The Arches Cluster lies deep in the heart of the Milky Way, hidden behind heaps of obscuring dust. But through the use of the European Southern Observatory's first adaptive optics system instrument, NACO, installed at the Very Large Telescope array in Chile, scientists now have a better view of this unique star cluster. Huge quantities of absorbing dust lie between Earth and the Arches cluster near the center of our galaxy, so using near-infrared imaging was necessary to see through that thick curtain.
The NACO adaptive optics remove as much of the blurring effect of Earth's atmosphere as possible to provide the crispest images possible. Even so, the resulting image still has a hazy appearance. The stars in the Arches Cluster appear as bright cores surrounded by faint diffuse halos. This is common for images obtained by adaptive optics instruments. The halo is light that was not fully corrected from the blurring effects of Earth’s atmosphere.
Discovery in the Arches Cluster
The Arches Cluster is located 25,000 light-years away toward the constellation of Sagittarius the Archer. (However, the name of the cluster comes not from the constellation but from arched filaments seen in radio maps of this region.) Approximately three light-years across with more than a thousand young, massive stars packed into each cubic light-year, the Arches Cluster has a density a million times greater than clusters seen elsewhere. Stars of less than 2.5 million years of age dominate the cluster. The cluster is ten times heavier than the average young star cluster found scattered throughout the Milky Way because it is enriched with chemical elements heavier than helium. Astronomers were surprised by the total mass of the cluster, about 30,000 times that of the sun, which is much heavier than they expected.
Astronomers wanted to learn how massive stars live in the extreme conditions resulting from being so close to the Milky Way's core, where they are influenced by powerful, opposing forces from stars, gas, and a supermassive black hole. The data show that, despite the violent neighborhood in which it lives, the Arches Cluster has the same proportions of low- and high-mass young stars as is found in quieter locations in the Milky Way. Although the Arches Cluster is the densest cluster of massive young stars, it still is liberally populated with less massive stars. Astronomers found that higher mass stars in the cluster are less common than their less massive brethren, and their relative numbers are the same everywhere, following a universal law.
"With the extreme conditions in the Arches Cluster," says Pablo Espinoza, a lead researcher on the project from the University of Arizona, "one might imagine that stars won't form in the same way as in our quiet solar system neighborhood. However, our new observations showed that the masses of stars in this cluster actually do follow the same universal law."
Fernando Selman of the ESO says, "The most massive star we found has a mass of about 120 times that of the sun. 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."
Enjoy a video journey to the Arches Cluster at the center of the Milky Way.
Source: European Southern Observatory (ESO)
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