Sagittarius A : What is The Galactic Center?
An original essay by Hubble Space Telescope Astronomer Lauretta Nagel.
Lauretta Nagel
August 2008
John Michell and Pierre-Simon Laplace first proposed the existence of ‘black holes’ in the late 18th century, but they were not called ‘black holes’ until John Archibald Wheeler came up with the term in 1967. Black holes were defined as spots in the space-time continuum that were so distorted that not even light could escape. These regions would be caused by the collapsing of very massive stars. Many theories abounded about just how massive the stars would have to be in order to trigger this kind of collapse; where they could be found and how they might be found. After all, if the hole is BLACK, how does one see it – especially if one is restricted to using technology that only detects light? Over the years, humankind tried different observations in different wavelengths to test the theories – trying to find proof of the existence of black holes. It is only in the last decade that we found this proof.
In order to understand how we find black holes, one must understand the black hole process. Imagine an object – it could be anything with mass: a rocket ship, a clock, or a lot of gas and dust. If it is traveling slowly, it will fall into the black hole, just like a meteor would fall through the Earth’s atmosphere. Meteors burn – often completely dissolving- but what happens in the case of the black hole is very different. The huge gravitational field causes something called ‘time-dilation.’ To the outside observer, the objects falling into a black hole move slower and slower (the clock’s hands would slow), until they reach the ‘event horizon.’ When astronomers talk about the size of a black hole, they mean the size of an event horizon. Inside the event horizon, nothing – not mass, not energy, not light, NOTHING escapes. Take a step back now and think about those slowly moving objects doomed to be captured by the black hole. There are usually a lot of them, bumping, jostling, and getting hotter because of friction and collisions. They give off energy in the form of light – specifically X-ray and radio light. These about-to-be-captured objects can be seen by the astronomers using X-ray and radio telescopes. Therefore, what we see is not the black hole itself, but the matter swirling around the black hole and those objects are what we name as a “source.”
There are different kinds of black holes in evidence and they differ by size or mass. There are the ‘stellar black holes’ that were created by a massive star exploding (a supernova) and collapsing down into and black hole – they are roughly at least ten times the mass of our Sun and 30 km at the event horizon. Amongst the other kinds, there is the ‘supermassive black hole.’ This kind is ten thousand to 100 million times the mass of our Sun and 0.001 – 10 Astronomical Units at the event horizon – and one of these is believed to exist at the center of most galaxies. The supermassive black hole at the center of our galaxy has been located by the X-ray source Sagittarius A. Supermassive black holes are different from stellar black holes in that the gravitational forces are weaker around the event horizon and the density inside is much lower – perhaps lower than the density of our air. These characteristics follow because of the comparatively large size of the supermassive black hole – at least compared to its stellar cousins. All this is prediction as we cannot see INTO the black hole. There are several models for the formation of a supermassive black hole, all involving collapse and destruction of matter, and work is continuing on them. We know that the supermassive black holes exist – we just do not know how they formed, exactly.
What about white holes? Is there a corresponding white hole for the black hole in Sagittarius A? The short answer is ‘very probably not.’ The long answer goes like this – we use the language of mathematics to define/explain/explore Physics and Astronomy, and Mathematics has a basic structure that demands symmetry. If Math were grammar, you could say, “Every sentence must have a Subject and an Object – with Verbs in the middle.” When exploring the theory of Black Holes with Math, solutions for White Holes show up in the equations. These White Holes are defined as the reverse of the Black Hole. The problem is – they cannot really exist. As soon as you add an object – no matter how small – to the interior of the black hole, the white hole phenomenon disappears. It would be ‘nice’ to have such a solution – but the universe does not always do ‘nice.’
Some would wonder if Earth is in danger from Sagittarius A; the truth is Earth and its solar system are in a stable orbit circling the center of the Milk Way Galaxy – along with a lot of other star systems in the Milky Way’s spiral arms. There is a very very very small probability that the Milky Way might collide with another galaxy that has a supermassive black hole in its center and Earth might be affected. If this were to happen, we on Earth would have hundreds of thousands of years warning before that black hole would ‘get us.’ Galactic collisions are not like car crashes – they take a loooooonnnnngggg tiiiimmmee.
Lauretta Nagel received her BA in astronomy from Boston University and was an astronomer on the Hubble Space Telescope Project for 18 years, with many research interests including astrometrics and variable stars. She left the Hubble project in 2007 to open her own bookstore, Constellation Books