Classifying Stars

1. 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

2. 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

3. 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

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

5. 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

6. 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)

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

8. 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

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

10. 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

11. Red Supergiants

12. Supernova Explosion

13. 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)

14. White Dwarf  Nova Image

15. 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

16. Black Dwarf

17. 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