Nothing really new for those with some interest for astronomy but the 16-years research at Max-Planck-Institut für extraterrestrische Physik (Germany) have confirmed that there is a black hole at the center of the Milky Way. After long years of observations, the astronomers involved have concluded that there is no other possible explanation for the behaviour of the stars near Sagittarius A*. They found that there is an inner group of very fast rotating stars with randomly inclined orbits and an outer one of stars orbiting the black hole in an ecliptic-like plane.
The most important evidence has come, for what I could infer, from the orbit of S2, a star that moves so quickly that has completed one whole orbit around the GC black hole in those 16 years of watch. In their own words:
Observations by our group and by the UCLA group of the velocities and accelerations of stars in the Galactic Center over the course of the last decade have already provided strong evidence that SgrA* is in fact a black hole of about 2.6 million solar masses. However, these measurements could not rule out some alternatives to the black hole model. Two such alternatives are a ball of massive, degenerate fermions, like neutrinos, or a cluster of dark astrophysical objects, such as stellar mass black holes or neutron stars. These two alternative explanations can now be excluded by analyzing the orbit of S2:
In spring 2002 S2 was passing with the extraordinary velocity of more than 5000 km/s at a mere 17 light hours distance -- about three times the size of our solar system -- through the perinigricon, the point of closest approach to the black hole. By combining all measurements of the position of S2 made between spring 1992 and summer 2002, we have obtained enough data in order to determine a unique keplerian orbit for this star, presented in Figure 1. It is highly elliptical (eccentricity 0.87), has a semimajor axis of 5.5 light days, a period of 15.2 years and an inclination of 46 degrees with respect to the plane of the sky. From Kepler's 3rd law we can determine the enclosed mass in a straightforward manner to be 3.7±1.5 million solar masses. Therefore at least 2.2 million solar masses have to be enclosed in a region with a radius of 17 light hours. It is not possible to explain this result with a neutrino ball model because the required neutrino masses would be too large, or with a dense cluster of dark astrophysical objects because such a cluster would have the extremely short lifetime of at most a few hundred thousand years. Compared to the lifetime of our Galaxy of the order of 10 billion years this configuration is highly improbable. The only remaining alternative to the black hole model is a ball of bosons, which would be hard to distinguish from a black hole because of its small size. However, eventually, after some time such a ball would eventually collapse to a black hole after having accreted enough matter from its surroundings. Hence, the observed keplerian orbit of S2 around SgrA* provides compelling evidence for the existence of a massive black hole at the center of our Galaxy.
The inner cluster of fast rotating stars, artistic reconstruction
In the media:
- Science Daily
Water and CO2 in extrasolar planet:
This also called my attention and has been mediatically relevant: an extrasolar "Jupiter" shows clear signs of having CO2 and water, substances that we immediately associate with life (though this extreme is, of course, unconfirmed yet).
(At BBC, at SD)