Classifying Stars The Hertzsprung-Russell Diagram (H-R Diagram) – Graph plotting the surface temperatures of stars against their luminosity (total energy given off each second) – Diagonal band that goes from upper left to lower right is the MAIN-SEQUENCE STARS – The Sun is a main sequence star

Classifying Stars continued … Supergiants: very luminous, large stars; red supergiants are the largest stars Giants: highly luminous, large stars Dwarfs: small, dim stars – White dwarfs: very faint, very dense, very small stars

Classifying Stars continued … Variable stars: vary in brightness over regular cycles – Pulsating stars: change in brightness as they expand (cool, dim) and contract (hot, bright) – Cepheid variables: the longer their cycle is the larger their absolute magnitude is – Eclipsing binary: 2 stars of unequal brightness that revolve around each other and appear to change brightness Pulsars: emits bursts of radio waves and light – They are neutron stars formed in supernovas – They produce radio pulses because they rotate very rapidly

LT 9: I can describe how a protostar becomes a star.

Begins as a nebula (a cloud of dust and gas) Diffuse nebula: lit up by bright light from nearby star (ex: Great Nebula in Orion) Dark nebula: show up as dark patch (ex: Horsehead Nebula in Orion Birthplace of Stars

Making a Star Cloud starts compressing and particles are pulled closer to each other by gravity and temperature rises Hot areas start to glow (protostars) and continue contracting Eventually they contract enough to trigger fusion and hydrogen into helium which gives off huge amounts of energy (star) When the energy released counterbalances gravity, the star stops contracting (main-sequence star)

LT 10: I can explain how a main-sequence star generates energy.

Stable State Nuclear fusion is what fuels a star’s energy – Hydrogen fuses to Hydrogen to form Helium When Hydrogen in core is used up, gravity causes the core to collapse Temperature rises making the outer shell of the star super expand The surface temperature drops and it is now a huge, bright, red aging star

LT 11: I can describe the evolution of a star after is main-sequence stage.

Red Giant (Red Supergiant) When all the fuel for fusion is used up the giant star collapses into a small, dense, dying star of low luminosity and high temperature Red Supergiants have a mass 7 times the Sun’s mass – When a red supergiant starts to collapse, there is a huge explosion (supernova); everything except the star’s core is blown out into space – What is left of the core becomes a very dense, invisible pulsar – This pulsar eventually stops and becomes a neutron star

Red Supergiants

Supernova Explosion

White Dwarf Cools, turns dull red and shines its leftover heat energy into space May be hit by another star causing it to flare up temporarily (Nova)

White Dwarf  Nova Image

Black Dwarf Final Stage in a star’s life Neutrons collapse into a very small volume with huge gravitational forces Cold, dark dead star Nothing can escape Ex: Cygnus X-1

Black Dwarf

The Sun An average medium- sized star Stable main-sequence star for approximately 5 billion years Should continue to shine steadily for another 5 billion years before its hydrogen supply is used up