You’ve probably heard there’s a supermassive black hole at the center of the Milky Way—and also that near the center of our galaxy there are a lot more stars. But did you ever think hard about what the Galactic Center is like?
I didn’t, until recently. As a kid I read about it in science fiction—like Asimov’s Foundation trilogy, where the capital of the Empire is near the Galactic Center on the world of Trantor, with a population of 40 billion. That shaped my impressions.
But now we know more. And it turns out the center of our galaxy is a wild and woolly place! Besides that black hole 4 million times the mass of our Sun, it’s full of young clusters of stars, supernova remnants, molecular clouds, weird filaments of gas, and more.
It’s in the constellation of Sagittarius, abbreviated ‘Sgr’. Let me go through the various features named above and explain them.
Sgr A contains the supermassive black hole called Sgr A*, which is worth a whole article of its own. Surrounding that is the Minispiral: a three-armed spiral of dust and gas falling into the black hole at speeds up to 1000 kilometers per second.
Also in Sgr A, surrounding the Minispiral, there is a torus of cooler molecular gas called the ‘Circumnuclear Disk’:
The inner radius of the Circumnuclear Disk is almost 5 light years. And inside this disk there are over 10 million stars. That’s a lot! Remember, the nearest stars to our Sun are 4 light years away.
Even weirder, among these stars there are lots of old red giants—but also many big, young stars that formed in a single event a few million years ago. These include about 100 OB stars, which are blue-hot, and Wolf-Rayet stars, which have blown off their outer atmosphere and are shining mainly in the ultraviolet.
Nobody knows how so many stars were able to form inside the Circumnuclear Disk espite the gravitational disruption of central black hole, and why so many are young. This is called the ‘paradox of youth’.
Stars don’t seem to be forming now in this region. But some predict that stars will form in the Circumnuclear Disk, perhaps causing a starburst in 200 million years, with many stars forming rapidly, and supernovae going off at a hundred times the current rate! As gas from these falls into the central black hole, life may get very exciting.
As if this weren’t enough, a region of Sgr A called Sgr A East contains a structure is approximately 25 light-years in width that looks like a supernova remnant, perhaps created between 35 and 100 thousand years ago. However, it would take 50 to 100 times more energy than a standard supernova explosion to create a structure of this size and energy. So, it’s a bit mysterious.
Moving further out, let’s turn to the Radio Arc, called simply ‘Arc’ in picture at the top of this article. This is the largest of a thousand mysterious filaments that emit radio waves. It’s obvious that the Galactic Center is wild, but these make it ‘woolly’. Nobody knows what causes them!
Here is the Radio Arc and some filaments:
Behind the Radio Arc is the Quintuplet Cluster, which contains one of the largest stars in the Galaxy—but more about that some other day.
Sgr B1 is a cloud of ionized gas. Nobody knows why it’s ionized. Like the filaments, perhaps it was heated up back when the black hole was eating more stars and emitting more radiation. Sgr B1 is connected to Sgr B2, a giant molecular cloud made of gas and dust, 3 million times the mass of the Sun.
The distance from Sgr A to Sgr B2 is 390 light years. That gives you a sense of the scale here! The whole picture spans a region in the sky 4 times the angular size of the Moon.
The two things called SNR are supernova remnants—hot gas shooting outwards from exploded stars. For example, in the top picture at lower right we see SNR 359.1-0.5, which looks like this close up:
The filament at right is called the Snake, while the Mouse at left is actually supposed to be a runaway pulsar. It looks like the Mouse is running away from the Snake! But that’s probably a coincidence.
Sgr D is another giant molecular cloud, and Sgr C is a group of molecular clouds.
So, a lot is going on in our galaxy’s center! Out here in the boondocks it’s more quiet.
Let me show you the first picture in all its glory without the labels. Click to enlarge:
It’s almost impossible to see the Galactic Center in visible light through all the dust, so this is an image in radio waves, made by the MeerKAT array of 64 radio dishes in South Africa. It was made by Ian Heywood with color processing by Juan Carlos Munoz-Mateos.
Here are two other versions of the same image, processed in different ways:
Click to enlarge!
Azimuth 
I’ve read that, just as there’s a habitable zone for stars, there’s a habitable zone for galaxies. Too far out there aren’t enough metals, too close in it’s too energetic (or as you say, “wild and wooly”). Our region may be more quiet, but it’s comfortably quiet!
(BTW: The link to the penultimate image — …MeerKAT_2.jpg — isn’t working.)
Interesting! Yes, I can see how it would be too energetic near the galactic center. I hadn’t thought about how there might not be enough metals near the rim of our galaxy. I guess these are ‘metals’ in the astronomer’s sense, which includes everything but hydrogen and helium. I’ve love to see a graph illustrating rough densities of various elements as a function of distance from the galactic center!
Thanks for catching that bug… I fixed it.
Yeah, astronomers are funny about “metals” but I did mean the heavier elements made only in supernovae and neutron star collisions (everything above iron). Also, IIRC, too close to the galactic center, and things are too metal-rich because of all the activity.
Per a comment I made on a previous post about chances of intelligent life evolving, one can also wonder about the chance of being near enough to old supernovae or neutron star collisions for there to be sufficient amounts of heavier elements that may be necessary for complex life to evolve. But not near any new events that would sterilize the planet.
As I understand it, the Solar system is in a bubble thought to be created by an ancient supernova, and that bubble may also be part of the evolutionary picture.
Arguments about “galactic habitable zones” are about “metallicity” in the usual astronomical sense: all elements heavier than helium. The main idea is: how likely is it to form planets when you form a star? Since the Earth is primarily iron, silicon, oxygen, and magnesium, this is a question about elements lighter than iron (+ iron itself); elements heavier than iron aren’t really relevant. They’re generally not relevant to life, either. (Minor exception: long-lived radioisotopes may affect things like volcanic activity and plate tectonics, and thus might be relevant to the long-term habitability of a planet.)
“Being near enough to old supernovae or old neutron star collisions” is not an issue, because the heavy elements produced in such explosions diffuse through the gas disk over time (azimuthally more than radially). When a star and its accompanying planets form, the amount of heavy elements depends overwhelmingly on the cumulative history of explosive nucleosynthesis at or near that Galactic radius, not on any recent local explosions.
“the Solar system is in a bubble thought to be created by an ancient supernova”
This is probably the Local Bubble, in which the hypothesized supernova(e) happened between ten and twenty million years ago. Which is “ancient” in human terms, but not in terms of the Solar System’s 4.6 billion-year history.
Thanks for that Wiki link! My sense is that it supports the argument about habitable zones. An astronomical argument about planet formation may indeed be about the astronomical meaning of metallicity, but an argument about the formation of intelligent life is about elements heavier than iron. Which are important for life as we know it. The very heavy radioisotopes may play a role in tectonics (as well as in keeping Earth warmer) but may also have played a role in the genetic mutations necessary for evolution.
I don’t know to what degree elements created by supernovae or neutron star collisions do diffuse throughout the putative “habitable zone” torus, but FWIW the Wiki article mentions that “the concentration and ratios of these vary throughout the galaxy.”
The Local Bubble, as you say, is too recent to have played a role in the formation of the Solar System, but my comment was about the evolution of intelligent life. Homo sapiens evolved roughly only 300,000 years ago. That said, the role of the LB is very speculative.
One criticism I noted in the Wiki article is about stars whose distance from galactic center varies by tens of thousands of LY. But who’s to say those stars are capable of supporting evolution to complex life? By the habitable galactic zone argument, they wouldn’t be!
I’ve long wondered about the Fermi Paradox, and my putative resolution is that intelligent life is extraordinarily rare. A simplified version of the Drake Equation speculates that, if at least five conditions or events with a probability of 1:10^4 are required for intelligent life, then the odds are 1:10^20 compared to a stellar population in the Local Group on the order of 10^14 or so.
I suspect elements heavier than iron are not necessary for Earth-like life. Yes, life on Earth uses them, but we should expect life to try using anything that’s around. Probably life could even do without iron.
Perhaps. It plays an important role in the magnetic field that shields us from the Solar wind. It’s possible we owe our large hot iron core to contributions from Theia, so that might be another bit of luck for us.
It’s possible that CHON-based life (like us) is a sweet spot for chemistry and biology. Silicon-based life is a pop favorite, but a putative waste product, silicon dioxide, is a solid and, more importantly, very stable. Carbon dioxide, in contrast, a nice gas that is broken down by available chemical reactions.
As I mentioned to Peter Erwin, it’s an interesting question to me whether complex life is an inevitable attractor or entropically very surprising excursion into the phase space.
Re metals and “heavy” elements: I think there’s some confusion here. Supernovae produce almost all elements heavier than boron, including things like oxygen and sodium and phosphorus and also many that are heavier than iron. (See, e.g. this version of the periodic table.) It’s only for (most but not all) elements heavier than molybdenum that neutron-star mergers are the dominant source. So there isn’t a meaningful distinction between elements lighter and heavier than iron.
From the Wikipedia article you linked to, it appears that the only ubiquitous/essential element that is produced mostly in NS mergers is bromine. (Whether that’s generically essential for life everywhere would be really hard to say.)
Expanding stellar winds (which contribute some metals) and explosions (e.g., supernovae and NS mergers) diffuse and turbulently mix into the existing interstellar gas; some is temporarily ejected out of the Galactic disk in “fountains” and then rains down elsewhere later on. All of this is further churned by the differential rotation of the disk and the gravitational influence of spiral arms. Such processes have been going on the the disk for roughly eight billion years. So, yes, elements produced by stars and stellar explosions do get thoroughly mixed azimuthally, and somewhat mixed radially.
As for mutations: keep in mind that mutations come from many sources, including cosmic rays, UV light from the Sun, chemical mutagens, and errors in DNA replication and repair. (And something like a quarter or a third of the Earth-based radioactivity humans are exposed to is from K-40, which is not “heavier than iron”.) I don’t think it makes sense to think that the evolution of life (let alone the evolution of “intelligent life”) depends in any meaningful sense on the abundance of uranium and thorium, aside from the possible role in vulcanism and plate tectonics.
I’ll admit I don’t see how the Local Bubble is supposed to be relevant to the evolution of life, intelligent or otherwise, in any way. (The Sun has been passing into and out of similar bubbles throughout its life, but the “metal” content of the Earth was basically set at its formation 4.6 billion years ago.)
Supernovae and neutron star collisions come into the picture in contributing elements stellar fusion doesn’t. I mention both novae and collisions only because they have different contribution profiles. (Iron only comes up here as the cutoff for fusion.) The habitable zone just assumes stronger diffusion along the azimuth than radially so there exists at some distance a most favorable blend of elements from supernovae, neutron stars, and stellar fusion. A Goldilocks combo.
I think it depends on whether, given the phase space of stellar evolution, planetary formation, and abiogenisys, complex (intelligent) life is an attractor — making it inevitable — or is highly improbable — “uphill” against great odds. If it’s the latter, one then speculates about necessary conditions. I know many view complex life as an attractor where many initial conditions lead to complex life. They may well be right.
OTOH, maybe we beat enormous odds due to just the right conditions. All the right elements, because a habitable zone and/or nearby events providing the right concentration. There are heavy elements that may not be essential for our lives now, but which may have been catlysts or otherwise participated in evolution. Or which are necessary in the food chain or other important chemical cycles. Just the right genetic mutation rate might be a factor. The right star, atmosphere, radioactives in the crust, these may all matter. The radioactives are, I think, important to the extent they keep the core hot. Maybe the Local Bubble provides a “quieter” neighborhood for evolution. All very speculative — on many counts, I admit — but I don’t think entirely ruled out?