A star with the shortest known orbit around Sgr A*, the supermassive black hole at the center of our galaxy, has been found: it takes just four years to orbit the giant once.
Named S4716, the star is part of a relatively recently discovered cluster of massive stars centered on the black hole at the heart of the Milky Way. As these stars move in their orbits over time, their positions can be measured to determine their orbits, and from there key features of the black hole itself can be found.
Sgr A*, as you may recall, was the subject of a recent series of observations using radio telescopes around the world to get a picture of matter in a swirling accretion disk. This material is only about 60 million kilometers from the black hole – not much further than Mercury is from the Sun. However, the stars in the cluster are many billions of kilometers from their closest, ranging to a tenth of a light-year, or about a trillion kilometers.
The individual stars in the so-called S cluster are difficult to observe for many reasons. One is simply because of how close they are to each other and how far they are from Earth. Even with our largest telescopes, stars can overlap and cause confusion. In addition, a lot of dust swirls around in the galactic center and obscures the view. Because of this, astronomers tend to use telescopes designed to see in infrared light, which can penetrate the dust and give us a clearer view. Even then, observations are like this difficult.
To make matters worse, S2, the brightest star in the cluster, tends to drown out light from nearby stars. Therefore, it is difficult to find stars that are fainter and close to the black hole.
Using different cameras on the giant Keck and Very Large Telescopes, a team of astronomers examined data collected from Sgr A* over the past two decades. [link to paper]. Using sophisticated image cleaning and sharpening techniques, they see a previously undiscovered star they call S4716 in images taken over 16 different observation periods. The star moves in a decently elliptical orbit (about 0.75 eccentricity for you geometry nerds) with a period of about 4 years around the black hole.
It passes as close as 1.5 billion kilometers from Sgr A*, which is quite close – about Saturn’s distance from the Sun – and is furthest about 10 billion kilometers, about twice Neptune’s distance from the Sun .
Given the black hole’s tremendous gravity, this means that at closest approach, the star is moving at a staggering 28 million kilometers per hour: that’s 2.6% the speed of light. Yegads.
Amazingly, this isn’t the fastest star orbiting Sgr A*! A star found a few years ago called S4714 is in a very elliptical orbit that accelerates it to a speed of about 85 million km/h, or 8% of the speed of light. The orbit of this star stretches very long, taking it much farther from the black hole, and its period is closer to 12 years. Thus, S4716 currently holds the record for the shortest and most compact orbit of any star around Sgr A*.
S4716 is a large star, about four times the Sun’s mass and about 130 times brighter; Good thing, otherwise it would be impossible to detect. Its orbit depends on the mass of the black hole and its distance, so using the centuries-old equations for orbital motion they can calculate the mass of Sgr A*, getting a value of 4.023 ± 0.087 million times the mass of the Sun, consistent with previous measurements.
You can also find the distance to the black hole and find that it is 26,170 ± 650 light years from us. This also agrees with previous measurements. That’s good to know. And these numbers are one of the reasons astronomers are so eager to discover these stars, as they can limit our knowledge of the black hole.
The S cluster itself is also a mystery. It’s not clear how it formed, as gas near the black hole would be heated too much to collapse and form massive stars. It’s likely that they formed further out and got closer to Sgr A* through close gravitational encounters with other stars, a process called process mass separation. The more stars in the cluster that are found and analyzed, the better we understand its history.
Observations like these are at the limit of what can currently be done. It’s possible that JWST can help here; As an infrared telescope, it can see through the dust, and its sharp vision can help separate the jumble of stars swirling in the galactic core.
The center of the Milky Way is a maelstrom of gas, dust, stars, powerful magnetic fields, and of course, a giant black hole at the heart of it all. But with perseverance and patience, order is teased out of the chaos.
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