BY DR EMILY BALDWIN
Posted: 8 June, 2009
Despite the extreme forces in operation near the Galaxy’s central black hole, the resident Arches Cluster displays curiously normal conditions.
The densely populated cluster, located some 25,000 light years away towards the constellation of Sagittarius, contains around one thousand young, massive stars less than 2.5 million years old and is ten times heavier than typical young star clusters, enriched with chemical elements heavier than helium.
This new composite infrared image of the Arches Cluster was achieved using the adaptive optics instrument at ESO's VLT. The stars appear as bright cores surrounded by faint diffuse halos, typical of adaptive optics instruments. Image: ESO/P. Espinoza.
Curiously, despite the extreme conditions common in such a location – strong opposing forces from the stars, gas and black hole – astronomers using ESO’s Very Large Telescope (VLT) found the same proportions of low- and high-mass young stars in the cluster as are found in more tranquil locations in our Milky Way.
The study, which used the NACO adaptive optics instrument on the VLT to remove the blurring effects of the atmosphere, confirms the Arches Cluster as about three light years across, packed with thousands of massive young stars per cubic light year – a million times more dense than in the Sun’s neighbourhood.
This image from the Digitized Sky Survey 2 shows the heart of our Milky Way Galaxy. The Arches Cluster is located in the centre of the image, but its stars are hidden behind large amount of dust. The bright star at the top of the image is 3 Sagittarii, while the cluster of stars seen at the bottom left is NGC 6451. Image: ESO, Digitized Sky Survey 2 & S. Guisard.
In general, astronomers studying clusters of stars have found that higher mass stars are rarer than their less massive siblings, and their relative numbers are the same everywhere, following a universal law. For many years, the Arches Cluster seemed to be a striking exception. “With the extreme conditions in the Arches Cluster, one might indeed imagine that stars won’t form in the same way as in our quiet solar neighborhood,” says Pablo Espinoza, the lead author of the paper reporting the new results in the Astrophysical Journal. “However, our new observations showed that the masses of stars in this cluster actually do follow the same universal law”.
The new image also allowed astronomers to study the brightest stars in the cluster. “The most massive star we found has a mass of about 120 times that of the Sun,” says co-author Fernando Selman. “We conclude from this that if stars more massive than 130 solar masses exist, they must live for less than 2.5 million years and end their lives without exploding as supernovae, as massive stars usually do.”
The new data suggests that the total mass of the cluster is roughly 30,000 times that of the Sun, much more than was previously thought.