Stars A star is a ball of burning gas. 1
Stars: The Hertzsrung-Russell diagram (HR) If we were to graph all of the stars in the sky, we would find a graph like this one. Comparing stars temperature to its brightness. Another word for brightness, is luminosity. Stars 2
Stars We would find that 90% of all stars in the sky are just average average stars. They are not too bright, not too hot, not too large, just average. Those average stars are called Main Sequence Stars. Stars What about the other 10% of stars in the sky? Where do giant and supergiant stars come from? What is a white dwarf? To answer these questions we have to look at the lifecycle of a star. 3
Life Cycles A star goes though a life cycle just like humans would. Birth Life Death Life Cycles : Birth 1. A shockwave causes nebula cloud to compress due to gravity. 4
Life Cycles: Birth 2. As the cloud compresses and shrinks due to gravity, it begins to heat up inside (compression creates heat). 3. It gets so hot that fusion begins inside the core of the cloud (a protostar forms). The cloud starts to collapse. A protostar forms. 4. The high heat and pressure from the fusion reaction overcomes the forces of gravity and the protostar stops compressing and starts to expand. 5. Eventually, a balance between gravity wanting to compress the protostar and heat and pressure wanting to expand the protostar is met. 6. A star is born and burns happily. Life Cycles: Happy life 1. This stable balance between gravity and heat and pressure from fusion must be kept in order for a star to live peacefully. 2. If a star does not burn enough hydrogen, then gravity will overcome it and it will start to compress in on itself again. 3. If a star makes too much energy, it will expand too much, thus shutting down the fusion reactions, ultimately causing the star to compress again until fusion starts back up. A DELACATE BALANCE BETWEEN HEAT, PRESSURE, AND GRAVITY FOR SURE! 5
Life Cycles: Happy life A star will continue to live happily as long as it has hydrogen to burn in its core. Fusion for a star, is like food for us. With it, we are sustained and can live. Without it, we die. Implication: The more massive a star is the harder it has to try to overcome the force of gravity working to compress it. The only way a star can overcome the compressing force of gravity, is through the expanding heat and pressure of fusion. More massive stars need to burn more hydrogen than less massive stars. Life Cycles: Happy life What happens when a star eats all its food? When it s hydrogen inside its core runs out? The star changes and is no longer a main sequence star. 6
Life Cycles: Death At this time, gravity and heat and pressure are no longer balanced. The star begins to compress as it can not overcome gravity without fusion. What happens after this point depends on the mass of the star. Life Cycles: Death Medium Mass Stars (Such as the Sun): 1. The stars core continues to compress. 2. It compresses so much that the enormous amount of heat produced causes leftover hydrogen in a cloud outside the core and He in the core to burn. H envelope core H He H 3. This burning causes the surrounding cloud to expand to a red giant and eventually be wisped off into space, leaving just the small core behind. 4. This dense core is called a white dwarf. 5. A white dwarf is about the size of Earth, but much more dense.. It gives off no light. 7
Fun with White dwarfs Surface gravity is 300,000 times stronger than on Earth! Earth White dwarf Life Cycles: Massive Stars High Mass Stars 1. Hydrogen runs out in the core much more quickly because of its high mass and the core compress. 2. It compresses so much that the inside heats up much more than inside a medium mass star. 3. High mass stars are so much hotter, fusion of elements heavier than H and He take place, causing the star to expand rapidly into a super-giant star. 4. Eventually, iron begins to form in the core causing fusion to shut down. 5. The star compresses violently in on itself and then explodes outward forming a supernova explosion. 6. However, the core remains. 8
Supernova From a Supernova now what? If core is 1.4-3.0 times as massive as the sun. 1. Core will compress to about the size of a small city. 2. Pressure from neutrons in the core stop the core from compressing further. 3. A neutron star forms. If core is more than 3.0 times as massive as the sun. 1. Core will compress to about the size of a small city. 2. Pressure from neutrons in the core cannot stop the core from compressing. 3. Core shrinks down to a point called a black hole. 9
Neutron Stars Small Extremely Dense - teaspoon weighs a mountain Stupendous surface gravity Rotate 1000 times per second (pulsar) marshmallow neutron star 3-megaton explosion Black Holes Massively dense core of a dead star compressed to a speck. There is an imaginary sphere around the black hole called an event horizon. Anything that goes into the event horizon can not escape its stupendous gravity. 10
Black Holes Not even light can escape from a black hole. event horizon BH Black Holes Black holes are NOT cosmic vacuum cleaners If Sun became black hole (not possible) Orbits of planets would be unchanged 11
Black Holes Tidal distortion ( spaghettification ) Gas, stars falling towards BH are tidally distorted even outside e.h. Distortion heats up stuff to millions of degrees and emit X-rays strongly Feeding BHs should be very bright in X-rays X Black Holes: Finding a Black Hole Look for a normal star with a binary companion that is: invisible in optical light bright in X-rays X 3 times as massive as the sun. 12