Black holes are probably among the scariest things in the universe, with gravitational forces powerful enough to warp the fabric of spacetime itself. Red dwarfs, on the other hand, are amongst the smallest of stars, shining dimly in the darkness — not exactly the sort of pairing which you might expect to be make dancing partners.
All the same, that’s exactly the pairing you’ll find in a star system known as MAXI J1659-152. This system contains just such an odd couple, locked in a tight orbit where a red dwarf is speeding around it’s heavier companion at an astonishing two million kilometers per hour (1.2 million mph)!
So fast is this orbit that it’s set a record for the fastest binary orbit ever discovered. The reason for this blistering speed is how close these two objects are to each other; the star is just one million kilometers (600 thousand miles) away from the black hole. That’s a little under three times the distance from Earth to the moon which, I can safely say, is a lot closer to a black hole than I’d ever like to be!
Being so close together, this doomed red dwarf is slowly being devoured as material is sucked from its surface into an accretion disk that is feeding the black hole. It was first discovered by NASA’s Swift space telescope in 2010, which mistook it for a gamma ray burst. It wasn’t until later observations by the Japanese MAXI telescope aboard the ISS found a bright x-ray source in the same spot.
X-ray sources in astronomy are a signpost, pointing to some of the most energetic things we can see in the universe around us. A black hole certainly qualifies. As material is torn away from the surface of this star into the black hole’s disk, it gets caught up in intense magnetic and gravitational fields. This heats it to incredible temperatures, eventually tearing apart the very atoms that once belonged to the red dwarf, causing that disk to shine brightly with x-rays.
Of course, the black hole itself emits no detectable light. It will hungrily consume anything which falls into its event horizon, including photons. The disk around a black hole, on the other hand, is actually able to convert matter into energy more efficiently than any star can, which makes it a tremendously powerful source of energy.
In the MAXI J1659-152 system, what we see is essentially a massive and slightly frightening version of the transiting exoplanets that NASA’s Kepler telescope has been hunting for. The black hole’s disk is almost edge-on from our point of view, and when the red dwarf passes between us and that disk, there’s a slight dip in the x-rays we can see. ESA’s XMM-Newton x-ray observatory picked up on this. Staring at this black hole for an uninterrupted 14.5 hours, it saw 7 of these dips, allowing the red dwarf’s orbital period to be measured at a record breaking 2.4 hours (the previous record holder was the Swift J1753.5–0127 system, with an orbit of 3.2 hours).
Oddly, the couple are high above the plane of the Milky Way, a trait that only two other black hole binary systems share. The reason for this weird placement may be down to how the black hole formed.
Stellar-mass black holes are created in supernovae, when massive stars explode. The star which created it would have been huge. A giant blue star, probably over 20 times as massive as the sun. But the most massive stars are also the shortest lived. This luminous blue star was to greedily use up all of its fuel, before swelling into a red supergiant and eventually exploding.
The kick from that explosion launched this pair high up, out of the plane of the galaxy. The end result is a black hole weighing in at around three solar masses. Its unfortunate stellar companion probably had some of its outer layers blasted away by the supernova, and with a gluttonous black hole slowly eating it, it’s currently just 20 percent the mass of the sun.
We can’t really say for certain what this star’s fate may be. Swift spotted it because of an outburst, likely as a huge mass ejection from the red dwarf was swallowed up by the black hole, causing a blast of gamma rays. But as the black hole feeds, the orbit of its undersized companion is going to become smaller and smaller. It probably doesn’t need to get too much closer before it’s literally torn apart. If that should happen, then we’ll see some real fireworks.
Image: Artists impression of the MAXI J1659-152 system. Credit: Markus Hammonds/supernovacondensate.net