发信人: kuux (kuux), 信区: Astronomy
标 题: 银河系中发现超大星团
发信站: BBS 水木清华站 (Wed Mar 23 09:26:31 2005), 站内
Super Star Cluster Discovered in Our Own Milky Way
Summary - (Mar 22, 2005) The Milky Way has several star clusters; collections
of stars pulling each other into a tight group. But now astronomers have
located a super star cluster, containing hundreds of thousands of stars
in a region only 6 light-years across. It's called Westerlund 1, and nobody
discovered it before now because it's hidden behind thick clouds of dust
. Astronomers used several of European Southern Observatory's infrared telescopes
to peer through the dust and see the super cluster's true size.
Full Story - Super star clusters are groups of hundreds of thousands of very
young stars packed into an unbelievably small volume. They represent the
most extreme environments in which stars and planets can form.
Until now, super star clusters were only known to exist very far away, mostly
in pairs or groups of interacting galaxies. Now, however, a team of European
astronomers [1] have used ESO's telescopes to uncover such a monster object
within our own Galaxy, the Milky Way, almost, but not quite, in our own
backyard!
The newly found massive structure is hidden behind a large cloud of dust
and gas and this is why it took so long to unveil its true nature. It is
known as "Westerlund 1" and is a thousand times closer than any other super
star cluster known so far. It is close enough that astronomers may now probe
its structure in some detail.
Westerlund 1 contains hundreds of very massive stars, some shining with a
brilliance of almost one million suns and some two-thousand times larger
than the Sun (as large as the orbit of Saturn)! Indeed, if the Sun were
located at the heart of this remarkable cluster, our sky would be full of
hundreds of stars as bright as the full Moon. Westerlund 1 is a most unique
natural laboratory for the study of extreme stellar physics, helping astronomers
to find out how the most massive stars in our Galaxy live and die.
From their observations, the astronomers conclude that this extreme cluster
most probably contains no less than 100,000 times the mass of the Sun, and
all of its stars are located within a region less than 6 light-years across
. Westerlund 1 thus appears to be the most massive compact young cluster
yet identified in the Milky Way Galaxy.
Super Star Clusters
Stars are generally born in small groups, mostly in so-called "open clusters
" that typically contain a few hundred stars. From a wide range of observations
, astronomers infer that the Sun itself was born in one such cluster, some
4,500 million years ago.
In some active ("starburst") galaxies, scientists have observed violent episodes
of star formation (see, for example, ESO Press Photo 31/04), leading to
the development of super star clusters, each containing several million stars
.
Such events were obviously common during the Milky Way's childhood, more
than 12,000 million years ago: the many galactic globular clusters - which
are nearly as old as our Galaxy (e.g. ESO PR 20/04) - are indeed thought
to be the remnants of early super star clusters.
All super star clusters so far observed in starburst galaxies are very distant
. It is not possible to distinguish their individual stars, even with the
most advanced technology. This dramatically complicates their study and
astronomers have therefore long been eager to find such clusters in our neighbourhood
in order to probe their structure in much more detail.
Now, a team of European astronomers [1] has finally succeeded in doing so
, using several of ESO's telescopes at the La Silla observatory (Chile).
Westerlund 1
The open cluster Westerlund 1 is located in the Southern constellation Ara
(the Altar). It was discovered in 1961 from Australia by Swedish astronomer
Bengt Westerlund, who later moved from there to become ESO Director in Chile
(1970 - 74). This cluster is behind a huge interstellar cloud of gas and
dust, which blocks most of its visible light. The dimming factor is more
than 100,000 - and this is why it has taken so long to uncover the true
nature of this particular cluster.
In 2001, the team of astronomers identified more than a dozen extremely hot
and peculiar massive stars in the cluster, so-called "Wolf-Rayet" stars.
They have since studied Westerlund 1 extensively with various ESO telescopes
.
They used images from the Wide Field Imager (WFI) attached to the 2.2-m ESO
/MPG as well as from the SUperb Seeing Imager 2 (SuSI2) camera on the ESO
3.5-m New Technology Telescope (NTT). From these observations, they were
able to identify about 200 cluster member stars.
To establish the true nature of these stars, the astronomers then performed
spectroscopic observations of about one quarter of them. For this, they
used the Boller & Chivens spectrograph on the ESO 1.52-m telescope and the
ESO Multi-Mode Instrument (EMMI) on the NTT.
An Exotic Zoo
These observations have revealed a large population of very bright and massive
, quite extreme stars. Some would fill the solar system space within the
orbit of Saturn (about 2,000 times larger than the Sun!), others are as bright
as a million Suns.
Westerlund 1 is obviously a fantastic stellar zoo, with a most exotic population
and a true astronomical bonanza. All stars identified are evolved and very
massive, spanning the full range of stellar oddities from Wolf-Rayet stars
, OB supergiants, Yellow Hypergiants (nearly as bright as a million Suns)
and Luminous Blue Variables (similar to the exceptional Eta Carinae object
- see ESO PR 31/03).
All stars so far analysed in Westerlund 1 weigh at least 30-40 times more
than the Sun. Because such stars have a rather short life - astronomically
speaking - Westerlund 1 must be very young. The astronomers determine an
age somewhere between 3.5 and 5 million years. So, Westerlund 1 is clearly
a "newborn" cluster in our Galaxy!
The Most Massive Cluster
Westerlund 1 is incredibly rich in monster stars - just as one example, it
contains as many Yellow Hypergiants as were hitherto known in the entire
Milky Way!
"If the Sun were located at the heart of Westerlund 1, the sky would be full
of stars, many of them brighter than the full Moon", comments Ignacio Negueruela
of the Universidad de Alicante in Spain and member of the team.
The large quantity of very massive stars implies that Westerlund 1 must contain
a huge number of stars. "In our Galaxy, explains Simon Clark of the University
College London (UK) and one of the authors of this study, "there are more
than 100 solar-like stars for every star weighing 10 times as much as the
Sun. The fact that we see hundreds of massive stars in Westerlund 1 means
that it probably contains close to half a million stars, but most of these
are not bright enough to peer through the obscuring cloud of gas and dust
". This is ten times more than any other known young clusterin the Milky
Way.
Westerlund 1 is presumably much more massive than the dense clusters of heavy
stars present in the central region of our Galaxy, like the Arches and Quintuplet
clusters. Further deep infrared observations will be required to confirm
this.
This super star cluster now provides astronomers with a unique perspective
towards one of the most extreme environments in the Universe. Westerlund
1 will certainly provide new opportunities in the long-standing quest for
more and finer details about how stars, and especially massive ones, do
form.
... and the Most Dense
The large number of stars in Westerlund 1 was not the only surprise awaiting
Clark and his colleagues. From their observations, the team members also
found that all these stars are packed into an amazingly small volume of
space, indeed less than 6 light-years across. In fact, this is more or less
comparable to the 4 light-year distance to the star nearest to the Sun,
Proxima Centauri!
It is incredible: the concentration in Westerlund 1 is so high that the mean
separation between stars is quite similar to the extent of the Solar System
.
"With so many stars in such a small volume, some of them may collide", envisages
Simon Clark. "This could lead to the formation of an intermediate-mass black
hole more massive than 100 solar masses. It may well be that such a monster
has already formed at the core of Westerlund 1."
The huge population of massive stars in Westerlund 1 suggests that it will
have a very significant impact on its surroundings. The cluster contains
so many massive stars that in a time span of less than 40 million years,
it will be the site of more than 1,500 supernovae. A gigantic firework that
may drive a fountain of galactic material!
Because Westerlund 1 is at a distance of only about 10,000 light-years, high
-resolution cameras such as NAOS/CONICA on ESO's Very Large Telescope can
resolve its individual stars. Such observations are now starting to reveal
smaller stars in Westerlund 1, including some that are less massive than
the Sun. Astronomers will thus soon be able to study this exotic galactic
zoo in great depth.
More information
The research presented in this ESO Press Release will soon appear in the
leading research journal Astronomy and Astrophysics ("On the massive stellar
population of the Super Star Cluster Westerlund 1" by J.S. Clark and colleagues
). The PDF file is available at the A&A web site. A second paper ("Further
Wolf-Rayet stars in the starburst cluster Westerlund 1", by Ignacio Negueruela
and Simon Clark) will also soon be published in Astronomy and Astrophysics
. It is available as astro-ph/0503303.
A Spanish press release issued by Universidad de Alicante is available on
the web site of Ignacio Negueruela.
Note
[1]: The team is composed of Simon Clark (University College London, UK),
Ignacio Negueruela (Universidad de Alicante, Spain), Paul Crowther (University
of Sheffield, UK), Simon Goodwin (University of Wales, Cardiff, UK), Rens
Waters (University of Amsterdam) and Sean Dougherty (Dominion Radio Astrophysical
Observatory).
Original Source: ESO News Release |
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