While astronomers have long understood that stars and planets form from the collapse of a cloud of gas, the question of the main causes of this process has remained open. One option is that the cloud cools, gravity gets the upper hand, and the cloud falls in on itself. The other possibility is that a "trigger" from some external source — such as radiation from a massive star or a shock from a supernova — initiates the collapse. Some previous studies have noted a combination of triggering mechanisms in effect. By combining observations of the star-forming cloud Cepheus B from the Chandra X-ray Observatory and the Spitzer Space Telescope, researchers have taken an important step in addressing this question. Cepheus B is a cloud of mainly cool molecular hydrogen located about 2,400 light years from Earth. There are hundreds of very young stars inside and around the cloud — ranging from a few million years old outside the cloud to less than a million in the interior — making it an important testing ground for star formation. This particular type of triggered star formation had previously been seen in small populations of a few dozen stars, but the latest result is the first time it has been clearly observed in a rich population of several hundred stars.
While slightly farther away than the famous Orion star-forming region, Cepheus B is at a better orientation for astronomers to observe the triggering process. The Chandra observations allowed the astronomers to pick out young stars within and around Cepheus B. Young stars have turbulent interiors that generate highly active magnetic fields, which, in turn, produce strong and identifiable X-ray signatures. The Spitzer data revealed whether the young stars have a disk of material (known as "protoplanetary" disks) around them. Since they only exist in very young systems where planets are still forming, the presence of protoplanetary disks — or lack thereof — is an indication of the age of a star system. The new study suggests that star formation in Cepheus B is mainly triggered by radiation from one bright, massive star outside the molecular cloud. According to theoretical models, radiation from this star would drive a compression wave into the cloud-triggering star formation in the interior, while evaporating the cloud's outer layers. The Chandra-Spitzer analysis revealed slightly older stars outside the cloud, and the youngest stars with the most protoplanetary disks in the cloud interior — exactly what is predicted from the triggered star formation scenario. A paper describing these results was published in the July 10 issue of the Astrophysical Journal.