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

Text:

“Discovering the Essential Universe, Fifth Edition”

by Neil F. Comins

Course WWW Pagehttp://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2012.html

Note the underline: … ast101_fall2012.html …

Also check out Nick Strobel’s Astronomy Notes:

http://www.astronomynotes.com/

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/Announcements

•Homework due Tuesday, October 9: Question 5, Chapter 4 (Describe four methods for discovering exoplanets)

Next:Comparative Planetology

• Outline and introduction to the Solar System

• Planets around other stars

Quick Concept Review

• Some useful concepts:– Density– Albedo

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.

The Planets

• Why solar system planets are special:

The Planets

• Why solar system planets are special: Planets are resolved when seen through

telescopes (i.e. you can see the disk, surface features, etc.).

The Planets

• Why solar system planets are special: Planets are resolved when seen through

telescopes (i.e. you can see the disk, surface features, etc.).

You can also send spacecraft to visit them.

The Planets

• Why solar system planets are special: Planets are resolved when seen through

telescopes (i.e. you can see the disk, surface features, etc.).

You can also send spacecraft to visit them. Stars always appear pointlike, even in the

largest telescopes.

The Planets

• Why solar system planets are special: Planets are resolved when seen through

telescopes (i.e. you can see the disk, surface features, etc.).

You can also send spacecraft to visit them. Stars always appear pointlike, even in the

largest telescopes. Also, they are so far away that we cannot send probes to study them.

The Solar System

• The Solar System refers to the Sun and the surrounding planets, asteroids, comets, etc.

The Solar System

• The Solar System refers to the Sun and the surrounding planets, asteroids, comets, etc.

• Do not confuse “solar system” with “galaxy”:– The solar system is the local collection of planets

around the Sun.– A galaxy is a vast collection of stars, typically a

hundred thousand light years across.

The Solar System Census:

• There were 5 planets known since antiquity:– Mercury – Venus– Mars– Jupiter– Saturn

The Solar System Census:

• There were 5 planets known since antiquity:– Mercury – Venus– Mars– Jupiter– Saturn

• Since the 1600s (Kepler, Galileo, Newton), the Earth was considered a planet as well.

New Members

• Uranus: discovered in 1781 by William Herschel.

New Members

• Uranus: discovered in 1781 by William Herschel.• Neptune: discovered in 1846 by Johann Galle

(based on the predictions of John C. Adams and Urbain Leverrier).

New Members

• Uranus: discovered in 1781 by William Herschel.• Neptune: discovered in 1846 by Johann Galle

(based on the predictions of John C. Adams and Urbain Leverrier).

• Pluto: discovered in 1930 by Clyde Tombaugh.

New Members

• Uranus: discovered in 1781 by William Herschel.• Neptune: discovered in 1846 by Johann Galle

(based on the predictions of John C. Adams and Urbain Leverrier).

• Pluto: discovered in 1930 by Clyde Tombaugh.• Asteroids: thousands, starting in 1801.

New Members

• Uranus: discovered in 1781 by William Herschel.• Neptune: discovered in 1846 by Johann Galle

(based on the predictions of John C. Adams and Urbain Leverrier).

• Pluto: discovered in 1930 by Clyde Tombaugh.• Asteroids: thousands, starting in 1801.• Kuiper Belt Objects: Dozens, starting in the

1980s.

Pluto “Demoted”!• The definition of a “planet” was changed recently:

– Planets: The eight worlds from Mercury to Neptune.

– Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."•

– Small Solar System Bodies: All other objects orbiting the Sun.

http://www.space.com/scienceastronomy/060824_planet_definition.html

The Solar System

• The planets orbit more or less in the same plane in space. Note the orbit of Pluto.

• This view is a nearly edge-on view.

Classifications of Solar System Objects

The Solar System

• The Solar System refers to the Sun and the surrounding planets, asteroids, comets, etc.

• The scale of things:– It takes light about 11 hours to travel across the Solar

system.

The Solar System

• The Solar System refers to the Sun and the surrounding planets, asteroids, comets, etc.

• The scale of things:– It takes light about 11 hours to travel across the Solar

system. This is 0.001265 years.

The Solar System

• The Solar System refers to the Sun and the surrounding planets, asteroids, comets, etc.

• The scale of things:– It takes light about 11 hours to travel across the Solar

system. This is 0.001265 years.– It takes light about 4.3 years to travel from the Sun to

the nearest star.

The Solar System

• The Solar System refers to the Sun and the surrounding planets, asteroids, comets, etc.

• The scale of things:– It takes light about 11 hours to travel across the Solar

system. This is 0.001265 years.– It takes light about 4.3 years to travel from the Sun to

the nearest star.– It takes light about 25,000 years to travel from the

Sun to the center of the Galaxy.

Scale Model Solar System• Most illustrations of the

solar system are not to scale.

Scale Model Solar System• Most illustrations of the

solar system are not to scale.

• Usually, the size of the planets shown is too large.

Scale Model Solar System

• Build your own scale model of the solar system:http://www.exploratorium.edu/ronh/

solar_system/

http://pages.umpi.edu/~nmms/solar/

Scale Model Solar System

• Build your own scale model of the solar system:http://www.exploratorium.edu/ronh/

solar_system/http://www.umpi.maine.edu/info/nmms/solar/index.htm

• Conclusion: the solar system is pretty empty.

Scale Model Solar System

• Most depictions of asteroids in the movies are wrong…

The Scale Model Solar System

• Most depictions of asteroid fields are also not to scale. Image from Star Trek Voyager.

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 Terrestrial Planets

• The terrestrial planets are Mercury, Venus, Earth (and Moon), and Mars.

The Giant Planets

• The giant planets are Jupiter, Saturn, Uranus, and Neptune.

The Giant Planets

• The radii are between about 4 and 11 times that of Earth.

• The masses are between 14 and 318 times that of Earth.

The Giant Planets

• The radii are between about 4 and 11 times that of Earth.

• The masses are between 14 and 318 times that of Earth.

• However, the densities are between 0.7 and 1.8 grams/cc, and the albedos are high.

The Giant Planets

• The radii are between about 4 and 11 times that of Earth.

• The masses are between 14 and 318 times that of Earth.

• However, the densities are between 0.7 and 1.8 grams/cc, and the albedos are high.

• The planets are composed of light elements, mostly hydrogen and helium.

The Gas Giants

• The composition of the giant planets, especially Jupiter, is close to that of the Sun.

• The internal structures of these planets is completely different from that of the Earth. In particular, there is no hard surface.

• These planets are relatively far from the Sun (more than 5 times the Earth-Sun distance), so heating by the Sun is not a big factor.

Next:

• The formation of the Solar System

Star Formation• The starting point is a giant molecular cloud.

The gas is relatively dense and cool, and usually contains dust.

• A typical cloud is several light years across, and can contain up to one million solar masses of material.

• Thousands of clouds are known.

Side Bar: Observing Clouds

• Ways to see gas: By “reflection” of a nearby light source. Blue light

reflects better than red light, so “reflection nebulae” tend to look blue.

By “emission” at discrete wavelengths. A common example is emission in the Balmer-alpha line of hydrogen, which appears red.

Side Bar: Observing Clouds

• Ways to see dust: If the dust is “warm” (a few hundred degrees K) then

it will emit light in the long-wavelength infrared region or in the short-wavelength radio.

Dust will absorb light: blue visible light is highly absorbed; red visible light is less absorbed, and infrared light suffers from relatively little absorption. Dust causes “reddening”.

Giant Molecular Clouds

• This nebula is in the belt of Orion. Dark dust lanes and also glowing gas are evident.

Giant Molecular Clouds

• Interstellar dust makes stars appear redder.

Giant Molecular Clouds

• This images shows dust obscuration, an emission nebula, and a reflection nebula.

Giant Molecular Clouds

• Inside many nebula one finds very dense cores called Bok globules that are ready to collapse…

Gravity and Angular Momentum

• There are two important concepts to keep in mind when considering the fate of giant molecular clouds:– Gravity: pulls things together– Angular momentum: a measure of the spin of

an object or a collection of objects.

Gravity

• There are giant clouds of gas and dust in the galaxy. They are roughly in equilibrium, where gas pressure balances gravity.

Gravity

• There are giant clouds of gas and dust in the galaxy. They are roughly in equilibrium, where gas pressure balances gravity.

• Sometimes, an external disturbance can cause parts of the cloud to move closer together. In this case, the gravitational force may be stronger than the pressure force.

Gravity

• Sometimes, an external disturbance can cause parts of the cloud to move closer together. In this case, the gravitational force may be stronger than the pressure force.

• As more matter is pulled in, the gravitational force increases, resulting in a runaway collapse.

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)

Conservation of Angular Momentum

• If an interstellar cloud has some net rotation, then it cannot collapse to a point.

Conservation of Angular Momentum

• If an interstellar cloud has some net rotation, then it cannot collapse to a point. Instead, the cloud collapses into a disk that is perpendicular to the rotation axis.

Coming up:

• Chapter 5 (The Earth)

• Chapter 6 (Other Planets and Moons)