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Recent Podcast
Tour of Sagittarius A
Tour of Sagittarius A
One of the biggest mysteries in astrophysics today is figuring out where mysterious particles called neutrinos come from. (2014-11-13)

Peering into the Heart of the Milky Way

NASA: We have booster ignition and liftoff of Columbia, reaching new heights for women and X-ray Astronomy.

Martin Elvis: The main thing Chandra does is take these superb, sharp images.

Cady Coleman: Nothing as beautiful as Chandra trailing off on its way to work

Narrator: Humanity has long sought to learn about the Milky Way, our home galaxy. Even after the advent of optical telescopes, the Milky Ways center, some 26,000 light years from Earth, remained mysterious because gas and dust blocks most visible light along our line of sight. Fortunately, X-ray telescopes, like NASAs Chandra X-ray Observatory, can detect higher-energy radiation that penetrates this veil of galactic debris. Dr. Frederick K. Baganoff of the Massachusetts Institute of Technology has been using Chandra to study our Galactic Center for several years. He explains why X-ray telescopes like Chandra are so crucial for this pursuit.

Fred Baganoff: X-rays are important for studying the center of our galaxy because the Milky Way, our galaxy, is a spiral galaxy, and that means it has this sort of frisbee-shaped disk, and then a bulge in the center. And in the disk are lots of dust-clouds. Because our sun is in the disk, we have to look through these dust-clouds to see towards the center of our galaxy, and the dust acts like smog and it blocks our view. So we actually can't see the center of our own galaxy in optical light, or ultraviolet light. We can only look at longer wavelengths like radio and infrared, or the shorter wavelengths like X-rays and gamma rays.

Narrator: What does Chandra see when it looks at the heart of the Galaxy? It finds a sea of hot gas enveloping thousands of white dwarfs, neutron stars and black holes. Chandra also finds regions aglow with the light of massive young stars. And thats not all. At the very core of the Milky Way lies a supermassive black hole that has a mass of nearly four million suns. This black hole is named Sagittarius A*, or Sgr A* for short, due to its location in the direction of the constellation Sagittarius. Sgr A* is continually growing as it pulls in gas. Infalling gas is heated to millions of degrees and gives off X-rays, which have been detected by Chandra. Sometimes gas outside the black hole can become super-heated and expelled from the region at millions of miles per hour.

Narrator: Fred Baganoff explains how Chandra has been used to observe Sgr A* as it erupts into flares.

Fred Baganoff: We looked at Sgr A* with Chandra in the first year and we saw pretty much just this quiescent-level emission. It was a low, steady source. In the next year, in 2000, we looked again. All of a sudden, in an hour and a half, it increases by a factor of 45 in X-ray luminosity, and then in ten minutes it dropped by a factor of five. We were able to use the fact that it dropped by a factor of five in ten minutes, with the fact that the speed of light is constant, to figure out the size of the region that was emitting these flares. And the size of the region is about ten times the radius of the event horizon of the black hole, the event horizon being the point at which even light can't escape. If you get too close to the black hole, gravity becomes so strong that eventually even light can't escape. That's called the event horizon. So this X-ray flare was coming from a region only about ten times that size, and that would be equivalent to about the Earth's orbit around the sun. That small of a region was producing all of this energy.

Narrator: What do scientists think cause these outbursts from Sgr A*?

Fred Baganoff: There are two possibilities. One is that a lot of extra mass was suddenly just dumped on to the accretion region. Second hypothesis is that it's more like a solar flare. Solar flares are related to magnetic fields, and so, the second possibility is: these X-ray flares were produced due to magnetic energy being turned into acceleration of particles.

Narrator: Clearly, there is much to explore and learn about our home galaxy, the Milky Way, and Chandra will continue to be an important tool in that effort.

Narrator: For more information about the Chandra X-ray Observatory, visit our website at

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