3 Copyright – Anglo Australian Observatory Light and Telescopes Astronomy: The Science of Seeing

3 How do you do Astronomy? How do Chemists do Chemistry? –Make solutions, mix chemicals … How do Biologists do Biology? –Breed fruit flies, (and whatever else biologists do). They devise and conduct experiments in their labs. But how do you do that for astronomy?

3 Light Astronomy is a “passive” science. We can’t (yet) go to the stars or other galaxies. The Universe must come to us. We rely on light exclusively!

3 But what does it look like from the back?

3 What you see is all you get! So you need to squeeze EVERY last drop of information out of the light we get. This semester we’ll see how we can use light to: 1.Weigh a planet. 2.Take a star’s temperature. 3.Tell what’s in the center of a star a thousand light- years away. 4.Tell what our Galaxy look like from the outside.

3 Goals What is light? What are the different types of light? What is the purpose of a telescope? What can we see with a telescope?

3 The “Visible” Spectrum When you think of “light”, what do you think of?

3 What is Light? Light is a wave of energy. Moves through a vacuum. Travels at the speed of light (a CONSTANT): c = 3 x cm/s The wavelength ( ) and frequency ( ) are related: c =

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3 What’s the Wavelength Kenneth? Arrow 93.1 FM 93.1 MHz (Mega Hertz) = 93.1 x 10 6 cycles/sec c = 3 x cm/sec = x 93.1 x 10 6 cycles/sec = (3 x cm/sec)/(93.1 x 10 6 cycles/sec) = 322 cm =3.22 m How big is your radio antenna?

3 To Sum Up… Radio waves, microwaves, rainbows, UV waves, x-rays, etc are ALL forms of light (electromagnetic waves). They ALL travel through space at the speed of light. c The higher the frequency, the shorter the wavelength. c =

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3 Radio Optical and infrared X-ray UV  -ray

3 Types of Telescopes

3 What is the Purpose of a Telescope? 1.Increase the amount of light we see. Sensitivity is proportional to Collecting Area (area of the circular opening of the telescope). S = constant times  D 2 If D increases, then S increases by D 2 If your telescope is 3 times bigger than mine, then your telescope can see 3 2 = 9 times fainter objects than mine. Can you read a book at night? What’s the faintest star you can see with your naked eye?

3 Sensitivity

3 What is the Purpose of a Telescope? 2. Increase the detail (resolution) we see. Resolution is inversely proportional to Telescope Diameter.  constant times 1/D  Diffraction Limit If D increases then  decreases by the same amount. If your telescope is 3 times bigger than mine then you can see 3 times smaller angles (3 times smaller objects or detail). Can you read a street sign a block away? Can you see the binary star in the Big Dipper with your naked eye?

3 Resolution

3 Angles The sky is 360 arc degrees around. 60 arcminutes = 1 arc degree –The Full Moon is about half an arc degree = 30 arcminutes. 60 arcseconds = 1 arcminute –Mars is about 2 arcminutes now milliarcsecond = 1 arcsecond –Polaris is 3 milliarcseconds in diameter –An astronaut on the Moon is 2 milliarcseconds tall!

3 Angular Size Angular size: How big does something look as viewed from the Earth? –During a solar eclipse, the Moon looks big enough to cover the Sun. The Sun is a million times larger than the Earth. The Moon is a fourth the size of the Earth. The distance from the Earth determines their ANGULAR SIZE.

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3 Atmospheric Seeing

3 Beat the Seeing Seeing degrades resolution Can put a telescope in space (Hubble Space Telescope)

3 Hubble Space Telescope Ground - KPNO 4.0m – Copyright NOAO/AURA/NSF Space - HST – 1.0m

3 Beat the Seeing Seeing degrades resolution Can put a telescope in space (Hubble Space Telescope) –Expensive! Interferometers (NPOI, VLA)

3 Interferometry Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope. VLA - radio NPOI - optical

3 Beat the Seeing Seeing degrades resolution Can put a telescope in space (Hubble Space Telescope) –Expensive! Interferometers (NPOI, VLA) –Complicated! Adaptive optics can recover resolution

3 HST Resolution and Seeing Neptune with the Palomar 200-inch reflector and HST

3 Seeing and Magnification Larger than a few inches, a telescope’s resolution stops getting better due to seeing. Don’t be fooled by advertisements claiming huge magnification increases! –“Amazing 500X magnification!” But sensitivity ALWAYS increases with bigger telescopes.

3 40-inch 100-inch 200-inch 400-inch

3 Homework #3 For Wednesday 9/18: Read Ty8, B6.4 – 6.5 Do B6: Review Question 15, Problems 3, 4, 5