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Light and Telescopes Astronomy: The Science of Seeing Copyright – Anglo Australian Observatory 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 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 lightyears 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 1010 cm/s • The wavelength (l) and frequency (n) are related: c = ln 3 3 What’s the Wavelength Kenneth? • Arrow 93.1 FM • 93.1 MHz (Mega Hertz) = 93.1 x 106 cycles/sec c = ln 3 x 1010 cm/sec = l x 93.1 x 106 cycles/sec l = (3 x 1010 cm/sec)/(93.1 x 106 cycles/sec) l = 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 = ln 3 3 Radio g-ray UV Optical and infrared X-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 D2 If D increases, then S increases by D2 • If your telescope is 3 times bigger than mine, then your telescope can see 32 = 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. • 1000 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. 3 3 Atmospheric Seeing 3 Beat the Seeing • Seeing degrades resolution • Can put a telescope in space (Hubble Space Telescope) 3 Hubble Space Telescope Space - HST – 1.0m Ground - KPNO 4.0m – Copyright NOAO/AURA/NSF 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. NPOI - optical VLA - radio 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 Resolution and Seeing HST 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. 3 100-inch 40-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 3