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 聳 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聮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 聳 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聮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.