In the constellation Pegasus
Approximately 1500 to 3,000 times the mass of the Sun
Diameter roughly the size of Mars to the size of Earth
The debate over what lies at the heart of the star cluster Messier 15 is the intellectual equivalent of a tennis match: One side serves, the other returns, and the game is on. So far, there's no clear winner.
M15 is a globular cluster, which is a tightly bound collection of hundreds of thousands of old stars. The cluster is about 13.2 billion years old, which means its stars were born when the universe itself was only about 500 million years old. Like most of the 150 or so other globulars in the Milky Way, it resides in the galaxy's halo, which extends far outside its bright, flat disk.
Over the decades, observations of M15 showed that the stars in its core were packed more tightly than the cores of most other globulars. In astronomical parlance, the cluster's core collapsed, with half of its stars falling into a central region only about 20 light-years across.
The collapsed core suggests that something heavy and dense but dark lurks in the center of the cluster, but the nature of that dark heart was unclear.
Serve: The first suggestion of a black hole in its core came in 1975. Early space-based X-ray telescopes had discovered X-rays coming from several globular clusters. Two teams of astronomers suggested the source could be accretion disks around intermediate-mass black holes roughly 1,000 times the mass of the Sun. Such disks are extremely hot, so they produce lots of X-rays. M15 was one of several globulars detected by the X-ray telescopes, so it became a candidate for a central black hole.
The following year, a team led by Barry Newell posited that a black hole of roughly 800 times the Sun's mass could account for the unusual brightness of M15's core. The gravity of the black hole would attract stars, making the center look brighter.
IMBHs are controversial, though. Several candidates have been discovered, all of which are in the cores of star clusters. But unlike the supermassive black holes at the centers of galaxies, which are so heavy but compact that there is no other reasonable explanation, astronomers can offer other possibilities for the densities at the centers of star clusters. And in 1977, two astronomers did just that.
Return: Garth Illingworth and Ivan King made the first counter-argument. They suggested that the density of stars in M15's core could be explained by a tight cluster of stellar remnants -- the "dead" stars known as neutron stars and white dwarfs. Such stars are small but heavy, and they produce little light, so they are difficult to see in the crowded confines of a globular cluster. Yet as the cluster ages, these dense remnants should settle in the center (the gravitational equivalent of the "bottom" of the cluster), just as the heavier ingredients settle to the bottom of a bowl of soup.
Several teams continued to debate the issue over the following 15 years, at first using observations from ground-based telescopes, which have a difficult time seeing individual stars because of the blurring effect of Earth's atmosphere, and later with observations through the clear eye of Hubble Space Telescope (HST).
In the mid 1990s, HST confirmed that the core of M15 is unusually dense. And Hubble data also set up the next big announcement in the M15 match.
Volley: In 2002, a team of astronomers using HST reported that it had solved the mystery. By plotting the orbits of stars in the very heart of M15, the astronomers calculated the mass of the central dark mass. Their numbers showed that this object was an intermediate-mass black hole (IMBH) about 4,000 times as massive as the Sun.
Return: The year after the HST results were published, however, a team led by Holger Baumgardt, using the same observations but its own models of stellar orbits, reported that a cluster of stellar remnants was a more likely explanation.
This team said that its simulations showed that a cluster of such remnants would explain the high-speed orbits of the stars around them. In addition, this team pointed out an error in the published results of the team that claimed the black hole discovery.
Volley: The initial team acknowledged the error and reduced the estimated mass of the black hole to between 1,700 and 3,200 times the mass of the Sun, but pointed out that the second team had assumed that all of the neutron stars ever born in the cluster would remain there. Yet many neutron stars are observed zipping through the galaxy at breakneck speeds as the result of a "kick" from the supernova explosions in which they are created, so M15 certainly would have lost many of its neutron stars over the eons.
Return: The second team then recalculated the orbits of stars in the center of M15 using a smaller number of neutron stars and massive white dwarfs. Their numbers suggested that a cluster of dark remnants would still fit the observations even if M15 lost 95 percent of its neutron stars, leaving about 1,600.
Models by other researchers tend to support the idea of a cluster of stellar remnants at the middle of M15, although they don't disregard the possibility of an IMBH. Most of their calculations, however, show that its mass can't exceed about 1,000 times the mass of the Sun.
Their models also limit the way in which such a black hole can form.
One possibility is that as massive stars fall toward the cluster's center they merge, forming a large black hole. The other is that a smaller "seed" black hole, formed from the collapse of a massive star's core, forms in the center of the cluster and glows slowly through gradual mergers with infalling stars.
Some astronomers, however, believe that the first scenario is unlikely because encounters among infalling stars would kick many of them away from the cluster's heart, preventing the growth of a large black hole. These astronomers suggest that if a black hole formed, it was through the gradual growth of a seed black hole.
The debate isn't settled, and is far from over. Better observations of stars even closer to the cluster's center, perhaps provided by a new generation of super-sized ground-based telescopes, may eventually provide the answer. Until then, the game continues.
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This document was last modified: February 13, 2012.