The Milky Way is a barred spiral galaxy around 13.6 billion years old with large pivoting arms stretching out across the cosmos. Its disk is about 100,000 light-years and just 1000 light-years thick, according to Las Cumbres Observatory (opens in new tab).
Just as Earth orbits the sun, the solar system orbits the center of the Milky Way. Despite hurtling through space at speeds of around 515,000mph (828,000kmph) it still takes our solar system approximately 250 million years to complete a single revolution, according to Interesting Engineering (opens in new tab). The last time our planet was in this position, dinosaurs were just emerging and mammals were yet to evolve.
If the center of the Milky Way were a city, we would be living in suburbia, about 25,000 to 30,000 light-years from the city center. Life in the outskirts is good, we find ourselves nestled in one of the smaller neighborhoods, the Orion-Cygnus Arm, sandwiched between larger Perseus and Carina-Sagittarius arms. If we were to travel inwards towards the city center we would find the Scutum-Centaurus and Norma arms.
On a clear night, void of light pollution, we can catch a glimpse of the bright lights of the galactic city streaking across the night sky. Our window into the universe, this milky white band of stars, dust and gas is where our galaxy gets its name.
Lying at the very heart of the Milky Way is a supermassive black hole called Sagittarius A*. About 4 million times the mass of the sun, this beast consumes anything that strays too close, gorging on an ample supply of stellar material enabling it to grow into a giant. Though we cannot directly view this glutton at the core of our galaxy, scientists can suggest its presence by investigating its effect on nearby matter.
Why is our galaxy called the Milky Way?
According to the American Museum of Natural History (opens in new tab) (AMNH), our galactic home is called the Milky Way after its apparent milky white appearance as it stretches across the night sky. In Greek mythology, this milky band appeared because the goddess Hera sprayed milk across the sky.
Around the world, the Milky Way is known by different names for example in China it is called "Silver River" and in the Kalahari Desert in South Africa it's called the "Backbone of Night".
The Great Debate of 1920
We are constantly building on our wealth of knowledge of the Milky Way, though up until relatively recently astronomers believed that all the stars in the sky belonged to our galaxy.
"The Great Debate" in 1920 saw astronomers Herber Curtis and Harlow Shapley argue the scale of the universe and the prospect of "island universes" (galaxies), according to the National Academy of Sciences (opens in new tab). On one side of the debate, Shapley believed the Milky Way was much larger than previous estimates and that we weren't at the center. He also claimed that "spiral nebulae" such as Andromeda were a part of the Milky Way. On the other side of the debate, Curtis did not dispute Shapley's claims of a far larger Milky Way, he did however argue that there were large island universes (galaxies) such as Andromeda, that lay beyond the boundaries of the Milky Way. The dispute was resolved when Edwin Hubble's measurements of Cepheid variable stars proved Andromeda was located far outside the Milky Way. Modern estimates suggest the Andromeda galaxy, our nearest galaxy neighbor is 2.5 million light-years away.
We now know that the Milky Way resides within the Local Group of galaxies, made up of over 30 galaxies including Andromeda, Triangulum and Leo I to name but a few. It turns out that it's pretty good to know who your neighbors are, as they may be closer than you think. The Milky Way is currently hurtling towards Andromeda at 250,000mph (400,000 km/h). Though there is no need to worry just yet, this crash of cosmic proportions is not due for another 4 billion years.
The Milky Way: Size, structure and mass
– Galaxy type: Barred spiral
– Age: 13.6 billion years (and counting)
– Size: 100,000 light-years across
– Number of stars: about 200 billion
– Rotation time: 250 million years
Studying the Milky Way used to be notoriously difficult. Astronomers sometimes compare the effort to attempting to describe the size and structure of a forest while being lost in the middle of it. From our position on Earth, we simply lack an overview. But two ground-breaking space telescopes launched since the 1990s have helped usher in the golden age of Milky Way research. Major strides have been made, especially since the 2013 launch of the European Space Agency's (ESA) Gaia mission.
Telescopes enabled astronomers to distinguish the basic shape and structure of some of the closest galaxies before they knew they were looking at galaxies. But reconstructing the shape and structure of our own galactic home was slow and tedious. The process involved building catalogs of stars, charting their positions in the sky and determining how far from Earth they are.
Dutch astronomer Jan Oort, sometimes dubbed the master of the galactic system, was the first to realize that the Milky Way isn't motionless but rotates, and he calculated speeds at which stars at various distances orbit around the galactic center. It also was Oort who determined the position of our sun in the vast galaxy. (The Oort Cloud, a repository of trillions of comets far from the sun, was named after him.)
Gradually, a complex picture emerged of a spiral galaxy that appears quite ordinary.
At the center of the Milky Way sits a supermassive black hole called Sagittarius A*. With a mass equal to that of four million suns, the black hole, discovered in 1974, can be observed in the sky with radio telescopes close to the constellation Sagittarius.
Everything else in the galaxy revolves around this powerful gateway to nothingness. In its immediate surroundings is a tightly packed region of dust, gas and stars called the galactic bulge. In the case of the Milky Way, this bulge is peanut-shaped, measuring 10,000 light-years across, according to ESA. It harbors 10 billion stars (out of the Milky Way's total of about 200 billion), mostly old red giants, which formed in the early stages of the galaxy's evolution.
Beyond the bulge extends the galactic disk. This feature is 100,000 light-years across and 1,000 light-years thick, and it's home to the majority of the galaxy's stars, including our sun. Stars in the disk are dispersed in clouds of stellar dust and gas. When we look up to the sky at night, it's the edge-on view of this disk extending toward the galactic center that takes our breath away.
Stars in the disk orbit around the galactic center, forming swirling streams that appear to emanate like arms from the galactic bulge. Research into the mechanisms that drive the creation of spiral arms is still in its infancy, but the latest studies suggest that these arms form and disperse within relatively short periods of up to 100 million years (out of the galaxy's 13 billion years of evolution).
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 Milky Way disk is not flat but warped, according to ESA. As it rotates, it precesses like a wobbling spinning top. This wobble, essentially a giant ripple, circles the galactic center much more slowly than the stars in the disk, completing a full rotation in about 600 to 700 million years. Astronomers think this ripple may be a result of a past collision with another galaxy.
Sprinkled around the disk and the bulge are globular clusters, collections of ancient stars, as well as approximately 40 dwarf galaxies that are either orbiting or colliding with the larger Milky Way according to a statement from ESA (opens in new tab).
All of that is surrounded by a spherical halo of dust and gas, which is twice as wide as the disk. Astronomers believe that the entire galaxy is embedded in an even larger halo of invisible dark matter. Since dark matter doesn't emit any light, its presence can only be inferred indirectly by its gravitational effects on the motions of stars in the galaxy. Calculations suggest that this puzzling stuff makes up to 90% of the galaxy's mass.
"Even though we know the dark matter should be there, [and] we think it should be there, the ratio of dark matter to luminous matter in particular galaxies may be under debate," Gwendolyn Eadie, a Ph.D. candidate in astrophysics at McMaster University in Ontario, Canada, and co-author on the research, told Space.com.
The mass of the Milky Way, dark matter included, equals 1.5 trillion solar masses, according to recent NASA estimates (opens in new tab). The galaxy's visible matter is distributed between its 200 billion stars, their planets and the massive clouds of dust and gas that fill the interstellar space.
Where is the sun in the Milky Way?
The sun orbits about 26,000 light-years from the black hole Sagittarius A*, roughly in the middle of the galactic disk. Traveling at the speed of 515,000 mph (828,000 kph), the sun takes 230 million years to complete a full orbit around the galactic center.
The sun sits near the edge of the Local Arm of the Milky Way, one of the two smaller spiral arms of the galaxy. In 2019, using data from the Gaia mission (opens in new tab), astronomers found that the sun is essentially surfing a wave of interstellar gas that's 9,000 light-years long, 400 light-years wide and undulates 500 light-years above and below the galactic disk according to ESA.
Planets of the solar system do not orbit in the plane of the galaxy but are tipped by about 63 degrees.
"It's almost like we're sailing through the galaxy sideways," Merav Opher, an astrophysicist at George Mason University in Virginia, told Space.com.
Milky Way formation and evolution
The evolution of the Milky Way began when clouds of gas and dust started collapsing, pushed together by gravity. First stars sprung up from the collapsed clouds, those that we see today in the globular clusters. The spherical halo emerged soon after, followed by the flat galactic disk. The galaxy started small and grew as the inescapable force of gravity pulled everything together.(opens in new tab)
The galaxy's evolution is, however, still shrouded in mystery. A discipline called galactic archaeology is slowly unraveling some of the puzzles of the Milky Way's life thanks to the Gaia mission, which released its first catalog of data (opens in new tab)in 2018.
Gaia measures (opens in new tab) the exact positions and distances of more than 1 billion stars, as well as their light spectra, which enables scientists to understand the stars' composition and age, according to ESA. The position data allow astronomers to determine the speeds and directions in which the stars move in space. As things in space follow predictable trajectories, astronomers can reconstruct the paths of the stars billions of years into the past and future. Combining these reconstructed trajectories into one stellar movie captures the evolution of the galaxy over eons.
There is also evidence that the Milky Way collided with several smaller galaxies during its evolution. In 2018, a team of Dutch astronomers found a group of 30,000 stars (opens in new tab) moving in sync through the sun's neighborhood in the opposite direction to the rest of the stars in the data set. The motion pattern matched what scientists had previously seen in computer simulations of galactic collisions. These stars also differed in color and brightness, which suggested they came from a different galaxy.
Remnants of another, slightly younger, collision were spotted a year later. The Milky Way continues devouring smaller galaxies to this day. A galaxy called Sagittarius (not to be mistaken with the black hole) currently orbits close to the Milky Way and has likely smashed through its disk several times (opens in new tab) in the past 7 billion years. Using Gaia data, scientists found that these collisions triggered periods of intense star formation in the Milky Way and may even have something to do with the galaxy's trademark spiral shape. The study suggests that our sun was born during one of those periods some 4.6 billion years ago.
The future of the Milky Way research
Since the beginning of its operations, the Gaia mission has provided three updates to its massive stellar catalog. Astronomers from all over the world continue analyzing the data in search of new patterns and revelations. Gaia data currently generates more research papers than even the famous Hubble Space Telescope. In a recent "space telescope tournament", a jokey poll run by astronomers on Twitter (opens in new tab), Gaia beat the veteran Hubble by five votes as the current darling of the astronomical community.
Gaia will continue charting the galaxy until at least 2022, and the catalog it has compiled will keep astronomers busy for decades to come.
Before Gaia, the largest dataset about positions and distances of stars in the Milky Way came from a mission called Hipparcos, after an ancient Greek astronomer who began charting the night sky 150 years before Christ. Hipparcos only saw about 100,000 of the brightest stars in the sun's neighborhood, compared to Gaia's one billion. The data was also less precise.
Even though Gaia sees less than 1% of stars in the galaxy, astronomers can expand their findings and model the behavior of the entire Milky Way.
Discover more about the Milky Way and other galaxies with this free learning material from the Open University (opens in new tab). Explore the Milky Way in virtual reality (opens in new tab) with ESA's Gaia mission. Tour the Milky Way with Gaia Sky (opens in new tab), a real-time, 3D, astronomy visualization software that uses ESA's Gaia mission data. Learn why it was so difficult to study the Milky Way before Gaia in this article from ESA (opens in new tab).
Xiang, M., Rix, HW. "A time-resolved picture of our Milky Way's early formation history (opens in new tab)". Nature 603, 599–603 (2022). https://doi.org/10.1038/s41586-022-04496-5
Robin, Annie C., et al. "A synthetic view on structure and evolution of the Milky Way. (opens in new tab)" Astronomy & Astrophysics 409.2 (2003): 523-540.
Dehnen, Walter, and James Binney. "Mass models of the Milky Way. (opens in new tab)" Monthly Notices of the Royal Astronomical Society 294.3 (1998): 429-438.
Helmi, Amina. "Streams, substructures, and the early history of the Milky Way. (opens in new tab)" Annual Review of Astronomy and Astrophysics 58 (2020): 205-256.