Lecture 2

Lecture 2

Since 1784, John Michell proposed the idea of a black star, yet it was only until the last century that astronomers identified these objects. Their current name was invented by John Archibald Wheeler. The existence of black holes had been predicted by Einstein’s Theory of General Relativity. They appear when enough material is confined into a small volume and do not let any light escape. The curvature of time and space created by dense matter keeps everything inside and nothing can escape. Black holes are very small; if the sun was a black hole it would have a radius of only 1.9 miles. To prove an object is a black hole, one has to measure small distances from an astronomical distance. The evidence proving their existence is overwhelming and, according to Caleb Scharf, they direct the evolution of the universe. At the center of our galaxy, the Milky Way, there is a black hole four million times the mass of the sun. Near the hole, one can observe the stars moving close to the speed of light. At the edge of the Milky Way, one sees three solar masses produced every year. According to Guillermo Gonzalez, Donald Brownlee, and Peter Ward, we live in the Circumstellar Habitable Zone, because we are in the best spot in the galaxy, as we are not close enough to be sucked in by the black hole nor so far out that we’d be destroyed by newly forming stars.

Lesson 1.2

On a one-to-one billion scale the sun is the size of a large grapefruit(14 centimeters), while Earth is the size of the tip of a ballpoint pen, at a distance of fifteen meters away. The Milky Way is one of 100 billion galaxies in the universe, as many as the grains of dry sand on all the beaches on Earth. The Milky Way is 100,000 light years big. If the life of the universe was compressed to one year, human civilization is just a few seconds old.

Chapter 1.3 Spaceship Earth

Contrary to our perception that Earth is not standing still, it is rotating. Earth orbits the sun once every year at an average distance of 1 AV which is approximately 150 million km. Earth’s axis is tilted by 23.5 degrees and rotates in the same direction it orbits, counter clockwise as viewed from the North Pole. Our Sun moves randomly relative to other stars in the local solar neighborhood at typical relative speeds of more than 70,000 km/hr, but stars are so far away that we cannot easily notice their motion. The sun orbits the galaxy every 230 million years. More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy. Most of the Milky Way’s light comes from the distant bulb, but most of the mass is in the halo. How do galaxies move within the universe? The galaxies are carried along with the expansion of the universe. Hubble discovered that all galaxies within our local group are moving away from us. The more distant the galaxy, the faster it is racing away. The conclusion to this is that we are living in an expanding universe.