How do we know what the Milky Way actually looks like, when we're inside it? I asked Mark Reid, Senior Radio Astronomer at the Harvard-Smithsonian Center for Astrophysics.
For more with Mark Reid on the Milky Way, check out the Q&A below.
NOVA: Do we know what the Milky Way looks like?
Reid: The answer really is no. At this point, we do know it is a spiral galaxy, we know it has spiral patterns, but we don't really know where these spiral arms lie. There's even debate between astronomers about whether there are two or four arms. I would say within the next few years when we've been able to map the positions of young stars that trace out these arms, then we'll know exactly where the arms are, and how many there are. We have good preliminary evidence to show there are four arms. And we know a little bit more about how tightly bound or open the arms are, there's been some debate about that, and it looks like it's a fairly loosely bound spiral, but to go beyond that it'll take a lot more observations and data.
NOVA: What are some of the challenges to studying the Milky Way?
Reid: The dust and gas in the Milky Way really makes it difficult for astronomers who use optical telescopes to see very far at all. And so the idea of measuring the distance to very distant stars with optical telescopes and making a map out of all that data won't work very well. You just can't see far enough. So you really have to use other techniques. You could use infrared light, or you could use radio light. What I use is radio light.
NOVA: Radio light?
Reid: Radio waves are light, they're not sound. They can be used to carry sound if you want, as people do with radio stations, but radio waves are still light. Your eyes just can't see. Radio telescopes collect radio light in much the same way that optical telescopes collect optical light. The ones I'm using are called the Very Long Baseline Array; there are 10 of them, all across the US from New Hampshire to Hawaii to the Virgin Islands, and a lot of them in between. By a technique called very long baseline interferometry, I'm able to make an image of what the sky would look like if your eyes could see radio waves, and if they were as big as the earth.
NOVA: Why use radio light?
Reid: The one advantage we have with radio light--with all these telescopes all across the earth--is it gives us incredible resolving power. We can measure very, very small shifts in angle as the earth goes around the sun, and from that we can calculate the distance to the star. That's a very powerful technique which you can't really do with any other wavelength at the moment. It's directly analogous to a surveyor measuring out a plot of land. A surveyor will look at object against a background, and will move to a different angle and measure again. If you know the baseline and angles you can calculate the distance. We've just extended that technique by a factor of a billion or more.
We can not only measure where things are, we measure how fast they're moving. So for every star forming region in the Milky Way, we look at how they're moving, so we know how the Milky Way rotates. From our observations we can tell how fast the milky way spins, and it tells us how much mass in the Milky Way--we figured out 15% faster than thought, and the mass of the galaxy is 50% more than before.
NOVA: What do you look for?
Reid: We look at very young stars. You can't detect stars with radio waves, but you can detect the clouds of gas around them because they just formed. Most of the gas is hydrogen, but there are trace amounts of water. The water molecules can act like a MASER, which is a radio wavelength version of laser light, which lets use measure position very well. Now there are probably only 500-1,000 of these very bright very young stars that have formed in the Milky Way at any one time, but the nice thing is they're very bright and trace out the spiral arms very nicely, much like in other galaxies.