This composite image, combining data from NASA's Chandra X-ray Observatory
and Spitzer Space Telescope shows the star-forming cloud Cepheus B,
located in our Milky Way galaxy about 2,400 light years from Earth. A
molecular cloud is a region containing cool interstellar gas and dust left
over from the formation of the galaxy and mostly contains molecular
hydrogen. The Spitzer data, in red, green and blue shows the molecular
cloud (in the bottom part of the image) plus young stars in and around
Cepheus B, and the Chandra data in violet shows the young stars in the
field.
The Chandra observations allowed the astronomers to pick out young stars
within and near Cepheus B, identified by their strong X-ray emission. The
Spitzer data showed whether the young stars have a so-called
"protoplanetary" disk around them. Such disks only exist in very young
systems where planets are still forming, so their presence is an
indication of the age of a star system.
These data provide an excellent opportunity to test a model for how stars
form. The new study suggests that star formation in Cepheus B is mainly
triggered by radiation from one bright, massive star (HD 217086) outside
the molecular cloud. According to the particular model of triggered star
formation that was tested -- called the radiation-driven implosion model
-- radiation from this massive star drives a compression wave into the
cloud triggering star formation in the interior, while evaporating the
cloud's outer layers.
Different types of triggered star formation have been observed in other
environments. For example, the formation of our solar system was thought
to have been triggered by a supernova explosion. In the star-forming
region W5, a "collect-and-collapse" mechanism is thought to apply, where
shock fronts generated by massive stars sweep up material as they progress
outwards. Eventually the accumulated gas becomes dense enough to collapse
and form hundreds of stars. The radiation-driven implosion model mechanism
is also thought to be responsible for the formation of dozens of stars in
W5. The main cause of star formation that does not involve triggering is
where a cloud of gas cools, gravity gets the upper hand, and the cloud
falls in on itself.