PHILADELPHIA, JAN. 16 -- Thanks in part to images provided by the Hubble Space Telescope, astronomers have discovered that many of our galaxy's near neighbors are unexpectedly eccentric, including "born-again" pulsars, "eggbeater" energy fields and a large star that appears on the verge of exploding.
Among the findings announced today at the annual meeting of the American Astronomical Society was the discovery that the activity of pulsars -- stars that release intermittent bursts of radio waves -- inside a globular star cluster known as M15 is producing an "eggbeater" effect that is preventing the collapse of matter into a black hole.
There are about 150 globular clusters such as M15, each containing around 1 million stars, perched around the periphery of our Milky Way galaxy like suburbs around a large city.
These clusters, where stars and other material are packed about a million times more tightly than in our part of the Milky Way, "are believed to be the most ancient stellar units in our galaxy," said Shrinivas Kulkarni of the California Institute of Technology.
Pulsars are believed to arise from highly compressed neutron stars that contain "the densest material you can get in the universe," Kulkarni said, "almost a black hole."
As pulsars spin, they emit beams of radio waves along their magnetic axes, producing an effect like holding a flashlight and making a small circular motion with the wrist. As a result, the beam points toward Earth every few milliseconds, producing the pulsar's trademark oscillation.
In all, some 30 pulsars have been identified so far in all the globular clusters surrounding our galaxy. "But this is just the tip of the iceberg," Kulkarni said. "There are probably somewhere between 2,000 and 10,000."
Ordinarily, such pulsars are thought to gradually "spin down" -- that is, to exhaust their rotational momentum thanks to the braking effect of their own magnetic fields. Eventually, their enormous collective density would cause them to collapse even closer together, producing a black hole. And, indeed, astronomers had expected to find evidence of a black hole at the center of M15.
But according to new observations from the Hubble's Wide Field/Planetary Camera and observers on the ground, it appears that the implosion has been reversed. Instead of collapsing together, many of these pulsars have formed into binary stars -- two objects that get trapped in each other's gravitational fields -- and "one steals mass from the other," said Brian Murphy of the University of Utrecht in the Netherlands.
By cannibalizing its companion, a neutron star can form what Kulkarni called "born-again" pulsars with "an amazing amount of energy." Meanwhile, as the binary pairs form, their gravitational energy is communicated to surrounding stars and gas, halting the collapse of matter.
Each of the pairs "acts as an eggbeater," said Sandra Faber of the University of California at Santa Cruz. As a result, the globular cluster that once seemed doomed to collapse has apparently begun to bounce back and spread out again.
The Hubble, using an instrument called the Goddard High Resolution Spectrograph, also has provided the clearest picture to date of what scientists say may be the most massive single star yet discovered.
Called Melnick 42, with a mass about 100 times the sun's, it lies in the relatively nearby galaxy called the Large Magellanic Cloud.
Hubble's Wide Field/Planetary Camera also has revealed new findings about what Jeff Hester of Caltech called "a star that's been trying to blow up": a highly unstable star called eta Carinae about 8,000 light years away that is widely thought to be the best candidate to explode and become a giant supernova visible on Earth.