(in case you missed it yesterday ) Merger of Two Neutron Stars!

Transcription

1 (in case you missed it yesterday ) Merger of Two Neutron Stars! More on this later in the semester

2 Lecture #14: Plan Light (cont d) Atomic physics Formation of spectra Doppler effect Telescopes

3 Light: Inverse-Square Law B = L / (4 π d 2 ) B = apparent brightness L = luminosity (= total energy output rate) d = distance from light source (e.g., star)

4 You see this every day!

5 The Nature of Matter: Atoms Atomic structure: Nucleus ( nm) orbiting electrons ( nm) Electrons are negatively charged Nucleus (> 99.9% of the mass!): neutrons (no charge) protons (positively charged) Chemical Elements: 1 = Hydrogen 2 = Helium 6 = Carbon 8 = Oxygen

6 Atoms Electrons are only allowed to move in fixed orbits about nucleus Energy levels are distinct for each element

7 Conservation of Energy Energy can never be created or destroyed. It can only be changed in form Forms of energy: Motion (mechanical / kinetic) Gravitational Electrical / Magnetic Light

8 Atomic Energy Levels Energy High State Low State High State Energy Low State

9 Atoms: Origin of Light Emission of light: Electrons move from higher to lower orbits Absorption of light: Electrons move from lower to higher orbits

10 Absorption

11 Emission

12 Types of Spectra Hot, high density gas, liquid, solid gives a continuous spectrum light at all λ s Hot, low density gas gives an emission line spectrum light only at some λ s Cool gas between observer and continuous light source gives an absorption line spectrum light missing or dimmer at some λ s

13 Emission Spectra

14 Absorption Spectra

15 Types of Spectra? Sun interior, stove burner, carbon filament,? Solar atmosphere, cool interstellar clouds,? Solar corona, aurora, sodium street lamps,?

16 Spectra of Astronomical Objects

17 Doppler Shift: Sound (Fire Engine Siren) Higher pitch when approaching, Lower pitch when moving away

18 Doppler Shift: Light Observer sees wavelength emitted or absorbed by moving source shifted to: Longer λ s if source and observer are moving apart (redshift) Shorter λ s if source and observer are moving toward each other (blueshift) Sound Light

19 Doppler Shift: Light Velocity of source, V, is given by: V = c (λ λ 0 ) / λ 0 λ = wavelength measured by observer λ 0 = wavelength emitted or absorbed by source (rest wavelength) c = speed of light

20 Example: Wise-guy speeder vs Smart cop λ = 500 nm (green) [observed color] λ 0 = 700 nm (red) [actual color, seen by cop] V/c = (λ λ 0 ) / λ 0 = ( ) / 700 = 200 / 700 = 2/7 [approaching] V = (2/7) c = (2/7) (3 x 10 5 km/s) = 86,000 km/s = 2 x 10 8 mph! No ticket for running the red light But a huge speeding ticket! e.g. 1 / mph over 100 mph $ 2,000,000!!!

21 Telescopes

22 Types of Telescopes Refracting Telescopes or Refractors Use lenses to collect light Reflecting Telescopes or Reflectors Use mirrors to collect light

23 Refractors

24 Refracting Telescopes Disadvantages: Large lenses are expensive Large lenses sag under gravity Many lens materials absorb ultraviolet Chromatic aberration due to wavelength dependence of refraction

25 Refracting Telescopes

26 Reflecting Telescopes

27 Different styles of reflectors

28 Multiple Mirrors Reflectors can be made very large if multiple mirrors are used as the primary mirror Example: Keck Telescopes (diameter of 10 meters! Made of 36 x 1.8 meter hexagonal segments)

29 Multiple Mirrors: Future (~ meters) GMT ELT

30 Collecting Area Light bucket : the bigger the area of the telescope s mirror or lens, the more photons it collects Collecting Area ~ (Diameter of telescope) 2

31 Collecting Area Example: Eye (D = 5 mm) vs Keck (D = 10 m) Collecting power (Keck) / Collecting power (eye) = D(Keck) 2 / D(eye) 2 = [D(Keck) / D(eye)] 2 = [10 / 5 x 10-3 ] 2 = [2 x 10 3 ] 2 = 4 x 10 6 Keck is 4,000,000 x more powerful than our eye!

32 Resolving Power The bigger the size of the telescope, the better it is at discerning fine details Resolving power ~ (Diameter of telescope) / λ

33 Radio Telescopes are big! (radio! longer λ, so larger D is needed to get the same resolving power as optical telescopes)

34 Radio Telescopes: Interferometry (Combine the signal from an array of radio telescopes to increase effective D)