1. Review: Which of the following element is the main constituent of a star? a.Hydrogenc. nitrogen b.neond. oxygen Answer: hydrogen

2. STARS: BIRTH & LIFE CYCLE

3. How do you describe how bright a star is? Luminosity 2 factors: a.The star’s brightness or luminosity and b.Its distance from Earth - a measure of the total amount of energy radiated by a star per second -Stars can be 10 000x less luminous than the Sun or 30 000x more (e.g. Alpha Centauri A, Sirius, Vega)

4. "what you see is what you get" magnitude the brightness of a star as seen from earth NO consideration given to how distance influences the observation The scale goes from -30 (the sun = -26) to +30 (Hubble space telescope = +29) Apparent magnitude

5. Absolute magnitude - "true" brightness, with the distance dependence factored out -Defined as the apparent magnitude that a star would have if it were (in our imagination) placed at 32.6 light years from the Earth -Sun = 4.7  Sun is not that bright compared to other stars parallax

6. Colour and Temperature of Stars A star’s colour can give us an idea of how hot that star is: – Blue  21,000-35,000  C – Bluish-white – Yellow  our Sun is yellow (photosphere ~ 6,000  C) – Orange – Red  3,300  C Which color indicates the hottest stars in the universe?

7. Hertzsprung-Russell (HR) Diagram Turn to p343 Used to compare properties of stars Shows luminosity versus temperature and colour

8. Main sequence The phase in which 90% of the stars are in The phase in a star’s life cycle in which the process of nuclear fusion - hydrogen to helium – has stabilized. seen as the diagonal band running from the top left to the bottom right on the Hertzsprung- Russell (HR) Diagram Top left = hot, luminous, massive stars Bottom right = cool, dim, low mass stars

9. Who was right? http://www.youtube.com/watch?v=5az0W4Y1nuU

10. Life cycle of a star- A comparison

11. 3 categories of stars 1 solar mass = 1.98892 × 10 30 kg 1. Low Mass Stars (or red dwarfs) 2. Medium Mass Stars: 3. High Mass Stars 0.5 solar mass or less Consume hydrogen over 100 billion years White dwarf 0.5 solar mass – 10 solar masses e.g. The sun Consume hydrogen in 10 billion years Red giants White dwarf 10 solar masses or larger supernova A star’s mass determines how it dies Higher mass stars burn fuel faster and therefore die faster. Neutron star if between 10-40x the mass of sun Black hole if 40x more than the mass of sun

12. The gradual build-up of heavy elements in the star’s centre causes the core to collapse sending out shockwave called a supernova supernovae = star explosions Releases many heavy elements which can help form new stars, planets, or other bodies The elements in your body were created (fused together) in the cores of old stars! As the star rips apart, a nebula is formed Recall: what is a nebula? SupernovaNeutron stars Life cycle of a star recap: http://www.youtube.com/watch?v=mzE7VZMT1z8&feature=relatedtp://www.youtube.com/watch?v=mzE7VZMT1z8&feature=related the core made up of densely packed neutrons The densest material known. Found in centre of the Crab nebula

14. Learning checkpoint Choose from the following hypotheses regarding length of star life: 1) The bigger a star is, the longer it will live. 2) The smaller a star is, the longer it will live. Answer: The smaller a star is, the longer it will live. Because larger stars burn fuel faster than smaller stars

15. Pulsars - Pulsar: a celestial object, thought to be a rapidly rotating neutron stars, that emits regular pulses of radio waves that can be detected on Earth Neutron stars : incredibly dense remains of massive collapsed stars Discovered by Bell and Hewish (1968) Pulsars = pulsing stars

16. Homework Today’s lesson HW 1. Read p341-349 2. Answer p342 #1-3; p347 #5,6; p349 #1,3 Reminder: Space quiz #2 Dec 4 2015