Astronomy is truly fascinating: Just when you think you have understood and nailed a phenomenon, another discovery will shake the very grounds on which it is built, forcing you to reconsider and to tweak the theoretical grounds on which it’s based; that’s just how science works.
New study has located a black hole about 70 times the mass of our Sun, in our own Milky Way galaxy, however according to our current stellar models and its evolution, it seems to be a “glitch in the matrix”, meaning a black hole of this mass cannot be formed from our current theoretical constructs regarding stellar evolution, at least in our galaxy.
Analyzing the spectrum of the stars in our galaxy, we can deduce that most of the mass, thus elements making it up, is expelled from the stars before they undergo a core collapse to form a black hole.
Really massive stars (130 – 150 Solar masses) form a black hole in a process called pair-instability supernova, where the mutual production/annihilation of electron and positron pairs in the presence of high energy gamma photons, reduces the internal pressure that supports the star against gravitational collapse, thereby creating a runaway thermonuclear explosion, smashing the stellar core to smithereens, leaving nothing behind. So astronomers are at their wits end trying to decipher how this particular black hole, LB-1, got so heavy and huge.
Jifeng Liu, of the National Astronomical Observatory of China, pithily remarks that black holes of such mass should not even exist in our galaxy. It’s twice as massive as they thought possible and hence the theorists are faced with explaining this anomalous behavior.
Black holes, as their name suggests, are completely black and don’t emit any radiation that we can detect (Hawking radiation is too faint for us to detect at this distance), unless they accrete matter from a companion star in a binary system.
This method of detecting black holes was first proposed by John Michell (it was called dark stars then until John Wheeler coined the term black holes). The trick was to find the companion star that does emit light and observing the influence of the black hole on it – the companion star would be tugged by the powerful gravity of the black hole.
This method, known as the radial velocity method, is used to detect both exoplanets and black holes as we can detect their gravitational influence on the stars.
Liu and his colleagues used the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), to identify these tiny gravitational perturbations to zero in on a main sequence blue giant star. It also took the follow up observations from the Gran Telescopio Canarius in Spain and the Keck Observatory in the US to confirm it.
The companion star of the black hole, estimated to be around 35 million years old and having 8 times the mass of our Sun, has a circular orbit, opposed to the normally elliptical orbit that’s usual.
The first gravitational wave detected by LIGO in 2015, given the name GW150914, also had a black hole in the mass range of what has been detected now, around 62 solar masses, which provides a way for a black hole of such mass to be formed however LB-1 still has its companion star whizzing around it.
One theory is that the black hole was formed from a collision of two black holes and the companion star may have been captured later on, but that would not explain the nearly circular orbit, as the black hole capturing another star would result in a highly elliptical orbit; the orbit can get smoothed out, but the timescales would vastly exceed the age of the star.
Another possibility is where the material ejected from the supernova could be captured by the star, directly leading to the formation of a black hole. This is possible only when the conditions are proper and we have not had any experimental confirmation of the same. Researchers are wondering if this could be the evidence for it.
Though it’s still a mystery how LB-1 formed, researchers are taking up the gauntlet to lift the veil to delve into the secrets of its formation.
This discovery would force us to re-examine our current theories of stellar evolution, observed LIGO director David Reitze.