Black Hole Flings Companion Star at 2,000,000 Kilometers Per Hour!

By Phil Plait

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Posted Wednesday, March 20, 2013, at 7:30 AM

Astronomical artist David Hardy's phenomenal drawing of a  star orbiting a collapsed companion. Not originally meant to represent the system described here, it nonetheless paints a good picture of it. Literally. Image credit: David Hardy

Life near a black hole is…unpleasant. Especially if you’re a star, and especially if you’re prone to motion sickness. Case in point: A black hole was recently discovered that has a star orbiting it at an incredible two million kilometers per hour (1.2 million mph). Even though the star is about million kilometers (600,000 miles) away from the black hole—a little more than twice the distance of the Earth to the Moon—it screams around its orbit in just 2.4 hours.

Just thinking about it makes me want to hurl.

But it’s still pretty exciting, and just this little bit of knowledge tells us a lot about the system. And it has quite a history.

First, this record-breaking pair is at least 1000 light years away—an exact distance is pretty hard to determine, and it could be more than 20 times farther away than that—and was discovered in 2010 simultaneously by NASA’s Swift and the Japanese MAXI satellites (which is why the system is named MAXI J1659−152; after the satellite and the system’s coordinates on the sky).

Black holes can emit fiercely bright X-rays as they gobble matter down: The material is heated tremendously and gets whipped up by the ridiculously strong magnetic fields around the black hole and in the material itself. It can reach temperatures of millions of degrees and blast out X-rays that can be visible from Earth. In this case, matter is being drained off the companion star, which falls into a disk around the black hole before taking The Final Plunge. It’s the disk that is so hot and bright.

The European X-ray observatory XMM-Newton stared at the duo for over 14 hours, and confirmed what was first seen by Swift: a very regular dip in the light every 2.4 hours. From Earth we see the system revolving close to edge-on, and as the star and black hole orbit each other a small irregularity in the disk is probably blocking the X-rays, causing the dip. That’s how astronomers determined the orbital period. This video from the ESA should help:

The thing is, there’s no way the companion star—a red dwarf about 1/4^th the mass of the Sun—could’ve formed that close to a black hole. So how did it get there?

Here’s how this all may have happened. I’m guessing with some of this, so fairly warned be thee, says I, but it’s based on studying supernovae and knowing a bit about how systems like this evolve. Caveat lector.

A long time ago, perhaps billions of years ago, a binary star was born. One was a modest star, red/orange and cool, somewhat less massive than the Sun. The other was a monster, a massive blue hot-head probably 20 times the Sun’s mass or more. They were separated by quite a ways, tens or hundreds of millions of kilometers.

The blue star ate its fuel up pretty rapidly. By the time it was only a few million years old it was already dying. It swelled up into a red supergiant, expanding so hugely that if you were to replace the Sun with it, the surface of the star would stretch out well past the orbit of Mars! At this point the red supergiant would have been blasting out a super-solar wind, draining away its outer layers.

Still, the star swelled up so immensely it’s entirely possible that the dwarf star companion literally found itself inside the red supergiant’s atmosphere. You might think that would slow it to a stop quickly, but in reality space is big, and red supergiant atmospheres whisper thin. But—although it would take a long time—eventually friction would win and the dwarf would slowly spiral in toward the core of the star.

Supernova painting by my old friend and fantastic space artist Dan Durda.
Image credit: Dan Durda

Then, disaster. The core of the supergiant ran out of fuel and collapsed. This would start a chain reaction that ends in a supernova, one of the most powerful explosions in the Universe. The outer layers of the star are ripped away in the explosion, and the core collapsed down into a black hole.

At this point, the smaller star was much closer in to the core of the star, and suffered the brunt of the blast. It may have lost mass itself as the fury of the supernova blew past it, but it would survive, and due to a quirk of physics wouldn’t have been ejected unless the primary star lost a bit more than half its mass in the supernova event. Since we still see the star there, we know this didn’t happen. That means the smaller star remained bound, now on an elliptical orbit that dipped close in to the newly-formed black hole.

Over time, the gravity from the black hole would force the orbit to become circular. If the star were close enough, the black hole’s gravity would be able to strip material from the outer part of the star, forming a bright, hot disk…and leaving us where we are now.

A lot of the details of the history of the system are important. How massive was the smaller star to start with? That would help determine the age of the system. How close did it get to the core of the bigger star before the latter exploded? How much mass did it lose? We’re not even sure of the mass of the black hole, though it’s probably between 3 and 20 or so times the mass of the Sun—a pretty normal mass for a stellar black hole. A lot of what we see in the system now depends on what the two stars were originally like, and that information may be lost forever.

But what of the future? MAXI J1659−152 was discovered because it had an outburst, a sudden flare of brightness. It may be that the smaller star is undergoing some paroxysms as it feeds the black hole material. Over time it will lose more mass to the black hole and slowly spiral in toward it. There will come a day, inevitably, when the fierce tides of its more massive master will tear it apart. When that happens, the outburst will make what it’s doing now look downright weak in comparison. We’ve seen what happens when a black holes tears apart a star (with followups here and here) and it’s a fairly dramatic event. And by that I mean really, really, really dramatic. Read those posts; I won’t spoil it here. But holy yikes. After all, a whole star is getting torn apart by a black hole.

A computer simuilation, based on real physics, of a black hole tearing apart a star. Click to schwarzschildenate.
Image Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)

I have to say: Looking over what I just wrote, and assuming I’m in the ballpark, there’s not one thing about this binary that isn’t weird and awesomely cool. Seriously, every step of the way is just flippin’ amazing, right down to the idea of a black hole being able to toss around a whole star at speeds hundreds of times faster than a rifle bullet.

And yet we only happened to twig on to it because a celestial hiccup caught our attention a few years ago. As always, I have to wonder: What the heck else is out there just waiting to be found?