Review: Which of the following element is the main constituent of a star?

a. Hydrogen c. nitrogenb. neon d. oxygen

Answer: hydrogen

STARS: BIRTH & LIFE CYCLE

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)

• "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

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

Colour and Temperature of StarsA star’s colour can give us an idea of how hot that star is:

– Blue 21,000-35,000C– Bluish-white– Yellow our Sun is yellow

(photosphere ~ 6,000C)– Orange– Red 3,300C

Which color indicates the hottest stars in the universe?

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

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 starsBottom right = cool, dim, low mass stars

Who was right?

http://www.youtube.com/watch?v=5az0W4Y1nuU

Life cycle of a star- A comparison

3 categories of stars 1 solar mass = 1.98892 × 1030 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 massese.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 ifbetween 10-40x the mass of sun

Black hole if40x more than the mass of sun

• 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?

Supernova Neutron stars

Life cycle of a star recap: http://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

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

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

Homework

Today’s lesson HW1. Read p341-3492. Answer p342 #1-3; p347 #5,6; p349 #1,3

Reminder: Space quiz #2 Dec 4 2015