10 Astonishing Things You Should Know About the Milky Way

Anne Dirkse, Flickr // CC BY-SA 2.0
Anne Dirkse, Flickr // CC BY-SA 2.0

Our little star and the tiny 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 and what all the fuss is about. 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.


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. So it's big. 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: One recent estimate says 2 trillion.


artist's illustration of the milky way galaxy and its center
An artist's concept of the Milky Way and the supermassive black hole Sagittarius A* at its core.
ESA–C. Carreau

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, but that is OK. There is definitely life here, but everywhere else is a question mark. For all we know, this might be the galactic Paris.


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, a professor of astronomy at the University of Virginia and the principal investigator on the Apache Point Observatory Galactic Evolution Experiment (APOGEE), tells Mental Floss. "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."


On the other hand, he tells 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. If they exist, the creatures in Andromeda, the galaxy next door (see #9), probably feel the same way about their own.


John McSporran, Flickr // CC BY 2.0

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 says. "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 further stymies astronomers. "That dust blocks our light, our view of the more distant parts of the Milky Way," Majewski says. "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.


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 says. "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 says. "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."


andromeda galaxy
The Andromeda galaxy
ESA/Hubble & NASA

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 says.


There are countless spacecraft and telescopes studying the Milky Way. Most famous is the Hubble Space Telescope, while other space telescopes such as Chandra, Spitzer, and Kepler are also returning data to help astronomers unlock the mysteries of our swirling patch of stars. The next landmark telescope in development is NASA's James Webb Space Telescope. It should finally launch in 2019. Meanwhile, such ambitious projects as APOGEE are working out the structure and evolution of our spiral home by doing "galactic archaeology." APOGEE is a survey of 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. 

5 Fast Facts About the Spring Equinox


The northern hemisphere has officially survived a long winter of Arctic temperatures, bomb cyclones, and ice tsunamis. Spring starts today, March 20, which means warmer weather and longer days are around the corner. To celebrate the spring equinox, hear are some facts about the event.

1. The spring equinox arrives at 5:58 p.m.

The first day of spring is today, but the spring equinox will only be here for a brief time. At 5:58 p.m. Eastern Time, the Sun will be perfectly in line with the equator, which results in both the northern and southern hemispheres receiving equal amounts of sunlight throughout the day. After the vernal equinox has passed, days will start to become shorter for the Southern Hemisphere and longer up north.

2. The Equinox isn't the only time you can balance an egg.

You may have heard the myth that you can balance on egg on its end during the vernal equinox, and you may have even tried the experiment in school. The idea is that the extra gravitational pull from the Sun when it's over the equator helps the egg stand up straight. While it is possible to balance an egg, the trick has nothing to do with the equinox: You can make an egg stand on its end by setting it on a rough surface any day of the year.

3. Not every place gets equal night and day.

The equal night and day split between the northern and southern hemispheres isn't distributed evenly across all parts of the world. Though every region gets approximately 12 hours of sunlight the day of the vernal equinox, some places get a little more (the day is 12 hours and 15 minute in Fairbanks, Alaska), and some get less (it's 12 hours and 6 minutes in Miami).

4. The name means Equal Night.

The word equinox literally translates to equal ("equi") and night ("nox") in Latin. The term vernal means "new and fresh," and comes from the Latin word vernus for "of spring."

5. The 2019 spring equinox coincides with a supermoon.

On March 20, the day the Sun lines up with equator, the Moon will reach the closest point to Earth in its orbit. The Moon will also be full, making it the third supermoon of 2019. A full moon last coincided with the first day of spring on March 20, 1981, and it the two events won't occur within 24 hours of each other again until 2030.

A Full Pink Moon Is Coming in April

Ana Luisa Santo, Flickr // CC BY-ND 2.0
Ana Luisa Santo, Flickr // CC BY-ND 2.0

Mark your calendars for Friday, April 19 and get ready to snap some blurry pictures of the sky on your way to work. A full pink moon will appear early that morning, according to a calendar published by The Old Farmer's Almanac.

Considering that the full moon cycle is completed every 29.5 days, the April full moon will be the fourth full moon of 2019. Despite its name, the surface of the moon doesn't actually appear rosy. The name refers to the wild ground phlox, a type of pink wildflower, that tends to sprout in the U.S. and Canada around this time of year. It's also sometimes called an egg moon, fish moon, or sprouting grass moon.

What does the Full Pink Moon mean?

The April full moon might be a bit of a misnomer, but it still plays a pretty important role in the Christian tradition. The date on which the full pink moon appears has historically been used to determine when Easter will be observed. The holiday always falls on the Sunday following the first full moon that appears after the spring equinox. However, if the full moon falls on a Sunday, Easter will be held the following Sunday.

This rule dates back to 325 C.E., when a group of Christian churches called the First Council of Nicaea decided that the light of the full moon would help guide religious pilgrims as they traveled ahead of the holiday. Since the full moon will be visible on April 19 this year, Easter will be held on April 21.

When to see the full pink moon

The best time to view this April full moon is around 4:12 a.m. on the West Coast and 7:12 a.m. on the East Coast. The exact time will vary depending on your location. For a more specific estimate, head to the Almanac's website and type in your city and state or ZIP code.

If you happen to miss this spectacle because you're enjoying a full night’s sleep, don't fret too much. A full flower moon will be arriving in May.