August 26, 2012
The Black Hole That (Didn't) Devour the Galaxy
When I was a kid, black holes had come into fashion. The term "black hole" had been coined recently (1967) and the term captured the public's imagination as black holes have continued to fascinate to this day.
At the time black holes were depicted as unstoppable, ravening monsters that ate everything. As a child when I thought about the end of the planet, I was less worried about giant asteroids destroying the Earth than I was about black holes eating it. There was a time when I thought the fate of the galaxy --- even of the universe --- was to be swallowed by black holes. The black holes would eat until the entire galaxy would be nothing but black holes and then the black holes would turn on each other and swallow one another until all that would be left was one giant black hole per galaxy..........and that would be the end of the universe. (Current thought is that black holes can merge to create even larger black holes -- so I had that much correct. This merging is thought to be part of how supermassive black holes in the center of galaxies form.)
So scientists like to sensationalize. And black holes were a good topic to do that with. However, eventually I couldn't help but notice that black holes had been around for quite a while and the galaxy was still here. In fact, instead of being rare, it turns out that black holes are fairly common. One out of every thousand stars is big enough to end up a stellar black hole. The Milky Way galaxy alone is supposed to have a hundred million stellar black holes. At the center of the Milky Way is a different kind of black hole, the supermassive black hole, the biggest black hole in the galaxy plus the center of the galaxy is packed with stellar black holes and yet the center of the galaxy is packed with mass ----- it's the brightest, densest part of the galaxy. If the highest concentration of black holes in the galaxy can't even manage to suck in all that rich, juicy matter surrounding it, the galaxy as a whole was safe as black holes were not the ultimate power I was afraid of as a child. I only wish the HubbleSite had been around back then to comfort me and let me know, "A black hole can only capture objects that come very close to it. They're more like Venus' Flytraps than cosmic vacuum cleaners"1 as I would have been saved some nightmares. (Latest theory is that black holes are self-limiting. As the black hole "feeds" radiation from the material being sucked in "blasts the black hole's neighbourhood free of additional sustenance."2)
Yet, the popular perception of black holes ----- fed by science programs featuring real astronomers ---- is still of that ravening monsters. A black hole will destroy anything that gets near it. "Near it".........a phrase that gets me irritated with its imprecision. "Too near it" would be more accurate it.
I once spent a single, glorious day visiting Niagara Falls (the Canadian side). There's a tourist attraction where you can go behind Niagara Falls. It's a tunnel through the cliff the falls go over. As I stood there looking at this gray wall of water rushing in front of me, the roar of it pounding in my ears, I marveled to be near so much power yet to be completely safe and protected from its effects. Niagara Falls, like a black hole, is the power of gravity in action. Near was safe. Too near it and I was dead.
Same thing for black holes. The theoretical model people love to talk about it is being just outside of the event horizon. So let's assume you had a ship powerful enough to survive just outside of the event horizon (where the black of the black hole begins). The event horizon is considered the outer limit of the actual black hole as it's the "point of no return," the spot where escape becomes impossible. (Though the reality is that you start experiencing very strong gravitational forces before you cross the event horizon --- while you're still in the surrounding accretion disk made up of material being visibly pulled into the black hole. Close to the event horizon you start to get stretched or "spaghettified;" how much of that you can take and still live, no one knows.) Stellar black holes range from about 10 to 100 solar masses, giving them a radius of from 30 to 300 kilometers. Taking an average of that gives a black hole of 55 solar masses and a radius of 160 kilometers. That's a 100 miles. One hundred miles from the black hole's center to the event horizon. In astronomical terms 100 miles is sitting in something's lap. Yet in this scenario, near as you were, you'd be so safe that you could drop trou and moon the black hole through a port. You could scan your butt and send the image out in a signal into the black hole.
But, as I said, that was putting yourself just outside the event horizon. If you wanted to be outside the black hole's accretion disk, that distance is much more variable, but, depending of the size of the black hole, that could be as little as 110 miles (six radii). If you only have to be six radii from our hypothetical average black hole of 55 solar masses, that would be 600 miles which is still sitting in something's lap when it comes to space and distance. I'll leave it to the HubbleSite to sum up the dangerousness of getting "near" a black hole with "It is possible to be near a black hole without falling into it, provided you move rapidly."3
Now having established how wimpy black holes are, recently, I happened to tune in toward the end of an episode of Deep Space Marvels (ep "Survival") depicting our Solar System being devoured by a black hole......the nightmare from my childhood. But I'm no longer that child. Instead of being frightened, I was merely puzzled. No black holes have been discovered in the vicinity. The nearest black hole found so far, V4641 Sgr, is a whopping 1600 light years from Earth. Even if the closest star to Earth, our very own Sun, became a black hole (which it cannot as it's too small), according to NASA, "If our Sun was suddenly replaced with a black hole of the same mass, Earth's orbit around the Sun would be unchanged."4 In fact, the Earth would have to be an incredibly close ten miles from the Sun-as-black-hole before it would be in danger of being drawn in.5
According to the Cornell University's "Curious About Astronomy? Ask an Astronomer" website, even a supermassive black hole (the nearest is 27,000 lightyears from us) would be "within a few hundred AU (1 AU = distance between the Earth and the Sun) of us, it would start to seriously disrupt the orbits of the planets in our solar system, including the Earth....Once the SBH came within about 1 AU of the Earth, the gravitational tidal forces would rip the Earth itself apart....Then the pile of rubble that was once the Earth would fall into the SBH."
So for the Sun-as-a-black-hole, the Earth would have to be within 10 miles to be drawn in. Even for a supermassive black hole, the Earth would have to be within 1 AU to be drawn in. (One AU is about 93 million miles.) That gives us a range for a stellar black hole (the kind the galaxy is filled with) as having to be between 10 miles to 93 milliion miles in order to draw in the Earth. Since supermassive black holes are over a billion solar masses while stellar black holes are in the range of a hundred solar masses, you can bet the Earth will have to be much closer to the 10 mile end of that range, not the 93 million end. That range is the best estimate that I could find.
So I decided to ask. After all, they do call the website "Curious About Astronomy? Ask an Astromer." I posited the example I used above of a stellar black hole of 55 solar masses and radius of 160km (100 miles) and asked how close the Earth would have to be to it in order to get drawn into the black hole. Here's the answer I received:
"Hmmmm, interesting question. If the Earth were orbiting a stellar black hole of 55 solar masses at the same distance that it orbits the Sun, it would have to be moving much faster. Otherwise it would have a very elliptical orbit, but it would still not be in danger of falling into the black hole.
In order to fall into a black hole, you have to be very close to its event horizon. It turns out that there is an inner boundary, called the ISCO (inner stable circular orbit), that marks the inner most point at which objects can orbit with a circular path. Objects can orbit interior to the ISCO, but their orbits must be elliptical. However, the ISCO is only a few times the radius of the black hole (depending on the spin of the black hole). So, you still have to be at only a few times the radius of a black hole to be in danger of falling in.
However, if you were ever nearly that close to a black hole of 55 solar masses, you'd be torn apart due to the tidal gravitational forces of the black hole. So the Earth and everything on it would be torn apart if it ever approached even near the ISCO."7
A few times the radius of the black hole...........meaning only a few hundred miles. It's an understatement to say there's a lot we don't know about the universe. But if there were a stellar black hole that close to Earth ----- that we'd know. I understand shooting the bull and theoretical speculation, but since this was a science program I was watching, an explanation of how this event they were depicting could happen would have been appropriate.....How could a stellar black hole get here? I kept watching, and it called the black hole a "rogue." My understanding of rogues is rouges are caused when something with enough gravity passed by the object to send it out of its orbit. What, I wondered, could have had enough gravity to disturb a black hole and send it traveling into our system? I figured the program must have covered that in the part I missed. I checked and the program repeated later that night. So I set the autotune and made sure to watch it.
No, the program, filled with great graphics and comments from actual astronomers, told about black holes and how powerful they are and then, out of the blue, depicts some black hole from nowhere that without explanation somehow ends up at the heart of the Solar System, managing to pass close to every one of the planets in the process, devouring the entire Solar System one body at a time until there's nothing left but the black hole in empty space where our home used to be.
I just had to shake my head. No wonder I used to have nightmares about black holes when I was kid. And apparently they're still trying to give this generation of kids nightmares too.
1"How do black holes grow?," Hubble Site, (accessed July 2012).
2David Shiga, "How big can a black hole grow," New Scientist, September 2008, (accessed July 2012).
3"Can I safely orbit a black hole?," Hubble Site, (accessed July 2012).
4 "Black Holes," NASA, (accessed July 2012).
5"How do black holes grow?," Hubble Site, (accessed July 2012).
6 Christopher Springob, "What would happen if a supermassive black hole came close to the Earth?," Curious About Astronomy? Ask an Astronomer, March 2003, (accessed August 2012).
7Jim, "Ask an Astronomer question" to me via "Curious About Astronomy? Ask an Astronomer,"Aug 24, 2012.
"Black Holes," NASA Science, (accessed July 2012).
"How many black holes are there?," Hubble Site, (accessed July 2012).
Robert Roy Britt, "Milky Way's Center Packed with Black Holes," Space.com, January 11, 2005, (accessed July 2012).
"Fall into a black hole," Hubble Site, (accessed July 2012).
"What is the size of the Solar System," Cornell, (accessed July 2012).
"Black Holes," Journey Through the Galaxy, January 11, 2006, (accessed July 2012).
"How big is a black hole?," ThinkQuest Library, (accessed July 2012).
Ori Sontata, "Can Black Holes Move or Do They Just Stay at Their Position?," Scienceray, April 16, 2009, (accessed July 2012).
"Are any black holes close to Earth?," StarDate, (accessed July 2012).
Tega Jessa, "Where is the Nearest Black Hole," Universe Today, October 14, 2010, (accessed July 2012).
Marek A. Abramowicz and Odele Straub, N, "Accretion discs," Scholarpedia, (accessed July 2012).
Christopher Wanjek, "Ring Around the Black Hole," NASA, February, 21, 2011, (accessed July 2012).
"Size of accretion disks around supermassive black holes," Bad Astronomy and Universe Today forum, April 2009, (accessed July 2012).
For more on astronomy, read "Star Wars, Einstein and When Lucas Got It Right" and it's addendum, "Difficulty Levels of Death Star Versus Various Astronomical Bodies."
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