Lifecycle of Stars

• How are stars born?

  Dense clouds of gas and dust are the birthplace of stars. Gravity pulls the gas and dust into clumps. If the clump is massive enough, a star forms- increased pressure and temperature cause its core to ignite, initiating nuclear fusion. Lower mass objects such as brown dwarfs, planets and asteroids form along with stars. A protostar is a contracting mass of gas which represents an early stage in the formation of a star, before nucleo-synthesis has begun.
• Stars live and die

  After billions of years of hydrostatic equilibrium, a star will run out of fuel in its core and begin to die. What happens next depends on the mass of the star
• Stability of stars

  Star remains stable in this phase for millions or billions of years. The nuclear reaction continues and keeps the star burning hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. Gravity pulls smaller amounts of outer dust and gas together, which form planets in order around the star. Our Sun and Solar System are in this stable phase. Our sun is a type of star called a yellow dwarf. It has been shining for nearly 5 billion year, and has enough hydrocarbon fuel to last another 5 billion years
• Low mass stars (up to 8 solar masses)


  Outer layers swell into a red giant. Star then ejects its outer layers, while the interior collapses into a white dwarf. Takes billions of years for the white dwarf to cool down. Ninety-nine percent of stars end their lives like this
• High mass star (8-20 solar masses)


  Becomes a red supergiant and begins to shed stellar matter. Star collapses in on itself, causing it to explode as a supernova, ejecting even more matter. Its core becomes a neutron star, which takes millions of years to cool down
• Most massive stars (over 20 solar masses)


  Form red or yellow supergiant's, and then explode in super-nova's, forming black holes in their centres. Blacks holes are so dense that not even light can escape their gravity
• Cycle continues


  As a star dies, it ejects matter out into space that provides raw material for new stars, planets, and other celestial objects
• Brown Dwarfs and Planets

  Brown dwarfs are neither stars nor planets. Smaller than all stars (less than 0.08 solar masses), they have enough mass to generate limited nuclear fusion, fusing only deuterium (heavy hydrogen) for a period of million of years. After this fuel runs out, the brown dwarf simply cools down
• As a star's life cycle goes on, fusion in the core produces heavier and heavier elements. The process continues until IRON (26 protons) is produced
• Older stars build up layers of heavier and heavier elements. Eventually, fusion in the core stops and gravity suddenly collapses the star. The collapse produces a supernova explosion
• In a supernova the conditions are so extreme that nuclei fuse to produce all of the element heavier than iron which are then blasted into space, later to form new stars and planets
• iron to uranium

  A) Stellar Nebula

  B) Average star

  C) Massive star

  D) Red giant

  E) Planetary Nebula

  F) Supernova

  G) White dwarf

  H) Neutron star

  I) Black hole

  J) Red supergiant