Life Cycle of a Star

Astronomy UnitEarth/Environmental

Nebula Stage

• Nebula – dark, cool clouds composed of dust and gases are the birthplace of stars

• In the Milky Way nebulas are: 92 % hydrogen, 7 % helium and less than 1% of heavier elements

• Nebula becomes dense enough to begin to contract, gravity squeezes particles which pulls everything towards the center

Protostar Stage• The temperature of this gaseous body slowly

rises until it is hot enough to radiate energy • Protostar – is a developing star not yet hot

enough to engage in nuclear fusion (occurs when less massive nuclei combine into more massive nuclei, releasing energy)

• When the core of a protostar has reached about 10 million K(Kelvin), pressure within is so great that nuclear fusion of hydrogen begins, and a star is born

Main-Sequence Stage

• Stable main-sequence star is balanced– Gravity, which is trying to squeeze it into a smaller

sphere– Gas pressure, which is trying to expand it

• Generally hydrogen fusion continues for a few billion years

• Average star spends 90% of its life in Main-Sequence stage

Red Giant Stage• Occurs because the zone of hydrogen fusion

continually moves outward• Once all of the star’s fuel has been used the core

begins to contract• As the core contract it grows hotter by converting

gravitational energy into heat energy, increasing the hydrogen fusion in the star’s outer layer

• Result is a giant body hundreds to thousands of times larger

• As the star expand, its surface cools

Burnout and Death

• Stars eventually run out of fuel and collapse due to gravity

• Low-Mass Stars– Small, cool red stars NEVER evolve into Red Giants– After they have used all of their fuel they collapse

into a White Dwarf• Medium-Mass Stars(similar mass to our Sun)• Massive Stars

Burnout and Death

Medium-Mass Stars(similar mass to our Sun)• Red Giants White Dwarfs

- cast off their large outer layer, creating an expanding cloud ring

• The remaining hot, central White Dwarf heats the gas cloud causing it to glow

Planetary Nebula

Burnout and Death

• Massive Stars(3 times that of the Sun)– End their lives in a brilliant explosion called a

supernova– Supernova becomes million times brighter than its

prenova stage– None have been observed in our galaxy since the

invention of the telescope

Crab Nebula

Hertzsprung-Russell Diagram

• Hertzsprung- Russell (H-R Diagram) represents the color and absolute magnitude of the star at various stages in its evolution

H-R Diagram

White Dwarfs

• Remains of low-mass and medium- mass stars– High densities– Electrons have been displaced and are closer to

the atom’s nucleus– Eventually they turn into a black dwarf– Smallest White Dwarfs are the most massive and

the largest are the least massive

Neutron Stars

• Are the remnants of a Supernova event• The electrons are forced to combine with

protons to produce neutrons• Can be thought of as large atomic nuclei• Overall all small in size• Strong magnetic field

Veil Nebula

Supernovae

• The outer layer of the star is ejected, while the core collapses into a very hot neutron star about 20 KM in diameter

• Pulsar- small spinning star that radiates short bursts or pules of radio energy(remains of the supernova of 1054)

Black Hole

• During a supernova event, remnants of stars 3 times more massive than the sun apparently collapse into objects even smaller and denser than a neutron star

• Very hot, their gravity is so strong that not even light will escape