Galaxies are scary places.
Space itself is scary enough—dark, vast, cold, and empty—but within galaxies lurk all sorts of terrifying beasts. Most of these astrophysical monsters are stars with various behavioral problems such as explosive supernovae or ridiculously powerful(and stormy) magnetars. Getting too close to one of these star tantrums guarantees very bad times.
On top of that, the galaxies themselves can be terrifying. In a recent column for The UniverseI wrote about particularly nasty cosmic nightmares called active galaxies. You see, all large galaxies have a supermassive black hole haunting their heart. Some of these black holes have billions of times the mass of the Sun.. If matter falls into one of these black holes at high speed, that falling material heats up to alarming temperatures and emits high-energy radiation. It also generates strong winds of subatomic particles that are carried away at high speeds, wreaking havoc around the galactic core. It's all bad enough, but A black hole's spin can also fire twin matter beams called jets, which can zip through space like death rays, frying everything in their path for many thousands of light years.
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This doesn't sound like a gentle environment! After publishing the article, I received many requests from readers asking alarming questions: how dangerous is it to live in an active galaxy and is the Milky Way one?
As usual, the answer is: it depends. Supermassive black hole of the Milky Way. called Sergeant A*It seems fairly quiet at present, but we don't know how active our galaxy was in its youth. However, we can see echoes of previous events around the galactic center; From the heart of the Milky Way extend two huge bubbles of hot gas, glowing softly with gamma rays. It is unclear whether these bubbles, called Fermi bubblesblown away from Sagittarius A* material after a snack or if a burst of star formation has created enough high-mass stars to dislodge the gas-expanding winds.
And as for the more fundamental question of hazards in active galaxies, it's not very well understood, but I found a couple of papers that addressed the question in limited circumstances. The results, unsurprisingly, show that the two most important factors are how powerful the active galaxy is and how far away you are from it.
In extreme cases, even being outside the active galaxy will not provide you with safety. A striking example of this is the pair of galaxies 3C321.where a jet from one galaxy crashes into another at a distance of about 20,000 light years. Being in the path of all that high-energy radiation must be tough, but the jet's impact also causes clouds of gas in the second galaxy to collapse, triggering bursts of star formation. This, in turn, means the creation of many high-mass stars that explode as supernovae in just a few million years.
Opinions vary on how close a planet can get to an exploding star and survive, but a few hundred light years is a conservative estimate. On a galactic scale this is very close, so on average it is unlikely to affect any particular world. On the other hand, on a galactic scale, a few million years is an instant, so all these supernovae ignited by the jets will explode almost simultaneously, which cannot be good for any worlds in the immediate vicinity. I would advise you to stay away from airplanes.
What about other effects? Subatomic particle wind is a problem, two problems actually. First, if wind hits a planet like Earth, it can transfer energy into the atmosphere, warming it. This causes the air to expand, making it more likely to be lost into space. Second, when these particles enter the atmosphere, they can cause several different chemical reactions, including the formation of nitrogen oxides, the main components of smog. This has several consequences, but one of the most damaging is that smog destroys ozone, a molecule made up of three oxygen atoms. Ozone absorbs the energetic ultraviolet light from the parent star, which could otherwise reach the surface and harm life.
In an article published earlier this year in Monthly Notices of the Royal Astronomical Society, team of astronomers modeled these two effects from an active galaxy. Researchers first suggested that the galaxy contains a black hole similar to Sgr A* in our Milky Way, with a mass of four million suns. They then calculated the wind's energy and found that it could destroy the atmosphere of an Earth-like world up to 1,300 light-years away, or simply deplete the planet's ozone if it was within about 3,000 light-years. These distances may seem dauntingly large, but the radius of the Milky Way is 60,000 light-years, and our solar system is about 26,000 light-years from the center, so we are safe from nasty winds.
This is cute. Even if some event causes a large amount of material to be dumped into our local supermassive black hole, we will get a light show but must survive.
However, not every galaxy is so lucky. Sgr A* is actually lighter than a galaxy our size, and many galaxies have supermassive black holes that are much larger—large enough, according to researchers, to potentially sterilize the whole galaxy when they burst into activity. I am glad that our local animal is short.
Another article published in 2022 in the journal Astrophysical Journal, looked at ultraviolet exposure about the habitability of the galaxy. What the authors discovered is interesting: UV radiation coming from an active galaxy can actually promote the growth of protective ozone in the planet's atmosphere, provided that there is enough oxygen for its formation. So, oddly enough, an active galaxy may help create the conditions for life, at least in this very narrow case.
However, like the relatively localized dangers posed by supernovae, the UV dead zone of any supermassive black hole would still be largely confined to the galactic core and thus would affect only a small percentage of all stars. In a spiral galaxy like ours, which has a flat disk filled with opaque dust that absorbs UV radiation.the effects would be virtually nil for a planet like Earth, which was on the outer edges of the disk.
This does raise the question of how habitable our galaxy as a whole is. This has indeed been studied, although again the conclusions vary. An early attempt at this in 2004 discovered that there was a “galactic habitable zone”. similar to those surrounding the starsin which life can arise. There are too many exploding stars too close to the center in the early stages. (Also, younger galaxies tend to be more active.) Too far away, stars tend to be too old, depleted of heavy elements such as iron and carbon, which are essential for life as we know it. This initial work depicted a galactic habitable zone that extends approximately 22,000 to 30,000 light-years from the center, essentially right where we are. But later studies expanded on this; one team of astronomers has discovered that the life-friendly region of the Milky Way extends from nearly 7,000 to more than 40,000 light-years, far beyond the extended disk of our galaxy.
However, I wonder—and worry—about what future opportunities await us. If the huge Andromeda Galaxy collides with the Milky Way, which could happen in a few billion yearsa lot of matter could be dumped into Sagittarius A*, as well as into the much more massive black hole at the center of Andromeda. This could briefly revive both galaxies as active. Any planets within a few thousand light years of either planet's core could be in dire straits.
Either way, it's all an eerie reminder that the universe tolerates us, but doesn't necessarily support a stable environment for life. If you need to feel a chill down your spine this Halloween, look to the dark sky while you go trick-or-treating. Take a look at The eerie glow of the Milky Way and marvel at how hospitable our galaxy really is.
