So close, yet so far away.
The black hole at the center of our Milky Way galaxy is by far the nearest such supermassive gravitational monster to our sun, yet observing this space-time maelstrom is a challenge because it is 27,000 light-years away and obscured in visible light by intervening star clouds and dust lanes in the galactic plane.
The region of space profoundly warped by the black hole is a little more than a light-year across.
Astronomers have been able to closely track the swift elliptical orbits of stars trapped in the black hole’s grasp. This stellar pinball offers the best evidence to date that the black hole is for real — jamming the mass of 4 million suns into a region of space smaller than Earth’s orbital radius.
Stellar forensics show that several million years ago a giant cloud of cold molecular hydrogen fell toward the black hole and flattened into a disk when it was caught in the gravitational whirlpool. An estimated 10,000 stars quickly formed within the disk, in what Jessica Lu of the University of Hawaii calls a “crazy environment.”
Astronomers are now following stars on the inner edge of the disk that plunge toward the black hole in comet-like elliptical orbits.
But new insights into the core-fireworks come from the most unlikely place: by looking far outside the plane of our galaxy.
In 1940, a young blue star was found among the ancient stars inhabiting the vast halo of our Milky Way. It is barreling through the halo at speeds several times faster than the staid halo population. Astronomers first hypothesized that a runaway star was in a binary system and ejected though some sort of gravitational interaction with a third star entering the system, or perhaps propelled by a supernova explosion. In 1988 the gravitational slingshot effect from the galaxy’s central black hole was hypothesized as the propulsion source.
In recent years, more of these so-called hypervelocity stars have been found zooming far away from our galaxy. In a survey of the northern sky five especially bright short-lived hypervelocity stars have been identified. They are all under 200 million years old.
But if these wayward stars were ejected from the disk of our galaxy they would be randomly distributed on the sky. Instead they are found far above the galaxy’s northern pole and clustered in a patch about 1/8th the area of the northern sky.
A supermassive black hole “launcher” is the simplest explanation because the concentration suggests that the hypervelocity stars follow an escape trajectory, perhaps perpendicular to the plane of stars encircling the black hole. If this is true, then a survey of the southern sky for high velocity stars should uncover a mirror-image concentration on the sky.
Researchers propose that every 10,000 years our supermassive black hole knocks a hypervelocity star out of the ballpark. An idea is that one star in a binary system near the black hole loses momentum and falls toward the black hole. This momentum is transferred to the binary companion that is then accelerated to escape velocity from our galaxy. The single giant black hole propulsion theory is supported by observations that show the stars seem spaced sequentially, like a series of fired cannonballs.
Besides providing clues to the behavior of the central black hole, these stars are “test particles” for probing the gravitational field of the entire galaxy and how gravity’s major source, the Milky Way’s invisible dark matter halo, may be shaped.
What’s especially intriguing is that a cluster of young blue stars also surrounds the supermassive black hole in the heart of the neighboring Andromeda galaxy. This means bursts of star formation around black holes in the hubs of galaxies must be frequent and common in the universe. If the black hole theory is correct, space telescopes could go looking for a cone shaped pattern of high-velocity stars in the Andromeda galaxy.
Alien astronomers living in a runaway star system would have a curious but grand overhead view of their parent galaxy. But putting together a coherent theory of star birth and evolution would be difficult without having powerful telescopes to peruse the starry tapestry beneath them.
Image credit: NASA