The supermassive black hole at the center of our galaxy, called Sagittarius A*, or Sgr A*, is estimated to contain about 4.5 million times the mass of our sun. (Photo: NASA/CXC/Stanford/I. Zhuravleva et al. [Public domain]/Wikipedia Commons)
"I would rather walk with a friend in the dark, than alone in the light."
When she expressed that thought back in the early 20th century, American author Helen Keller was probably referring to "like-hearted" people.
But it turns out, those may be words to live by — quite literally — for black holes.
Consider Sagittarius A*, or simply Sgr A* — the Milky Way galaxy's supermassive black hole-in-residence. Cramming a mass of around 4 million suns into a region about the size of the space between the Earth and the sun, it lives squarely at the heart of our galaxy,
But it was assumed that, aside from the retinue of stars that surround it, this galactic carnivore lived in solitude.
After all, who wants to hang out with a black hole?
They're unrepentant gluttons at the dinner table. And definitely not fun at parties.
But someone may put up with them. And that someone could be another black hole.
According to a scholarly article published in arXiv, the star cluster surrounding Sgr A* indicates another dark influence at work — the gravitational pull of a smaller, but still massive, void lurking in the shadows.
"Based on what we've seen, my colleagues and I show that if there is a friend there, it might be a second black hole nearby that is at least 100,000 times the mass of the Sun," study co-author Smadar Naoz, an astrophysicist at the University of California, writes in LiveScience.
That would make the heart of our galaxy — already a place of high intrigue and drama — even more complicated. The black holes would be locked into a kind of gravitational dance. They would orbit each other, while simultaneously tugging at nearby stars in their orbits.
The tell-tale heart of our galaxy is actually a star known as S0-2. It's close enough to Sgr A* to provide scientists with all kinds of information about its all-consuming neighbor — without actually being on the menu. Instead, S0-2 whips around Sgr A* at speeds of more than 16 million miles per hour. When the star passes too close to the black hole, its light stretches and shifts into the red part of the electromagnetic spectrum — a validation of Einstein's theory of general relativity.
Check out the video below to see how exactly S0-2 and other nearby stars bend to Sgr A*'s gravitational sway.
By studying that shift in light, Naoz and her colleagues might be able to detect another hand pulling S0-2's strings. But it would be a much smaller hand. Because, so far, they haven't been able to note its effect on SO-2's orbit.
"But that doesn't mean that a smaller companion black hole cannot still hide there," Naoz notes. "Such an object may not alter the orbit of SO-2 in a way we can easily measure."
Instead, the research team is focusing on gravitational waves that would be emitted by two massive black holes orbiting each other. But to pick up on those extremely low-frequency ripples in space, they will have to wait until the space-based detector known as LISA, or Laser Interferometer Space Antenna, is launched in 2034.
Then we might know for certain if these cold, remote chasms in space and time have any friends.