“Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” –Douglas Adams
Here inside the Milky Way galaxy, all you need are some dark skies and a decent set of eyes, and you’ll be greeted of a spectacular, close-up view of the galaxy you inhabit.
But getting a handle on the size and scale of our Milky Way? Perhaps paradoxically, that’s a question we’re still figuring out the answers to! Which is remarkable, when you think about it, because in many ways it was so much easier to learn these things about galaxies located many billions of light years away! The reason behind this? It’s the same reason you’d never know your own eye color if you never saw your own reflection: we don’t have a clear view of our Milky Way from inside it!
The best we can do is make careful measurements from within it, mapping out the location of stars, pulsars and gas clouds through the dust and haze in a myriad of wavelengths, and try to reconstruct exactly what we’d look like to an outsider. The acquisition of knowledge has been painstakingly slow, and it’s only relatively recently that we’ve determined some of the most basic properties of our galaxy.
But this would be easy for anyone outside of our Milky Way. We know this because, for all the other galaxies, where we’re the outsider, these characteristics are easy to see.
Above, the galaxy NGC 6744 is often referred to as the Milky Way’s twin, and for many good reasons. Much like our own galaxy, there’s a small, central bar around the galactic center, there are two main spiral arms that curl around the galaxy multiple times and with many branches, making it a flocculent galaxy. There are deep dust lanes (appearing dark) and a few regions of intense star formation (in pink), and there are small, dwarf satellites (like the one on the upper left) being gobbled up.
And much like our own galaxy, this one is about 100,000 light years in diameter. All of this, for a distant galaxy, we can learn just from a glance through a telescope across the Universe, some 30 million light years away. Indeed, many galaxies have some of these similarities to our own.
Most of them are not directly face-on to us; they typically, like NGC 2841, above, come inclined at some angle. And in extremely serendipitous circumstances, we’ll get to view a galaxy that appears much the same as our own galaxy appears from inside of it, 26,000 light years away from the center: edge-on. The beautiful NGC 4565, below, appears much as our Milky Way would if it were edge on, but nearly 50 million light years distant!
We may know that our galaxy is 100,000 light years across, but much less well-known is how thick our galaxy’s disc is. Yes, we’re thicker at the center, with that same spheroidal bulge that NGC 4565 has, but the bulk of the disc, such as out where we are, is a roughly uniform thickness. But what is that thickness?
If you ask wikipedia, they’ll send you here, which as of today — I’m sorry to say — gets it wrong by quite a bit. Our galaxy is much thicker than the 1,000 light years figure they quote. But how much thicker? Even among scientists today, that’s up for debate. And the reason that’s up for debate is that you get different answers depending on how you look. It might occur to you, for instance, to simply look at stars.
Take a look around our Solar neighborhood, look out of the plane of the galactic disc, and measure the star density directly. Our equipment is good enough, these days, that a group has done exactly this. Their findings, lifted from figure 3 from their paper, below, show us how thick the disc is, at least in terms of stars.
They find that, although the density of stars is greatest about 300 parsecs (1,000 light years) in either direction about the center of the galactic plane, we continue to have a solid abundance of stars out to about 900-1000 parsecs (more like 3,000 light years) in either direction, giving us a thickness estimate for the galactic disc of 6,000 light years.
A team led by Bryan Gaensler did just this in 2008, by looking at a set of pulsars of varying distances and varying galactic latitudes. What they intended to do was to determine how much intervening gas there was between these pulsars and us, and thus to determine the extent of the gas and how it was distributed.
One can do this, because pulsars emit electromagnetic radiation of different frequencies, but the different frequencies are affected in different ways by the warm gas. And when they deduced where this gas was, what they found was — at least, based on where the stars are — a bit shocking! (You’ll want to read the graph that says “H” to see the height of the gas above/below the galactic plane, in parsecs.)
Amazingly, they found that the gas is double the thickness of the stars in the galaxy. If you want to be safe, you can say that the thickness of the Milky Way is somewhere between 6,000 and 12,000 light years, based on the stars and the warm ionized gas as the lower- and upper-limits, respectively.
But after all this time, living in our galaxy and watching the skies, the Milky Way is still one of the least understood galaxies, all because we’re right in the thick of it. Or, if you want to be more accurate, right in the thin of it! However you slice it, it’s still a remarkable story, and I hope you have a better appreciation for how hard it is to figure out something so simple: the shape and size of our galaxy!