Supermassive Black Holes: The Extremes of Accretion and Ejection

Tuesday 17th September, 2019

Our speaker for the evening was Dr Andrew Lobban from Keele University who came to talk to us about "Supermassive Black Holes: The Extremes of Accretion and Ejection". He began by saying that he would shortly be leaving Keele University to take up a new post in November as a research fellow with ESA in Madrid, Spain, and then gave us a short introduction to what a black hole is and when the term was first introduced.

He explained that a black hole is an object in space with so much mass that its gravity traps anything that comes near it. This even includes light itself, which is why we cannot "see" a black hole. The term "black hole" was first recorded in a letter published by the clergyman and astronomer John Michell in November 1784.It was not until 1915 that the physicist Karl Schwarzschild, whilst working on a solution to Albert Einstein's equations relating to the Theory of General Relativity, showed that these strange objects could, at least in theory, exist.

The first black hole to be discovered was Cygnus X-1 in 1964 and it is one of the strongest X-ray sources near to Earth. This black hole is actually in orbit around another huge blue star and is pulling material off its partner and forming a disk around itself. The stolen gas and dust gradually spiral in towards the black hole and friction heats it to a point that X-rays are given off. Not all the infalling material is swallowed by the black hole, though, as some of it gains enough momentum to be ejected in two jets perpendicular to the disk. Luckily for Earth, this all happens at a distance of over 6,000 trillion miles away and so we are nowhere near the chaos and hazardous radiation.

Dr Lobban continued by saying a black hole forms when a star that has about 5 times the mass of our Sun runs out of the fuel for its nuclear reactions at its centre. It is this outward pressure generated by the nuclear reactions that stop the star collapsing in on itself as gravity tries to pull the star's material inwards. With nothing to keep the star inflated gravity wins and the material implodes with nothing to stop it, forming an unbelievably dense object where the laws of physics, as we understand them, just break down.

He then mentioned that there is a much larger black hole at the centre of our own galaxy, the Milky Way. This has a mass of 4 million times the mass of our own Sun and is known as a supermassive black hole. It was identified and "weighed" by observing the motion of stars hurtling around it as it cannot be seen directly. He added that there are intermediate mass black holes but most are either star-sized or huge ones at the centre of all galaxies.

The latest breakthrough in studying black holes came this year in April when a team of astronomers, using radio telescopes across the world, published the first image of a black hole at the centre of a galaxy named M87. In fact, as the actual black hole is invisible, the image showed an orange doughnut-shaped ring with a black central region. The actual black hole is a much smaller area at the middle of the inner dark area and the orange ring in the image represents the infalling material around the black hole.