A simulation showing the positions and orbits of stars orbiting the supermassive black hole in the center of the Milky Way. Credit: ESO/L. Calçada/spaceengine.org
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A simulation showing the positions and orbits of stars orbiting the supermassive black hole in the center of the Milky Way. Credit: ESO/L. Calçada/spaceengine.org

The fastest star in the galaxy has been found, and yeah, it’s really *really* fast

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Aug 17, 2020, 9:29 AM EDT (Updated)

While peering deep into the heart of the galaxy, astronomers have found the fastest star ever seen. This thing is tearing through space; at one point in its orbit, it’s moving at a terrifying 24,000 kilometers per second — over 85 million kilometers per hour!

That’s 8% the speed of light.

Holy relativistic acceleration! How can any star be screaming through space that quickly?

Well, it helps to plunge down to within 1.9 billion kilometers of a supermassive black hole.

Oh yeah, there’s a story here.

In the center of our galaxy, the Milky Way, is an enormous black hole with a mass four million times that of the Sun. All big galaxies have one.

This black hole, called Sgr A* (literally, “Sagittarius A-star”) isn’t alone. Surrounding it are a variety of objects, including dozens of massive stars. These orbit the black hole pretty close in, and the enormous gravity of the black hole swings them around at fiercely high velocity. The move so rapidly that changes in their positions can be seen over a matter of mere months.

Not that it’s easy to see that. We’re about 27,000 light years from the Milky Way’s center, so even though these stars are luminous, distance reduces their brightness. Also, there’s a lot of dust between us and the galactic center, making them even tougher to observe. One way to get around that is to look in infrared light, which pierces the dust better than optical light, the kind we see.

It also helps if you’re using one of the enormous Very Large Telescope 8.2-meter behemoths in Chile. Using a pair of extremely sensitive and sharp-eyed cameras (called SINFONI and NACO), astronomers have been mapping these stars for many years. One in particular, called S2, has an orbit so close in and so well determined that it was used to measure the mass of the black hole in the first place.

The orbits of several black-hole-divebombing stars in the very center of the Milky Way as seen on the sky (the center is the position of Sgr A*, the vertical axis is north/south, horizontal is east/west). S4714 is marked by the red arrow, and has an orbit similar to S62. The dots are their positions in 2007. Credit: Peissker et al.

 

But several stars have now been spotted on even tighter orbits. One, S62, recently passed very close to the black hole — less than 3 billion kilometers, closer than Neptune is to the Sun! — producing all sorts of fun observations and science. For comparison, the black hole itself is about 24 million kilometers across.

In the new paper, astronomers announce a recently discovered star called S4714. Its orbit is similar to S62’s: Highly elliptical, and dropping it down extremely close to the black hole. But it gets even closer than S62, coming within about 1.9 billion kilometers*. That’s closer than Uranus is to the Sun.

When S62 flies past the black hole it moves at about 6.7% the speed of light, but S4714 has it beat, just absolutely cannonballing past Sgr A* at 8% lightspeed.

I won’t lie to you. That makes the hair on the back of neck stand up. That’s fast.

I mean, that’s fast enough to cross the continental United States in a fifth of a second. Fast enough to get from the Earth to the Moon in about 16 seconds (it took the Apollo spacecraft more than three days to get that far). The Parker Solar Probe, when it’s closest to the Sun. moves at over 100 kilometers per second, the fastest spacecraft ever. This star is moving well over 200 times faster.

Ye, and may I add, gads.

This is more than just a cool record. For one thing, that close to the black hole relativity rears its head. There’s an effect called frame dragging that shifts the orientation of the orbit, causing it to rotate. The orbits of these stars have been seen to do this, which helps us better understand relativistic effects of the black hole on nearby space. Because the orbit of S4714 is only 12 years long, that means astronomers can observe this effect in near real time, too.

The positions of 25 stars close to Sgr A* (marked with a black X) using an image from 2008. S4714 is number 19 (upper right). As you can see, tracking these stars is difficult. Credit: Peissker et al.

S4714 is so close to Sgr A* and so faint that even the world’s biggest infrared telescopes have a hard time picking it out of the crowd of stars on that spot in the sky. Still, it implies that there may be stars even closer still, perhaps just less massive and too faint to spot. There’s a limit to how close they can get, though, without getting torn apart. It’ll be interesting to find out just how close these stars get. Hopefully future, gigantic telescopes will have an even better view.

I remember when the first stars were seen close in to the black hole, and how their movement was used to measure its mass and our distance from it. It wasn’t that long ago, but we’ve still come a long way since. It makes me wonder what else we’ll see in this curious menagerie. And what records will be broken next.


* The orbit of the star isn’t perfectly determined, and has a large uncertainty. The closest approach is actually 1.9 billion kilometers ± 1.4 billion, so it may actually get closer… or may not get as close. For this article I assume a closest approach of 1.9 billion.


Tip o’ the ergosphere to Bryan Gaensler.

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