10 Astonishing Things You Should Know About the Milky Way

Our sun and the planets that circle it are part of a galaxy called the Milky Way. Its name comes from the Greek galaxias kyklos (“milky circle”) and Latin via lactea (“milky road”). Find a remote area in a national park, miles from the nearest street light, and you'll see exactly why the name makes sense. Above is not a sky of black, but a luminous sea of whites, blues, greens, and tans. Here are a few things you might not know about our spiraling home in the universe.

1. The Milky Way is gigantic.
2. The Milky Way is jam-packed with celestial objects.
3. It’s typical for a spiral galaxy ...
4. ... But in one specific way, our galaxy is unique.
5. Figuring out the Milky Way’s structure from the inside is a challenge.
6. Interstellar dust blocks our view of some parts of the galaxy.
7. Calculating the Milky Way’s spin doesn’t add up.
8. Dark matter is a big problem in galactic studies.
9. The Milky Way is on a collision course with Andromeda.
10. The Hubble Space Telescope is just one of the spacecraft collecting data on the Milky Way.

The Milky Way is gigantic.

The Milky Way galaxy is about 1,000,000,000,000,000,000 kilometers (about 621,371,000,000,000,000 miles) across. Even traveling at the speed of light, it would still take you well over 100,000 years to go from one end of the galaxy to the other. It’s not quite as big as space itself, which is “vastly, hugely, mind-bogglingly big,” as Douglas Adams wrote, but respectably large. And that's just one galaxy. Consider how many galaxies there are in the universe: a recent estimate suggests there are between 100 and 200 billion galaxies in the observable universe.

The Milky Way is jam-packed with celestial objects.

The Milky Way is a barred spiral galaxy composed of an estimated 300 billion stars, along with dust, gas, and celestial phenomena such as nebulae, all of which orbits around a hub of sorts called the Galactic Center, with a supermassive black hole called Sagittarius A* (pronounced “A-star”) at its core. The bar refers to the characteristic arrangement of stars at the interior of the galaxy, with interstellar gas essentially being channeled inward to feed an interstellar nursery. There are four spiral arms of the galaxy, with the sun residing on the inner part of a minor arm called Orion. We're located in the boondocks of the Milky Way. There is definitely life here, but everywhere else is a question mark.

It’s typical for a spiral galaxy ...

If you looked at all the spiral galaxies in the local volume of the universe, the Milky Way wouldn't stand out as being much different than any other. “As galaxies go, the Milky Way is pretty ordinary for its type,” Steve Majewski, an astronomer at the University of Virginia, told Mental Floss in 2018. “It’s got a pretty regular form. It’s got its usual complement of star clusters around it. It’s got a supermassive black hole in the center, which most galaxies seem to indicate they have. From that point of view, the Milky Way is a pretty run-of-the-mill spiral galaxy.”

... But in one specific way, our galaxy is unique.

On the other hand, he told Mental Floss, spiral galaxies in general tend to be larger than most other types of galaxies. “If you did a census of all the galaxies in the universe, the Milky Way would seem rather unusual because it is very big, our type being one of the biggest kinds of galaxies that there are in the universe.” From a human perspective, the most important thing about the Milky Way is that it definitely managed to produce life—something no other galaxy is known to have done (so far).

Figuring out the Milky Way’s structure from the inside is a challenge.

We have a very close-up view of the phenomena and forces at work in the Milky Way because we live inside of it, but that internal perspective places astronomers at a disadvantage when it comes to determining a galactic pattern. “We have a nice view of the Andromeda galaxy because we can see the whole thing laid out in front of us,” Majewski said. “We don't have that opportunity in the Milky Way.”

To figure out its structure, astronomers have to think like band members during a football halftime show. Though spectators in the stands can easily see the letters and shapes being made on the field by the marchers, the band can't see the shapes they are making. Rather, they can only work together in some coordinated way, moving to make these patterns and motions on the field. So it is with telescopes and stars.

Interstellar dust blocks our view of some parts of the galaxy.

Interstellar dust further stymies astronomers. “That dust blocks our light, our view of the more distant parts of the Milky Way,” Majewski said. “There are areas of the galaxy that are relatively obscured from view because they are behind huge columns of dust that we can't see through in the optical wavelengths that our eyes work in.” To ameliorate this problem, astronomers sometimes work in longer wavelengths such as radio or infrared, which lessen the effects of the dust.

Calculating the Milky Way’s spin doesn’t add up.

Astronomers can make pretty reasonable estimates of the mass of the galaxy by the amount of light they can see. They can count the galaxy's stars and calculate how much those stars should weigh. They can account for all the dust in the galaxy and all of the gas. And when they tally the mass of everything they can see, they find that it is far short of what is needed to account for the gravity that causes the Milky Way to spin.

In short, our sun is about two-thirds of the way from the center of the galaxy, and astronomers know that it goes around the galaxy at about 144 miles per second. “If you calculate it based on the amount of matter interior to the orbit of the sun, how fast we should be going around, the number you should get is around 150 or 160 kilometers {93–99 miles} per second,” Majewski said. “Further out, the stars are rotating even faster than they should if you just account for what we call luminous matter. Clearly there is some other substance in the Milky Way exerting a gravitational effect. We call it dark matter.”

Dark matter is a big problem in galactic studies.

“In the Milky Way, we study it by looking at the orbits of stars and star clusters and satellite galaxies, and then trying to figure out how much mass do we need interior to the orbit of that thing to get it moving at the speed that we can measure,” Majewski said. “And so by doing this kind of analysis for objects at different radii across the galaxy, we actually have a fairly good idea of the distribution of the dark matter in the Milky Way—and yet we still have no idea what the dark matter is.”

The Milky Way is on a collision course with Andromeda.

But there’s no need to panic. Sometime in the next 4 or 5 billion years, the Milky Way and Andromeda galaxies will smash into each other. The two galaxies are about the same size and have about the same number of stars, but there is no cause for alarm. “Even though there are 300 billion stars in our galaxy and a comparable number, or maybe more, in Andromeda, when they collide together, not a single star is expected to hit another star. The space between stars is that vast,” Majewski said.

The Hubble Space Telescope is just one of the spacecraft collecting data on the Milky Way.

Countless spacecraft and telescopes have studied the Milky Way. Hubble is the best known, while other space telescopes such as Chandra and the James Webb are sending data to help astronomers unlock the mysteries of our swirling patch of stars. Ambitious projects like APOGEE have studied the structure and evolution of our spiral home by doing “galactic archaeology.” APOGEE surveyed the Milky Way using spectroscopy, measuring the chemical compositions of hundreds of thousands of stars across the galaxy in great detail. The properties of stars around us are fossil evidence of their formation, which, when combined with their ages, helps astronomers understand the timeline and evolution of the galaxy we call home. 

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A version of this story was published in 2018; it has been updated for 2025.