What are Black Holes?


Black holes are celestial bodies that capture the imagination, playing roles in both science fact and science fiction. They are created during the death of massive stars and weight about 3 to 100 times the mass of our Sun, yet they can be difficult to see as their extreme gravitational pull attracts everything straying too close, even light. One way to observe them is to watch as material falls in (a process known as accretion). Some of the material’s gravitational potential energy is lost as heat and light. It is this ‘light’ that allows us to observe these fascinating objects across the electromagnetic spectrum (mainly in the X-ray). 

Although we observe black holes via the light (emission/ radiation) from accretion, we are still struggling to understand the physical processes occurring in these systems. The standard idea is that , as material falls, it spirals around the black hole creating a disc. If it were just a simple disc we would expect to see the same emission all the time. This is not the case; the emission from these objects can change from one day to the next. This suggests that something else is going on. New ideas state that the geometry of these systems may change depending on the amount of material falling in. These different geometries are known as accretion states.

Black holes do not just come in one size, we have also found much heavier objects in the centre of galaxies. These objects are known as supermassive black holes because they are a million to a billion times heavier than our Sun. While they are much larger, we do see the same accretion geometries or states. We also find that they increase in brightness (luminosity) as their mass increases.

One such supermassive black hole can be found in the centre of our own galaxy, the Milky Way. However, it is is the lowest possible accretion state, a state known as quiescence (meaning at rest or inactive). The accretion rate is so low that we cannot see any disc of in-falling material. We have only been able to detect this huge object by its impact on nearby stars. By taking images of the centre of our galaxy over the last 10 to 15 years, we have been able to track the movement of stars. We see these stars revolving, tracking out orbits, in the same way that the Earth, the Moon, and the other planets in our solar system do. Just as gravity keeps us revolving around our Sun, these stars must be held by gravity too. You can see these orbits in the three planes shown ???. Although we cannot see the object holding these stars in their orbits we can see (and measure) it's effects. By doing this we find that the black hole is about 2,000,000 times the mass of our Sun.

Black holes also have smaller cousins, these are known as Neutron Stars. These objects are also created as a large star dies, but their smaller mass means that they do not collapse completely during death, but that some of the stars core (the material at it's centre) remains. This leaves behind the densest material known to man. Only one teaspoon of this stuff can weigh more than your car (more than a ton)! This is heavy stuff! These 'compact objects' weigh about one and a half times our Sun, but are squashed into ball only 10 km across. This means that the of gravity around these neutron stars is so strong that they can also pull material from nearby stars, resulting in an accretion disc.

The work I do focuses, mainly, on the first of these objects, black holes. I am currently looking at a strangely bright group of these objects, they emit more light in X-rays than was thought possible for a stellar mass black hole. One idea is that these are larger, or more massive, that we are looking at something so far undetected. In this case we would be looking at an object that is intermediate in luminosity (brightness) between stellar mass and supermassive black holes, could this be an intermediate mass black hole??? The other option is that we are looking at a new, again unknown, extremely bright accretion state. If this is true, then we. Would be observing new physics. In order the answer this question, I and using a number of parts of the electromagnetic spectrum to try and weigh these black holes.

If you would like to know more about observing these and other types of compact objects, please click here.