Posted: August 26, 2008
In celebration of Spitzer’s five years in orbit, a stunning new image reveals the family portrait of a star-forming inferno studded with multiple generations of brilliant stars, and provides dramatic new evidence that massive stars can trigger the birth of stellar newborns through their savage winds and radiation.
"Triggered star formation continues to be very hard to prove," says Xavier Koenig of the Harvard Smithsonian Centre for Astrophysics, "but our preliminary analysis shows that the phenomenon can explain the multiple generations of stars seen in the W5 region."
The Spitzer image was taken over a period of 24 hours and is a three-colour composite showing infrared observations from two Spitzer instruments. Blue represents 3.6-micron light and are older stars in the star-forming cloud as well as unrelated stars behind and in front of the cloud, and green shows light of 8 microns, highlighting dense clouds, both captured by Spitzer's infrared array camera. Red is 24-micron light detected by Spitzer's multiband imaging photometer and shows heated dust that pervades the region's cavities. White knotty areas are where the youngest stars are forming. Image: NASA/JPL-Caltech/Harvard-Smithsonian CfA.
W5 spans an area of sky equivalent to four full moons and is about 6,500 light-years away in the constellation Cassiopeia. Like other massive star-forming regions such as Orion and Carina, W5 hosts vast cavities that were carved out by blistering radiation and ferocious winds from the region's most massive stars. According to the theory of triggered star-formation, the sculpting of these cavities forces gas together, causing it to ignite into successive generations of new stars along the cavities' expanding rims. The result is a radial family tree of stars, with the oldest in the middle of the cavity, and younger stars further out.
Koenig and colleagues set out to test the triggered star-formation theory by studying the ages of the stars in W5. They used Spitzer's penetrating infrared vision to peer through the dusty clouds and view the stars' various stages of evolution. They found that stars within the W5 cavities (blue dots) are older than stars at the rims (pink and white dots) and even older than stars further out past the rim that are embedded in the elephant trunk-like pillars of gas. This ladder-like separation of ages provides some of the best evidence yet that massive stars do, in fact, give rise to younger generations.
"Our first look at this region suggests we are looking at one or two generations of stars that were triggered by the massive stars," says co-author Lori Allen. However, it is possible that the younger stars just happen to be near the edge of the cavities and were not triggered by the larger stars and so the research team plan to follow up with even more detailed measurements of the stars' ages to find out if there is a distinct time gap between the stars just inside and outside the rim.
And in a serious twist of fate, the massive stars in W5 will eventually die in tremendous explosions, wiping out some of the very stars that they triggered into existence millions of years previously.