John Nobel Wilford
New pictures from the Hubble space telescope have given astronomers striking views of stellar birth triggered by the collision of two galaxies – and a glimpse of what could be in store for the Milky Way in about five billion years.
The Earth-orbiting telescope has produced the most detailed pictures yet of a galactic collision, showing the blue light of hot stars bursting to life like the brilliant finale to a Fourth of July fireworks display. As the two galaxies – called the Antennae – come together, the merging of gravitational forces compresses clouds of hydrogen gas, which then collapse into dense spheres glowing with thermonuclear energy: stars, millions of them.
Astronomers reported that they counted more than 1 000 clusters of newborn stars in the centre of the merging galaxies. The brightest of the clusters probably contains at least one million stars, most of them relatively young. Ground-based telescopes are only able to see the brightest of these clusters, and not in any detail.
“The sheer number of these young star clusters is amazing,” said Dr Bradley Whitmore, an astronomer at the Space Telescope Science Institute in Baltimore. He and Dr Francois Schweizer of the Carnegie Institution of Washington announced the findings at a news conference at the National Aeronautics and Space Administration in Washington this week.
They and other astronomers said the observations should provide new insights into the turbulent early history of the universe and the formation and evolution of large structures like galaxies, congregations of billions of stars.
Before these pictures, said Dr Bruce Margon, an astronomer at the University of Washington in Seattle, scientists relied primarily on computer simulations to depict the turbulent forces and creation of new structures as a result of galactic mergers. The Hubble pictures showed a surprisingly chaotic environment, he said, that “tells us how naive the computer simulations were.”
Other Hubble telescope pictures have shown that nearly one-third of the most distant galaxies, the ones that existed earliest in time, are in the midst of similar collisions or mergers. This is not surprising, because the universe was denser then and has been expanding ever since.
But most of these distant galactic mergers are occurring too far away to be studied in detail, and collisions are rarer in the more attenuated universe today. Still, astronomers focused the Hubble telescope on two nearby colliding groups of stars, called the Antennae because the pair of long tails of luminous matter formed by the collision resembles an insect’s whiskery antenna. The Antennae are close by cosmic standards, 63- million light-years away in the southern constellation Corvus.
Schweizer said the observations should help clarify how especially large star clusters, known as globular clusters, are formed from giant hydrogen clouds in space. Most globular clusters appear to be among the oldest objects in the universe. Now it seems that some are emerging out of recent galactic collisions.
Examining the new pictures, astronomers could not resist the notion that they might be seeing into the distant future. They have long known that the Milky Way and the Andromeda galaxy could be moving on a collision course.
“It’s pretty speculative,” said Whitmore. “But there’s a reasonable chance that it might happen, and we should know in about 10 years.”
The Milky Way and Andromeda are approaching each other with a speed of 480 000km an hour. They are now 2,2-million light years apart, which is roughly 20 times the diameter of the Milky Way.
Whether the two eventually collide depends on the angle at which Andromeda is heading this way. If it is coming straight at the Milky Way, there will be a head-on collision in five billion years. A direct collision would lead to a grand merger between the two that would take several billion years to complete; these are not hit-and-run events. Andromeda might only sideswipe the Milky Way or miss it altogether.
In 2005, Nasa plans to launch the Space Interferometry Mission, a spacecraft carrying an array of telescopes, which should quickly determine, among other things, the exact angle of Andromeda’s approach.
Not that an impending galactic smash-up would ever be a matter of urgency for human beings. By then, the Sun would have burned out and the Earth been reduced to a lifeless cinder. With their wide separations, stars and any of their planetary systems might escape direct hits.
But the sky would change, astronomers said. New stars would flare with blue light, as in the Antennae galaxies. Their lives would probably be short, as they self-destructed in a few million years as supernovas popping off like firecrackers. New patterns of stars and dust clouds would take shape, and gone would be the familiar arc of dust and light in the night sky, a major belt of the Milky Way. The old and new stars of the two galaxies, once spiral-shaped like pinwheels, would become a single elliptical-shaped galaxy.
— Galactic crash: News and images of two galaxies colliding are available on the Internet at: http://oposite.stsci.edu/pubinfo/PR/97/34/
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