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ASTRO 101. Principles of Astronomy. Instructor: Jerome A. Orosz (rhymes with “ boris ” ) Contact:. Telephone: 594-7118 E-mail: orosz@sciences.sdsu.edu WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ Office: Physics 241, hours T TH 3:30-5:00. Homework/Announcements.
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ASTRO 101 Principles of Astronomy
Instructor: Jerome A. Orosz (rhymes with “boris”)Contact: • Telephone: 594-7118 • E-mail: orosz@sciences.sdsu.edu • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00
Homework/Announcements • Homework due Tuesday, March 5: Question 5, Chapter 4 (Describe four methods for discovering exoplanets)
Homework/Announcements • Homework due Tuesday, March 12: Question 4, Chapter 5 (Why is Earth’s surface not riddled with craters as is that of the Moon?).
Comets! • http://earthsky.org/space/comet-panstarrs-possibly-visible-to-eye-in-march-2013 • Comet PANSTARRS should be viewable in the western skies starting March 7. It will be close to the crescent Moon on March 12. • http://earthsky.org/space/big-sun-diving-comet-ison-might-be-spectacular-in-2013 • Comet ISON might be very spectacular in December, 2013, provided it survives its close encounter with the Sun.
Coming up: • Chapter 5 (The Earth) • Chapter 6 (Other Planets and Moons)
Quick Concept Review • Some useful concepts: • Density • Albedo • Gravity • Atmospheres • Phases of matter • Surface gravities • Escape velocities
Density and Albedo • The concepts of density and albedo are useful in planetary studies. • Density = mass/volume • The density of water is 1 gram per cubic cm. • The density of rock is 3 grams per cubic cm. • The density of lead is 8 grams per cubic cm. • The density of an object can give an indication of its composition.
Density and Albedo • The concepts of density and albedo are useful in planetary studies. • Albedo = % of incident light that is reflected. • A perfect mirror has an albedo of 100% • A black surface has an albedo of 0%. • The albedo of an object is an indication of the surface composition.
Surface Gravities • The surface gravity of a planet is an indication of the force of gravity at the surface. • The surface gravity of a planet depends on its mass and on its radius. http://www.exploratorium.edu/ronh/weight/index.html
Surface Gravities and Escape Velocities • The surface gravity of a planet is an indication of the force of gravity at the surface. • The surface gravity of a planet depends on its mass and on its radius. • The escape velocity of a planet is how fast you have to go to escape the planet’s gravity. • The escape velocity depends on the surface gravity.
Atmospheres • An atmosphere is a thin layer of gas bound to a planet by gravity. • The existence (or lack thereof) an atmosphere has a profound effect on the surface features of a planet: • The nature of a planetary atmosphere (if any) depends on the type of gas and on the size of the planet.
Phases of Matter • Matter has three “phases”
Phases of Matter • Matter has three “phases” • Solid. Constant volume and constant shape. • Liquid. Constant volume but variable shape. • Gas. Variable volume and variable shape.
Phases of Matter • Material can change its phase under certain conditions: • Water turns into ice by lowering the temperature. • Water turns into steam by heating it. • Ice can melt under pressure. This is how an ice skater glides across the ice. • CO2 gas can be made into a solid by cooling it.
The Gas Phase • In a gas, the atoms and/or molecules are widely separated and are moving at high velocities: • Relatively heavy molecules such as CO2 move relatively slowly. • Relatively light molecules like H2 move relatively quickly. • The average velocities of the gas particles depend on the temperature of the gas.
Heating a Gas • The velocity of a gas particle depends on the mass of the particle and its temperature. Image from Nick Strobel (http://www.astronomynotes.com)
Atmospheres • In general, a gas will expand to fill its container. In the case of a planetary atmosphere, gravity is the “container”: • The gas particles will have a characteristic velocity depending on the temperature at the surface of the planet and on the nature of the gas. • Depending on the planet, there is a limit on how fast something can go before it escapes the gravitational pull: remember the escape velocity!
Escape Velocities and Atmospheres • An atmosphere is a thin layer of gas bound to a planet by gravity. The gas particles will have some typical velocity depending on the temperature and type of gas. • The velocity of gas particles can exceed the escape velocity of the smaller planets.
Escape Velocities and Atmospheres • An atmosphere is a thin layer of gas bound to a planet by gravity. The gas particles will have some typical velocity depending on the temperature and type of gas. • The velocity of gas particles can exceed the escape velocity of the smaller planets. This means the planet will not have an atmosphere.
Temperature vs. Gravity • Heavier gasses tend to stay closer to the surface than lighter gasses. • If the gravity is weak, even the heavier gasses may escape. Image from Nick Strobel (http://www.astronomynotes.com)
Planetology • Some things we want to know about a planet: • What are the surface features like? • What is the interior like? • What is the atmosphere like (if any)? • How did it form? • Is there (or was there) life?
Planetology • Some things we want to know about a planet: • What are the surface features like? • What is the interior like? • What is the atmosphere like (if any)? • How did it form? • Is there (or was there) life? • An understanding of other planets may lead to a better understanding of our own Earth.
Next: The Terrestrial Planets
Two Types of Planets • Planets come in two types: • Small and rocky. • Large and gaseous. Or • Terrestrial • Jovian
The Terrestrial Planets • The terrestrial planets are Mercury, Venus, Earth (and Moon), and Mars. • Their densities range from about 3 grams/cc to 5.5 grams/cc, indicating their composition is a combination of metals and rocky material.
The Earth • The place where we keep all of our stuff.
The Earth • The density is 5.5 g/cc, indicating a substantial amount of dense elements, probably iron.
The Earth • The density is 5.5 g/cc, indicating a substantial amount of dense elements, probably iron. • There is a substantial atmosphere (mostly nitrogen), and water in its three phases (liquid, solid, and gas).
The Earth • The density is 5.5 g/cc, indicating a substantial amount of dense elements, probably iron. • There is a substantial atmosphere (mostly nitrogen), and water in its three phases (liquid, solid, and gas). • Owing to erosion, the surface features are relatively young.
The Earth’s Atmosphere • The Earth’s atmosphere is useful in at least three ways: • It keeps the Earth warmer than it would otherwise be. • It keeps most of the harmful UV and X-ray radiation from reaching the ground. • It allows us to breathe.
The Earth’s Atmosphere • The Earth’s atmosphere today consists of mostly nitrogen (N2) and oxygen (O2), plus water vapor (H2O) and carbon dioxide (CO2). • There has been considerable evolution of the atmosphere • The first atmosphere was mostly H and He • The second came from the Earth in the form of CO2, H2O, and N2 • The oceans absorbed much of the CO2, and early life converted it into O2.
The Earth’s Atmosphere • The temperature of the atmosphere has a complex dependence on the height:
The Greenhouse Effect Image from Nick Strobel (http://www.astronomynotes.com)
The Greenhouse Effect • The Sun heats the Earth. Some of the energy is scattered, and some heats the ground and water. • The Earth tries to cool at night, but the atmosphere traps much of the radiation.
The Greenhouse Effect • Because of the Atmosphere, the Earth is about 30 degrees Celsius warmer than it would otherwise be.
The Greenhouse Effect • Because of the Atmosphere, the Earth is about 30 degrees Celsius warmer than it would otherwise be. • During the last 50 years, the concentration of CO2 and other gasses has risen dramatically owing to human activity.
The Greenhouse Effect • The buildup of CO2 is leading to a general warming trend, known as “global warming.”
Global Warming • The concentration of CO2 in the atmosphere has risen steeply in recent years as a result of human activity. • There is substantial evidence of climate change taking place as a result of this increased level of CO2. • The future consequences of this climate change could be dramatic, e.g. a major shift in the ocean currents, melting of polar ice leading to rising sea levels, etc.
The Interior of the Earth • The Earth has a radius of about 6400 km. With current technology, we can drill only a few km deep. Therefore we must use indirect techniques to study the deep interior of the Earth…
The Interior of the Earth • We can use earthquakes to study the interior of the Earth. There are two types of waves: • P-waves, which travel through solids and liquids. • S-waves, which only go through solids. Image from Nick Strobel (http://www.astronomynotes.com)