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

Homework Homework due September 25: Question 4 from Chapter 3 (What are the three main functions of a telescope?) Write down the answer on a sheet of paper and hand it in before the end of class on September 25.

Homework Go to a planetarium show in PA 209: Wednesday, September 12:12:00 PM -- 12:30 PM Thursday, September 13: 12:00 PM – 12:30 PM AND 12:30 PM – 1:00 PM Friday, September 14: 12:00 PM – 12:30 PM AND 12:30 PM – 1:00 PM Monday, September 17: 12:00 PM – 12:30 PM AND 12:30 PM – 1:00 PM Thursday, September 20: 12:00 PM – 12:30 PM AND 12:30 PM – 1:00 PM AND 4:00 PM – 4:30 PM Friday, September 21: 12:00 PM – 12:30 PM AND 12:30 PM – 1:00 PM Get 10 points extra credit for homework part of grade. Sign up for a session outside PA 209. Hand in a sheet of paper with your name and the data and time of the session.

Announcements Tuesday, September 25: wrap-up and review Thursday, September 27: exam 1 Office hours for today are cancelled

Next: Chapter 3: Light and Telescopes

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

The 4 “Forces” of Nature There are 4 “fundamental forces” in nature: 1.Gravity: relative strength = 1, range = infinite. 2.Electromagnetic: rel. str. = 10 36, range = infinite. 3.“Weak” nuclear: rel. str. = 10 25, range = meter. 4.“Strong” nuclear: rel. str. = 10 38, range = meter.

The 4 “Forces” of Nature There are 4 “fundamental forces” in nature: 1.Gravity: relative strength = 1, range = infinite. 2.Electromagnetic: rel. str. = 10 36, range = infinite. 3.“Weak” nuclear: rel. str. = 10 25, range = meter. 4.“Strong” nuclear: rel. str. = 10 38, range = meter. Gravity is an attractive force between all matter in the Universe. The more mass something has, the larger the net gravitational force is.

The 4 “Forces” of Nature There are 4 “fundamental forces” in nature: 1.Gravity: relative strength = 1, range = infinite. 2.Electromagnetic: rel. str. = 10 36, range = infinite. 3.“Weak” nuclear: rel. str. = 10 25, range = meter. 4.“Strong” nuclear: rel. str. = 10 38, range = meter. The electromagnetic force can be repulsive (+,+ or -,-) or attractive (+,-). Normal chemical reactions are governed by this force.

The 4 “Forces” of Nature There are 4 “fundamental forces” in nature: 1.Gravity: relative strength = 1, range = infinite. 2.Electromagnetic: rel. str. = 10 36, range = infinite. 3.“Weak” nuclear: rel. str. = 10 25, range = meter. 4.“Strong” nuclear: rel. str. = 10 38, range = meter. The weak force governs certain radioactive decay reactions. The strong force holds atomic nuclei together.

The 4 “Forces” of Nature There are 4 “fundamental forces” in nature: 1.Gravity: relative strength = 1, range = infinite. 2.Electromagnetic: rel. str. = 10 36, range = infinite. 3.“Weak” nuclear: rel. str. = 10 25, range = meter. 4.“Strong” nuclear: rel. str. = 10 38, range = meter. Gravity is the most important force over large scales since positive and negative charges tend to cancel.

A Thought Experiment How does your vision work? –Do your eyes send out a “scanning” signal? –Do your eyes receive information from outside? How can you tell?

What is Energy? What is light, and what can it tell us?

Energy is the ability to do “work.” “Work” is done when something is moved.

Forms of energy Energy of motion (e.g. moving bodies):  For a given velocity, a more massive object has more energy.  For a given mass, a faster moving body has more energy. Potential energy:  Chemical energy.  Nuclear energy.  Gravitational energy.

Forms of energy Thermal (or heat) energy. Electromagnetic energy.

Forms of energy Thermal (or heat) energy. Electromagnetic energy. Mass, as in E=mc 2.

The conservation of energy:

The conservation of energy: Energy is neither created nor destroyed, but may be changed in form.

Energy changing form: Potential energy in gasoline turns into energy of motion of a car, along with heat and noise. The energy of motion of a falling body creates an impact crater. Matter in turned into energy at the center of the Sun.

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

Light is a form of energy.

Light is a form of energy. Why is this important?

Light is a form of energy. Why is this important? With very few exceptions, the only way we have to study objects in Astronomy is via the light they emit.

What is the nature of light?

What is the nature of light? Light can be thought of as a wave in an electric field or as discrete particles of energy…

What is the nature of light? Light can be thought of as a wave. The wavelength (usually denoted with a ) is the distance from crest to crest. Image from Nick Strobel’s Astronomy Notes (

What is the nature of light? Light can be thought of as a wave. The frequency (usually denoted with  is the number of crests that pass a given point each second. Image from Nick Strobel’s Astronomy Notes (

What is the nature of light? Light can be thought of as a wave. The frequency (usually denoted with  is the number of crests that pass a given point each second.

What is the nature of light? The velocity of the wave is the wavelength times the frequency: The velocity of light in vacuum is constant for all wavelengths, regardless of the relative velocities of the observer and the light source.

What is the nature of light? The velocity of light is not infinite.

What is the nature of light? Although the velocity of light is large, it is not infinite. c = 300,000 km/sec or c = 186,000 miles/sec

What is the nature of light? Although the velocity of light is large, it is not infinite. c = 300,000 km/sec or c = 186,000 miles/sec Ordinary matter cannot travel faster than the speed of light.

What is the nature of light? The above animation shows waves with different wavelengths moving with the same speed. Their frequencies are different. Image from Nick Strobel’s Astronomy Notes (

What is the nature of light? Light can be thought of as a wave in an electric field or as discrete particles of energy…

What is the nature of light? Light can also behave like discrete particles called photons. The energy of a photon depends on the frequency (or equivalently the wavelength): The value of h is constant for all situations.

What is the nature of light? Photons of higher energy have higher frequencies and shorter wavelengths, since

What is the nature of light? The above animation shows waves with different wavelengths moving with the same speed. Their frequencies are different. Image from Nick Strobel’s Astronomy Notes (

Intensity vs. Energy A photon’s energy depends on the frequency. The intensity of a source refers to the number of waves or photons from that source. Image from Nick Strobel’s Astronomy Notes (

Different “types” of light. What light can tell us.

Visible light White light is made up of different colors

Visible light Different colors correspond to different frequencies (or wavelengths). The colors of the rainbow are ROY G BIV: red orange yellow green blue indigo violet.

Visible light In the visible,  red has the longest wavelength, the smallest frequency, and the lowest energy.  violet has the shortest wavelength, the highest frequency, and the highest energy.

The Electromagnetic Spectrum Visible light is only a tiny fraction of the Electromagnetic Spectrum. For example, there is invisible radiation with wavelengths longer than red light that heats the thermometer.

The Electromagnetic Spectrum As we go to wavelengths slightly longer than visible (i.e. smaller frequencies and lower energies), we find infrared radiation, which is basically perceived as heat.

The Electromagnetic Spectrum As we go to wavelengths slightly longer than visible (i.e. smaller frequencies and lower energies), we find infrared radiation, which is basically perceived as heat. As we go to longer wavelengths still, we find microwave radiation, which is often used to pop popcorn.

The Electromagnetic Spectrum At the longest wavelengths, corresponding to the smallest frequencies and the lowest energies, we have radio waves, including AM/FM, shortwave, TV, etc.

The Electromagnetic Spectrum Visible light is only a tiny fraction of the Electromagnetic Spectrum. If we go to shorter wavelengths (higher frequencies and energies), we find ultraviolet light. With higher energies, UV photons can damage skin cells.

The Electromagnetic Spectrum As we go even shorter in wavelength (higher in frequency and energy), we get X- rays. With their high energies, X-rays can be used to image our insides.

The Electromagnetic Spectrum As we go even shorter in wavelength (higher in frequency and energy), we get X- rays. With their high energies, X-rays can be used to image our insides. As the shortest wavelengths and the highest energies, we have gamma rays. Gamma rays are sometimes used to sterilize food.

The Electromagnetic Spectrum Visible light is only a tiny fraction of the Electromagnetic Spectrum.

The Electromagnetic Spectrum Gamma rays, X-rays, UV light, visible light, infrared radiation, microwaves, and radio waves are all different manifestations of electromagnetic energy. The range in wavelengths typically encountered span a factor of All forms of electromagnetic radiation travel with the same velocity.

The Earth’s atmosphere is transparent to visible light, some infrared, and the radio. It is opaque to UV, X-rays, and gamma rays.

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