New Hubble observations of the massive globular cluster NGC 2808 provide evidence that it has three generations of stars
instead of one as current theories predict.
This is the Hubblecast!
News and Images from the NASA/ESA Hubble Space Telescope. Travelling through time and space with our host Doctor J a.k.a. Dr Joe Liske.
Welcome to the Hubblecast!
Globular clusters are the homesteaders of our Milky Way Galaxy because they were born during our Galaxy’s formation.
They are compact swarms of typically hundreds of thousands of stars that are being held together by gravity.
Now astronomers have long thought that globular clusters experience a single baby boom of star formation
at the beginning of their lives,and then settle into a rather quiet existence.
New observations by the NASA/ESA Hubble Space Telescope is showing that this idea may be too simple.
The Hubble analysis of the massive globular cluster NGC 2808 is providing evidence that instead of having one baby boom of star formation,
star birth went “boom, boom, boom,” creating three generations of stars early on in the cluster’s life.
We were really struck by the results when we saw for the first time the same team we said wow because we were not expecting anything like that.
The picture we had in mind was that globular clusters were simple stellar systems.
Simple because they are all stars which formed all together at the same time from the same material,
that these were the same composition, and stars which are all at the same distance.
I think this is going to change our view of globular clusters. In this sense it’s a cornerstone result.
The astronomers used Hubble’s Advanced Camera for Surveys to measure the brightness -
seen along this axis - and the colour of the cluster stars – seen here, with blue to the left and red to the right.
The measurements showed three distinct populations, with each successive generation appearing slightly bluer.
This colour difference suggests that successive generations contain a slightly different mix of some chemical elements.
We don’t really know how it happened but it may be that the cluster formed a first generation that expelled a lot of material
that only later fell down forming a second generation of stars with completely different chemical properties.
That may be just the explanation and may be consistent with the fact that NGC 2808 is one of the most massive clusters in the galaxy, so able to retain all this gas.
Astronomers commonly believed that globular clusters produced only a single stellar generation,
because the energy from the first batch of stars cleared out the remaining gas needed for more stars.
But a hefty cluster like NGC 2808, which is two to three times more massive than typical globular clusters,
may have enough gravity to hold onto that gas. Although the astronomers have searched only two globular clusters for multiple stellar generations,
they say this may be a typical occurrence in other massive clusters as well.
Now no one is taking the radical step of suggesting that previous work on other clusters is no longer valid.
But this discovery does show that the study of stellar populations in globular clusters may be heading in a new direction.
The team plans to use ESO’s Very Large Telescope in Chile to study the chemical composition of NGC 2808.
This may offer further evidence that the stars were formed at different times and it may yield clues as to how they formed.
The team will also will use Hubble to hunt for multiple generations in about 10 more hefty globular clusters.
This is Dr J signing off for the Hubblecast.
Once again, you've guessed it, nature has surprised us beyond our wildest imagination …
Hubblecast is produced by ESA/Hubble at the European Southern Observatory in Germany.
The Hubble mission is a project of international cooperation between NASA and the European Space Agency.