Life Cycles of Stars How Stars Form The

Life Cycles of Stars � How Stars Form: -The space around stars contains gas/dust � A nebula is a large cloud of dust/gas, some nebulas glow lit by other stars and some are cold and dark that block the light from distant stars behind it

How Stars Form � Stars are created by gravity. � Gravity pulls the nebula’s dust/gas into a denser cloud, as it contracts it heats up � A contracting cloud of dust with enough mass to form a star is called a proto-star � A star is formed when a contracting cloud of dust/gas becomes so dense and hot that nuclear fusion begins

Adult Stars � Spend about 90% of their lives in main sequence (converting hydrogen to helium) � A star’s mass determines the star’s place on the main sequence and how long it will stay there � The amount of gas/dust when the star forms determines the mass of the star

Adult Stars � High mass stars become the brightest/bluest, about 300, 000 times brighter than sun � Because they are bigger and burn so brightly, they only last for a few million years � A yellow star like our sun will remain stable for 10 billion years � A red sequence star may stay in sequence for more than 100 billion years (Formed from small nebulas, 1/10 the sun’s mass)

The Death of a Star � When a star runs out of fuel gravity starts to compress the core and causes it to shrink � Then the star will begin to fuse helium into carbon/oxygen/and other heavier elements � Once all fuel is gone it causes the star to die and turn into a white dwarf, neutron star, or black hole

Low/Medium Mass Stars � Can be 8 times as massive as the sun will eventually turn into white dwarfs � The dying star is surrounded by a glowing cloud of gas called a planetary nebula (Because the 1 st ones found looked like planets viewed through a small telescope) � Once it blows off most of its mass, only its hot core remains (Size of Earth but still about the same mass as before) � Once it cools its called a black dwarf (Takes 20 billion years so the universe has not been around long enough for this to occur)

High Mass Stars � More than 8 times the mass of the sun, grow into super-giants creating new elements , the heaviest being iron � Dies quickly because it consumes its fuel very rapidly � Once the star runs out of fuel the star collapses and explodes as a supernova (Becomes brighter than an entire galaxy) � Produces elements heavier than iron -Rare Earth Elements -Uranium, Platinum, Gold, Etc

High Mass Stars � The heavier elements in our solar system, including the atoms in your body, came from a supernova that occurred in our galaxy billions of years ago � As the supernova spews material into space, its core continues to collapse � If the remaining core has a mass less than 3 times the sun’s mass, it will become a neutron star (This star is dense, electrons/protons are crushed together by the enormous gravity to form neutrons)

High Mass Stars � Neutron stars are much smaller and denser than white dwarfs � A spoonful of a neutron star would weigh nearly a billion tons on Earth � Can spin hundreds of times a second � If a neutron star gives off strong pulses of radio waves it is called a Pulsar

High Mass Stars � If a star’s core after a supernova explosion is more than 3 times the sun’s mass, gravity causes the collapse beyond the neutron star stage � The pull of gravity increases and the speed required to escape the star’s core reaches the speed of light

High Mass Stars � Beyond this point, nothing can escape and a black hole is formed � A black hole is an object whose surface gravity is so great that not even electromagnetic waves, traveling at the speed of light, can escape from it