Download Topic 3: Astronomy

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Topic 3
Earth in the Universe
rotation: the turning of an object on its axis
revolution: the movement of a body in orbit around an object
 Models of the Universe
Geocentric (“Earth-centered”) models proposed by Aristotle, Ptolemy
- the Earth is located at the center of the universe and does not move
- the stars are fixed on a transparent sphere that rotates once each day
- the Sun, Moon, and planets are carried on separate spheres which also rotate
- this model explains the general features of the apparent motions of the stars, Sun, Moon, and planets,
but does not allow for accurate predictions or apparent changes in size and speed, or apparent backward
motion of the planets, or terrestrial observations
Heliocentric (“Sun-centered”) model proposed by Copernicus
- the Sun is located at the center of the solar system and does not move
- the stars are fixed onto an unmoving sphere
- the planets, including Earth, move in circles around the Sun
- the Moon moves in a circle around the Earth
- the Earth rotates on its axis once each day
- this model explains most observations except the apparent cyclic variations in size and speed of orbiting
bodies
Improving the Heliocentric Model -- Kepler, Galileo, Newton
Galileo saw Venus experience phases like the Moon, and saw bodies like our Moon orbiting Jupiter, which
supported the heliocentric theory
Kepler’s Laws:
1. The orbit of every planet is an ellipse with the Sun at one focus
ellipse: a flattened circle with 2 foci (focal points)
eccentricity: amount of flattening of an ellipse
focal points
perihelion
aphelion
distance between foci
eccentricity = -----------------------------length of major axis
major axis
2. Planets move faster when they’re closer to the Sun (at perihelion) and slower when they are farthest
away (at aphelion)
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3. The farther away a planet’s orbit is from the Sun, the longer its period of revolution
Isaac Newton’s Law of Universal Gravitation: The force of attraction between two objects is greater
with increased masses of the objects and greater with decreased distance between them.
 Terrestrial Observations
1. Foucault Pendulum
- a free swinging pendulum that changes the direction of its swing, 15 for every hour
2. Coriolis Effect
- the apparent deflection of free-moving objects (fluids) over Earth’s surface, due to the different
rates at which the Earth rotates
Northern Hemisphere -- deflection is to the right of intended path
Southern Hemisphere -- deflection is to the left of intended path
 The Solar System
[A lot of details and statistics are available on the ESRT]
Terrestrial / Inner Planets -- close to the Sun, mostly solid, dense, small, few or no moons, no rings
 Mercury
- difficult to see in the sky because it’s so close to the Sun
- extreme temperatures
- similar to our Moon -- no atmosphere, lots of surface craters
 Venus
- brightest object in nighttime sky, appears green
- rotates in the opposite direction of most planets
- revolves faster than it rotates
- dense atmosphere of CO2 and acids produce extreme greenhouse effect for constant high temperatures
 Earth
- rich nitrogen and oxygen atmosphere
- liquid water on surface
- one moon
 Mars
- appears as a red star in nighttime sky
- believed to have once had water
- surface has craters, largest volcano in solar system, dried up river beds
- very thin atmosphere
- two small moons
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Jovian / Outer Planets / Gas Giants -- far from the Sun, mostly gaseous, more massive but less dense,
most have rings and many moons
 Jupiter
- largest planet, one faint ring
- rotates the fastest
- great red spot believed to be a huge storm
 Saturn
- lowest density
- largest planet with thickest rings
- one of its many moons (Titan) has a nitrogen atmosphere and is being studied extensively
 Uranus
- tilt is almost equal to the plane of its orbit
- can’t be seen with the naked eye
 Neptune
- methane atmosphere
- structurally similar to Uranus
- sometimes the farthest planet from the Sun
 Pluto
- smallest planet
- more “terrestrial-like” (rocky, solid)
- one very large moon
- most eccentric planetary orbit, so it’s not always the farthest planet
- many scientists still debate if Pluto should be classified as a planet or an asteroid
 Interplanetary Bodies
 Asteroids -- solid bodies having no atmosphere with a well-determined orbit. Many are located in a belt
between Mars and Jupiter
 Meteoroids -- chunks of matter that circle the Sun in orbits that cross planetary orbits; could be from
broken up asteroids or comets
- meteor: streak of light produced if the chunk enters Earth’s atmosphere (also called “fireballs”,
“shooting stars”)
- meteorite: any chunk that hits the Earth. These tells us a lot about the composition of Earth and the
solar system
- meteor showers occur whenever Earth crosses the path of a clump of meteoroids
 Comets -- solid chunks of debris and ice with highly elongated elliptical orbits. Many come from beyond
Pluto in the Oort Cloud
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 Stars
- constellation: a group of stars that appears to form a pattern that looks like a familiar object or
character
- asterism: a distinct star group that is part of a constellation (EX/ the Big Dipper is an asterism that is
part of a larger constellation called Ursa Major)
- as Earth rotates, constellations appear to circle Polaris, moving east to west at 15/hr
- stars that never set below the horizon (this depends on your latitude) are called circumpolar stars
Brightness of Stars
- absolute magnitude: the amount of light actually given off by a star (luminosity)
- apparent magnitude: the amount of light from a star received on Earth, which depends on luminosity and
distance
Distance to Stars
- light year: the distance light travels in one year; 9.5 trillion km (light travels 300,000 km/sec)
- our nearest star, after the Sun, is 4.2 LY away
Star Classification
- based on temperature and brightness using the H-R Diagram [see ESRT]
- the color of a star depends on its surface temperature
hot stars = blue = 30,000 C
cool stars = red = 3,000 C
our Sun = yellow = 5,500 C
Star Energy
- nuclear fusion: four hydrogen atoms fuse together to form one helium atom, giving off huge amounts of
energy in the process. This is continually happening in the Sun.
Star Evolution & Origin
- Stars begin as a large cloud of gas and dust called a nebula, which contracts due to gravity when it gets
large enough. As temperatures increase, nuclear fusion begins and light is given off. Now it’s a star.
- Most of its life is spent as a main sequence star -- an average size, average temperature star -- using
up its hydrogen fuel. Then the outer layers expand and it becomes a giant (or, if massive enough, a
supergiant)
- If the original mass was low, a hot dense core will be left behind called a white dwarf, then slowly die
out to become a black dwarf.
- If the original mass is large, it will explode into a supernova and then rapidly collapse, forming either a
neutron star or a black hole
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 Galaxies
- galaxy: a large group of stars, gas, and dust held together by gravity
- three basic galaxy shapes: spiral, elliptical, and irregular
- galaxies are grouped into clusters, clusters are grouped into superclusters
- our galaxy, the Milky Way, is part of the Local Group cluster
 about 200 billion stars in the Milky Way
 spiral-shaped with the Sun in one of the spiral arms, about 30,000 LY from center
 about 100,000 LY in diameter
 all stars orbit the center (our Sun has a 200 million year orbit)
 The Universe
cosmology: the study of the origin, evolution, and fate of the universe
observations indicate that the universe is expanding
Doppler effect: a shift in wavelength caused by the motion of an observer toward or away from a source
 If the source is moving toward the observer, the result is a blue shift
 If the source is moving away from the observer, the result is a red shift
The Big Bang Theory
Approximately 15 billion years ago, the universe began expanding out of an enormous explosion, forming
hydrogen and helium gases as matter cooled and then began collecting into clumps which formed galaxies,
which then formed individual solar systems.
The Search for Extra-Terrestrial Intelligence
There are three basic limitations to space travel:
1) distance -- the closest star to our solar system is 4.2 LY away and scientists don’t think it has planets
2) speed -- many physicists believe we can’t go faster than light speed (so even if we could go as fast as
light speed, it would take more than four years to get to the closest star)
3) fuel -- if we could go that fast, could we afford the fuel that would be necessary for such a long
journey and the return trip home?
Radio transmissions have been sent from Earth since 1974 with a message for anyone “listening”
Several projects (including SETI) have been listening for similar signals being sent to Earth from space