A team of scientists from China and the United States have discovered how relatively young stars are able to make their way into dense collections of older stars known as globular clusters.
While it was initially thought that these clusters form their stars all at the same time, it has been revealed that they are also capable of producing thousands of second and even third generations of sibling stars.
In a study featured in the journal Nature, researchers from Peking University and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) in China and the Adler Planetarium and Northwestern University (NU) in the U.S. have found that globular clusters are capable of taking in gas from outside sources, which can then lead to the formation of new stars.
This discovery contradicts an earlier notion that it is the aging stars themselves that shed gas in order to trigger the creation of newer stars.
Formation Of New Stellar Populations
Chengyuan Li, a researcher from Peking's Kavli Institute for Astronomy and Astrophysics (KIAA) and lead author of the study, explained that their research offers new perspective on how several stellar populations are able to form as part of star clusters.
Their findings suggest that the gas from which new stars are formed likely originates from outside globular clusters rather than from the inside.
This event can be compared to how some people choose to adopt kids instead of having biological children of their own with their partners.
Globular clusters are capable of producing their own progeny of stars, but it appears that they would rather "adopt" young stars, or at the very least the materials with which new stars can be formed.
"Our explanation that secondary stellar populations originate from gas accreted from the clusters' environments is the strongest alternative idea put forward to date," KIAA astronomer Richard de Grijs said.
"Globular clusters have turned out to be much more complex than we once thought."
Magellanic Clouds
The Milky Way is known to contain hundreds of spherical and densely packed globular clusters at its outskirts. A large number of these clusters are already quite old, which is why Li and his colleagues chose to focus their study on younger clusters.
The team found their target clusters in two dwarf galaxies known as the Magellanic Clouds.
Through the use of data gathered from observations of the Hubble Space Telescope, the researchers were able to identify three particular globular clusters: NGC 1696 and NGC 1783 found in the Large Magellanic Cloud and NGC 411 found in the Small Magellanic Cloud.
In the NGC 1783 star cluster, Li and his team identified an initial stellar population that is already 1.4 billion years old, as well as two other stellar populations that are 450 million years old and 890 million years old.
The difference in the ages of the star populations was first thought to be because of their ability to retain dust and gas enough to produce several generations of stars. However, this appears to be unlikely according to the researchers.
NU astronomer Aaron M. Geller said that after massive stars are formed, they only have an estimated 10 million years before they meet their end in powerful supernovae that can eliminate any remaining dust or gas in the surrounding area.
After the explosion, lower-mass stars would then trigger an accumulation of dust and gas in the area once again.
The researchers believe globular clusters take up material from stray dust and gas as they move about their host galaxies.
Li and his colleagues are now planning to extend their study to globular clusters in the Milky Way other than Magellanic Cloud.
