Assignments. For Mon. 1 st Midterm is Friday, Oct. 12. Read Ch. 6 Optionally do MT1-sample-problems

Transcription

1 Assignments For Mon. Read Ch. 6 Optionally do MT1-sample-problems 1 st Midterm is Friday, Oct. 12

2 Chapter 5 Light: The Cosmic Messenger

3 Thermal Radiation 1. Hotter objects emit photons with a higher average energy. 2. Hotter objects emit more light at all frequencies per unit area.

4 Temperature dependence of wavelength Wien s law observed color λ max = λ max,sun T sun T at peak intensity depends on temperature 6000K = 500nm T

5 A star s spectrum has it peak at a wavelength of 300 nm. About what is its surface temperature? A K B K C K D K

6 A star s spectrum has it peak at a wavelength of 250 nm. About what is its surface temperature? A K B K C K D K T = T sun λ peak,sun λ peak = 6000K 500nm 250nm = 12000K

7 Stefan-Boltzman law Temperature dependence of total energy flux flux=power/area F = σ T 4 depends on temperature σ = Stefan-Boltzman constant

8 Luminosity luminosity=total power=flux*area depends on temperature and radius L = F A= σt 4 4π R 2 Using the sun as reference gives: L = L sun T T sun 4 R R sun 2

9 A star has the same radius as the sun but is twice the temperature. What is the star s luminosity in L sun? A. 2 L sun B. 4 L sun C. 8 L sun D. 16 L sun E. 64 L sun

10 A star has the same radius as the sun but is twice the temperature. What is the star s luminosity in L sun? A. 2 L sun B. 4 L sun C. 8 L sun D. 16 L sun E. 64 L sun

11 A star has the same temperature as the sun but has a radius that s twice as big. What is the star s luminosity in L sun? A. 2 L sun B. 4 L sun C. 8 L sun D. 16 L sun E. 64 L sun

12 A star has the same temperature as the sun but has a radius that s twice as big. What is the star s luminosity in L sun? A. 2 L sun B. 4 L sun C. 8 L sun D. 16 L sun E. 64 L sun

13 Telescopes: Portals of Discovery All of this has been discovered and observed these last days thanks to the telescope that I have [built], after having been enlightened by divine grace. Galileo Galilei ( ) Astronomer & Physicist

14 The Bending of Light Focus to bend all light waves coming from the same direction to a single point Light rays from different directions converge to form an image.

15 Angular Resolution Ability to distinguish two objects. Angle between two objects decreases as your distance to them increases. The smallest angle at which you can distinguish two objects is your angular resolution.

16 Two Fundamental Properties of a Telescope 1. Light-Collecting Area think of the telescope as a photon bucket its area: A = π D 2 /4

17 A. 2 times i-clicker If one doubles a telescope s diameter, by what factor does its light collection area increase? B. 4 times C. 16 times D. It depends on the wavelength of the light

18 A. 2 times i-clicker If one doubles a telescope s diameter, by what factor does its light collection area increase? B. 4 times A ~ D 2 C. 16 times D. It depends on the wavelength of the light

19 Two Fundamental Properties of a Telescope 1. Light-Collecting Area think of the telescope as a photon bucket its area: A = π D 2 /4 2. Angular Resolution smallest angle that can be seen a = 1.2 λ/ D 180 o /π = 70 o λ/ D = 4 (λ/μm)/(d/m) recall angular size α = s/d = size/distance

20 A. 2 i-clicker If one doubles a telescope s diameter, by what factor does its angular resolution change? B. 1/2 C. 4 D. 1/4

21 A. 2 i-clicker If one doubles a telescope s diameter, by what factor does its angular resolution change? B. 1/2 a ~ 1/D C. 4 D. 1/4

22 Telescope Types Refractor focuses light using lenses Reflector focuses light using mirrors used exclusively in professional astronomy

23 Refractor Yerkes 40-inch telescope; largest refractor in the world

24 Reflector Gemini 8-m Telescope, Mauna Kea, Hawaii

25 Reflector -- Radio Heinrich Hertz Telescope Mt. Graham, AZ

26 Imaging Filters are placed in front of a camera to allow only certain colors to be imaged Single color images are superimposed to form true color images.

27 Spectroscopy The spectrograph reflects light off a grating: a finely ruled, smooth surface. Light interferes with itself and disperses into colors. This spectrum is recorded by a digital detector called a CCD.

28 Nonvisible Light Most light is invisible to the human eye. Special detectors/receivers can record such light. Digital images are reconstructed using falsecolor coding so that we can see this light. Chandra X-ray image of the Center of the Milky Way Galaxy

29 Seeing Through the Atmosphere Earth s atmosphere causes problems for astronomers on the ground. Bad weather makes it impossible to observe the night sky. Air turbulence in the atmosphere distorts light. That is why the stars appear to twinkle. Angular resolution is degraded. Man-made light is reflected by the atmosphere, thus making the night sky brighter. this is called light pollution

30 Adaptive Optics (AO) It is possible to de-twinkle a star. A star s light rays are deformed by the atmosphere. By monitoring the distortions of the light from a nearby bright star (or a laser), computer can deform the secondary mirror in the opposite way Angular resolution improves. These two stars are separated by 0.38 Without AO, we see only one star. AO mirror off AO mirror on

31 Atmospheric Absorption of Light Earth s atmosphere absorbs most types of light. good thing it does, or we would be dead! Only visible, radio, and certain IR and UV light make it through to the ground. To observe the other wavelengths, we must put our telescopes in space!

32 Space Based Telescopes Chandra X-ray Obs. Hubble Space Telescope

33 X-ray Telescopes X-rays will pass right through a mirror. But can be reflected/focused at shallow angles like skimming stones

34 Radio Telescopes The wavelengths of radio waves are long. So the dishes which reflect them must be very large to achieve any reasonable angular resolution. 305-meter radio telescope at Arecibo, Puerto Rico

35 Two (or more) radio dishes observe same object. Interferometry Signals are made to interfere Reconstruct image with angular resolution of single dish with size of distance between them. Light-collecting area still only sum of individual dish areas.

36 Very Large Array (VLA)

37 VLT (Very Large Telescope) European Southern Paranal Moutain, Chile Observatory