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

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! Comet PANSTARRS should be viewable in the western skies starting March 7. It will be close to the crescent Moon on March 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.

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” 1.Solid. Constant volume and constant shape. 2.Liquid. Constant volume but variable shape. 3.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.  CO 2 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 CO 2 move relatively slowly. –Relatively light molecules like H 2 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 (

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 (

Planetology Some things we want to know about a planet: 1)What are the surface features like? 2)What is the interior like? 3)What is the atmosphere like (if any)? 4)How did it form? 5)Is there (or was there) life?

Planetology Some things we want to know about a planet: 1)What are the surface features like? 2)What is the interior like? 3)What is the atmosphere like (if any)? 4)How did it form? 5)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: 1)It keeps the Earth warmer than it would otherwise be. 2)It keeps most of the harmful UV and X-ray radiation from reaching the ground. 3)It allows us to breathe.

The Earth’s Atmosphere The Earth’s atmosphere today consists of mostly nitrogen (N 2 ) and oxygen (O 2 ), plus water vapor (H 2 O) and carbon dioxide (CO 2 ). 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 CO 2, H 2 O, and N 2 –The oceans absorbed much of the CO 2, and early life converted it into O 2.

The Earth’s Atmosphere The temperature of the atmosphere has a complex dependence on the height:

The Greenhouse Effect Image from Nick Strobel (

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 CO 2 and other gasses has risen dramatically owing to human activity.

The Greenhouse Effect The buildup of CO 2 is leading to a general warming trend, known as “global warming.”

Global Warming The concentration of CO 2 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 CO 2. 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 (

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 (

The Interior of the Earth There is a solid inner core of iron, surrounded by a liquid iron core, surrounded by the mantle composed of silicates. A thin crust is on the outside.

The Interior of the Earth Natural radioactivity provides the energy source that heats the Earth’s interior. The thin crustal “plates” float on top of the liquid mantle. The motion of the crustal plates (a few cm per century) causes earthquakes and volcanoes.

The Interior of the Earth Convection causes motions of the crustal plates.

The Interior of the Earth Convection causes motions of the crustal plates. The spreading apart of two plates created this mountain range under the Atlantic.

The Interior of the Earth Convection causes motions of the crustal plates. The continental land masses constantly move relative to each other.

The Interior of the Earth Areas near plate boundaries are prone to earthquakes.

The Magnetic Field of the Earth The Earth has a “dipole” magnetic field, much like a bar magnet.

The Magnetic Field of the Earth The Earth has a “dipole” magnetic field, much like a bar magnet. This field helps protect us from the “solar wind”.

The Magnetic Field of the Earth The Earth has a “dipole” magnetic field, much like a bar magnet. This field helps protect us from the “solar wind”. The interaction between the solar wind and the field can produce the northern lights.

Next: The Moon