SC300Eric Bliss

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun.

If a star that massive undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose

gravitational forces, the remnant will collapse in on itself.

• The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity".

• Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region.

It is therefore called a black hole, and its surface is called the “event horizon ".

http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html

Since black holes are small, and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see. the photograph below shows the optical

companion star to the (invisible) black hole Cygnus X-1.

If a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can pull matter into itself.

What is the biggest obstacle to proving that black holes really do exist?

Astrophysicists generally agree that when the compact object in an X-ray binary system is shown to be more massive than about 3 times the mass of the Sun, then this compact object is a black hole beyond reasonable doubt.

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/011120a.html

What kinds of evidence are there that they do?

There is good observational evidence from X-ray observations and from the Hubble Space Telescope that there are massive black holes (with masses more than a million times that of the Sun) exist in the centers of some galaxies.

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970805b.html

Given what you know about scientific inquiry, why is the lack of direct evidence for black holes a problem for scientists?

Steps of the scientific method… 1. Name the problem or question 2. Form an educated guess (hypothesis) of the

cause of the problem and make predictions based upon the hypothesis

3. Test your hypothesis by doing an experiment or study (with proper controls)

4. Check and interpret your results 5. Report your results to the scientific

community

Why might black holes be important?

“If I have seen further, it is by standing on the shoulders of giants.” Sir Isaac Newton

Culture of exploration?

http://www.youtube.com/watch?v=EOi0xA9GvX8

How is the search for black holes similar to the quest to view microscopic objects?

As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms and they emit

x-rays The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity.

Thus we can see this X-ray emission.

• http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html