Astronomers
observe last light from mysterious blue star
By Diane
Ainsworth, Public Affairs
Posted June 7, 2000
It may be lights out soon for a massive bright blue star that has been flaring and fading over the last two years in the outskirts of a dwarf galaxy 25 million light years from earth.
Astronomers around the world have been monitoring the fading star in dwarf galaxy NGC 3432 since May 3, when Berkeley astronomers Weidong Li and Marina Papenkova captured its violent outburst using the Lick Observatory's 30-inch KAIT telescope atop Mount Hamilton. Li and members of the telescope team suspect this colossal blue twinkler is a "luminous blue variable" that blew off huge amounts of stellar soot from its outer shell before dimming to near invisibility.
"The super outburst of this star on May 3 might have been caused by the explosion of the outer shell," Li said. "It brightened by at least a factor of 15."
"This object is very rare and doesn't fall into any of our classifications," he said. "We've only seen three or four like it in the outer galaxies. They're brighter than normal stars but fainter than other supernovae. This one is analogous to eta Carinae, the most luminous star in our galaxy, which is found in the constellation Carinae, about 7,000 light years from earth, and which could explode as a supernova any time."
Luminous blue variables are hot massive stars with temperatures around 32,000 degrees Fahrenheit, which lose the equivalency in mass of one of our suns every 100,000 years. They have radii of about 200 times the sun, and are among the most massive and luminous stars in a galaxy. They appear "blue" because of their surface temperature, and "variable" because their photospheres, or outer shells, are very active as the stars expend their fuel and lose mass.
Astronomers using the fully automated Katzman Automatic Imaging Telescope are conducting a supernova search to learn more about the physics of exploding stars. The telescope, which can image a few thousand galaxies every three to five days, can detect objects 10,000 times fainter than other telescopes like it in a 30-minute sky survey, or objects that are a million times fainter than the human eye can see unaided.
"We are discovering essentially every exploding star within about 100 million light years of earth," Li said. "The only stellar outbursts we miss are those too near other bright objects, like galactic centers, or those that are buried in haze near the horizon."
Astronomer Alex Filippenko, professor in the Department of Astronomy and team leader of the Katzman Automatic Imaging Telescope, was first to categorize the mysterious object from spectra &endash; which are measurements of the amount of light at different colors or wavelenghts emanating from an object &endash; that were obtained with the Bok telescope at Steward Observatory in Arizona. The burst of light, he said, looked a lot like an extremely bright eruption from a bright blue variable, a rare type of very hot, bright and unstable stars that sometimes produce violent explosions.
Others observing the faint object agreed. The eruption was too faint to be considered a regular supernova but too bright to be a normal stellar explosion either. Rene Hudec of the Ondrejov Observatory in the Czech Republic suggested that the cosmic explosion might have been the optical afterglow of a lone "orphan" gamma ray burst that escaped detection. The afterglows of these bursts have been predicted by astronomical models which show that gamma rays strongly focused in two opposite beams will produce different light effects from ground-based observatories. When one of the beams is pointed in earth's direction, a gamma ray burst can be witnessed. If the beam is not pointed at earth, astronomers will only see the optical afterglow.
The only flaw in that theory, Shrinivas Kulkarni of the California Institute of Technology has reported, is that the star's spectrum doesn't match a typical afterglow.
Whatever it is, the clock is ticking for Li and his colleagues. No one knows how much longer the object will flicker before it disappears.
"The star is very faint right now, and observers around the world are watching it," Li said. "Our observations will tell us more about the ultimate fate of very massive stars, those that are 25 to 30 times the mass of the sun, and whether their cores collapse eventually to form black holes."
June 7 - July 11,
2000 (Volume 28, Number 34)
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