Infrared Images Reveal Formation of Stars
Infant stars swaddled in dust have been discovered as they emerge from a dark interstellar gas cloud, giving scientists a new model of stellar formation and suggesting that sunlike stars are born with the raw materials for planets.
The stars described by astronomers last week are among the youngest ever photographed, and were found by using a sensitive new infrared instrument able to penetrate the veil of dark gas and debris that blocks the view of forming stars.
In the past, scientists suspected that most stars formed in large nebulae, such as Orion, by churning out thousands of stars bound in clusters by gravity. The new discovery found that most stars in the Milky Way may have formed in “aggregates” or small stellar incubators.
Astronomer Karen M. Strom of the University of Massachusetts at Amherst and her husband, Stephen E. Strom, along with co-researcher K. Michael Merrill of the Kitt Peak National Observatory in Tucson, said their images appear to show girdles of dust particles condensing around each star to form asteroid-like “planetesimals.”
Astronomers, using the orbiting Infrared Astronomical Satellite (IRAS), have found the discs of dust around a few other stars such as Beta Pictoris and Vega. They concluded that perhaps one in four stars are formed with these protoplanetary discs.
Stephen Strom, however, argued that such discs are intrinsic to star formation. “All stars . . . must have discs. It’s necessary for their development. The question is, do they evolve differently (depending on the star and the way it was formed)? . . . That’s something that we’re trying to find out.”
He said he agrees with other astronomers’ estimate that these planetesimals may seed the condensation of planets around perhaps 10% to 20% of all stars.
No one, including the Stroms, has seen planets outside Earth’s solar system, although ample circumstantial evidence indicates that they should exist. Finding the first planets is a compelling goal for astronomers.
The Stroms said they decided to look for star formation in the interstellar gas cloud Lynds 1641 after IRAS saw several inviting infrared light sources there. After studying the cloud, which is 1,500 light-years from Earth, Karen Strom said she chose to investigate the five densest areas.
A light-year is a measure of distance based on how far light travels in one year, or roughly 5.8 trillion miles.
The areas chosen by the Stroms cannot be seen using an optical telescope because the cloud, a stew of fine dust particles, ionized hydrogen and other gases, blocks visible light. Longer infrared waves pass easily though the gas clouds, so the Stroms, with Merrill’s help, developed a new wide-field infrared camera at Kitt Peak.
In the areas of space surveyed, the astronomers said they found eight locations where aggregates, or groups of stars, were being born. Most groups held 10 to 50 individual stars, although Karen Strom said she counted 150 stars in the largest group.
Merrill said that, considering how widespread star formation now appears to be, it is not surprising that astronomers are getting their best view of it in a relatively “suburban” part of the galaxy.
“Imagine conducting a census by measuring body heat,” he said. “We moved out of the cities and into the suburbs, where we can see individuals better without worrying if three groups are overlapping one another.”
Karen Strom said that one reason she and other astronomers had never bothered to look for star formation in Lynds 1641 was because of the proximity of the spectacularly tumultuous Orion nebula.
“This area has been neglected because the Orion nebula is just above it and everybody looks for activity there. I did it myself,” she said. “I would start out at Orion nebula and work my way down, but run out of . . . time before getting to what I now believe is the most interesting area.”
Revolutionary technological advances in infrared astronomy over the last 10 years made it easier to penetrate the dark interstellar clouds that cloak many interesting objects, she added.
Even with Kitt Peak’s new infrared array, the astronomers were prevented by superdense clouds from glimpsing the birth of a star. But Stephen Strom said that some stars they did photograph are only a few hundred thousand years old--stellar newborns. The sun, a middle-aged star, is 4.5 billion years old.
He said the newborn stars emit very long infrared radiation when they emerge, indicating that the very hot core is cloaked in dusty gas.
That telltale infrared “signature” vanishes as stars age, shrink and move from the group, Strom said. Based on reports of a similar phenomenon elsewhere in the galaxy, he said that he believes the dust condenses into asteroid-size planetesimals that have not yet been observed.
Meanwhile, a second team of infrared astronomers at the same meeting here of the American Astronomical Society reported discovering distinctive emissions from massive, 12-billion-year-old gas clouds that appear ready to condense into new galaxies.
Robert L. Brown and Paul A. Vanden Bout said the discovery should help find answers to some of the most basic and puzzling questions about galaxies, which are communities of billions of stars: What created them, and why?
