ASTRO 101 Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: WWW: Office: Physics 241, hours T TH 3:30-5:00

Text: “Discovering the Essential Universe, Fifth Edition” by Neil F. Comins

Course WWW Page Note the underline: … ast101_fall2012.html … Also check out Nick Strobel’s Astronomy Notes:

Where: Room 215, physics-astronomy building. No appointment needed! Just drop by! When: Monday: 12-2, 4-6 PM Tuesday: 12-1 PM; 4-6 PM Wednesday: 12-2, 5-6 PM Thursday: 4-6 PM

Exam 1: N=12 (0 missing) Average = 63.2 low = 28.5, high = 96 A 90%--100% A- 85%--89% B+ 80%--84% B 75%--79% B- 70%--74% C+ 65%--69% C 60%--64% C- 50%--59% D 40%--49% F 0%--39%

Venus in the Geocentric View Venus is always close to the Sun on the sky, so its epicycle restricts its position. In this view, Venus always appears as a crescent.

Venus in the Heliocentric View In the heliocentric view, Venus orbits the Sun closer than the Earth does. We on Earth can see a fully lit Venus when it is on the far side of its orbit.

Venus in the Heliocentric View The correlation between the phases and the size is accounted for in the heliocentric view.

Homework/Announcements Homework due Tuesday, October 9: Question 5, Chapter 4 (Describe four methods for discovering exoplanets) Substitute lecturer Thursday October 4 Football!

Coming Up: The 4 forces of Nature Energy and the conservation of energy The nature of light –Waves and bundles of energy –Different types of light Telescopes and detectors Spectra –Emission spectra –Absorption spectra

The spectrum Definition and types: –Continuous –Discrete The spectrum and its uses: –Temperature –Chemical composition –Velocity

The spectrum A graph of the intensity of light vs. the color (e.g. the wavelength, frequency, or energy) is called a spectrum. A spectrum is probably the single most useful diagnostic tool available in Astronomy.

The spectrum A spectrum can tell us about the temperature and composition of an astronomical object. There are two types of spectra of concern here:  Continuous spectra (the intensity varies smoothly from one wavelength to the next).  Line spectra (there are discrete jumps in the intensity from one wavelength to the next).

The spectrum Continuous spectrum. Discrete or line spectra. Images from Nick Strobel (

Thermal Spectra The most common type of continuous spectrum is a thermal spectrum. Any dense body will emit a thermal spectrum of radiation when its temperature is above “absolute zero”: –The “color” depends only the temperature; –The total intensity depends on the temperature and the size of the body. This type of radiation is often called “black body” radiation.

Thermal Spectra The most common type of continuous spectrum is a thermal spectrum.

Black body radiation Sample spectra from black bodies of different temperatures. Note that the area under the curves is largest for the hottest temperature. There is always a well- defined peak, which crudely defines the “color”. The peak is at bluer wavelengths for hotter temperatures.

Important points The luminosity (energy loss per unit time) of a black body is proportional the surface area times the temperature to the 4th power: Hotter objects have higher intensities (for a given area), and larger objects have higher intensities.

Important points The peak of the spectrum is inversely proportional to the temperature (hotter objects are bluer): Hotter objects are bluer than cooler objects.

How light interacts with matter and the line spectrum.

What Things Are Made Of Atoms are the basic blocks of matter. The number of protons determines the element. For example hydrogen has 1 proton, helium has 2 protons, uranium has 92 protons, etc. In most cases, the number of electrons equals the number of protons.

How Light Interacts with Matter. Atoms are the basic blocks of matter. In the 1850s, it was discovered that an element, when burned, gave off a unique emission line spectrum.

How Light Interacts with Matter. An electron will interact with a photon. An electron that absorbs a photon will gain energy. An electron that loses energy must emit a photon. The total energy (electron plus photon) remains constant during this process.

How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Thus, only photons of certain energy can interact with the electrons in a given atom.

How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Thus, only photons of certain energy can interact with the electrons in a given atom. Image from Nick Strobel (

How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Each element has its own unique pattern of energies.

How Light Interacts with Matter. Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Each element has its own unique pattern of energies, hence its own distinct line spectrum. Image from Nick Strobel (

How Light Interacts with Matter. An electron in free space can have any energy. –It can absorb a photon of any energy –It can lose any amount of energy (  E) by emitting a photon with energy equal to  E An electron in an atom can only have very specific values of its energy E 1, E 2, E 3, … E N ) –The electron can absorb a photon with an energy equal to (E 1 - E 2 ), (E 1 -E 3 ), (E 2 -E 3 ), … and jump to a higher level –The electron can lose an amount of energy equal to a change between levels (E 1 -E 2 ), (E 1 -E 3 ), (E 2 -E 3 ), … and move down to a lower level

How Light Interacts with Matter. An electron in an atom can only have very specific values of its energy E 1, E 2, E 3, … E N ) –The electron can absorb a photon with an energy equal to (E 1 -E 2 ), (E 1 -E 3 ), (E 2 -E 3 ), … and jump to a higher level –The electron can lose an amount of energy equal to a change between levels (E 1 -E 2 ), (E 1 -E 3 ), (E 2 -E 3 ), … and move down to a lower level Since each element has its own unique sequence of energy levels (E 1, E 2, E 3, … E N ), the differences between the levels are also unique, giving rise to a unique line spectrum

Emission spectra and absorption spectra.

Emission and Absorption If you view a hot gas against a dark background, you see emission lines (wavelengths at which there is an abrupt spike in the brightness).

Emission and Absorption If you view a continuous spectrum through cool gas, you see absorption lines (wavelengths where there is little light).

Emission and Absorption Image from Nick Strobel (

The spectrum View a hot, dense source, get a continuous spectrum. View that hot source through cool gas, get an absorption spectrum. View that gas against a dark background, get emission spectrum.

The spectrum View a hot, dense source, get a continuou s spectrum.continuou s spectrum View that hot source through cool gas, get an absorption spectrum. View that gas against a dark background, get emission spectrum.

Tying things together: The spectrum of a star is approximately a black body spectrum.  Hotter stars are bluer, cooler stars are redder.  For a given temperature, larger stars give off more energy than smaller stars.

In the constellation of Orion, the reddish star Betelgeuse is a relatively cool star. The blue star Rigel is relatively hot.

Tying things together: The spectrum of a star is approximately a black body spectrum.  Hotter stars are bluer, cooler stars are redder.  For a given temperature, larger stars give off more energy than smaller stars.  However, a closer look reveals details in the spectra…

The Line Spectrum Upon closer examination, the spectra of real stars show fine detail. Dark regions where there is relatively little light are called lines.

The Line Spectrum Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength. The “lines” where there is relatively little light show up as dips in the curves.

The Line Spectrum Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength. The “lines” where there is relatively little light show up as dips in the curves. These dips tell us about what elements are present in the star!

Atomic Fingerprints Hydrogen has a specific line spectrum. Each atom has its own specific line spectrum.

Atomic Fingerprints These stars have absorption lines with the wavelengths corresponding to hydrogen!

Atomic Fingerprints The Sun (and other stars) have absorption lines with the wavelengths corresponding to iron.

Atomic Fingerprints. One can also look at the spectra of other objects besides stars, for example clouds of hot gas. This cloud of gas looks red since its spectrum is a line spectrum from hydrogen gas.

The Doppler Shift: Measuring Motion If a source of waves is not moving, then the waves are equally spaced in all directions.

The Doppler Shift: Measuring Motion If a source of waves is moving, then the spacing of the wave crests depends on the direction relative to the direction of motion.

The Doppler Shift: Measuring Motion Think of sound waves from a fast-moving car, train, plane, etc.  The sound has a higher pitch (higher frequency) when the car approaches.  The pitch is lower (lower frequency) as the car passes and moves further away.

The Doppler Shift: Measuring Motion If a source of light is moving away, the wavelengths are increased, or “redshifted”.

The Doppler Shift: Measuring Motion If a source of light is moving closer, the wavelengths are shortened, or “blueshifted”.

The Doppler Shift: Measuring Motion The size of the wavelength shift depends on the relative velocity of the source and the observer.

The Doppler Shift: Measuring Motion The size of the wavelength shift depends on the relative velocity of the source and the observer. The motion of a star towards you or away from you can be measured with the Doppler shift.

Using a Spectrum, we can… Measure a star’s temperature by measuring the overall shape of the spectrum (essentially its color). Measure what chemical elements are in a star’s atmosphere by measuring the lines. Measure the relative velocity of a star by measuring the Doppler shifts of the lines.

Next: Comparative Planetology Outline and introduction to the Solar System Planets around other stars