Portrait of a black hole

Sage-A* can only be observed in regions of the electromagnetic spectrum that are both far above and below the visible spectrum-these regions include microwaves (above or longer wavelengths) and x-rays and gamma rays (below or shorter wavelengths).

Sadge-A* is a highly active radio source at the bull's eye of our galaxy's core; it is 26,000 light years away from us. From Earth, it is located on the border of the constellations Sagittarius and Scorpius. And just like every galaxy we observe in deep space, our Milky Way has an immense gravity hole at its center.

How big is Sadge-A*? Well, it's big on a mind-numbing scale. The Milky Way's mother of all black holes is estimated to have a mass equivalent to 4 million suns. The stellar mass is compressed into a small volume-and that's essentially what makes the immense "dark star".

Sadge-A*'s event horizon, the boundary around the black hole, is 24 million kilometers (15 million miles) in diameter. It's so large that it is 20 microarcseconds across in the sky or one part per 10 billion of a circle. As seen from Earth, it's approximately the size of one of Apollo 14 astronaut Alan Shepard's golf balls left on the Moon in 1971.

Because the gravity of Sadge-A* bends light, it acts as a giant lens. So when the EHT array finally assembles the data to create an image of Sadge-A*, it will be more of a silhouette of the black hole, not the actual object. And the object will appear larger than it actually is.

Astronomers predict that Sadge-A* will look lopsided due to its hyper spinning; light rays approaching Earth will be brightest when compared to those traveling in the opposite direction.

What's in the Sky: Look for Comet Hartley 2 in the night sky this week. The 6th magnitude comet is visible in binoculars during the evening high in Perseus. On Oct. 20, the comet will be closest to Earth. For an online sky map of the comet's path see: http://www.aerith.net/comet/catalog/0103P/2010.html.

Lou Varricchio, M.Sc. was a science writer at the NASA Ames Research Center. He is a member of the NASA-JPL Solar System Ambassador program in Vermont.

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