Classifying StarsClassifying 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…Classifying Stars continued…

Supergiants: very luminous, large stars; red supergiants are the largest stars

Giants: highly luminous, large starsDwarfs: small, dim stars

– White dwarfs: very faint, very dense, very small stars

Classifying Stars continued…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: LT 9: I can describe how a protostar becomes a I can describe how a protostar becomes a star.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 StarsBirthplace of Stars

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

Stable StateStable StateNuclear fusion is what fuels a star’s energy

– Hydrogen fuses to Hydrogen to form HeliumWhen Hydrogen in core is used up, gravity

causes the core to collapseTemperature rises making the outer shell of

the star super expandThe surface temperature drops and it is now

a huge, bright, red aging star

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

Red Giant (Red Supergiant)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 SupergiantsRed Supergiants

Supernova ExplosionSupernova Explosion

White DwarfWhite 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 DwarfWhite Dwarf

Nova Image

Black DwarfBlack Dwarf

Final Stage in a star’s lifeNeutrons collapse into a very small volume

with huge gravitational forcesCold, dark dead starNothing can escapeEx: Cygnus X-1

Black DwarfBlack Dwarf

The SunThe 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