Here is an edge-on illustration of our Milky Way Galaxy. (Keep in mind that the disk actually stretches quite a bit farther out from the budge than is apparent in this illustration. Proportionally, its full diameter makes its thickness less than that of a pizza crust.)
What if I asked you to imagine what that central bulge would look like to us–lifeforms living inside the galaxy? What would you imagine?
Perhaps you’d imagine looking inwards toward a glowing ball of light. Perhaps you’d imagine a region of our sky unusually thick with stars and interstellar clouds. Or perhaps you’d imagine something entirely different.
But…would you imagine this?
I know I wouldn’t…at least, not at first.
“This” is an image of our central bulge, taken with observations in the radio range of the electromagnetic spectrum.
Radio wavelengths are quite long, so they can pass through the majority of thick, interstellar dust clouds. For the most part, radio observations are our only hope of peering inward, through the dusty spiral arms that stand between us and the core of our galaxy.
You can see this for yourself…sort of. You won’t see any of the detailed radio observations because your eyes can’t see radio wavelengths. But, in the summertime, try taking a peek at the constellation Sagittarius. See if you can catch a look on a clear night in a dark-sky location (such as the desert).
Oh…apparently it’s supposed to be an archer. I thought it was a teapot.
…Oh well. Easy mistake. The constellations really don’t look like the characters/objects they’re named for…
Anyway. Sagittarius and the nearby constellation Scorpius (I’m fairly certain that one’s a scorpion) are both very close to the galactic center.
This region is absolutely teeming with objects to look at–from the open clusters M6 and M7, to nebulae like M8 (Lagoon Nebula) and M20 (Trifid Nebula), to globular clusters like M69 and NGC 6681.
An actual crap ton of the objects here are globular clusters–just download a sky atlas app such as Sky Map, and with a low-power telescope or binoculars, you can find them all!
At the very heart of the galactic bulge is Sgr A, abbreviated from Sagittarius A. Sgr A is the strongest radio source in the bulge. It is, in fact, the location of our galactic nucleus.
There is, of course, a lot to pick apart from the radio image above–so let’s get started.
Notice all those circular patches? There’s a whole cluster up at the “top” (Sgr D SNR and Sgr B1 to name a couple), and some much larger ones down at the “bottom,” including SNR 359.0-0.9 and SNR 359.1-0.5.
Yeah…astronomers have a habit of coming up with very boring names for stuff. But on the bright side, these designations are actually more helpful for scientific research than creative names.
Anyway. The circular patches are distinct clouds of gas within the interstellar medium.
A few of these are HII regions, clouds of twice-ionized hydrogen. In simple terms, we’re talking about clouds of very hot hydrogen gas. Sgr D HII and Sgr B1 are both HII regions.
Here’s an example of an HII region that you and I can observe: M17.
Then, there are a few giant molecular clouds (GMCs) and smaller star-forming regions. Sgr B2 is a GMC that seems to be interacting gravitationally with Sgr B1, the nearby HII region.
Sgr C and Sgr E seem to be star-forming regions. Star birth in the central bulge of a galaxy is rare due to low numbers of the GMCs most commonly found in the spiral arms. But there is some–no doubt thanks to the chaotic nature of the galactic core!
If more wavelengths of light could reach our instruments from the central bulge, these star-forming regions would probably look something like this:
Above is an image from the constellation Cassiopeia. This one was caught by the Spitzer Space Telescope. It’s not a visible light phenomenon, so you and I wouldn’t see it if we looked in a telescope.
Other radio sources within the central bulge are supernova remnants, clouds of gas expelled during the death throes of massive stars. These clouds were once the atmospheres of the massive stars. Often, we can spot the remains of the stellar core in the center–in the form of a neutron star or black hole.
The supernova remnants in the central bulge are all abbreviated as “SNR.” As you can see, there are quite a few: Sgr D SNR, SNR 0.9 + 0.1, SNR 0.3 + 0.0, SNR 359.0-0.9, and SNR 359.1-0.5.
Many supernova remnants are visible-light objects you can see through a telescope, including the Crab Nebula and Veil Nebula.
The Crab Nebula is a very young supernova remnant. It’s still small enough to be captured by a single image. The Veil, on the other hand, is around 10,000 years old. When it formed, it was probably smaller than the Crab Nebula. But like an expanding shockwave, it has spread from the source of the supernova explosion.
The image above is only one faint segment of a greater ring-shaped phenomenon that appears in Earth’s sky as six times the diameter of the full moon. (This specific segment of the Veil is often called the Witch’s Broom).
The central bulge is also full of more creatively named objects: the Arc, Filaments, Cane, Snake, Pelican, Mouse, and Tornado.
Honestly, none of these look particularly like the objects they’re named for. In particular, I can’t make out any sort of vortex-shape in the Tornado, and the Mouse looks to me like a little comet–I don’t see any legs or ears!
The Tornado is, apparently, another supernova remnant. It’s a bit weird-looking, though. Almost like a lopsided lump of coal. Just look at SNR 359.1-0.5–see the obvious ring shape? The Tornado doesn’t look like that. It’s entirely possible it is being strongly gravitationally affected by the chaotic core, distorting its shape.
Astronomers aren’t certain how these various phenomena are created, especially the bright Arc near the center. They may be gas trapped in magnetic fields. Either way, I’d say there’s no doubt they’re spectacular.
But you might notice…there’s one feature of the central bulge I’ve left out thus far, and it’s the most spectacular one of all.
Meet Sagittarius A, a massive radio source at the heart of our galaxy.
Sagittarius A, abbreviated Sgr A, is undoubtedly the most mysterious part of our galaxy’s central bulge. This dense region, glowing bright at radio wavelengths, is the nucleus of our galaxy.
This region controls all gravitational activity in our galaxy. It holds more than 100 billion stars in orbit, along with countless interstellar clouds. It has a reach of at least 300,000 light years, necessary to hold the galaxy’s spherical halo.
Clearly, this region has very powerful gravity.
But what could explain that kind of powerful, far-reaching gravitational pull?
This is the central three parsecs of our galaxy, deep within the spherical radio source Sgr A.
Most of the space Sgr A occupies is strangely empty–this central phenomenon lies within a cavity of low-density gas. The cavity’s main occupants are a chaotic swirl of stars.
These stars have highly eccentric orbits–orbits that, unlike the tranquil orbits of our solar system, are highly elongated rather than nearly circular. Their orbits are also inclined to the galactic plane at steep angles, making a spherical system rather than a flat one.
Thanks to those orbiting stars, astromers can determine the mass of the central object, the intense radio source known as Sgr A*. (The asterisk * distinguishes it from the broader region of Sgr A.)
Observations indicate that Sgr A* is ridiculously massive.
Well, it would have to be, to hold an entire galaxy in orbit. Still–the numbers are astounding. Within a space less than 34 light-hours across is an object of 4 million solar masses.
Four million. That’s four million times the mass of our sun.
What could Sgr A* be? A cluster of massive stars or neutron stars, or even a cluster of stellar-mass black holes?
None of those hypotheses can explain how close these orbiting stars pass near Sgr A*.
Sgr A* must be a single black hole with a fairly tiny event horizon.
But that would mean that we’re dealing with a supermassive black hole–a black hole far more massive than even the most massive dying stars could produce.
How the heck do we explain such an object? How would a supermassive black hole get all the mass it needs to form in the first place?
The answer may lie with a mysterious concept I touched on earlier, in my post on the mass of the Milky Way: dark matter.
Again, it’s not quite time to explore dark matter yet. We have some more galaxy-exploring to do before we’re ready. But it’s coming up–soon.