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 Católica 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.
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