The previous post was a farewell to black holes in the class, not here on the blog. (And it was not quite a farewell there either, since the midterm yesterday was all about the properties of the Reissner-Nordström black hole, representing a black hole with an electric charge, and a nice computation involving cosmic censorship.)
There have been two rather notable discoveries in the black hole astrophysics world this week. The first is the discovery of what seems to be another case of an intermediate mass black hole (there was only one example known before). Not the supermassive ones that live at the centers of galaxies (tens to hundreds of millions of times the mass of our sun), and not stellar mass ones of a few times the mass of our sun. Instead in a nearby Globular Cluster (quotes from Hubble news site):
Astronomers have found evidence for a medium-
size black hole at the core of Omega Centauri, one of the largest and most massive globular star clusters orbiting our Milky Way Galaxy.
The intermediate-mass black hole is estimated to be roughly 40,000 times the mass of the Sun. The black hole was discovered with NASA’s Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile. The ancient cluster is located 17,000 light-years from Earth.
“This result shows that there is a continuous range of masses for black holes, from supermassive, to intermediate, to small, stellar types,” explained astronomer Eva Noyola of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, and leader of the team that made the discovery. “This finding also is important because the theory of formation for supermassive black holes requires seed black holes that are exactly in the mass range of the one we found. Such seeds have not been identified so far. If these types of intermediate-mass black holes happen to be common in star clusters, then they can provide numerous seeds for the formation of the supermassive black holes.”
Astronomers have debated the existence of moderately sized black holes because they have not found strong evidence for them, and there is no widely accepted mechanism for how they could form.
Please read the nicely written press release for more information about the evidence that they used to conclude the above.
The second piece of news* is about the smallest black hole found so far! From an article by Andrea Thompson in Space.com:
The low-mass black hole sits in a binary system in our galaxy known as XTE J1650-500 in the southern hemisphere constellation Ara. NASA’s Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in 2001, and astronomers soon realized that the system harbored a relatively lightweight black hole. But the black hole’s mass had never been precisely measured.
And what are its vital statistics?
The new lightweight record-holder weighs in at about 3.8 times the mass of our sun and is only 15 miles (24 kilometers) in diameter.
“This black hole is really pushing the limits,” said study team leader Nikolai Shaposhnikov of NASA’s Goddard Space Flight Center in Greenbelt, Md. “For many years astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question.”
More on that here.