Formation

• Stars are balanced between two forces: the expansion of their atoms due to nuclear fusion and internal gravity. These two forces remain in equilibrium throughout the star's lifetime. Eventually, fusion stops and gravity takes over, causing smaller stars to collapse into neutron stars. These smaller stars cannot collapse any further, because the particles cannot occupy the same space at the same time. Stars larger than three solar masses have internal gravity sufficient to override this principle, causing the star to collapse into a black hole.
  
  

Behavior

• Although gravity is commonly thought of as a force, Einstein's Theory of General Relativity holds that objects warp the fabric of space-time due to their mass, creating the perception of gravity. More mass causes more warping. A black hole creates such distortion that all space past the event horizon -- the effective range of a black hole's gravity -- is curved inward toward the singularity, or center point. Therefore, no matter how fast matter or energy moves inside the event horizon, all paths lead to the singularity. This means that once matter and energy enter the event horizon, there are lost to any outside observers. The name "black hole" comes from the fact that no light or other radiation can escape. In addition, the event horizon is constantly rotating, causing any gas and trapped matter approaching to form an accretion disc.
  
  

Relativity

• Intense gravity causes time to slow down to outside observers. At the event horizon of a black hole, gravity is effectively infinite, so time appears to stop. If you were to synchronize two clocks, hold one and drop the other in a black hole, you would notice the other clock's ticking slowing down as it approached the event horizon. From your perspective, it would never cross the event horizon. The reverse would occur if you journeyed into the black hole; time would seem to pass normally to you, but you would observe that the rest of the universe had sped up.
  
  

Wormholes

• One popular science-fiction theory about what lies inside a black hole is wormholes. Wormholes are hypothetical passageways that connect to points in space-time together instantly, like a worm traveling through an apple instead of around the surface, however, even if you went into one wormhole and survived the gravity and intense radiation, you would still likely be trapped inside the event horizon of the black hole on the other side of the tunnel.