Crowd of Stars Surprisingly Normal
This image of the Arches Cluster of young, massive stars was obtained with NACO on ESO’s Very Large Telescope. The field of view is 28 arcseconds.
Credit: ESO/P. Espinoza

A dense cluster of young stars situated near the supermassive black hole at the heart of the Milky Way is surprisingly normal, with stars of high and low masses roughly in the same proportions as clusters in more tranquil parts of the galactic neighborhood.

Using the European Southern Observatory?s Very Large Telescope (VLT), astronomers were able to get one of the sharpest views ever of this stellar assemblage, called the Arches Cluster.

The result: Contrary to what astronomers had thought, the dense cluster follows what seems to be a universal law regarding the distribution of stellar masses.

The massive Arches Cluster is located 25,000 light-years away (a light-year is the distance that light travels in a year, about 6 trillion miles or 10 trillion kilometers) toward the constellation of Sagittarius (the Archer). It contains about a thousand young, massive stars, all less than 2.5 million years old, plus other less massive stars.

Ideal lab

The name ?Arches? does not come from the constellation the cluster is located in, but because it is located next to arched filaments detected in radio maps of the center of the Milky Way.

The cluster is an ideal laboratory to study how massive stars are born in extreme conditions, as it is close to the center of our Milky Way where it experiences huge opposing forces from the stars, gas and the supermassive black hole that reside there.

The Arches Cluster is 10 times heavier than typical young star clusters scattered throughout our Milky Way, and it is enriched with chemical elements heavier than helium.

With the adaptive optics instrument on the VLT, astronomers could remove the blurring effects of Earth's atmosphere and were able to take images of the Arches Cluster that are even crisper than those obtained with telescopes in space.

The cluster was observed in infrared light because the gas and dust that lies between Earth and the cluster is impenetrable to visible light.

The new study, to be detailed in an upcoming issue of the journal Astronomy and Astrophysics, confirms the Arches Cluster to be the densest cluster of massive young stars known. It is about 3 light-years across with more than 1,000 stars packed into each cubic light-year ? an extreme density a million times greater than in the sun?s neighborhood. In fact the closest star to our sun is about 4 light-years away.

The sun is also though to have formed in dense starbirth region, however, when other massive stars exploded and put pressure on the surrounding clouds of gas and dust. The sun then got gravitationally booted out of its birth cluster and ended up a relative loner.

Striking exception?

Astronomers studying clusters of stars have found that higher mass stars are rarer than their less massive brethren, 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," said study team member Pablo Espinoza, who worked on the research as an undergraduate student at the Pontificia Universidad Cat?lica de Chile. "However, our new observations showed that the masses of stars in this cluster actually do follow the same universal law."

With the VLT image, astronomers could also observed the brightest stars in the cluster.

"The most massive star we found has a mass of about 120 times that of the sun," said team member Fernando Selman of the European Southern Observatory. "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 total mass of the cluster seems to be about 30,000 times that of the sun, much more than was previously thought.