• 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:

On a mountaintopMauna Kea, Hawai’i (~14,000 feet elevation)

Hubble Space Telescope

Or in Space…

Space Telescopes avoid the blurring effects of Earth’s Atmosphere.

The Spitzer Space Telescope, launched in 2003, observes infrared light

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

Red Hot Orange Hot Yellow Hot

Using Light to Measure Temperature

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

The SpectrographA spectrograph uses a prism to

split light up into different wavelengths (=colors!)

Modern spectra are recorded digitally as plots of intensity vs.

wavelength

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

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.

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.

Photon Emissione-

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 Absorptione-

Electron absorbs a photon, causing it to jump to a higher energy level

Photon

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

Low-Resolution Spectrum of the Sun

We can barely see some absorption lines

A very high resolution spectrum of the Sun

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.

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

Earth

The Sun – Our Star