The Milky Way

Have you looked up at a moonless night sky and noticed a band of stars snaking across? That’s the Milky Way, our home galaxy. The Earth, our Sun and every other star you can see are all inside the Milky Way. 

From Earth, we cannot see the full shape of the Milky Way. For a long time, people believed the Milky Way was shaped like a pancake. But astronomers have new data that puts a twist in the shape of our galaxy - literally!

Did you know?

Because of light pollution, over one-third of the people on Earth can no longer see the Milky Way.

What is a galaxy?

A galaxy is made up  of stars, planets and clouds of hydrogen gas.  Most galaxies have more stars in their centres and fewer towards their edges. The closer a star is to the centre of a galaxy, the stronger the force of gravity it feels. The force of gravity is like the glue holding the galaxy together. This is the same force that makes your pencil fall to the ground.

Where in the Milky Way are we?

Our solar system is in the Orion Spur of the galaxy, a quiet suburb some 26 000 light-years from the galaxy’s centre. A light-year is the distance travelled by light in one year. At the speed of light, you could travel between Halifax and Vancouver 48 times in 1 second! 

What shape is the Milky Way?

Did you know that galaxies come in various shapes? They may be spiral or elliptical (oval). If you’ve seen telescope photos of other galaxies like the Andromeda galaxy, you may have noticed their bright centre. That brightness is caused by the stars in that region. You may have also noticed their pancake-like shape. 

If you look closely at the Milky Way with a telescope, you will see a band of stars with a bulge in the centre. If you look at a pancake from the edge, you see something similar. Astronomers measured the speeds and distances of several stars to find the shape of this disc. It turns out that shape is a spiral. 

Imagine trying to draw an outline of Canada based on what you can see from your bedroom window. This would be very difficult, because you would not see very much! Astronomers trying to map the Milky Way have a similar problem. Figuring out the shape of the Milky Way is difficult because our sun, the Earth and every space probe we have are inside it.

Now imagine you were standing on a mountain and could see lighthouses far away. You could use the lighthouses on the Canadian coast to draw the shape of Canada. Of course, your shape would not be totally accurate. First of all, you wouldn’t be able to see every lighthouse from where you are standing. Also, places might not have lighthouses at all. For these places, you’d have to guess what their shape might be. 

Astronomers used a similar technique to figure out the shape of the Milky Way. Of course, they didn’t use lighthouses. They used special stars called Cepheid variables. Astronomers calculated the distances to 1339 Cepheid variables. Then, they plotted the locations and distances of these stars on a 3D map.

This 3D map shows that our Milky Way is not a flat pancake! It’s more of a spiral pancake with one edge pulled up and the opposite edge pulled down.  The further out from the centre you go, the more of a twist there is. You can see an animation of this at this link.

Is the Milky Way’s shape unique?

Astronomers know about other galaxies that are shaped like a bent spiral, though it appears this shape is rare. Astronomers don’t know the reason for the bent spiral, but they have some ideas.  It might be the pull from the stars in the centre of our galaxy. Maybe smaller galaxies merged with ours in the past. 

This time, astronomers found a twist in the Milky Way’s shape. As astronomers continue to map out more Cepheid stars, they may find “twists” in our understanding of the Milky Way, too!