While looking deep into the heart of the galaxy, astronomers have seen the fastest star ever. This thing scurries through space; at one point in the orbit it moves at a staggering 24,000 kilometers per second – more than 85 one million kilometers per hour!
That is 8% the speed of light.
Holy relativistic acceleration! How can any star scream that fast through space?
Well, it helps to explode inward 1.9 billion kilometers of a supermassive black hole.
Oh yeah, there’s a story here.
At the center of our galaxy, the Milky Way, is an enormous black hole with a mass four million times that of the Sun. All large galaxies have one.
This black hole, called Sgr A * (literally, “Sagittarius A-star”) is not alone. Around it are a variety of objects, including dozens of massive stars. These orbit the black hole fairly closely, and the enormous gravity of the black hole turns them around at extremely high speeds. The movement so fast that changes in their positions over a matter of just months can be seen.
Not that it’s easy to see that. We are about 27,000 light-years from the center of the Milky Way, so although these stars are bright, distance decreases their brightness. There is also a lot of dust between us and the galactic center, making them even harder to observe. One way to do this is to see in infrared light that the dust is better absorbed by optical light, the kind we see.
It also helps if you use one of the enormous Very Large Telescope 8.2 meter behemoths in Chile. Using a pair of extremely sensitive and sharp-eyed cameras (called SYMPHONI and NACO), astronomers have been mapping these stars for many years. One in particular, called S2, has a lane so close in and so well defined that it was used to measure the mass of the black hole in the first place.
But several stars have now been spotted on even tighter orbits. One, S62, recently went very close to the black hole – less than 3 billion kilometers, closer than Neptune is to the sun! – produce all kinds of fun observations and science. For comparison, the black hole itself is about 24 million kilometers long.
In the new paper, astronomers announce a recently discovered star named S4714. The orbit is equal to S62s: Very elliptical, and throws it down extreme near the black hole. But it comes even closer than S62, which comes 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 succeeded, just absolutely cannonballs past Sgr A * at 8% light speed.
I will not like you. This makes the hair on the back of the neck stand up. That is curse.
I mean, that’s fast enough to cross the continental United States in a fifth of a second. Fast enough to reach the moon in about 16 seconds from Earth (it took the Apollo spacecraft more than three days to get that far). The Parker Solar Probe, when it is closest to the sun. moves at more than 100 kilometers per second, the fastest spaceship ever. This star is moving well more than 200 times faster.
You, and may I add, gads.
This is more than just a cool plate. For one thing, that near the black hole is relativity’s head. There is called an effect frame drag which 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, it means that astronomers can also observe this effect in near real time.
S4714 is so close to Sgr A * and so faint that even the largest infrared telescopes in the world have difficulty picking it out of the multitude of stars in that place in the sky. Yet it implies that there may be even stars even closer, perhaps just less massive and too weak to spot. However, there is a limit to how close they can get without falling apart. It will be interesting to find out how close these stars get. Hopefully future giant telescopes will have an even better view.
I remember when the first stars were seen near the black hole, and how their motion was used to measure their mass and our distance from it. It was not so long ago, but we have come a long way. It makes me envious of what else we will see in this interesting menagerie. And what records will be broken next.
* The orbit of the star is not 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 maybe not get that close. For this article, I take a closer approach of 1.9 billion.
Touch the ergosphere Bryan Gaensler.
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