Earth will be long gone by then. Once our sun runs out of Hydrogen in about 4.5 billion years, it will expand greatly into a red giant. Our oceans will boil away and our orbit will collapse and spiral into the sun.

The story behind this discovery is actually one of the great ones in Science. An Indian student on his way to graduate school in England worked out the calculations on the boat ride to School. Subrahmanyan Chandrasekhar was age 19 when he figured out, that once a star burns up it’s fuel, it will become a white dwarf UNLESS it’s mass is over 1.4 times the mass of our sun. If it is heavier than this, it will collapse into a black hole.

He was later (much later) awarded the Nobel Prize for the discovery! Not bad for some “off the cuff” calculations on the way to graduate school! If that isn’t the definition of an over-achiever, I don’t know what is!

Over the last few years, astronomers have predicted and then discovered that galaxies have massive black holes at their center. Since black holes cannot be seen, spotting them is not very easy. Not impossible though. As material swirls into a black hole it heats up to millions of degrees and gives off x-rays. The Chandra X-Ray telescope has spotted many this way. You now also know why it’s called the Chandra Telescope! (Dr. Chandrasekhar went by Chandra)

The Universe looks even more interesting in infra-red and x-ray light. This image is from the Chandra telescope. These 4 galaxies have super-massive black holes at their center. The image on right is an artist conception.

The center of our galaxy cannot be seen in visible light. Too much gas and dust. Infrared light is another matter, and that is how we can look at the stars at the very center. Astronomers looked at the motions of stars at the center and saw something very strange.

The Milky Way- This generation of humans is the first in human history to live in a world, where it is not visible in the night sky. City lights block it out to most of us. Ctsy Max Planck Inst. and Yuri Beletsky

They made a movie out of dozens of images of the center taken with infra red telescopes. Look at what they saw below:

Stars orbiting a supermassive black hole at the center of our Milky Way Galaxy. Ctsy Max Planck Inst

A close-up view of stars at our galactic center. Ctsy Max Planck Inst

The black hole’s mass can be measured rather easily. If we can estimate the mass of the orbiting stars and observe how long it takes them to orbit, we can solve for the unknown mass of the black hole. Newton himself figured out the equations.

The answer is that there is a black hole with a mass of 4 million suns! This object is also only about 100 astronomical units across. (1AU= earth/sun distance) This is tiny compared to it’s mass. As Astronomer ALex Filippenko observed, “The most conservative guess is that it’s a black hole”.

Something to think about when next you find yourself under a really dark sky…

Dan

Sources: http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/galcenter.htm

Alex Filippenko and Jay Pasachoff- The Cosmos. NASA/Chandra and U. of AZ: http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/galcenter.htm

Also Astronomy Cast.