The Milky Way. Credit: ESO
At night, if it is dark enough, you can see a huge streak of faded white color stretching across the sky. This band of white is actually a band of stars and cosmic dust that we call the Milky Way. This is also the name of the galaxy that we live in. The word “galaxy” actually comes from the Latin word for milk, so even ancient peoples noticed this big white stream in the sky and thought it looked “milky." What we are actually seeing is the small part of one side of one spiral arm in our galaxy.
Illustration of the Milky Way Galaxy as seen from above.
We think that if you saw our galaxy from above, this is what you’d see. We think that this is like looking at a DVD from above: it is mostly round from the top, but would be thin and flat if you looked at it from the side.
This is what our galaxy would look like seen edge on.
Scientists believe that we are located about two thirds of the way out from the center of the galaxy, on one of the rotating spiral arms of the galaxy. While there are actually a bunch of stars rotating all around the Milky Way in a giant bubble, most of the stars are in the flat disc that we see in this illustration. The streak we see across the night sky is because we are looking at the galaxy from the side, where most of the stars are located. Since there are more stars here, it looks brighter, which also makes it easier to see the giant clouds of dust that are floating around the galaxy, mostly in this flat part, or “plane” of the galaxy.
A picture of a million stars, each one a star like our Sun, possibly with planets and maybe even life like ours.
Size and Scale of the Milky Way
There are literally billions of stars in the Milky Way galaxy. In fact, we now think that there are between 200 and 400 billion of them. This number is almost too big to imagine. Since it would take hundreds if not thousands of years just to count them all, we guess the number of stars by looking at different parts of the sky and then multiplying this by the distances we see around the galaxy. This is also how we can guess at the shape of the Milky Way. Since we are inside of this huge star city, we can’t see the edges or even the full shape of the galaxy. But by looking around and comparing what we see in our galaxy to other galaxies in space, we can guess at what the Milky Way would look like from the outside.
An Actual Picture of Galaxy Messier 83. By seeing how stars are organized in distant galaxies like this one we can guess at the shape of our own Milky Way.
While it may sound like 200 billion stars is a lot of stuff, the Milky Way is actually much, much, much bigger than this. In fact, it is mostly made up of empty space, with enormous distances between the stars. Imagine an airplane that could travel all the way around the Earth without stopping for fuel. Imagine if you could keep speeding up in that airplane until you could travel around the Earth in a day, then an hour. Finally, you are going so fast around the Earth that you could travel all the way around it in a second. It is close to 40,000 kilometers around the Earth, so you would be going around 40,000 kilometers per second! This is obviously very, very fast.
Now imagine that instead of travelling around the Earth you were going in a straight line at this speed. You would go an enormous distance in the space of an hour, or even in one day. Imagine doing this for just a day or two, and you already can imagine how far from the Earth you would be. Imagine how far you would be after a month, or even a year of travelling at this speed in a straight line.
Even going at this speed though, it would take you almost twelve years just to fly across our own solar system! This is absolutely nothing compared to the size of the Milky Way though. If you imagined the size of the solar system to be around the size of a DVD, then the Milky Way would be around the size of the entire Earth compared to this!
Can You Do the Math?
In our example above, we needed to figure out how long it would take to travel across the solar system in a ship that could circle the entire Earth in a second. Why is this interesting? Well, look up some night, and you may get the feeling that the sky is so close that you can almost touch it, or that the galaxy is full of stars. But by understanding distances, you can actually know that the Universe is a very, very big place. This is useful, since the Universe is also full of things that seem incredible at first, or might even seem impossible. By knowing that the Universe is a very big place, you can start to learn about things like Black Holes with the mass of millions of Suns, but also know that they are so far away that they will probably never get anywhere near to the Earth in its entire history. So math becomes a great way to discover the Universe and learn more about all the amazing things and places that are part of our world.
The comets we have measured we believe could orbit the Sun as far as 50,000 AU’s away! So, edge-to-edge, the solar system could be as much as 100,000 AU’s across, or around 14,959,787,070,000 kilometers! That's a quarter of the way to the nearest star !
Our answer is around twelve years (or, more accurately, 11.86 years). Can you reach that number as well? Take your time, and feel free to use a calculator if you need to (unless your teacher or parent wants you to try it without one)!
Dark Mysteries of the Milky Way
As you can see from the pictures of the Milky Way and other spiral galaxies, it looks like most of the stars and dust in these enormous galaxies are orbiting around the center of a disc, like a CD or DVD in a disc player. So what is it that keeps them all spinning around this center?
It is the force of gravity. As amazing as it sounds, it is the same force that keeps you stuck to your chair, or that makes you fall back down when you jump. If you stop and think about it, it is pretty incredible that without this force, you could just fly off into space just by pushing off the ground with your feet. Everything that we can observe that has mass seems to also have gravity. And the more mass it has, the more gravity it seems to have too. So while we all are stuck to the Earth, since the Earth is very big, the Earth is actually stuck to you too, because of your gravity pulling on the Earth! Since you are tiny compared to the whole planet, the Earth would have a much easier time getting unstuck from you than you would have getting off of the Earth!
We believe that the combined gravity of all the stars, dust, and other material in the Milky Way helps hold all of these objects together in a giant gravitational web that keeps them spinning around a common center of gravity. There is not enough gravity in the stars that we can see, however, to keep everything together, so we have looked even harder to see if we can find anything else.
Some recent discoveries might actually hold some of the answers.
At the center of the Milky Way, we have discovered something fascinating and very strange. It seems like the stars in the center of the Milky Way are all orbiting around a point in space where there is a lot of gravity. But no matter how hard we look, we can’t find anything there. So scientists now believe that what we are looking at, is something called a “black hole.”
This is an actual picture of the stars orbiting around the center of our galaxy! Believe it or not, but what we are looking at, but can’t see in the center of this picture, may actually be a black hole containing the gravity of millions and millions of Suns! Luckily it is very, very far away from us, but it must be pretty scary to anyone who might be living nearby!
Black holes are mysterious things, but they seem to be very real. We believe that they are the collapsed centers of really big stars. We think that when these stars run out of fuel, all the matter inside of them collapses into an extremely squeezed and compact object with amazing amounts of gravity. If this is true, then it would be so dense that even atoms would get squashed down into almost nothing, and the only thing we would have left is this weird object made almost entirely out of pure gravity floating around space. Even light would get crushed down into the center of this object, so it would be invisible and terrifying: a real “black hole” in space. Only a few years ago, we thought that these objects were just an imaginary story out of comic books or science fiction novels. But we now think that there are millions of them silently flying around the galaxy, right now, as you’re reading this. Luckily the galaxy is a huge place, so it is almost certain that we’ll never get close enough to one to get sucked in!
An artist’s impression of a black hole travelling through space.
But that is not the only mystery that we have found in our Milky Way.
In 1975, a young astronomer named Vera Rubin announced the results of some astonishing observations that she had made over the prior few years. It looked like the stars on the edge of the Milky Way were moving way too fast to explain. Similar movement had been discovered in some clusters, or big groups of stars, in 1933 by an astronomer named Fritz Zwicky, from California.
After looking for everything they could, both of these astronomers decided that nothing that we knew about could explain why everything was moving so fast. Though some scientists are getting close to figuring this out, we still don’t know what this stuff is!
Since it seems to have gravity, we call it “matter,” and since we can’t seem to find it, we call it “dark.” So we have decided that on top of everything else in our Universe that we can see, there is an enormous amount of this mysterious dark matter floating around in space and shaping the movement of the Universe! Maybe by observing the Universe you can find out what this mysterious stuff is. If you discovered a whole new kind of matter that no one had ever seen you would become one of the most famous scientists in the world!