Star Types & Lifecycle

1. Star Types & Lifecycle

2. Consider these questions: • Is our Sun an average star or a unique star? • Will the Sun and Earth go on forever?

3. Our Sun • Our Sun is an average medium sized star. • It has a lifespan of 10 billion years • It is about half way through its lifespan • Mercury, Venus, and Earth will be destroyed when the Sun becomes a “Red Giant” (in about 4 billion years) • All stars go through different stages • Small stars live very long quiet lives • Large stars live very short violent lives

4. Observed Properties of Stars • When we look into the sky, what properties of stars can we observe? • Brightness (Luminosity) • Stars in the sky have different brightness (called Magnitude) • Some brightness differences are the result of distance • But, even stars at the same distance have different brightness • Colour (Temperature) • Starlight can be split into a rainbow called a spectra. • Spectra tell us that stars have different temperatures and compositions. • E.g. Blue objects are hotter than red objects • Note: Stars are too distant to see their disks or surface features.

5. The Hertzsprung-Russell Diagram • Scientists love to plot data on graphs! • In the early 1900’s astronomers measured the luminosity and colour of 1000’s of stars. • When they plotted this data they found interesting relationships!

6. Life Cycle of Stars • They identified definite groupings (types) of stars: • Red Dwarfs • White Dwarfs • Red Giants • Blue Giants • Supernovas • Neutron Stars • Black Holes • Further research revealed that these types of stars are just different stages in the life cycle of stars. • Stars spend most of their life on the “main sequence” in the middle of the H-R Diagram

7. Activity – Part 1 • Complete the HR-Diagram in your handout based on the following notes and discussion.

8. 1. Protostars – New Stars • Properties: • Luminosity: Medium • Temperature: Cool • After a few million years the hydrogen gas and dust from the cloud has compressed enough to form stars. • Stars get their energy by converting hydrogen into helium and other elements. • This process is called nuclear fusion • It is the same principle as a hydrogen bomb • The Pleiades Cluster contains many stars that are close together. • After 1 million years these stars will have spread throughout the galaxy • Our Sun began this way

9. 2. Main Sequence – Adult Life • Properties - High Mass Stars • Luminosity: High • Temperature: Hot • Properties - Low Mass Stars • Luminosity: Medium • Temperature: Medium • Stars spend most of their life on the Main Sequence. • High mass stars live only a few million years (They burn through their hydrogen very fast) • Low mass stars live billions of years (They burn through their hydrogen very slowly) • Our Sun is a medium mass yellow star

10. 3. Brown Dwarfs • Properties: • Luminosity: Very Dim • Temperature: Cool • Small stars do not have enough gravity to create the conditions necessary to burn helium once their hydrogen is used up. • Small stars just fade away after 100 billion years. • The universe it too young to have any dead brown dwarfs yet. • Jupiter could have become a brown dwarf if it was just a few times bigger. • Most stars in the sky occur in pairs called binary stars.

11. 4a. Red Giants – Old Age • Properties – Low and Medium Mass Stars • Luminosity: Bright • Temperature: Cool • When a star runs out of hydrogen fuel… • It starts to burn helium • This causes the star to greatly expand • Our Sun will become a Red Giant in about 5 billion years • It will expand to near the orbit of Mars • Earth will be burnt to a crisp

12. 4b. Supergiant – Old Age • Properties – High Mass Stars • Luminosity: Very Bright • Temperature: Very Cool • When a star runs out of hydrogen fuel… • It starts to burn helium • This causes the star to greatly expand • Massive Blue stars become Supergiants • They expand to bigger than the orbit of Jupiter! • They are extremely unstable!

13. 5. White Dwarfs – End of Life • Properties: • Luminosity: Low • Temperature: Very Hot • Medium size stars end their life as a White Dwarf • This is what is left behind at the center of a Planetary Nebula • They the remains of the core of the original star • A white dwarf is extremely dense and hot • The Sun will end up as a White Dwarf Earth and White Dwarfare about the same size.

14. 5b. Planetary Nebula • Planetary nebulas are the cloud of dust and gassurrounding a white dwarf • After a few million years the red giant star will have used up all its helium. • What is left is Lithium and Carbon • These stars do not have enough gravity to create the conditions necessary to burn heavier elements • They puff their outer layers out like a “smoke ring” • These “smoke rings” are called Planetary Nebula • Planetary Nebula have nothing to do with planets • Also, They have nothing to do with Hydrogen Nebula • Our Sun will create a Planetary Nebula when it dies

15. 6. Neutron Stars & Black Holes • Properties: • Luminosity: Very Low • Temperature: Hot • These are created at the end of life of massive stars. • They are what is left after a supernova. • Neutron Star – A star made of solid neutrons • There is a neutron star at the center of the Crab Nebula • Black Hole – Extreme gravity, A “hole” in space • Nothing can escape (not even light) • There is a black hole at the center of our galaxy

16. 6b. Supernova - Death Same star • A massive explosion related to the death of high mass stars. • Occurs when the star has used up all of its fuel for nuclear fusion. • Gravitational collapse generates tremendous instantaneous energy and temperature increase. • Results in a temporary extreme brightening of the star. • Brightening lasts days or a few weeks. • After 100’s or 1000’s of years it results in a Supernova Nebula Cloud. • The Crab Nebula was observed a supernova star in 1054.

17. Activity – Part 2 • Complete the Stellar Lifecycle chart in your handout based on your notes and pages 341 to 349 of the textbook.