Once massive stars reach the red giant phase, the core temperature continues to increase as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull together the carbon atoms in the core until the temperature reaches 600,000,000 degrees Celsius. At this temperature, carbon atoms form heavy elements such as oxygen and nitrogen. The fusion and production of heavy elements continues until iron starts to form. At this point, fusion stops and the iron atoms start to absorb energy. This energy is eventually released in a powerful explosion called a supernova. A supernova can light the sky up for weeks. The temperature in a supernova can reach 1,000,000,000 degrees Celsius. This high temperature can lead to the production of new elements which may appear in the new nebula that results after the supernova explosion. The core of a massive star that is 1.5 to 4 times as massive as our Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves. If the radio waves appear to be emitted in pulses (due to the star's spin), these neutron stars are called pulsars. The core of a massive star that has 10 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It has now become a black hole which readily swallows any matter and energy that comes too near it. Some black holes have companion stars whose gases they pull off. As the gases are pulled down into the black hole, they heat up and give off energy in the form of X-rays. Black holes are detected by the X-rays which are given off as matter falls down into the hole.
Newton realized that the reason the planets orbit the Sun is related to why objects fall to Earth when we drop them. The Sun's gravity pulls on the planets, just as Earth's gravity pulls down anything that is not held up by some other force and keeps you and me on the ground.
