolve around anything?
Yes, it does. Everything moves in space, and so does our galaxy. The Milky Way and the Andromeda Galaxy orbit each other, around their center of mass.
The center of mass is the point in a system where the mass is equally distributed. It is affected by the mass of the objects and their distance from each other. In the Earth-Sun system, the Sun has the center of mass because it is much more massive than the Earth. Our planet orbits the Sun as a result.
The center of mass can be in space between two objects if they have about the same mass. This is the case for the Milky Way and the Andromeda Galaxy. They are very far from each other but they are linked by gravity. They orbit around their center of mass where they are expected to collide billions of years from now.
How can we see the Milky Way if we are in it?
We can only see parts of the Milky Way because we are inside it. The band of stars that we see is just a part of the whole.
Since we are in the Milky Way, we cannot have an “aerial view” of it just like other galaxies. For example, we have pictures of the entire Andromeda galaxy since we are outside it.
No one has gone out of our galaxy so we do not know what it exactly looks like from the outside. However, we get an idea of what it appears like by combining the bits of information we have about it.
Think of a house, for example. We cannot see the entire house if we are inside it. However, we get an idea of its structure and appearance by observing it from the inside. Observing the neighboring houses helps too.
With all that said, the pictures of the Milky Way that we see in its “full size” are from simulations, illustrations, and artistic renditions.
Milky Way Facts – Sources:
Milky Way Facts – Image Sources:
Milky Way galaxy:
Milky Way in 360°:
Harlow Shapley and Heber Curtis:
How big is it:
The Milky Way over Cabot Head Lighthouse:
Structure and galaxy type:
Edge-on view with structures:
The Local group:
The viewMilky Way before the merger:
Hera suckling the baby Heracles:
Zhinü and Niulang: