Hubble astronomers found something extraordinary at the heart of nearby globular cluster NGC 6397 –a concentration of smaller black holes lurking there instead of one monster, supermassive black hole. Ancient stellar jewelry boxes, globular star clusters are densely packed objects, glittering with the light of a million stars in a ball only about 100 light-years across dating back almost to the birth of the Milky Way.
Our home galaxy is surrounded by about 150 globular clusters, formed 2.3 billion years after the Big Bang, harboring hundreds of thousands, and sometimes millions of stars, formed very early in the vast halo surrounding the embryonic Milky Way before it flattened to form the spiral disc that exists today.
Orbits Outside Milk Way’s Starry Disk
NGC 6397 reports Hubble/ESA astronomers, “is one of the globular clusters that orbit outside of our Milky Way galaxy’s comparatively younger starry disc. These spherical, densely packed swarms of hundreds of thousands of stars are our galaxy’s first homesteaders. The cluster’s blue stars are near the end of their lives. These stars have used up their hydrogen fuel that makes them shine. Now they are converting helium to energy in their cores, which fuses at a higher temperature and appears blue. The reddish glow is from red giant stars that have consumed their hydrogen fuel and have expanded in size. The myriad small white objects include stars like our Sun.”
A Stellar Graveyard –“Core Collapse Phenomenon”
At the heart of globular cluster almost as old as the universe itself, NGC 6397 and one of the closest to Earth, some 7,800 light-years away, NGC 6397 is one of some 20 globular clusters of the Milky Way Galaxy that have undergone a core collapse meaning that the core has contracted to a very dense stellar agglomeration.
The amount of mass a black hole can pack away varies widely from less than twice the mass of our Sun to over a billion times our Sun’s mass. Midway between are intermediate-mass black holes weighing roughly hundreds to tens of thousands of solar masses.
Case of Supermassive Mistaken Identity
The astronomers at first thought the globular cluster hosted an intermediate-mass black hole– the long-sought “missing link” between supermassive black holes (many millions to billions of times our Sun’s mass) that lie at the cores of galaxies–and the vastly smaller stellar-mass black holes that form following the collapse of a single massive star.
“We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not ‘point-like’ (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster,” said Eduardo Vitral of the Paris Institute of Astrophysics (IAP) in Paris, France.
Velocity = Mass
To detect the elusive hidden mass, reports NASA/Hubble, Vitral and senior astronomer Gary Mamon, also of IAP, used the velocities of stars in the cluster to determine the distribution of its total mass –the mass in the visible stars– as well as in faint stars and black holes. The more mass at some location, the faster the stars travel around it.
The researchers used previous estimates of the stars’ tiny proper motions, which allow for determining their true velocities within the cluster. These precise measurements for stars in the cluster’s core could only be made with Hubble over several years of observation. The Hubble data were added to well-calibrated proper motion measurements provided by the European Space Agency’s Gaia space observatory which are less precise than Hubble’s observations in the core.
“Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated,” explained Mamon. These moderate-elongation orbital shapes limit what the inner mass must be.
“Dynamical Friction,”–Stellar Pinball
The researchers concluded, reports NASA/Hubble, “that the invisible component can only be made of the remnants of massive stars (white dwarfs, neutron stars, and black holes) given its mass, extent, and location. These stellar corpses progressively sank to the cluster’s center after gravitational interactions with nearby less massive stars. This game of stellar pinball is called “dynamical friction,” where, through an exchange of momentum, heavier stars are segregated in the cluster’s core and lower-mass stars migrate to the cluster’s periphery.”
“We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes,” said Mamon. Two other recent studies had also proposed that stellar remnants, in particular, stellar-mass black holes, could populate the inner regions of globular clusters. “Ours is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the center of a core-collapsed globular cluster,” added Vitral.
Source of Gravitational Waves?
In a coda, the astronomers noted that this discovery raises the possibility that mergers of these tightly packed black holes in globular clusters may be an important source of gravitational waves, ripples through spacetime, that could be detected by LIGO Observatory ( Laser Interferometer Gravitational-Wave) experiment.
Image credit: NASA, ESA, T. Brown, S. Casertano, and J. Anderson (STScI)