Ten things you don’t know about the Milky Way Galaxy

The entire universe in blog form
March 12 2008 9:33 PM

Ten things you don’t know about the Milky Way Galaxy

So you've lived here all your life -- in fact, everyone has -- but what do you really know about the Milky Way galaxy? Sure, you know it's a spiral, and it's 100,000 light years across. And of course, BABloggees are smarter, more well-read, and better looking than the average population, but be honest: do you know all ten of these things? Really?

Liar.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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So let's see if these really are Ten Things You Don't Know About the Milky Way Galaxy.

1) It's a barred spiral.

You might know that the Milky Way is a spiral galaxy, perhaps the most beautiful galaxy type. You've seen 'em: majestic arms sweeping out from a central hub or bulge of glowing stars. That's us. But a lot of spirals have a weird feature: a rectangular block of stars at the center instead of a sphere, and the arms radiate away from the ends of the block. Astronomers call this block a bar, and, you guessed it: we have one.

Is fact, ours is pretty big. At 27,000 light years end-to-end, it's beefier than most bars. Of course, space is a rough neighborhood. Who wouldn't want a huge bar located right downtown?

By the way, the image above is not a photograph, it's a drawing-- there's no way to get outside the galaxy and take a picture like this looking back. It would be a loooong walk home! Click the picture to embiggen and get more details (which is true for all the pictures in this post).

2) There's a supermassive black hole at its heart.

At the very center of the Galaxy, right at its very core, lies a monster: a supermassive black hole.

We know it's there due to the effect of its gravity. Stars very near the center -- some only a few dozen billion kilometers out -- orbit the center at fantastic speeds. They scream around their orbits at thousands of kilometers per second, and their phenomenal speed betrays the mass of the object to which they're enthralled. Applying some fairly basic math, it's possible to determine that the mass needed to accelerate the stars to those speeds must tip the cosmic scales at four million times the mass of the Sun! Yet in the images, nothing can be seen. So what can be as massive as 4,000,000 Suns and yet not emit any light?

Right. A black hole.

Even though it's huge, bear in mind that the Galaxy itself is something like 200 billion solar masses strong, so in reality the black hole at the center is only a tiny fraction of the total mass of the Galaxy. And we're in no danger of plunging into it: after all, it's 250,000,000,000,000,000 kilometers away.

It's thought now that a supermassive black hole in the center of a galaxy forms along with the galaxy itself, and in facts winds blown outward as material falls in affects the formation of stars in the galaxy. So black holes may be dangerous, but it's entirely possible the Sun's eventual birth -- and the Earth's along with it -- may have been lent a hand by the four million solar mass killer so far away.

3) It's a cannibal.

Galaxies are big, and have lots of mass. If another, smaller galaxy passes too close by, the bigger galaxy can rip it to shreds and ingest its stars and gas.

The Milky Way is pretty, but it's savage, too. It's currently eating several other galaxies. They've been ripped into long, curving arcs of stars that orbit the center of the Milky Way. Eventually they'll merge completely with us, and we'll be a slightly larger galaxy. Ironically though, the galaxies add their mass to ours, making it more likely we'll feed again. Eating only makes galaxies hungrier.

4) We live in a nice neighborhood...

The Milky Way is not alone in space. We're part of a small group of nearby galaxies called -- get ready to be shocked -- the Local Group. We're the heaviest guy on the block, and the Andromeda galaxy is maybe a bit less massive, though it's actually spread out more. The Triangulum galaxy is also a spiral, but not terribly big, and there are other assorted galaxies dotted here and there in the Group. All together, there are something like three dozen galaxies in the Local Group, with most being dinky dwarf galaxies that are incredibly faint and difficult to detect.

5) ... and we're in the suburbs.

The Local Group is small and cozy, and everyone makes sure their lawns are mowed and houses painted nicely. That's because if you take the long view, we live in the suburbs. The big city in this picture is the Virgo Cluster, a huge collection of about 2000 galaxies, many of which are as large or larger than the Milky Way. It's the nearest big cluster; the center of it is about 60 million light years away. We appear to be gravitationally bound to it; in other words, we're a part of it, just far-flung. The total mass of the cluster may be as high as a quadrillion times the mass of the Sun.

6) You can only see 0.000003% percent of it.

When you got out on a dark night, you can see thousands of stars. But the Milky Way has two hundred billion stars in it. You're only seeing a tiny tiny fraction of the number of stars tooling around the galaxy. In fact, with only a handful of exceptions, the most distant stars you can readily see are 1000 light years away. Worse, most stars are so faint that they are invisible much closer than that; the Sun is too dim to see from farther than about 60 light years away... and the Sun is pretty bright compared to most stars. So the little bubble of stars we can see around us is just a drop in the ocean of the Milky Way.

7) 90% of it is invisible.

When you look at the motions of the stars in our galaxy, you can apply some math and physics and determine how much mass the galaxy has (more mass means more gravity, which means stars will move faster under its influence). You can also count up the number of stars in the galaxy and figure out how much mass they have. Problem is, the two numbers don't match: stars (and other visible things like gas and dust) make up only 10% of the mass of the galaxy. Where's the other 90%?

Whatever it is, it has mass, but doesn't glow. So we call it Dark Matter, for lack of a better term (and it's actually pretty accurate). We know it's not black holes, dead stars, ejected planets, cold gas -- those have all been searched for, and marked off the list -- and the candidates that remain get pretty weird (like WIMPs). But we know it's real, and we know it's out there. We just don't know what it is. Smart people are trying to figure that out, and given the findings in recent years, I bet we're less than a decade from their success.

8) Spiral arms are an illusion.

Well, they're not an illusion per se, but the number of stars in the spiral arms of our galaxy isn't really very different than the number between the arms! The arms are like cosmic traffic jams, regions where the local density is enhanced. Like a traffic jam on a highway, cars enter and leave the jam, but the jam itself stays. The arms have stars entering and leaving, but the arms themselves persist (that's why they don't wind up like twine on a spindle).

Just like on highways, too, there are fender benders. Giant gas clouds can collide in the arms, which makes them collapse and form stars. The vast majority of these stars are faint, low mass, and very long-lived, so they eventually wander out of the arms. But some rare stars are very massive, hot, and bright, and they illuminate the surrounding gas. These stars don't live very long, and they die (bang!) before they can move out of the arms. Since the gas clouds in the arms light up this way, it makes the spiral arms more obvious.

We see the arms because the light is better there, not because that's where all the stars are.

9) It's seriously warped.

The Milky Way is a flat disk roughly 100,000 light years across and a few thousand light years thick (depending on how you measure it). It has the same proportion as a stack of four DVDs, if that helps.

Have you ever left a DVD out in the Sun? It can warp as it heats up, getting twisted (old vinyl LPs used to be very prone to this). The Milky Way has a similar warp!

The disk is bent, warped, probably due to the gravitational influence of a pair of orbiting satellite galaxies. One side of the disk is bent up, if you will, and the other down. In a sense, it's like a ripple in the plane of the Milky Way. It's not hard to spot in other galaxies; grab an image of the Andromeda galaxy and take a look. At first it's hard to see, but if you cover the inner part you'll suddenly notice the disk is flared up on the left and down on the right. Andromeda has satellite galaxies too, and they warp its disk just like our satellite galaxies warp ours.

As far as I can tell, the warp doesn't really affect us at all. It's just a cool thing you may not know about the Milky Way. Hey, that would make a good blog entry!

10) We're going to get to know the Andromeda galaxy a lot better.

Speaking of Andromeda, have you ever seen it in the sky? It's visible to the naked eye on a clear, dark, moonless night (check your local listings). It's faint, but big; it's four or more degrees across, eight times the apparent size of the Moon on the sky.

If that doesn't seem too big, then give it, oh, say, two billion years. Then you'll have a much better view.

The Andromeda Galaxy and the Milky Way are approaching each other, two cosmic steam engines chugging down the tracks at each other at 200 kilometers per second. Remember when I said big galaxies eat small ones? Well, when two big galaxies smack into each other, you get real fireworks. Stars don't physically collide; they're way too small on this scale. But gas clouds can, and like I said before, when they do they form stars. So you get a burst of star formation, lighting up the two galaxies.

In the meantime, the mutual gravity of the two galaxies draw out long tendrils from the other, making weird, delicate arcs and filaments of stars and gas. It's beautiful, really, but it indicates violence on an epic scale.

Eventually (it takes a few billion years), the two galaxies will merge, and will become, what, Milkomeda? Andromeway? Well, whatever, they form a giant elliptical galaxy when they finally settle down. In fact, the Sun will still be around when this happens; it won't have yet become a red giant. Will our descendants witness the biggest collision in the history of the galaxy?

That's cool to think about. Incidentally, I talk about this event a whole lot more, and in a lot more detail, in my upcoming book Death from the Skies! In case you forgot about that.

Until then, these Ten Things should keep you occupied. And of course, I only wanted to list ten things so I could give this post the cool title. But if there's something you find surprising about the Milky Way, leave a comment! I don't want to hog all the fun.