Ch.2, Sec.2 – The Life Cycle of Stars The Beginning and End of Stars - stars begin their lives by coming together through the force of gravity inside of nebulas - once there is enough gravity to pull hydrogen atoms together, they become extremely hot and fuse together through nuclear fusion - a star is born and is often called a “protostar”!

Ch.2, Sec.2 – The Life Cycle of Stars Different Types of Stars - stars are classified by size, mass, brightness, color, age, spectrum, & temperature - stars can be classified as main-sequence (like our Sun), giants, super giants, and white dwarfs - after stars form, they enter the longest phase of their lives called the “main sequence” - this is the stage where stars fuse hydrogen atoms into helium atoms, releasing massive amounts of energy!

Ch.2, Sec.2 – The Life Cycle of Stars - after stars have fused all of their hydrogen atoms, they enter the 3rd stage of their lives called red giant - red giants expand and cool, while their cores shrink and their atmospheres grow very large

Ch.2, Sec.2 – The Life Cycle of Stars - in the final stages of a star’s life cycle, a star that has the same mass as the sun or smaller can be called a white dwarf - white dwarfs have no hydrogen left for energy & can take millions of years to cool completely

Ch.2, Sec.2 – The Life Cycle of Stars A Tool for Studying Stars - two astronomers, Hertzsprung & Russell, developed a chart comparing stars’ surface temperatures & absolute magnitudes - the H-R Diagram also shows how stars change over time Ejnar Hertzsprung Henry Norris Russell

Ch.2, Sec.2 – The Life Cycle of Stars - most stars live in the main sequence stage of their lives!

Ch.2, Sec.2 – The Life Cycle of Stars When Stars Get Old - stars more massive than the sun can become stranger objects than red giants & white dwarfs! - when massive blue stars use up all their energy, they explode into supernovas - supernovas can end up being brighter than galaxies for several days

Ch.2, Sec.2 – The Life Cycle of Stars - after supernovas occur, the materials eventually succumb to the force of gravity to become stars 2x the mass of our sun - when these stars are created, all of their mass becomes neutrons and they’re called neutron stars

Ch.2, Sec.2 – The Life Cycle of Stars - if a neutron star is spinning, it is called a pulsar - pulsars send out pulses or beams of radiation as they spin

Ch.2, Sec.2 – The Life Cycle of Stars - neutron stars can sometimes succumb to the overwhelming force of gravity to become black holes, which are usually the center of galaxies - black holes have the greatest gravitational pull in the Universe - no light can escape the pull of black holes, so they are very difficult to find!

Ch.2, Sec.2 – The Life Cycle of Stars