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

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.
Birthplace of Stars

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
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 mediumsized 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