Today Summary of Chapter 3: Light All of Chapter 4: Spectra & Atoms Optional: Ast. Toolbox 4-2 Optional: Stephan-Boltzmann Law Next Homework Due Oct. 9 Coming up: The Sun (Chapter 10)
Resolving Power: Seeing Details Resolving Power (or resolution) is a telescope s ability to see small details. It is proportional to a telescope s size (diameter D)
Magnification makes things look larger However it does not improve resolution (seeing fine details).
The Best Location for a Telescope Far away from civilization to avoid light pollution Text
On a mountaintop Mauna Kea, Hawai i (~14,000 feet elevation)
Or in Space Space Telescopes avoid the blurring effects of Earth s atmosphere. Hubble Space Telescope
Space Telescopes also allow us to see light blocked by Earth s atmosphere: infrared, ultraviolet, X-rays & gamma rays. Spitzer (2003) Fermi Gamma Ray Observatory (2008)
Andromeda Galaxy Visible Light Infrared Light
A newer IR image from the WISE satellite
X-ray Astronomy X-rays are high energy light with very short wavelength They are emitted by very hot gas in the universe. Chandra X-ray Telescope Chandra Image of Supernova Explosion
The Largest Radio Telescope Since radio waves pass through Earth s atmosphere, we can build radio telescopes on the ground. The 300-m telescope in Arecibo, Puerto Rico is the largest in the world. It can hold 13 football fields!
Review of Chapter 3! To understand what s in space, we must understand light.! Light is an electro-magnetic wave with three properties:! Speed: c = 300,000 km/s! Frequency (f): number of light waves per second.! Wavelength (λ): distance. from one peak to the next! We see different wavelengths as colors! These three are related: c = f x λ! Many forms of light exist, most invisible to humans:! Infrared radiation and radio waves have longer wavelengths than visible! Ultraviolet, X rays, and gamma rays have shorter wavelengths! Visible light occupies only a small portion of the elctromagnetic spectrum.
Chapter 3 Review! Visible light waves are small: their wavelengths are measured in nanometers (1 nm = 10-9 m)! Visible light range: λ=400 nm (violet) to λ=700 nm (red)! Light s energy depends on its frequency! Telescopes: gather light, reveal details, and magnify images! The two main types:! Reflectors produce images using mirrors,! Refractors use lenses to focus light! Light gathering ability depends on the telescope mirrors s area.! The area of a circle is: A = π r 2
The Power of Starlight Chapter 4 By analyzing the light from a star, we learn about its: 1. Temperature 2. Composition 3. Motion
Using Light to Measure Temperature Red Hot Orange Hot Yellow Hot
Color and Temperature Stars have different colors, Rigel is blue Betelgeuse is red Our Sun is yellow. Betelgeuse The different colors of stars are due to their different temperatures. Rigel Orion
Astronomers use Kelvins (K) to measure temperatures. Temperature Example: The freezing point of water is: 32 F 0 C 273 K 0 K is Absolute Zero
Wavelengths of Light Light from any source has a variety of colors. Q: At which color (or wavelength) does the source emit the most light? To answer this question we use a spectrograph. A spectrograph produces readout of light intensity vs. wavelength called a spectrum
The Spectrograph A spectrograph uses a prism to split light up into different wavelengths (=colors!)
A Spectrum Modern spectra are recorded digitally as plots of intensity vs. wavelength
Challenge Question: What Color is the Sun? Are you sure? What about at sunset? The Sun produces light that is not yellow.
Brightness (or Intensity) Spectrum of the Sun Wavelength The Sun emits UV, Visible and Infrared light
Stellar Spectra BUT, the Sun emits more yellow (λ=570 nm) light than any other color. The Sun s spectrum peaks in the yellow part. We can use this fact to measure the temperature of the Sun, or any other star.
Peak Wavelength A spectrum of a typical star: Measure how much light is produced of each color/ wavelength More light is produced at one peak wavelength than any other. Call this wavelength: λ max
How to Measure a Star s Temperature Spectra of Three Stars The peak wavelength (λ max ) is longer for stars that are cooler....and shorter for stars that are hotter. This is called Wien s law: T K 3,000,000 nm / λ max or λ max 3,000,000 nm / T K λ max λmax λ max (where T K is the temperature in Kelvin).
Example of Wien s Law Wien s Law: T K = 3,000,000 nm / λ max Q: What is the Temperature of a star whose spectrum peaks at 1,000nm? Answer: λ max = 1,000 nm is given. T K = 3,000,000 nm / λ max = 3,000,000nm/ 1,000nm = 3,000 K The Sun s temperature is about 6,000 Kelvins, so this star is cooler than the Sun.
Stellar Spectra fall into three categories: 1. Continuum Spectrum -A rainbow in which all colors are represented
2. Absorption Spectrum -A rainbow from which some colors missing
3. Emission Spectrum - Mostly Dark, but a few bright emission lines are seen
We can compare these spectra to lab experiments. Very hot bulb Rainbow Colder gas Bright Lines Very hot bulb Rainbow with dark lines demo
A Spectral Mystery Most stars spectra are not perfect rainbows... They are missing light at certain wavelengths/colors. (they have an absorption spectrum.)...why? This light was produced by matter. Since matter is composed of atoms, we need to understand atoms & how light interacts with them.
Atoms An atom consists of a nucleus and a cloud of electrons surrounding it. The nucleus contains protons and neutrons. Almost all of the mass is contained in the nucleus Almost all the space is occupied by the electron cloud. Freaky Fact: The atoms that make us up... are mostly empty space!
Elements & Nuclei An Element is defined by the number of protons in its nucleus. Hydrogen (H), is the simplest element: one proton Helium (He), is next simplest: 2 protons 2 neutrons Helium
An Isotope of an element is a nucleus with a different number of neutrons than normal Deuterium is an isotope of Hydrogen Carbon-13 is an isotope of Carbon-12
Bohr Model of the Atom (Niels Bohr 1886-1962) Every atom consists of a nucleus plus electrons, which can be in different energy states. The farther the electron is from the nucleus, the higher its energy. Freaky Fact: Electrons can t be in any energy state. Allowable energy states are quantized (because an electron is a wave) This simple model can help us understand the mysterious spectra of stars
Bohr model of an atom Electron Nucleus The electron orbits the nucleus, in one of these possible levels
Atoms & Light Light interacts with atoms by exchanging energy with electrons An electron in a high energy level can jump down to a low energy level. It loses energy, and emits of a photon of light. The energy of this photon equals the energy lost by the electron. This is the energy difference between the levels.
Photon Emission e - Photon Electron jumps to a lower energy level causing it to emit a photon
Atoms & Light An electron can jump up from lower to higher energy levels But, it needs energy to do this. A photon of light can provide the energy. However not every photon can do the trick. If the photon s energy matches the energy difference between the levels, the photon will be absorbed. If not, it will fly through the atom.
Photon Absorption e - Photon Electron absorbs a photon, causing it to jump to a higher energy level
Because electrons exist only at specific energy levels, photons are emitted & absorbed with specific energies.
Absorption Spectrum The center of a star is very hot, but the outer layers are cooler. So light from a star is simlar to this experiment: Hot Cool Photons of different wavelength have different energy. So atoms only absorb certain wavelengths of light! That s why their spectra have dark lines.
Each element s electrons have different energy levels Hydrogen Helium Calcium Iron So each element has a different spectrum
Low-Resolution Spectrum of the Sun We can barely see some absorption lines
A very high resolution spectrum of the Sun
The Power of Starlight We have seen that by analyzing starlight we can determine: 1.Temperature - From Wien s Law 2.Composition - from spectral lines 3. Next: Motion - From Doppler Effect.
Doppler Effect
Doppler Effect Stationary source of waves Moving source of waves Nice Demo: http://www.astro.ubc.ca/~scharein/applets/
Doppler Effect: sound waves Stationary train As the train approaches, the sound waves get crunched together. The wavelength gets shorter. (higher pitch) As the train recedes, the sound waves get stretched apart. The wavelength gets longer. (lower pitch) demo Moving train
Doppler Effect: Light Waves Receding If a source of light is approaching, the waves of light will be crunched, and smaller. Smaller wavelength (λ) = blue. This is called blueshift. Approaching If a source of light is receding away, the waves will be stretched and the light will become redder. Longer wavelength (λ) = red This is called redshift.
Doppler Shift of Spectral Lines Not Moving We can determine if a star is moving toward us or away from us based on its spectral lines, which are shifted from their usual rest wavelength λ o
Doppler Effect Calculation The faster something moves, the bigger the change in wavelength. λ 0 = rest wavelength emitted by the source Δλ = wavelength change due to Doppler effect v r = radial velocity (speed) The light of a moving source is blueshifted or redshifted by Δλ/λ 0 = v r /c c = speed of light
Example: A certain spectral line (H α ) has a rest wavelength of 656 nm Suppose we observe a star s spectrum with the H α line at λ = 658 nm. Question: How Fast is this Star moving? Is it moving toward us or away?
Example: λ 0 = 656 nm (rest wavelength) λ = 658 nm (observed wavelength) The change in wavelength is: Δλ = λ λ 0 = 2 nm. v r /c = Δλ/λ 0 We find Δλ/λ 0 = 2nm/656nm = 0.003 = 3*10-3 v r /c = Δλ/λ 0 = 0.003 v r = 0.003 * c v r = 0.003 *(300,000 km/s) = 900 km/s. The star is receding from us at 900 km/s.
Doppler Effect: Applications The Doppler Effect can be used to measure how fast something is moving Police: Speeding Tickets Weather: Doppler Radar Astronomy: Motions of stars, including planet detection.
Chapter 4 Summary " Wien s Law: Measure Temperature " Bohr model " Atoms & Light " How a spectrum is formed. " Doppler Effect: Measure motion
Earth The Sun Our Star