Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young
super star cluster about 3.8
kpc (12,000 ly) away from Earth. It is thought to be the most massive young star cluster in the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
,
and was discovered by
Bengt Westerlund in 1961 but remained largely unstudied for many years due to high
interstellar absorption in its direction. In the future, it will probably evolve into a
globular cluster
A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...
.
The cluster contains a large number of rare, evolved, high-mass stars, including: 6
yellow hypergiant
A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually lumi ...
s, 4
red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Ant ...
s including
Westerlund 1-26
Westerlund 1-26 or Wd 1-26 is a red supergiant within the outskirts of the Westerlund 1 super star cluster. It is one of the List of largest known stars, largest known stars discovered so far although its radius is uncertain but is calcu ...
, one of the
largest known stars, 24
Wolf-Rayet stars, a
luminous blue variable, many
OB supergiants, and an unusual supergiant
sgB star">star which has been proposed to be the remnant of a recent
stellar merger
A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood ...
.
In addition,
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
observations have revealed the presence of the
anomalous X-ray pulsar
Anomalous X-ray pulsars (AXPs) are an observational manifestation of magnetars—young, isolated, highly magnetized neutron stars. These energetic X-ray pulsars are characterized by slow rotation periods of ~2–12 seconds and large magnetic fields ...
CXO J164710.20-455217, a slow rotating
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
that must have formed from a high-mass progenitor star. Westerlund 1 is believed to have formed in a single burst of star formation, implying the constituent stars have similar ages and compositions.
Aside from hosting some of the most massive and least-understood stars in our galaxy, Westerlund 1 is useful as a relatively nearby, easy to observe
super star cluster that can help astronomers determine what occurs within extragalactic super star clusters.
Observations
The brightest O7-8V
main sequence stars in Wd1 have
V-band photometric magnitudes around 20.5, and therefore at visual wavelengths Wd1 is dominated by highly luminous post-Main Sequence stars (V-band magnitudes of 14.5–18,
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...
s −7 to −10), along with less-luminous post-Main Sequence stars of
luminosity class
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
Ib and II (V-band magnitudes of 18–20). Due to the extremely high interstellar reddening towards Wd1, it is very difficult to observe in the U- and B-bands, and most observations are made in the R- or I-bands at the red end of the spectrum or in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
. Stars in the cluster are generally named using a classification introduced by Westerlund,
although a separate naming convention is often used for the Wolf-Rayet stars.
At X-ray wavelengths, Wd1 shows diffuse emission from interstellar gas and point emission from both high-mass, post-Main Sequence and low mass, pre-Main Sequence stars. The Westerlund 1
magnetar
A magnetar is a type of neutron star with an extremely powerful magnetic field (∼109 to 1011 T, ∼1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.War ...
is the most luminous X-ray point source in the cluster, with the sgB
star W9, the (presumed) binary W30a and the Wolf–Rayet stars WR A and WR B all strong X-ray sources. Approximately 50 other X-ray point sources are associated with luminous optical counterparts. Finally, at
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...
wavelengths the sgB
star W9 and red supergiants W20 and
W26 are strong radio sources, while the majority of the cool hypergiants and a few OB supergiants and Wolf–Rayet stars are also detected.
Age and evolutionary state
The age of Wd1 is estimated at 4–5
Myr from comparison of the population of evolved stars with models of
stellar evolution. The presence of significant numbers of both Wolf–Rayet stars ''and'' red and yellow supergiants in Wd1 represents a strong constraint on the age: theory suggests that red supergiants will not form until around 4 Myr as the most massive stars do not go through a red supergiant phase, while the Wolf–Rayet population declines sharply after 5 Myr. This range of ages is broadly consistent with infra-red observations of Wd1 that reveal the presence of
late-O main sequence stars, although a lower age of around 3.5 Myr has been suggested from observations of lower-mass stars in Wd1.
If Wd1 formed stars with a typical
initial mass function
In astronomy, the initial mass function (IMF) is an empirical function that describes the initial distribution of masses for a population of stars. The IMF is an output of the process of star formation. The IMF is often given as a probability d ...
then the cluster would have originally contained a significant number of very massive stars, such as those currently observed in the younger
Arches cluster
The Arches Cluster is the densest known star cluster in the Milky Way, about 100 light-years from its center in the constellation Sagittarius (The Archer), 25,000 light-years from Earth.
Its discovery was reported by Nagata et al. in 1995, and i ...
. Current estimates of the age of Wd1 are greater than the lifetimes of these stars, and stellar evolution models suggest that there would already have been 50–150
supernovae in Wd1, with a supernova rate of approximately one per 10,000 years over the last million years. However, to date only one definitive supernova remnant has been detected—the Westerlund 1 magnetar—and the lack of other
compact objects and
high-mass X-ray binaries
X-ray binaries are a class of binary stars that are luminous in X-rays.
The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', whi ...
is puzzling. A number of suggestions have been put forward, including high supernova
kick velocities that disrupt binary systems, the formation of slowly accreting (and therefore undetectable) stellar mass
black holes, or binary systems in which both objects are now compact objects, but the problem has yet to be resolved.
As the stars in Westerlund 1 have the same age, composition and distance, the cluster represents an ideal environment for understanding the evolution of massive stars. The simultaneous presence of stars evolving on to and off of the Main Sequence presents a robust test for stellar evolution models, which are also currently unable to correctly predict the observed distribution of Wolf–Rayet subtypes in Westerlund 1.
Binary fraction
A number of lines of evidence point to a high binary fraction amongst the high-mass stars in Wd1. Some massive binaries are detected directly
through photometry
and
radial velocity observations, while many others are inferred through secondary characteristics (such as high X-ray luminosity, non-thermal radio spectra and excess infra-red emission) that are typical of colliding-wind binaries or dust-forming Wolf–Rayet stars. Overall binary fractions of 70% for the Wolf–Rayet population
and in excess of 40% for the OB supergiants are currently estimated, although both may be incomplete.
Members
As well as documented members of the cluster, the
luminous blue variable MN44 is thought to be a
runaway star
In astronomy, stellar kinematics is the Observational astronomy, observational study or measurement of the kinematics or motions of stars through space.
Stellar kinematics encompasses the measurement of stellar Velocity, velocities in the Milky W ...
ejected from Westerlund 1 four to five million years ago.
References
External links
*
Artist’s impression of a magnetar X-ray satellites catch magnetar in gigantic stellar ‘hiccup’ ESA
, owners =
, headquarters = Paris, Île-de-France, France
, coordinates =
, spaceport = Guiana Space Centre
, seal = File:ESA emblem seal.png
, seal_size = 130px
, image = Views in the Main Control Room (120 ...
website, 2007
Image of Westerlund 1Neutron star found where black hole expected
{{DEFAULTSORT:Westerlund 1
Open clusters
Super star clusters
Ara (constellation)
277-12
Astronomical objects discovered in 1961