Science: Black holes
Conducting a survey of black holes
Black holes represent the ultimate end-state of matter. When a massive star explodes as a supernova, the innermost core collapses down into a ball just a few kilometers across. When an object's size decreases, the gravity at its surface increases. A black hole is formed when the gravity is so strong that not even light can escape its grasp.
And yet, even though black holes are notorious for swallowing everything nearby –even light– in reality, vast amounts of radiation can be emitted from right outside their effective surfaces, known as event horizons. As matter falls in towards the black hole, a swirling disk is formed. Friction and other forces heat that disk to millions of degrees, and matter that gets so hot pours out copious X-rays.
These X-rays (really just a high-energy form of light) give away the locations of the otherwise invisible black holes. By searching for X-ray sources, astronomers can find black holes, and even determine their masses. Black holes are known to have masses anywhere from a few times that of the Sun ("stellar black holes"), to monsters with a mass of millions or even billions of times the Sun's mass ("supermassive black holes"). The black hole in the center of our Milky Way Galaxy is estimated to be about 4 million solar masses. While its visible light is obscured by nearby dust and gas, the X-rays shine through.
One of NuSTAR's main goals is to look for black holes of all mass ranges. NuSTAR will spend several months finding black holes both in the distant Universe and within our own Milky Way Galaxy. NuSTAR will study regions where other telescopes (including the Chandra X-ray Observatory, the Hubble Space Telescope, and the Spitzer Space Telescope) have mapped the sky in lower-energy light, so that visible and infrared counterparts to the X-ray sources are already known. NuSTAR will also peer into the heart of the Milky Way. Previous X-ray telescopes have revealed many X-ray sources there besides the "monster in the middle", so this is a fertile hunting ground for stellar black holes and other exotic phenomena.