What's in a black hole

Black holes are endlessly fascinating for astronomers, they represent the ultimate fate for massive stars or relics of the very earliest stages of the universe.

A black hole is an astronomical body whose gravity is so strong that nothing not even light can escape from it. Obviously if nothing can escape them, we have to observe them indirectly, by seeing the effect they have on other objects in their vicinity or in the line of sight to us. We can detect the radiation that is emitted when material falls in to the black hole or we can see how the black hole bends the light that is emitted by an object that lies between it and our telescope.

Black holes are classified depending on their size. This ranges from black holes that are formed in the death of a star that is a few times the mass of our sun to the super massive black holes (SMBH) that lurk in the centre of almost every galaxy. These SMBHs can be millions or even billions of times as massive as our sun. These giant black holes provide us with a glimpse into the deep history of the galaxy they inhabit. The majority of galaxies are the result of mergers of smaller galaxies in the past and the black holes in the centre of these galaxies are still present. If we are lucky enough to observe them interact with normal stars or the dust and gas that lies between stars we can get valuable information about the formation of that galaxy.

This highlights a big problem with black holes of any size, they can only be seen when they interact with the normal matter in the universe. The closest big black hole is the one at the centre of our own galaxy. For the past twenty years the biggest telescopes in the world have regularly been tracking the motion of about 20 stars in orbit around it. By studying how fast these stars are moving and the shape of their orbits we can calculate the size of this black hole to be about 4 million times as massive as the sun. We can also tell from the way that gas clouds in its vicinity have been heated up that the black hole has flared up at least twice in the past 300 years. What causes these flare ups is unknown, it could be passing dust or gas that is pulled into the black hole.

It could also be asteroids or comets that are pulled away from one of the twenty stars that are orbiting the black hole, thankfully this is something we get to see from the very safe distance of 25,000 light years, which is how far out we are from the centre of the milky way.

Limerick Astronomy Club email limerickastronomyclub @gmail.com