Star May Be Biggest, Brightest Yet Discovered

A University of Florida-led team of astronomers may have
discovered the brightest star yet observed in the universe, a fiery behemoth
that could be as much as much as seven times brighter than the current record
holder.

But don’t expect to find the star — which is at least 5 million times brighter
than the sun — in the night sky. Dust particles between Earth and the star
block out all of its visible light. Whereas the sun is located only 8.3 light
minutes from Earth, the bright star is 45,000 light years away, on the other
side of the galaxy. It is detectable only with instruments that measure infrared
light, which has longer wavelengths that can better penetrate the dust.

In a National Science Foundation-funded study scheduled to be presented today at
the American Astronomical Society national conference in Atlanta, the team says
the star is at least as bright as the Pistol Star, the current record holder, so
named for the pistol-shaped nebula surrounding it. Whereas the Pistol Star is
between 5 million and 6 million times as bright as the sun, however, the new
contender, LBV 1806-20, could be as much as 40 million times the sun’s brightness.

"We think we’ve found what may be the most massive and most luminous star ever
discovered," said Steve Eikenberry, a UF professor of astronomy and the lead
author of a paper on the discovery that was recently submitted to the
Astrophysical Journal.

Eikenberry will discuss his findings in a news conference to be held by the
society at 12:30 p.m. today at the Courtland Room in the Hyatt Regency Atlanta,
where the conference is being held.

One longstanding problem with gauging the brightness of stars at great distances
is that what seems at first to be one amazingly bright star turns out on closer
examination to be a cluster of nearby stars. Don Figer, an astronomer at the
Baltimore-based Space Telescope Science Institute who led the team that
discovered the Pistol Star in 1997, said the high-quality data collected by the
UF-led team reduced but did not eliminate this possibility.

"The high-resolution data prove that the object is not simply a cluster of lower
mass stars, although it is possible that it is a collection of a few stars in a
tight orbit around each other," Figer said. "More study will be needed to
determine the distance and singularity of the object in order to establish
whether the object is truly the most massive star known."

Astronomers have known about LBV 1806-20 since the 1990s. At that time, it was
identified as a "luminous blue variable star" — a relatively rare, massive and
short-lived star. Such stars get their names from their propensity to display
light and color variability in the infrared spectrum.

Luminous blue variable stars are extremely large, with LBV 1806-20 probably at
least 150 times larger than the sun, Eikenberry said. The stars are also
extremely young by stellar time. LBV 1806-20 is estimated at less than 2 million
years old. The sun in our solar system, by contrast, is 5 billion years old.
Typical stars, such as the sun, live 10 billion years.

LBVs have "short and troubled lives," as Eikenberry put it, because "the more
mass you have, the more nuclear fuel you have, the faster you burn it up. They
start blowing themselves to bits."

Eikenberry’s team made several key advances that led to the estimate of the
star’s oversized mass and brightness, he said.

One, they sharpened infrared images obtained from the Palomar 200-inch telescope
at the California Institute of Technology’s Palomar Observatory using a camera
equipped with "speckle imaging," a relatively new technology for improving
resolution of objects at great distances.

"The shimmering that you see coming off a hot blacktop road in the summer — the
upper atmosphere kind of does that with star light," Eikenberry said. "Speckle
imaging kind of freezes that motion out, and you get much better images."

Composed of 17 astronomers and graduate students, the team also came up with an
accurate estimate for the distance from the Earth to the bright star. Team
members further determined its temperature and how much of the star’s infrared
light gets absorbed by dust particles as the light makes its way toward Earth.
The scientists relied on data collected by the Blanco 4-meter telescope at the
National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory
in Chile.

Each of these variables contributed to the estimate of the star’s remarkable
candlepower. "You correct for dust absorption, then you correct for temperature
of the star, you correct for distance of the star — all of those things feed
into luminosity," Eikenberry said.

One of the mysteries about LBV 1806-20 is how it got so big. Current theories of
star formation suggest they should be limited to about 120 solar masses, or 120
times as large as the sun, because the heat and pressure from such big stars’
cores force matter away from their surfaces. Eikenberry said one possibility is
that the big star was formed in a process called shock-induced star formation,
which occurs when a supernova blows up and slams the gaseous material in a
molecular cloud together into a massive star.

The star’s size is not its only distinguishing characteristic. It is located in
a small cluster of highly unusual or extremely rare stars, including a so-called
"soft gamma ray repeater," a freakishly magnetic neutron star that is one of
only four identified in the entire galaxy of 100 billion stars. With a magnetic
field hundreds of trillions of times more powerful than Earth’s magnetic field,
this type of star gets its name from its periodic bursts of gamma rays. The
cluster also apparently includes an infant or newly formed star.

"We’ve got this zoo of freak stars, all crammed together, really nearby, and
they’re all part of the same cluster of stars," Eikenberry said. "It’s really
kind of weird."

Also buried within the cluster is an extremely young infant star, Eikenberry
said. The presence of the infant star, the luminous blue variable and the soft
gamma ray repeater are vivid examples of an important emerging fact about
stellar evolution: All stars in a single cluster don’t form at the same time, he
said. "We’re seeing what I think is going to become a textbook example of the
fact that stars aren’t all born in an instant, even in a small cluster," he said.

Figer, the Pistol Star discoverer, said the research makes an important
contribution to astronomers’ understanding of the star formation process.

"The findings are significant because such massive stars are very rare and
define the upper limits of the star formation process," he said. "The team has
made a remarkable contribution to our understanding of the most extreme stars."

The team carrying out this work also included UF’s Jessica LaVine; Keith
Matthews, with the California Institute of Technology; Stephane Corbel, with the
Universite de Paris; John-David Smith, with the University of Arizona; John
Wilson, with the University of Virginia; Donald Barry, Michael Colonno and James
Houck, all with Cornell University; and undergraduate research students Shannon
Patel, Malia Jackson, and Dounan Hu of Cornell University; and Megan Garske of
Northwestern Nazarene University.

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IMAGE CAPTION:
[http://www.napa.ufl.edu/2004news/bigbrightstarph.htm]
A star that may be the biggest and brightest in the universe dwarfs the sun in
this artist’s depiction, drawn to scale to demonstrate the star’s massive size.
The star, dubbed LBV 1806-20, is at least 150 times larger and 5 million times
brighter than the sun. Located about 45,000 light years from Earth, it is
detectable only with infrared telescopes, because all of its visible light is
absorbed by dust particles in space. If it was located as near to the sun as
this image suggests, the star’s light would completely swamp that of the sun,
making it invisible and virtually undetectable. (University of Florida/Meghan
Kennedy)