The NASA/ESA Hubble Space Telescope has captured the best ever image of
the globular cluster Messier 15, a gathering of very old stars that
orbits the center of the Milky Way. This glittering cluster contains
over 100,000 stars, and could also hide a rare type of black hole at its center.
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| This cluster
of stars is known as Messier 15, and is located some 35 000 light-years
away in the constellation of Pegasus (The Winged Horse). It is one of
the oldest globular clusters known, with an age of around 12 billion
years. Both very hot blue stars and cooler golden stars can be seen
swarming together in the image, becoming more concentrated towards the
cluster's bright centre. Messier 15 is one of the densest globular
clusters known, with most of its mass concentrated at its core. As well
as stars, Messier 15 was the first cluster known to host a planetary
nebula, and it has been found to have a rare type of black hole at its
centre. This new image is made up of observations from Hubble's Wide
Field Camera 3 and Advanced Camera for Surveys in the ultraviolet,
infrared, and optical parts of the spectrum [Credit: NASA, ESA] |
This multi-colored firework display is a cluster of stars known as
Messier 15, located some 35,000 light-years away in the constellation of
Pegasus (The Winged Horse). It is one of the oldest globular clusters
[1] known, with an age of around 12 billion years.
Very hot blue stars and cooler golden stars are seen swarming together
in this image, becoming more concentrated towards the cluster's bright center. Messier 15 is one of the densest globular clusters known, with
most of its mass concentrated at its core.
However, this sparkling bauble has hidden secrets. Astronomers studying
the cluster with Hubble in 2002 found there to be something dark and
mysterious lurking at its heart. It could either be a collection of dark
neutron stars
[2], or an
intermediate-mass black hole. Of the two possibilities it is more likely
that Messier 15 harbors a black hole at its center, as does the
massive globular cluster Mayall II.
This is an animation of a rare and
exotic intermediate-mass black hole at the centre of a star cluster,
similar to the one thought to be at the centre of globular cluster
Messier 15. Studying these unusual black holes could tell us about how
such objects grow and evolve within both star clusters and galaxies
[Credit: NASA, ESA, and M. Kornmesser]
Intermediate-mass black holes are thought to form either from the
merging of several smaller, stellar-mass black holes, or as a result of a
collision between massive stars in dense clusters. A third possibility
is that they were formed during the Big Bang. In terms of mass they lie
between the more commonly found stellar-mass and supermassive types of
black hole
[3], and could tell us about how black holes grow and evolve within clusters like Messier 15, and within galaxies.
As well as this black hole, Messier 15 is known to house a planetary nebula, Pease 1
[4] -- and it was the first globular known to contain one of these objects
[5]. This nebula is visible as the bright blue object just to the left of the cluster's center.
This new image is made up of observations from Hubble's Wide Field
Camera 3 and Advanced Camera for Surveys in the ultraviolet, infrared
and optical parts of the spectrum.
Notes
[1] A globular cluster is a
roughly spherical group of stars that orbits the core of a galaxy. The
Milky Way has over 150 of these starry satellites -- including Messier
15. However, other galaxies have many more than this; a staggering
160,000 were recently discovered by Hubble in the galaxy cluster Abell
1689 (heic1317). Globular clusters contain some of the most ancient
stars in the Universe.
[2] A neutron star is formed
from the collapse of a massive star. They are very hot and very dense,
with an average mass of around two solar masses contained within a
radius of tens of kilometers.
[3] Stellar-mass black holes
have masses of a few to a few tens of times the mass of the Sun.
Supermassive black holes have masses ranging from hundreds of thousands
to billions of times the mass of the Sun.
[4] Pease 1 is also known as PN Ku 648, or Kürster 648.
[5] Since the discovery of Pease
1, only three other globular clusters have been found to host planetary
nebulae: Messier 22, NGC 6441, and Palomar 6. This number is so low
because planetary nebulae are a very brief, short-lived phase at the end
of the lives of low to moderate mass stars -- which are not common
within globular clusters.
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