We all must recall the great excitement of 2016 when the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the detection of gravitational waves.
This instrument felt space itself contract and expand in reaction to the collision of two large black holes a billion light years away. What sounded like science fiction to some even a handful of years ago is suddenly science fact. Yes, this is a huge distance in terms of light years (and even more in terms of miles). At the same time, it is small compared to the size of the universe. We call this intermediate distance one that is at ‘low redshift.’
In a previous article I called this the discovery of the century. It is an outstanding physics result which upholds Einsteins prediction of how gravity works from a century ago. It is also a Herculian engineering feat to succeed in building detectors that have sensitivities that meet the specs. As an astronomer, my job is to give an answer to what in the world are two 30 solar mass black holes doing in this low redshift galaxy?
Black holes of this size are formed by a stellar explosion, or supernova, formed from a star that was originally much more massive than the mass of the black hole. Although no one really knows for sure just how big the original star would have been, it probably weighed in at around 150 solar masses.
Stars in galaxies like the Milky Way produce stars of many sizes, but not stars more massive than about 100 solar masses. In fact, we have never found such a massive star in any galaxy ever, and we gather that the pressure induced by the radiation itself would blow the stars apart if they were much larger than 100 solar masses. Where the enormous stars would come from that led to the historic LIGO detection is the topic of next week’s article.