The Black Hole Gap(s)
It may seem like a simple question, but at what mass does a neutron star collapse further into a black hole ?
The limit for white dwarf collapse to Neutron stars is 1.4 solar masses is known as the Chandrasekhar limit, named after the astrophysicist Subrahmanyan Chandrasekhar. This can be observed by measuring the largest white dwarf mass, and the smallest neutron star, and it matches the limit.
So, you'd expect, if you simply measured the largest neutron star, and the smallest black hole, you'd find the same situation, except, that wasn't what was happening.
The smallest black hole that had been detected was around 5 solar masses, while the largest neutron star was around 2.1 solar masses. So what was happening between there and 5 ? This is what became known as the black hole mass gap.
That was, until LIGO showed up, and while it's been around now a few years, it's only in the last few that we've really started to get the data sets to start seeing things we simply couldn't see before.
LIGO works by using lasers and mirrors stretched out over miles, to detect the minute gravitational waves given off from objects that are quite frequently, billions of light years from us, most commonly, black holes merging.
LIGO helped us begin to locate more and more different sized black holes at the point of merger, and that included some less than 5.
On Aug 17, 2017 two neutron stars were detected to collide, briefly creating a large neutron star for a few hundred milliseconds before collapsing into a black hole. It turns out this particular merger created a new neutron star of 2.7-2.8 times the mass of the Sun, and it was that which collapsed. But what else was discovered was that the spin of the neutron star can effect the mass at which this collapse occurs, meaning there isn't a single limit, but more a range between 2.5 to 2.8 solar masses.
The reason for the gap was simple, we can only observe directly stellar black holes that are in a binary configuration with another star that keeps feeding it, and therefore, causing it to burp out x-rays and other EM signals, so there is a limited pool by which we can observe, and the large ones are more likely to be the ones pulling in material and making the noise, while smaller ones even if with binary companions could be sat in stable orbits for much longer periods of time.
Finally, there is another gap, and that's the gap of 20+ solar mass black holes, the so called intermediate range, and again, LIGO is helping to uncover these objects and proving their existence.
