Atomic Spectra• A satellite orbiting the Earth contain

gravitational potential energy.

• The satellite can orbit the Earth at any height. Or, it can contain any amount of gravitation energy—Its gravitational potential energy is continuous.

• Similar to the satellite, an electron orbiting the nucleus of an atom possesses electric potential energy. However, unlike satellites in orbits, the electrons can only stay in a finite number of discrete energy levels (or orbits). The energy levels of the atoms are

‘quantized’.

• There are only certain allowed Atomic energy levels

• Gravitational potential energy level can be anywhere along the line…

E

Selection Rules:There are rules about if a transition from level i to level j can occur or not!

i

j

• Different elements have different energy level structure…

Element A

Element B

How does light tell us what things are made off?

• Every chemical element has its own unit spectral fingerprint.

• We can study the chemical composition of an astronomical object by observing its absorption or emission spectrum.

Emission spectrum of Hydrogen

Absorption spectrum of HydrogenWith a thermal spectrum background.

Astronomical SpectraWe can learn a lot about a astronomical object by studying the spectra of the

object….There are three basic types of spectra:a. Thermal radiation Spectra: All objects with a finite temperature emit

thermal radiation.b. Absorption Line Spectra: c. Emission Line Spectra:

Click on image to start animation

Determining the Chemical Composition of Stars

• By studying the strength of the absorption spectra of stars, we can determine the chemical composition of stars

• The H-R diagram (Chapter 11) on the left shows how chemical composition is related to the temperature and luminosity of stars.

How do we measure the temperature of Stars (or any object)?

Everything with a temperature emit thermal radiation. We can measure the temperature of the stars or any object by studying the shape of their overall spectra.

Black BodyAn idealized perfect light absorber that absorbs all the photons that strikes it (no reflection). It re-emit the absorbed energy through thermal radiation, with a spectrum characterized by the blackbody spectrum.

• The shape of the blackbody spectrum is always the same, independent of its temperature.

• The peak position (in wavelength) of the blackbody spectrum depends only on the temperature, independent of the blackbody’s composition, or size, etc.

Click on image to start animation

Different Temperature Scales

• Celsius temperature measurement system is calibrated against the properties of water at sea level…

• Kelvin temperature scale uses the same temperature division as the Celsius system, but set the lowest possible temperature to 0 degree.

Temperature of Stars

• By analyzing the thermal spectrum of stars (at what color is the star light brightest?), we can determine their temperature.

• The H-R diagram (Chapter 11) shows how the temperature of the main sequence stars are related to their luminosity.

How do we study the motion of astronomical objects?

Doppler Shift

• The frequency (or wavelength) of a wave emitted by a moving source observed by a stationary observer is different from that when the source is at rest with respect to the observer.

• If the source is approaching the observer, than the observed wavelengths will be shorter than the ‘rest’ wavelength—blue shift…or higher pitch

• If the source is moving away from the observer, than the observed wavelengths will be longer than the ‘rest’ wavelength—red shift…or lower pitch

Line-of-Sight Velocity

• Doppler effect only tell us about the velocity along the line connecting the observer and the source—the line-of-sight velocity.

• If the object is moving in a direction perpendicular to the line-of-sight, then we will not see any Doppler effect.

Animation of Doppler Shift

Doppler Shift of Spectral Lines

• The wavelength of spectral lines that are close to each other will be changed by the (almost) the same amount, thus the distance between spectral lines does not change.

• By studying how much the pattern of spectral lines is shifted, we can measure the velocity of the light source with respect to us.

Click on image to start animation

Using Doppler Effect to Search for Extrasolar Planets

• A large planet orbiting a star cause the star to move in a circular motion also, producing Doppler Effect that can be observed (Chapter 6).

• It is very difficult to directly see light reflected or emitted by the planet, because it is overwhelmed by the star light.

Questions?

TelescopesThere are two different basic designs of telescope:

• Refracting Telescope: Light is focused by transparent glass with curved surfaces (the lens)

• Reflecting Telescope: Light is focused by reflecting curved mirror.

The primary mirror of Gemini is 8 meter in diameter

The 1-meter reflector at the University of Chicago’s Yerkes Observatory was the largest refracting telescope.

How Refracting Telescope Works• Light changes its propagation direction when it enters a

different media (e.g., apparent ‘bending’ of a stick partly submerged in water).

• The shape of the lens is precisely made such that light from a point source far away can be focused into another point.

• Drawback of Refracting Telescope:– No glass can transmit the entire electromagnetic spectrum.– Large glass with high quality is difficult to make.– The size of the lens of the largest refracting telescope is

about 1 meter.

Light from very distant point

Light from nearby point

Focal length of the lens

How Reflecting Telescope Works• With the correct shape, light from distant point can be focused into a point by

a curved reflecting mirror.

• A reflecting telescope works for all the wavelength…

• We can build much larger mirrors than lenses…

Keck Telescope on Mauna Kea

Why do we need BIG telescopes?Two important factors of astronomical observation depends on the size of the telescope—the larger the better.

• Sensitivity

The farther an object is, the fainter it becomes. A large telescope collects much more light from a distant object than a small one, allowing us to see very faint, distant object.

• Resolution:

The farther an object is, the more difficult it is to see the detailed structure of the object. The angular resolution of a telescope depends on its size. Thus, the larger the telescope is, the more details it can resolve.

Click on image to start animation

Car Headlight Simulation

How Earth’s Atmosphere Affect Astronomical Observations

Some undesirable things of Earth’s atmosphere…– Day-night cycle.

– Light pollution…

– Clouds, rains, snows…

– The motion of the atmosphere blurs the images formed by the telescopes on the ground.

– Some light (wavelengths) cannot reach the ground (gamma ray, X ray, UV, and IR)

Best Place for Observatories• Dark sky• Dry air (no clouds, rain, snow, low IR

absorption)• good seeing ( Calm atmosphere gives sharp

images)• High (less air to deal with)

Mauna Kea, Chile Space Moon?

The primary mirror of Hubble Space Telescope is only 1.5 meter, but it has produced much sharper images than the big telescopes on the ground because it is not affected by the undesirable effect of the atmosphere…especially seeing.

Adaptive OpticsWith new technology, the seeing effect of the atmosphere can be minimized.

• Adaptive optics (AO) compensate the blurring effects of the atmosphere by rapidly changing the shape of a mirror to focus the light better.

Image of sunspot with and without AO

AO corrected image of a binary system

Telescopes for X-Ray, Radio Wavelength

• Very-Large Array Radio Telescope in New Mexico…

• Chandra X-ray Telescope

Observations Across the Spectrum

X-ray visible Infrared

Radio

Crab Nebula observed in four different wavelengths

Light at different wavelengths are produced by different physical processes, and/or from different regions. Thus, we can get a complete picture of what’s going on only by observing the same object at all regions of the electromagnetic spectrum…

• What’s light?– Basic properties about light…

• What’s spectrum?– Wavelength, frequency, energy light carries…

• Matter– Atomic structure– How do we build different elements?

• How does light interact with matter?• How does light with different wavelength interact with matter?

– What can we see on the surface of the Earth?– What can’t we see on the surface of the Earth?

• How do we measure chemical composition?– Atomic spectra– Quantization of atomic spectra

• How do we measure speed?• How do we measure temperature?• What do we learn about an astronomical object observing with different

telescopes?• How do telescopes work?• Why do we need to put telescopes in space?