| My next topic I'm sure will interest all of you, |
| I want to talk to you now about Black Holes. |
| Simply speaking, a black hole is what's left after a large star dies. |
| You're already aware that a star is an energy producer, a nuclear fusion reactor, |
| its core is a gigantic nuclear fusion bomb that's trying to explode, |
| but its mass of surrounding gases is so large that its gravity contains the explosion, |
| and the balance that exists between the gravity and the fusion is what determines the star's size. |
| However, as a star gets older, as it ages, |
| its fuels get used up and its nuclear reactor slows down. |
| And then, its gravity gets the upper hand. |
| The star implodes. |
| Gravity pulls inward and compresses the stellar material into the star's center. |
| As it's compressed, the core heats up tremendously, |
| and then, at some point, a supernova, a great explosion, occurs, |
| and the stellar material and a lot of radiation are blasted out into space. |
| Only the extremely dense, extremely massive core is left. |
| Its gravitational field is so strong that nothing can escape it, not even light. |
| So it disappears from view: it's black. |
| It's now a black hole. |
| Now, the idea of a 'black hole', an object with so much gravity that it won't let light escape, |
| was first proposed more than two hundred years ago, in 1795, |
| by a French mathematician, Pierre LaPlace. |
| He used Newton's gravitational theory to calculate |
| that if an object was compressed small enough, |
| it would require an escape velocity of almost 300,000 kilometers per second, the speed of light. |
| More recently, the name of Stephen Hawking, the great British physicist, has become synonymous with black hole theory. |
| A black hole consists of two parts, a 'singularity' and an 'event horizon'. |
| Its 'singularity' is the point where its gravity is indefinitely strong |
| and its mass is indefinitely dense, |
| and this point is theoretically at the center of the black hole's core. |
| And its 'event horizon' is the perimeter around the core at the distance where its gravity is still strong enough to pull light into itself, |
| at the distance where escape velocity equals the speed of light, |
| and where nothing can escape its pull. |
| Both the singularity and the event horizon are intangible, of course, |
| but both of them can be calculated mathematically. |
| The distance of the event horizon from the core is called the 'Schwartzchild radius', |
| and this radius is equal to "two GM divided by C squared", |
| where G is Newton's gravitational constant, |
| M equals the mass of the core, |
| and C equals the speed of light. |
| Even though we can't see them, black holes do exist, |
| and we can prove their existence in three basic ways. |
| One way is to search for celestial objects that are very small but that have a very large mass. |
| For example, the astronomical feature called 'M87' is only about the size of our solar system, |
| but it weighs three billion times more than our Sun. |
| So it's a good bet that M87 is a black hole. |
| Another way to find a black hole is to search for matter that's accelerating, |
| because a black hole accelerates anything that approaches it. |
| As the matter gets sucked in, it speeds up and heats up, |
| and this superheated matter produces X-rays, which can be detected. |
| The star Cygnus X-1 is a strong X-ray source, |
| so there's a good possibility that there's a black hole in its neighborhood. |
| And finally, a black hole can be detected using Einstein's Theory of Relativity, |
| which tells us that gravity can actually bend space, warp space. |
| An object with a lot of gravity located between Earth and a more distant star |
| can bend that star's light like a lens or a prism does. |
| This is called the gravitational lens effect. |
| In 1996, a gravitational lens passed between Earth and MACHO-96-BL5, |
| and the temporarily brightened image was photographed by both the Hubble Space Telescope and ground observers. |
| 'Black holes' are a bit frightening, but if the idea of a black hole sucking in the rest of the Universe upsets you, |
| let's put them into perspective. |
| A black hole doesn't suck in everything in sight, |
| it only affects nearby material. |
| If a black hole with the same mass suddenly replaced our Sun, |
| then its Schwartzchild radius would be only three kilometers, |
| compared to our Sun's radius of 700,000 kilometers! |
| And since Earth is 150 million kilometers from the Sun, |
| it would be in no danger of being sucked in. |
| Without the Sun, though, Earth would be very cold and lifeless, I'm afraid! |