Constellation-X

Constellation-X is a planned X-ray space observatory operated by NASA. It will open a new window of X-ray astrophysics by allowing us to watch matter fall into super-massive black holes and in effect, make 'movies' of matter as it makes its last orbit around a black hole before plunging over the event horizon. These will not be movies in the classical sense of a series of images, but instead will be movies in the form of a series of X-ray spectra. These spectra will measure the Doppler shifts of the matter as it orbits and plunges into the black hole, and one can invert this series of spectra into a series of images by applying the equations of General Relativity (GR).

Because Constellation-X is a Great Observatory, it will impact all areas of Astrophysics, not just black hole studies. The black hole studies will be used to perform precision tests of general relativity, but Constellation-X will also study Dark Energy (and Dark Matter). The tests of GR will be done by measuring the path of matter as it orbits and plunges into the black hole and comparing those paths with those that GR predicts matter should follow. The Dark Matter/Energy studies will be done by measuring the X-ray properties of clusters of galaxies. These clusters of galaxies are the largest gravitationally bound systems in the Universe and so studies of them should help us understand Gravity on the largest scales. Together Dark Energy and Dark Matter make up 96% of the Universe, and we currently know almost nothing about their nature other than that they seem to exist.

As well as helping physicists understand the 'Dark Side' of the universe, Constellation-X will help us understand the normal 4% of the Universe that is not 'Dark' but instead is made of the same sort of matter the earth is made of (protons, electrons, quarks, etc). Of this 4%, approximately half is hidden from our view. This half is theorized to be in the form of a Warm-Hot Intergalactic Medium (WHIM). The WHIM is theorized to take the form of clouds and filaments between galaxy clusters, but these clouds are likely too thin and faint to be seen directly. Instead, Constellation-X may be able to see the shadow that they cast on the light of background objects. The Chandra X-ray Observatory has made preliminary, controversial detections of this shadow, but Constellation-X will be able to make completely unambiguous detections. Constellation-X will determine what happens to matter at extremely high densities (higher than we can achieve in any Earth-bound particle accelerator, but which do exist in space) by measuring mass-radius relation of neutron stars to determine the Equation of State (EOS) of ultra-dense matter. Compressing a neutron star slightly will turn it into a black hole - how much pressure must be applied is not known, but measuring its EOS will tell us.

Recent discoveries with the Hubble Space Telescope and the Chandra X-ray Observatory show that super-massive black holes are surprisingly important in the formation of galaxies and in the evolution of the Universe. Constellation-X will allow us to study the the evolution of super-massive black holes throughout the Universe, and study the effect these black holes have on their surroundings. These effects take the form of 'Cosmic Feedback' which has only recently been discovered with Chandra, and the way this feedback works is still not well understood. It seems that somehow the feedback manages to regulate the growth rate of galaxies to match the growth rate of the super-massive black holes at the center of galaxies. It may be that as the black holes grow by accreting matter from the surrounding galaxy, they periodically eject some of this matter in the form of very high energy collimated jets which then stop the matter from flowing into the black hole. Constellation-X will be a much larger telescope than any previous X-ray telescope, and it will have much higher spectral resolution. This will allow the Constellation X-ray Observatory to make major advances in all areas of astrophysics from solar system objects to distant quasars. Every decade the professional Astrophysics community, with the help of the National Academy of Sciences agrees on a set of priorities for the coming decade, and in the 2000-2010 decadal survey Constellation-X was ranked next in priority after the James Webb Space Telescope (which is the successor to the Hubble Space Telescope) Constellation-X is ready to be built and operating by 2017.