Astronomers Map a Hypergiant Star's Massive Outbursts (Forwarded)

Donna Weaver
Space Telescope Science Institute, Baltimore
(Phone: 410-338-4493)

Roberta Humphreys
University of Minnesota, Minn.
(Phone: 612-624-6530)

FOR RELEASE: January 8, 2007 12:20 PM (EST)

News Release Number: STScI-2007-03

Astronomers Map a Hypergiant Star's Massive Outbursts

Using NASA's Hubble Space Telescope and the W.M. Keck Observatory,
Kameula, Hawaii, astronomers have learned that the gaseous outflow from
one of the brightest super-sized stars in the sky is more complex than
originally thought.

The outbursts are from VY Canis Majoris, a red supergiant star that is
also classified as a hypergiant because of its very high luminosity. The
eruptions have formed loops, arcs, and knots of material moving at various
speeds and in many different directions. The star has had many outbursts
over the past 1,000 years as it nears the end of its life.

A team of astronomers led by Roberta Humphreys of the University of
Minnesota used NASA's Hubble Space Telescope and the W.M. Keck Observatory
to measure the motions of the ejected material and to map the distribution
of the highly polarized dust, which reflects light at a specific
orientation. The polarized light shows how the dust is distributed.
Astronomers combined the Hubble and Keck information to produce a
three-dimensional image of the matter emitted from VY Canis Majoris.

"We thought mass loss in red supergiants was a simple, spherical, and
uniform outflow, but in this star it is very complex," Humphreys said. "VY
Canis Majoris is ejecting large amounts of gas at a prodigious rate and is
consequently one of our most important stars for understanding the
high-mass loss episodes near the end of massive star evolution. During the
outbursts, the star loses about 10 times more mass than its normal rate.

"With these observations, we have a complete picture of the motions and
directions of the outflows, and their spatial distribution, which confirms
their origin from eruptions at different times from separate regions on
the star."

Humphreys and her collaborators presented their findings today (Jan. 8) at
the American Astronomical Society meeting in Seattle, Wash.

Astronomers have studied VY Canis Majoris for more than a century. The
star is located 5,000 light-years away. It is 500,000 times brighter and
about 30 to 40 times more massive than the Sun. If the Sun were replaced
with the bloated VY Canis Majoris, its surface could extend to the orbit
of Saturn.

Images with Hubble's Wide Field and Planetary Camera 2 revealed for the
first time the complexity of the star's ejecta. The first images provided
evidence that the brightest arcs and knots were created during several
outbursts. The random orientations of the arcs also suggested that they
were produced by localized eruptions from active regions on the star's
surface.

With spectroscopy obtained using the Keck Telescope, Humphreys and her
team learned more about the shape, motion, and origin of the star's
outflow. Line of sight velocities, measured from the spectra, showed that
the arcs and knots were expanding relative to the star. With recently
obtained Hubble images, the group measured the ejecta's motions across the
line of sight.

The team found that the numerous arcs, loops, and knots were moving at
different speeds and in various directions, confirming they were produced
from separate events and from different locations on the star.

The astronomers also used the measurements to determine when the outbursts
happened. The outermost material was ejected about 1,000 years ago, while
a knot near the star may have been ejected as recently as 50 years ago.

The arcs and knots represent massive outflows of gas probably ejected from
large star spots or convective cells on the star's surface, analogous to
the Sun's activity with sunspots and prominences associated with magnetic
fields, but on a vastly larger scale. Magnetic fields have been measured
in VY Canis Majoris's ejecta that correspond to field strengths on its
surface comparable to the magnetic fields on the Sun. These measurements
show that the supergiant star's magnetic fields would supply sufficient
energy for these massive outflows.

The astronomers used the measurements to map the velocity and direction of
the outflows with respect to the embedded star. When combined with the
dust distribution map, they also determined the location of the arcs and
knots, yielding the three-dimensional shape of VY Canis Majoris and its
ejecta.

"With these observations, we may have captured a short-lived phase in the
life of a massive star," Humphreys said. "The most luminous red
supergiants may all eventually experience high-mass loss episodes like VY
Canis Majoris before ending their lives."

The typical red supergiant phase lasts about 500,000 years. A massive star
becomes a red supergiant near the end of its life, when it exhausts the
hydrogen fuel at its core. As the core contracts under gravity, the outer
layers expand, the star gets 100 times larger, and it begins to lose mass
at a higher rate. VY Canis Majoris has probably already shed about half of
its mass, and it will eventually explode as a supernova.

Images and additional information about this research are available on the
Web at:
http://hubblesite.org/newscenter/arc...eases/2007/03/

The Hubble Space Telescope is an international cooperative project between
NASA and the European Space Agency. The Space Telescope Science Institute
is operated for NASA by the Association of Universities for Research in
Astronomy, Inc., Washington.