Ultraviolet Spotlight on Plump Stars in Tiny Galaxies

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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin@jpl.nasa.gov

Feature: 2011-122 April 21, 2011

Ultraviolet Spotlight on Plump Stars in Tiny Galaxies

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-122&cid=release_2011-122

Astronomers using NASA's Galaxy Evolution Explorer may be closer to knowing why
some of the most massive stellar explosions ever observed occur in the tiniest of galaxies.

"It's like finding a sumo wrestler in a little 'Smart Car,'" said Don Neill, a member of
NASA's Galaxy Evolution Explorer team at the California Institute of Technology in
Pasadena, and lead author of a new study published in the Astrophysical Journal.

"The most powerful explosions of massive stars are happening in extremely low-mass
galaxies. New data are revealing that the stars that start out massive in these little galaxies
stay massive until they explode, while in larger galaxies they are whittled away as they
age, and are less massive when they explode," said Neill.

Over the past few years, astronomers using data from the Palomar Transient Factory, a
sky survey based at the ground-based Palomar Observatory near San Diego, have
discovered a surprising number of exceptionally bright stellar explosions in so-called
dwarf galaxies up to 1,000 times smaller than our Milky Way galaxy. Stellar explosions,
called supernovae, occur when massive stars -- some up to 100 times the mass of our sun -
- end their lives.

The Palomar observations may explain a mystery first pointed out by Neil deGrasse Tyson
and John Scalo when they were at the University of Austin Texas (Tyson is now the
director of the Hayden Planetarium in New York, N.Y.). They noted that supernovae
were occurring where there seemed to be no galaxies at all, and they even proposed that
dwarf galaxies were the culprits, as the Palomar data now indicate.

Now, astronomers are using ultraviolet data from the Galaxy Evolution Explorer to
further examine the dwarf galaxies. Newly formed stars tend to radiate copious amounts
of ultraviolet light, so the Galaxy Evolution Explorer, which has scanned much of the sky
in ultraviolet light, is the ideal tool for measuring the rate of star birth in galaxies.

The results show that the little galaxies are low in mass, as suspected, and have low rates
of star formation. In other words, the petite galaxies are not producing that many huge
stars.

"Even in these little galaxies where the explosions are happening, the big guys are rare,"
said co-author Michael Rich of UCLA, who is a member of the mission team.

In addition, the new study helps explain why massive stars in little galaxies undergo even
more powerful explosions than stars of a similar heft in larger galaxies like our Milky
Way. The reason is that low-mass galaxies tend to have fewer heavy atoms, such as
carbon and oxygen, than their larger counterparts. These small galaxies are younger, and
thus their stars have had less time to enrich the environment with heavy atoms.

According to Neill and his collaborators, the lack of heavy atoms in the atmosphere
around a massive star causes it to shed less material as it ages. In essence, the massive
stars in little galaxies are fatter in their old age than the massive stars in larger galaxies.
And the fatter the star, the bigger the blast that will occur when it finally goes supernova.
This, according to the astronomers, may explain why super supernovae are occurring in
the not-so-super galaxies.

"These stars are like heavyweight champions, breaking all the records," said Neill.

Added Rich, "These dwarf galaxies are especially interesting to astronomers, because
they are quite similar to the kinds of galaxies that may have been present in our young
universe, shortly after the Big Bang. The Galaxy Evolution Explorer has given us a
powerful tool for learning what galaxies were like when the universe was just a child."

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science
operations and data analysis. NASA's Jet Propulsion Laboratory in Pasadena manages the
mission and built the science instrument. Caltech manages JPL for NASA. The mission
was developed under NASA's Explorers Program managed by the Goddard Space Flight
Center, Greenbelt, Md. Researchers sponsored by Yonsei University in South Korea and
the Centre National d'Etudes Spatiales (CNES) in France collaborated on this mission.

Graphics and additional information about the Galaxy Evolution Explorer are online at
http://www.nasa.gov/galex/ and http://www.galex.caltech.edu .

-end-

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