Review: Which of the following element is the main constituent of a star? a.Hydrogenc. nitrogen...
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Transcript of Review: Which of the following element is the main constituent of a star? a.Hydrogenc. nitrogen...
Review: Which of the following element is the main constituent of a star?
a. Hydrogen c. nitrogenb. neon d. oxygen
Answer: hydrogen
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
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