A University of Colorado at Boulder professor and several colleagues have identified what is
believed to be the youngest massive star cluster yet detected in the Milky Way Galaxy.
Known as W49, the cluster appears to contain about 100 type O stars - the hottest and most
massive stars in our galaxy -- that are estimated be less than 1 million years old. The
cluster also is thought to be peppered with thousands of lower mass, lower luminosity stars.
"These massive type O stars are in the birthing process," said Professor Peter Conti of
CU-Boulder's astrophysical and planetary sciences department. "Ten to 100 of the type O
stars would be a very luminous ensemble easily visible across the galaxy were it not for
the absorption of optical light by dust in the Milky Way."
The new results were announced at the 24th General Assembly of the International Astronomical
Meeting held Aug. 7 to Aug. 18 at the University of Manchester in Manchester, England.
The large amount of gas and dust swaddling this particular cluster is indicative of the
star formation process, Conti said. In the case of W49, the density of the material in
and around the star cluster makes the stars themselves invisible, not only at optical
wavelengths but also to infrared telescopes.
Such dark and dusty birthing regions surrounding type O stars - each of which has a surface
temperature up to 90,000 degrees Fahrenheit -- are known as charged hydrogen regions and
contain glowing hydrogen clouds, Conti said. The glow is strongly visible in the emission
lines of hydrogen, which appear throughout the electromagnetic spectrum.
Using 4-meter and 1.5-meter telescopes at the Cerro Tololo Inter-American Observatory in
the Chilean Andes, Conti and his colleagues recently imaged eight charged hydrogen regions
in the near infrared region of the electromagnetic spectrum that are believed to harbor
luminous type O star clusters. In all but one, type O star clusters were confirmed, he said.
But in the W49 region, located some 23 thousand light-years distant from Earth in the Milky Way,
stellar images of type O stars were not visible in the infrared, he said. Since other
scientists previously reported that stellar-like objects were visible in longer radio
wavelengths able to pass relatively unobscured through the darker, dustier regions of our
galaxy, Conti and his colleagues concluded W49 was harboring a cluster of infant type O stars.
"We knew something was there, but it was not showing up in the infrared," he said. "It
finally occurred to me that the cluster was so young and its individual O stars so hidden
within a charged hydrogen region as to be undetectable both in the visible and the infrared
portions of the light spectrum, but detectable in longer radio wavelengths. In a sense,
seeing no cluster was like a dog that didn't bark."
Conti made the observations with astronomer Robert Blum of the Cerro Tololo
Inter-American University and Augusto Damineli of the University of Sao Paulo, Brazil. Blum
is a former CU-Boulder postdoctoral researcher who worked under Conti, while Damineli was a
CU Visiting Scientist in 1987-1998 collaborating with Conti at JILA. JILA is a joint
program of CU and the National Institute of Standards and Technology on campus.
In a related discovery, astronomers from the University of Wisconsin, Madison, and
CU-Boulder obtained a glimpse of what may be some of the youngest massive star clusters
ever seen in 1999. The team observed five clusters of stars containing an estimated 500
to 1,000 massive stars each in Henize 2-10, a galaxy 32 million light-years from Earth
in the constellation Pyxis. They first reported on it at the January 2000 American Astronomical
Society meeting in Atlanta.
The massive clusters are believed to be only about 500,000 years old, which is analogous
to the first day of a human life, said UW astronomer and lead project scientist Henry
Kobulnicky and CU-Boulder doctoral student Kelsey Johnson of the APS department and JILA.
The discovery was made with the help of the Very Large Array, a huge and highly sensitive
radio telescope located near Socorro, N.M.
Packed into relatively small areas of space, such dense clusters of massive stars are
believed to evolve over billions of years into globular clusters like
the ones orbiting the Milky Way. Knowing about the first stages of their development
is important because it will provide insight into how such objects
-- which appear to be common in all galaxies -- come into being, said Johnson.
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