Why do young stars ap­pear in old clus­ters?


Many stars in the Uni­verse ex­ist in clus­ters. The ac­cepted view is that the stars in a par­tic­u­lar clus­ter share a com­mon ori­gin – they formed from the same cloud of gas and dust at about the same time, mean­ing that they’re about the same age.

But around a decade ago, as­tronomers were look­ing at ob­ser­va­tions from the Hub­ble Space Tele­scope and found some­thing rather sur­pris­ing. They dis­cov­ered that some ga­lac­tic glob­u­lar clus­ters con­tain stars with dif­fer­ing chem­i­cal com­po­si­tions, mean­ing that the stars couldn’t have formed at the same time. This dis­cov­ery chal­lenged the tra­di­tional idea that star clus­ters form in a sin­gle episode of star for­ma­tion.

It’s a co­nun­drum that I’ve been in­ves­ti­gat­ing by look­ing at old star clus­ters, in­clud­ing glob­u­lar clus­ters, in a com­pan­ion galaxy to the Milky Way called the Large Mag­el­lanic Cloud (LMC). In the glob­u­lar star clus­ter com­mu­nity, two ex­pla­na­tions have been pro­posed for what Hub­ble ob­served. One is that star for­ma­tion in clus­ters is a very long process. And the other, which is caus­ing much de­bate, is that what Hub­ble recorded was just an ob­ser­va­tional ef­fect, and not real.

To try to solve this puz­zle, my col­leagues and I started look­ing for a young star in its very early stages of life, just be­fore it burns hy­dro­gen to evolve into a main se­quence star. Such stars have a very short life­time, so if we were able to find young stel­lar ob­jects in a star clus­ter, it would be di­rect ev­i­dence that star for­ma­tion is cur­rently tak­ing place, to sup­port the first ex­pla­na­tion.

To try to find these stars, we’ve been us­ing data from the Euro­pean Space Agency’s Her­schel Space Ob­ser­va­tory, an in­frared tele­scope that was able to see into the LMC’s dust and tell us the po­si­tion of all the young stel­lar ob­jects. Her­schel al­lowed us to match the lo­ca­tions of sev­eral thou­sand young stars with the lo­ca­tions of stel­lar clus­ters and among them we found 15 can­di­dates that were much younger than other stars within the same clus­ter. Now we need to un­der­stand how young stars are be­ing born in these old clus­ters. In other words, we need to find out what is fu­elling this sec­ond gen­er­a­tion of star for­ma­tion? One pos­si­bil­ity is that the fuel might be gas en­ter­ing the clus­ters from in­ter­stel­lar space, but ob­ser­va­tions us­ing ra­dio tele­scopes have shown no cor­re­la­tion be­tween this gas and the lo­ca­tion of the clus­ters we’ve been study­ing. Hence we’re look­ing at a sec­ond pos­si­bil­ity: that an old star ex­pels a lot of gas into the in­ter­stel­lar medium, which then ac­cretes to form a sec­ond gen­er­a­tion of stars. This is the cur­rent line of thought we’re pur­su­ing. You may ask why we’re not ex­am­in­ing at clus­ters in our own Galaxy. The rea­son is be­cause some of the clus­ters are hid­den be­hind the ga­lac­tic plane and the dust and gas in the Milky Way. The LMC is eas­ier to study. The next thing we have to do is pro­duce some sim­u­la­tions to check whether old stars are ca­pa­ble of eject­ing enough ma­te­rial to fuel star for­ma­tion. Then we can com­pare the re­sults we get from these sim­u­la­tions with the chem­i­cal com­po­si­tions of the real stars that have been ob­served with our ground-based tele­scopes. NASA is set to launch the James Webb Space Tele­scope, the suc­ces­sor to Hub­ble, next year. What we’d like to do once the James Webb is op­er­a­tional is un­der­take a sim­i­lar study of clus­ters, but this time in a galaxy much fur­ther away, such as M31, the An­dromeda Galaxy. Look­ing at stel­lar pop­u­la­tions in other gal­ax­ies will help tell us whether or not the pro­cesses ob­served in the LMC are unique to that dwarf galaxy.

The Large Mag­el­lanic Cloud may con­tain clus­ters in which both old and, con­trary to con­ven­tional wis­dom, young stars re­side

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