Normal stellar black holes are believed to be the final destinations of massive stars. However, there may exist supermassive ones containing mass up to billions of solar masses. Black holes are scattering in the Universe, and some may even have lurked somewhere near our Solar System to wait for the next preys! Black hole is “black” because its monstrously strong gravitational force can engulf everything. Even the fastest moving thing in the world, light, can have no chance to escape once falls within the boundary of a black hole called “event horizon”. Black holes also distort the surrounding spacetime and consequently lead to many bizarre phenomena.

(Credit: J. A. Biretta et al., Hubble Heritage Team (STScI /AURA), NASA)

M87and its jets - The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. ！

(Credit: NGST, NASA/CXC/SAO)

The new sky show “Exploring the Black Holes” introduces the formation and nature of these invisible celestial objects. You could follow the footsteps of astronomers to find these cosmic destroyers and explore their mind-boggling characteristics.

Chandra X-ray Observatory - Chandra X-ray observatory, launched by Space Shuttle in July 1999, is the most sophisticated X-ray observatory built to date. Chandra X-ray observatory is designed to observe X-rays from high-energy regions of the Universe.

(Credit: SOHO(ESA & NASA))

Hydrostatic equilibrium  - Hydrostatic equilibrium refers to the balancing of pressures and gravity of a star. Striking an equilibrium between two kinds of pressure thrusting outward, namely, gas pressure and radiation pressure, and the gravitational force which turns things inward, a main sequence star can remain stable for a long period of time. Gas pressure stems from the materials of a star. With higher temperature or greater density, the gas pressure is greater. On the other hand, radiation pressure originating from photons will also increase as the temperature rises. Gravitational force has a direct relationship with the distance and mass of materials: the closer the materials, the stronger the gravitational force; and the more massive the materials, the stronger the gravitational attraction. Disequilibrium of these forces will lead to collapse at the core of a star which will finally become a white dwarf, neutron star or black hole.

Structure of a Black Hole - For a non-rotating and uncharged simple black holes, its boundary is known as the “Event Horizon” where even light cannot escape from it. The radius of Event Horizon is equal to the “Schwarzschild Radius”. As nothing can escape beyond the event horizon, we will never learn about anything happening within such boundary. The centre of a black hole is called “singularity”, a point into which all masses are concentrated and to which laws of physics as known today are completely inapplicable. Outside the event horizon, there is a spherical region called “Photon Sphere”. If light passes the sphere tangentially, it will be captured by the gravitational force of the black hole and, from then on, will keep on revolving around the black hole forever like a satellite orbiting a star.