• Summary of Chapter 3: Light
Today
•All of Chapter 4: Spectra & AtomsOptional: Ast. Toolbox 4-2Optional: Stephan-Boltzmann Law
•Coming up: The Sun (Chapter 10)
• Next Homework Due March 6
1.) Wed. Feb. 28, 7 pm, "When Mars Was Like Earth: Five Years of Exploration
with NASA's Curiosity Mars Rover" by Dr. Ashwin Vasavada, (NASA's JPL) @Smithwick Theater at Foothill College, in Los Altoshttp://www.foothill.edu/news/maps.php
WED. FEB 21 at SF City College
2.) Simulating the Center of the Galaxy, by Richard Anantua, (UC
Berkeley) 5 - 6pm MUB 140
3.) “Monks Under the Moon” by Vivian White, (Ast. Soc. Pacific) 6 - 7pm MUB 140
4.) 7:15 – 9:00pm Star Party: Observe the Moon, Planets, Nebulae, Star Clusters! On the Roof (via Science Hall 405)
Extra Credit Astro-talks:
Hubble Space Telescope
Or in Space…
Space Telescopes avoid the blurring effects of Earth’s Atmosphere.
Infrared Space Telescopes
Spitzer’s view of Andromeda in Infrared Light
Spitzer can see infrared light that is blocked by Earth’s atmosphere
Typical view of the Andromeda galaxy in Visible Light
A new NASA space telescope WISE, can see even longer IR
waves.
X-ray AstronomyX-rays are high energy light with very short wavelength
They are emitted by very hot gas in the universe.To observe X-rays, NASA launched the Chandra X-ray
Observatory in 1999
Chandra Telescope Chandra Image of Supernova Explosion
The Largest Radio TelescopeSince 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! They 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.”
Review of Chapter 3! 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 has energy that is proportional to its frequency
! Telescopes: gather light, reveal details, and magnify images! There are 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 StarlightChapter 4
By analyzing the light from a star, we learn about its:
1. Temperature2. Composition3. Motion
Color and Temperature
Orion
Betelgeuse
Rigel
Stars have different colors,
Rigel is blue
Betelgeuse is red
Our Sun is yellow.
The different colors of stars are due to their different temperatures.
TemperatureAstronomers use Kelvins (K) to measure temperatures.
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
Challenge Question: What Color is the Sun?
Are you sure? What about at sunset?
The Sun produces light that is not yellow.
Spectrum of the Sun
The Sun emits light at UV, Visible and Infrared wavelengths
Wavelength
Brig
htne
ss (o
r Int
ensi
ty)
Spectra
• A spectrum (pl. spectra) is just a record of how much light there is at different wavelengths.
• We can learn a lot about a star from its spectrum.
• The overall shape of the spectrum tells us the temperature of the star.
• The detailed features tell us the composition of the star (which elements it’s made of.)
Thermal Radiation(“Radiation”=Light)
A spectrum of a typical star:
There is a strong peak in the light at one wavelength,
called λmax
This is called a thermal or black body spectrum.
How to Measure a Star’s Temperature
This is called Wien’s law:
TK ≈ 3,000,000 nm / λmax
or
λmax ≈ 3,000,000 nm / TK
(where TK is the temperature in Kelvin).
λmaxλmax
λmax
The peak wavelength (λmax) is longer for stars that are cooler.
...And shorter for stars that are hotter.Spectra of Three Stars
Example of Wien’s Law
TK = 3,000,000 nm / λmax
Q: What is the Temperature of a star whose spectrum peaks at 1,000nm?
Answer:
λmax = 1,000 nm is given.
TK = 3,000,000 nm / λmax = 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.
Wien’s Law:
1. Continuum Spectrum
Spectra fall into three categories:
-A rainbow in which all colors are represented
We can compare these spectra to lab experiments.
Rainbow
Bright Lines
Rainbow with dark lines
Colder gas
Very hot bulb
Very hot bulb
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.
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.
Because electrons exist only at specific energy levels, photons are emitted & absorbed with specific energies.
Absorption Spectrum
Photons of different wavelength have different energy.So atoms only absorb certain wavelengths of light!That’s why their spectra have dark lines.
The center of a star is very hot, but the outer layers are cooler.
So light from a star is simlar to this experiment:
HotCool
Iron
Hydrogen
Helium
Calcium
So each element has a different spectrum
Each element’s electrons have different energy levels
The Power of StarlightWe 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.
Stationary source of waves Moving source of waves
Doppler Effect
Nice Demo: http://www.astro.ubc.ca/~scharein/applets/
Doppler Effect: sound waves
As the train approaches, the soundwaves get crunched together. Thewavelength gets shorter. (higher pitch)
As the train recedes, the soundwaves get stretched apart. Thewavelength gets longer. (lower pitch)
Stationary train
Moving traindemo
Doppler Effect: Light Waves
•If a source of light is approaching, the waves of light will be crunched, and smaller. •Smaller wavelength (λ) = blue.•This is called blueshift.
•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.
Approaching
Receding
Doppler Shift of Spectral Lines
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” λo
Not Moving
Doppler Effect Calculation
The light of a moving source is blueshifted
or redshifted by
Δλ/λ0 = vr/cλ0 = rest wavelength emitted by the source
Δλ = wavelength change due to Doppler effect
vr = radial velocity (speed)
The faster something moves, the bigger the change in wavelength.
c = speed of light
Example:A certain spectral line (Hα) has a rest wavelength of 656 nm
Suppose we observe a star’s spectrum with the Hα line at λ = 658 nm.
Question: How Fast is this Star moving?
Is it moving toward us or away?
Example:
λ = 658 nm (observed wavelength) The change in wavelength is: Δλ = λ− λ0 = 2 nm.
vr/c = Δλ/λ0
We find Δλ/λ0 = 2nm/656nm = 0.003 = 3*10-3
vr/c = Δλ/λ0 = 0.003 vr = 0.003 * c vr = 0.003 *(300,000 km/s) = 900 km/s.
The star is receding from us at 900 km/s.
λ0 = 656 nm (rest wavelength)
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