March 7, 2007 -- Almost 13 billion years ago, just 1 billion years after the Big Bang, globular clusters crammed with massive stars appeared in the universe. These clusters, which were formed in huge numbers about the time galaxies were created, are incredibly dense. A globular cluster whose radius is the distance between the Sun and the nearest star, Alpha Centauri, could contain as many as 2 million stars. The origin of these clusters remains mysterious.
“We are a very long way from understanding how the stars in these clusters or the clusters themselves formed,” says Kelsey Johnson, an assistant professor of astronomy at U.Va. Johnson studies the contemporary formation of massive stars, 10 or more times as big as our Sun, as a way to shed light on these ancient formations.
Johnson’s quest is important because massive star formation from this era supplied most of the energy as well as much of the heavier elements in the universe. The stellar winds they generated as they burned and the supernovae they produced when they died created enormous amounts of mechanical energy, and their ultraviolet radiation heated much of the warm dust in the galaxies. “Massive stars are a building block of the universe,” notes Johnson, “but we know very little about how they form.”
A decade ago, scientists began to discover adolescent globular clusters with the Hubble Space Telescope. This was a revelation because, until that time, they assumed that globular clusters were formed only in the early days of the universe. This observation, however, revealed little about the creation of the massive stars within them. “By the time they are apparent in visible light,” says Johnson, “these stars are dying.”
Johnson decided to look for proto-globular clusters using other wavelengths. In 2001, she earned a National Science Foundation (NSF) fellowship, and took it to the Very Large Array in New Mexico, one of the world’s premier astronomical radio observatories. Johnson was given some of the director’s discretionary time for her search. She targeted the local universe, selecting galaxies relatively close to ours, so that she could observe them in what is considered real-time, at least on an astronomical scale. Eventually, Johnson located a handful of these natal clusters.
“Once we find one, we try to go after it with every tool we have,” Johnson says. “Radio waves can give us clues to its size, density, and pressure, while infrared observations can help us understand the thermal characteristics of the cocoons of dust out of which massive stars are formed.” With the help of a Faculty Early Career Development grant from the NSF, Johnson will take her observations and use them to construct computer models that enable her to test her theories about massive star formation.
Although our knowledge of massive star formation is in its infancy, Johnson believes that major discoveries are in the offing. She is particularly excited about discoveries that will be made possible with the Atacama Large Millimeter Array, the most technologically advanced and expensive telescope on the planet. The National Radio Astronomy Observatory (NRAO), located in Charlottesville, is overseeing construction of this observatory on behalf of its North American partners. “With NRAO just up the road, we are in an ideal position to understand what the new facility can do,” Johnson says.