PHYS 182 Lecture Notes - Lecture 14: Main Sequence, Radiation Zone, Super Giant
PHYS182: Our Evolving Universe
2017-10-26 LEC 14
Essay: Nature article on astronomy
- 5-page maximum summary
- Put in perspective with class material
- Due before the final, check webpage for instructions
Ch. 15-17: Surveying the Stars
Review of Last time
- Apparent brightness to determine distance
- Absolute magnitude (M) of a star
o By definition, M of the sun is 4.8
o The magnitude increases if the star decreases in brightness (! magnitude = fainter star)
o M is the magnitude a star has at a distance (from us) of 10pc
o All M values are relative to our sun
- Luminosity (L): is the amount of energy emitted per unit time
o L1/L2 = [(100)1/5]M2-M1
o A magnitude difference of 5 = a luminosity difference of 100
- Apparent luminosity (l) and apparent magnitude (m)
o l1/l2 = (1001/5)m2-m1
Luminosity & Distance
- If all stars were the same size, decreased luminosity would mean further away, but it doesn’t
- Range of stars’ intrinsic brightness:
o 10-4LO<L<106LO
o 15 < M < 15
- All we can measure is the apparent magnitude and apparent brightness
Stellar Properties
- Surface temperature: easy to measure via the star’s
spectrum
o Pass the light of the star through the prism,
look for the wavelength of peak intensity,
which is related to temperature
- Spectral sequence: classification of star temperature
based on where their spectrum peaks
o So by measuring the colour of the star you
can find the surface temperature
- Radius:
o L = 4πr2øT4
o ø = 5.7 x 10-8 watt/m2K2
§ L is hard to measure, but T is easy to measure
- Mass: can measure in binary star systems with Kepler’s laws
Binary Star Systems
- Visual binaries: can track the position of the 2 stars in the sky orbiting each other
- Eclipsing binaries: periodic change in luminosity because one star covers another star so you have
lower intensity of light
Document Summary
Due before the final, check webpage for instructions. Luminosity (l): is the amount of energy emitted per unit time: l1/l2 = [(100)1/5]m2-m1, a magnitude difference of 5 = a luminosity difference of 100. Apparent luminosity (l) and apparent magnitude (m) l1/l2 = (1001/5)m2-m1. If all stars were the same size, decreased luminosity would mean further away, but it doesn"t. Range of stars" intrinsic brightness: 10-4lo
