Transcript here
ASTRO 101 Principles of Astronomy Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: • Telephone: 594-7118 • E-mail: [email protected] • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00 Text: “Discovering the Essential Universe, Fifth Edition” by Neil F. Comins Course WWW Page http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2013.html Note the underline: … ast101_fall2013.html … Also check out Nick Strobel’s Astronomy Notes: http://www.astronomynotes.com/ Homework • Homework due September 19: 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 19. Homework • Go to a planetarium show in PA 209: • • • • • • • • The days and times of the shows will be (all shows last less than 1 hour): Thursday, September 5 : 5 PM Friday, September 6 : 2 PM Monday, September 9 : 1 PM and 5 PM Tuesday, September 10 : 1 PM and 5 PM Wednesday, September 11 : 5 PM Thursday, September 12 : 5 PM Friday, September 13 : 3 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. Coming Up • This week: Chapter 3 (Telescopes and light) • Tuesday, September 24: wrap-up, review • Thursday, September 26: Exam #1 Fall 2013 No appointment needed! Just drop by! Where: Room 215, physics-astronomy building (PA-215). When: All semester long, at the following days and times: • Monday: 12 – 2 PM; 5 – 6 PM • Tuesday: 12 – 2 PM; 5 – 6 PM • Wednesday: 12 – 2 PM; 5 – 6 PM • Thursday: 1 – 2 PM; 3 – 6 PM Isaac Newton (1642-1727) http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Newton.html Isaac Newton (1642-1727) • Isaac Newton was born the year Galileo died. • He was professor of mathematics at Cambridge University in England. (Steven Hawking currently hold’s Newton’s Chair at Cambridge). • He was later the Master of the Mint in London, where first proposed the use of grooved edges on coins to prevent shaving. Isaac Newton (1642-1727) • Newton was perhaps the greatest scientist of all time, making substantial contributions to physics, mathematics (he invented calculus as a college student), optics, and chemistry. • His laws of motion and of gravity could explain Kepler’s Laws of planetary motion. Newton’s Laws of Motion 1. 2. 3. A body in motion tends to stay in motion in a straight line unless acted upon by an external force. The force on an object is the mass times the acceleration (F=ma). For every action, there is an equal and opposite reaction. (For example, a rocket is propelled by expelling hot gas from its thrusters). What is Gravity? • Gravity is a force between all matter in the Universe. • It is difficult to say what gravity is. However, we can describe how it works. What is Gravity? • The gravitational force between larger bodies is greater than it is between smaller bodies, for a fixed distance. What is Gravity? • As two bodies move further apart, the gravitational force decreases. The range of the force is infinite, although it is very small at very large distances. Newton’s Laws • Using Newton’s Laws, we can… Derive Kepler’s Three Laws. Measure the mass of the Sun, the Moon, and the Planets. Measure the masses of distant stars in binary systems. Laws of Physics • The models of Aristotle and Ptolomy were based mainly on beliefs (i.e. that motion should be on perfect circles, etc.). • Starting with Newton, we had a physical model of how the planets moved: the laws of motion and gravity as observed on Earth give a model for how the planets move. • All modern models in Astronomy are based on the laws of Physics. Newton’s Laws and Orbits • Newton realized that since the Moon’s path is curved (i.e. it is accelerating), there must be a force acting on it. Newton’s Laws and Orbits • If you shoot a cannonball horizontally, it follows a curved path to the ground. The faster you launch it, the further it goes. Newton’s Laws and Orbits • If you shoot a cannonball horizontally, it follows a curved path to the ground. The faster you launch it, the further it goes. • If it goes really far, the Earth curves from under it… Newton’s Laws and Orbits • Why doesn’t the Moon fall down on the Earth? Newton’s Laws and Orbits • Why doesn’t the Moon fall down on the Earth? • Because of angular momentum… Angular Momentum • Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin. • I = (mass).(radius).(spin rate) Angular Momentum • Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin. • For a given distance to the rotation axis, more mass means more angular momentum. Angular Momentum • Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin. • For a given distance to the rotation axis, more mass means more angular momentum. • For a given mass, a larger distance means more angular momentum. Angular Momentum • Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin. • For a fixed mass and distance, a higher rate of spin means a larger angular momentum. Angular Momentum • Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin. • I = (mass).(radius).(spin rate) • The angular momentum in a system stays fixed, unless acted on by an outside force. Conservation of Angular Momentum • An ice skater demonstrates the conservation of angular momentum: Conservation of Angular Momentum • An ice skater demonstrates the conservation of angular momentum: • Arms held in: high rate of spin. • Arms extended: low rate of spin. • I = (mass).(radius).(spin rate) (angular momentum and mass are fixed here) Angular Momentum • Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin. • I = (mass).(radius).(spin rate) • The angular momentum in a system stays fixed, unless acted on by an outside force. • An orbiting body will not move towards the other body unless there is an external force. Weight and Mass Weight and Mass • In Physics, we distinguish between weight and mass: Weight and Mass • In Physics, we distinguish between weight and mass: Weight is a force due to gravity. Weight and Mass • In Physics, we distinguish between weight and mass: Weight is a force due to gravity. Mass is a measure of the amount of matter in an object. Weight and Mass • In Physics, we distinguish between weight and mass: Weight is a force due to gravity. Mass is a measure of the amount of matter in an object. The units of weight are pounds in the British system or newtons in the metric system. Weight and Mass • In Physics, we distinguish between weight and mass: Weight is a force due to gravity. Mass is a measure of the amount of matter in an object. The units of weight are pounds in the British system or newtons in the metric system. The units of mass are stones in the British system or kilograms in the metric system. Weight and Mass • Your weight depends where you are (e.g. on the Earth, on the Moon, in outer space, etc.). • Your mass is the same no matter where you are. • In most cases on Earth, we can use the terms weight and mass interchangeably. Weight and Mass • The mass is used in Newton’s Gravity formula: 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. 2. 3. 4. Gravity: relative strength = 1, range = infinite. Electromagnetic: rel. str. = 1036, range = infinite. “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. “Strong” nuclear: rel. str. = 1038, range = 10-15 meter. The 4 “Forces” of Nature • There are 4 “fundamental forces” in nature: 1. 2. 3. 4. • Gravity: relative strength = 1, range = infinite. Electromagnetic: rel. str. = 1036, range = infinite. “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. “Strong” nuclear: rel. str. = 1038, range = 10-15 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. 2. 3. 4. • Gravity: relative strength = 1, range = infinite. Electromagnetic: rel. str. = 1036, range = infinite. “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. “Strong” nuclear: rel. str. = 1038, range = 10-15 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. 2. 3. 4. • • Gravity: relative strength = 1, range = infinite. Electromagnetic: rel. str. = 1036, range = infinite. “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. “Strong” nuclear: rel. str. = 1038, range = 10-15 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. 2. 3. 4. • Gravity: relative strength = 1, range = infinite. Electromagnetic: rel. str. = 1036, range = infinite. “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. “Strong” nuclear: rel. str. = 1038, range = 10-15 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=mc2. 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? Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Light can be thought of as a wave. The wavelength (usually denoted with a l) is the distance from crest to crest. What is the nature of light? Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Light can be thought of as a wave. The frequency (usually denoted with n) is the number of crests that pass a given point each second. What is the nature of light? Light can be thought of as a wave. The frequency (usually denoted with n) 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? Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) The above animation shows waves with different wavelengths moving with the same speed. Their frequencies are different. 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? Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) The above animation shows waves with different wavelengths moving with the same speed. Their frequencies are different. 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 (http://www.astronomynotes.com) 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 Xrays. 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 Xrays. 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 1014. • 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