There’s a supermassive black hole in the middle of our galaxy known as Sagittarius A* (SgrA*). Hundreds of stars closely orbit it, and astronomers have been tracking their motion. Following these stars and calculating their orbits is how they know that SgrA* has a mass of about 4 million suns. Observations have become so precise now that the researchers can even test whether our understanding of black holes is accurate.
The most studied star is known as S2. It orbits the black hole every ten years. It was the star with the smallest orbit until recently when the discovery of S62 was made. Now, S62 is the star with the smallest orbit, and it’s the fastest. It’s twice as massive as our sun and orbits the black hole every ten years.
S62 is so close to SgrA* that it doesn’t follow a Keplerian orbit (a simple ellipse path). Instead, it follows a spirograph motion that shifts its orbit by about 10 degrees with each cycle. At its closest approach to SgrA*, it moves at over 8% the speed of light. At speeds like that, the star makes visible the relativistic phenomena of time dilation and length contraction. To illustrate, it’s fast enough to cross the United States in a fifth of a second or get from the Earth to the Moon in about 16 seconds!
The motion of the stars that have large orbits can be described by Newtonian gravity and Kepler’s laws of motion – which is most of them. However, some orbit so close to the black hole that their motion can only be accurately described by Einstein’s theory of general relativity.
When two objects are moving at different speeds, they measure the passage of time differently. Take S62 and Earth, for example, an hour at that star would last about a hundred Earth minutes. Meaning, astronomers watching S62 will see it take 100 minutes to reach its destination, whereas someone on the star would experience the journey as one hour.
The phenomenon happens at every speed but only becomes visible when you approach the speed of light. The time dilation effect for S62 is just barely noticeable at 8% the speed of light. But, the fact that its noticeable at all makes the star an excellent tool for studying special relativity in action.
S62 will make its next close approach to SgrA* in the Fall of 2022. Astronomers will then be able to test the effects of relativity more precisely than ever before, and likely unravel more of the mysteries surrounding black holes.
