Black Hole Drags Star to Dusty Death

Gulp. Burp.

And so it goes in the cruel and carnivorous universe according to Einstein.

Astronomers said on Thursday that they had seen a giant black hole in a nearby galaxy rip apart an unfortunate wayward star and spread half of it into a messy blaze of light and heat swirling toward doom. The other half was spit outward in a fiery high-energy jet at a quarter of the speed of light.

All this happened out of human sight, deep in the dusty heart of a pair of colliding galaxies known collectively as Arp 299, about 150 million light years from here in the constellation Ursa Major. An international team of astronomers, led by Dr. Seppo Mattila of the University of Turku in Finland and Miguel Perez-Torres of the Astrophysical Institute of Andalusia in Spain, teased out the story of what happened from observations of infrared, or heat, radiation and radio waves that can penetrate the dust and leak out to the rest of the universe. They published their report Thursday in the journal Science.

Black holes are gravitational pits, trap doors to eternity, predicted by Einstein’s theory of gravity, general relativity. They are so deep and dense that nothing, not even light can escape them. Every galaxy seems to have a supersize version of one of these monsters squatting at its core. The black hole in the center of one of the colliding lobes in Arp 299 has the mass of about 20 million suns.

When Mattila and his colleagues first saw a bright burst of heat coming from the region of that black hole in January 2005, they thought they had discovered a supernova, a cataclysmic explosion in which a massive star ends its life.

When galaxies collide, as the two conglomerations of stars that make up Arp 299 are doing, Mattila explained in an email, large clouds of gas fall into the central regions. That triggers a burst of star formation and then a subsequent burst of supernova explosions as the most massive of these stars quickly burn out and die.

High-resolution radio measurements revealed that the object they observed coincided almost spot on with the black hole, but it was behaving in a way very unlike a supernova. Over the course of the next decade, it expanded into a long jet of radio energy whose head by now has traveled some 3 light years from its origin.

Supernovas don’t do this, but a star falling into a black hole could, in what is called a tidal disruption event.

Mattila and his colleagues concluded that a star passing too close to the black hole had been stretched and ripped by fierce tidal forces into a stream of hot gas. About half the gas, according to theoretical models, would wind up in a dense hot doughnut called an accretion disk, glowing with energy as it swirls around the hole to its eventual doom, like water around a drain.

Meanwhile, pressure from the swirling material would squeeze matter and energy out the poles of the doughnut, making the characteristic jet that radio astronomers can see erupting from the hearts of many galaxies and quasars.

Although the process has been studied in theoretical models and computer simulations, this destruction of a star has rarely been recorded.

From the energy released — about as much as in a regular supernova — the astronomers calculated that the original doomed star was about twice the mass of the sun, or more.

“This is the first time scientists have witnessed by direct imaging a rapidly expanding outflow, a so-called jet, launched by a tidal disruption of a star by a massive black hole at the center of a galaxy,” Mattila said in an email.

But, he went on, the violent disruption in Arp 299 could just be “the tip of the iceberg” of a hidden population of star disruptions we can’t see because they happen deep in the invisible depths of distant, dusty galaxies.

More dark deeds done in secret way out there in time and space.

“Such events,” Mattila wrote in the email referring to star disruptions, “can be expected to be more frequent in the distant universe where luminous infrared galaxies and galaxy mergers were much more common.”