Star: huge ball of gas held together by gravity
Generates light, heat, energy, through nuclear reactions
Source of energy
4 billion years old
100x wider than the earth, 1,000,000x larger, 300,000x as massive
1 AU = average distance between the Earth and Sun
≈ 150 million km ≈ 93 million miles
Light Year → distance
(ly) 10 trillion km in one year
“look-back” time; it‟s how far back in time we are seeing
The Celestial Sphere
• Model of the sky
• Zenith: the point of the sky that is straight overhead.
• Nadir: the point on the sphere directly below you and opposite the zenith.
• Celestial Poles” two points that do not move - lie exactly above the North and South oles of the Earth.
• Polaris: “North Star” - lies close to the direction of true north.
• Celestial Equator: lies directly above the Earth’s equator.
• Stars on the celestial equator rise due east and set due west.
• The pattern of stars is NOT a constellation - asterisms
• 88 constellations
• Constellations - stars
• Celestial Sphere divided by: Lines of declination (East → West) and
Lines of ascension (North → South)
• Astronomy → study of the universe
• Planet → Earth → size defined the fundamental unit of the metric
system: the meter
• Moon = Earth’s satellite
• 1/25 the size of Earth
• Surface = craters
• Less internal heat than Earth
• ✷No atmosphere → not protected from impacting objects
• ↳ no erosion or geological activity = no surface change ever Planets
• Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus →
• Merc: craters, airless surface
• Ven: clouds of sulfuric acid droplets
• Ear: white clouds, blue oceans, green jungles, red deserts
• Mar: canyons, deserts
• Jup: atmospheric storms with lightning
• Sat: icy fragments orbit, form bright rings
• Ura: tipped axis, blue
• Nep: methane clouds, deep blue
Beyond the Solar System
• Gravity: a force that attracts every object toward every other object
• Milky Way Galaxy: cloud of several hundred billion stars with a flattened
• 100x the diameter of the solar system.
• 2 largest galaxy in the Local Group after Andromeda Galaxy
• ↑ • Collection of galaxies
• Dark Energy: all-persuasive energy
The Light Year (ly)
Light Year → distance
(ly) 10 trillion km in one year
“look-back” time; it‟s how far back in time we are seeing
The Parsec (pc)
Distance to an object whose parallax motion measures 1 sec of arc in 6 months of
observation time on Earth.
1 pc = 3.26 ly
Parallax - apparent shift in position of some object because of change in position of
General procedures scientists use to construct their ideas about how nature (everything)
↓ Scientific Theory/ Model/ Law
Explains observable things
Must be testable & disprovable
Concerns observable and measurable things
Protons (positively charged) and neutrons (uncharged) make up the nucleus at the center
of an atom.
Electrons (negatively charged particles) are found relatively far from the nucleus.
Forces in Nature
Force between objects with mass
Infinite in range, but weakens with distance
Force between charged bodies
Infinite in range, but weakens with distance
Like charges repel
Holds atomic nuclei together
Very short range - 10^-15 meters
Short range Annual Motion of the Sun
As the Earth revolves around (orbits) the Sun, the Sun appears to move through 13
constellations on a belt around the Celestial Sphere called the Ecliptic.
When the sun in the sky is in “front” of a particular constellation, we say that the Sun is
“in” that constellation.
The Ecliptic “belt” on the Celestial Sphere is tipped relative to the Celestial Equator due
to the 23.5∘inclination of the Earth‟s rotational axis.
Analemma - pattern formed by taking a picture of the Sun at the same time on different
days of the year.
The Earth‟s inclination is ultimately responsible for the change in seasons.
The Sun‟s gravity adds a pull that causes it to “wobble”
Wobble means the axis of the Earth is rotating or precessing with a 26,000 year period
Nutation - extra wobble
Pull by moon
Because of this precession, Polaris has not always been and will not always be “The North Star”
One Solar Day is 24 hours - takes 24 hours for the Sun to return to the same spot
overhead on successive days.
Our clocks are based on the Solar Day
One Sidereal Day is 23 hours and 56 minutes
Phases of the Moon
29.5 day cycle
Half of it‟s surface is always lit
We (n Earth) only see the illuminated portion
Full Moon: Earth is between the Moon & Sun ← see illuminated side
New Moon: Moon is between Earth & Sun ← see none of illuminated
Harvest Moon: first full moon following the equinox
Waxing - brighter
Waning - darker
Same side of the moon always faces us thanks to Tidal Lock:
Tidal locking (or captured rotation) occurs when the gravitational gradient makes one
side of an astronomical body always face another, an effect known as synchronous
rotation. For example, the same side of the Earth's Moon always faces the Earth. A
tidally locked body takes just as long to rotate around its own axis as it does to revolve
around its partner. This causes one hemisphere constantly to face the partner body.
Usually, at any given time only the satellite is tidally locked around the larger body, but if the difference in mass between the two bodies and their physical separation is small, each
may be tidally locked to the other, as is the case between Pluto and Charon. This effect is
employed to stabilize some artificial satellites.
Solar Eclipses: At new moon, the moon is between the Earth and the Sun. Sometimes, the
alignment is just right allowing the Moon to block the light from the Sun on the surface
of the Earth → Solar Eclipse
Umbra: darkest part of the shadow, directly behind the body of the Moon.
Penumbra: part of the shadow where the light from the Sun is only partially blocked
Lunar Eclipse:As the moon passes behind the Earth relative to the Sun, the Earth can cast
a shadow on the surface of the Moon.
Midterm 7:00-8:20 → Gilbert 224
Kepler‟s First Law
Planets move in Elliptical orbits with the Sun at one focus of the Ellipse
Sun-centered model (heliocentric)
Shape of the Ellipse is described by it‟s semi-major and semi-minor axis
Elliptical is more likely Kepler‟s Second Law
Orbital speed of a planet varies so that a line joining the Sun and planet will sweep out
equal areas in equal time intervals.
Planets move faster when near the Sun and slower when farther from the Sun.
Kepler‟s Third Law
The amount of time a planet takes to orbit the Sun (it‟s period) P is related to its orbit‟s
size, a, by P² = a³
Describes the shape of a planets orbit, it‟s orbital period, and how far from the Sun it is.
Galileo did not invent telescope - improved it
Newton discovered laws that govern the motion of all (large) bodies
Invented calculus to do it
Vector - a physical quantity that requires both a magnitude (size) and a direction to
fully define it.
Scaler - a physical quantity that can be fully defined by magnitude (size) only.
A state of free fall is equivalent to a state of weightlessness.
Newton‟s Law of Gravitation - Every mass exerts a force of attraction on every other mass.
Larger masses have larger gravity.
Objects close together pull more on each other than objects farther apart.
For planets, the accelerating (2 Law) force that keeps the planets moving on an elliptical
path is the gravitational force.
Moon exerts a gravitational force on the Earth, and on the water in the oceans.
The effect of the various forces acting on the ocean‟s water causes the water to create
tidal bulges both beneath the Moon and on the opposite side of the Earth (from the Moon)
at the same time.
When the Sun and the Moon line up, higher tides, called “spring tides” are formed.
When the Sun and the Moon are at right angles to each other smaller “neap tides” result.
Nothing to do with seasons
Law of Conservation of Energy
The energy in a closed system may change form, but the total amount of energy does not
change as a result of any process.
Energy can be neither created nor destroyed but only changed in form.
The energy of motion
Mass and speed contribute to it. Thermal Energy
Energy associated with heat.
Is really just a form of kinetic energy.
Stored energy - energy due to position
Gravitational potential energy - the energy an object has due to its position in a
GPE is transformed when the object is allowed to fall, becoming the kinetic energy of the
Linear Momentum: p = m x V. If no external forces are acting on an object, then its
linear momentum is conserved.
Angular Momentum: the rotational equivalent of linear momentum.
If no external forces (torques) are actin on an object, then its angular momentum is
L = m x V x r = constant
Ptolmey Opposition of mars
Phases of the Moon
Primary tool in learning about the universe is from the electromagnetic radiation - light.
Is radiant energy
Travels very fast - 300,000 km/sec, 186,000 miles/sec
Has dual nature - can be described either as a wave or as a particle traveling through
Has 0 mass
As a wave:
A disturbance in an electric field creates a magnetic field, which in turn creates an
electric field, and so on, a self-propagating electromagnetic wave.
Light waves can constructively or destructively interfere
Color of light is determined by its frequency
The energy is also determined by frequency
As a particle:
Particles of light (photons) travel through space
These photons have very specific energies, that is, light is quantized.
Photons strike your eye (or other sensors) like very small massless “balls” (maybe bb‟s),
and are detected.
Light as a Wave Versus Mechanical Waves
Wave - transfer of energy without the transfer of matter. Wave phenomena - refraction, diffraction, constructive and destructive interference,
super positioning, Doppler shift.
Measurable wave characteristics - amplitude, wavelength, frequency, period.
Mechanical Waves - water, sound - must have some physical matter - a medium - in
which to exist and travel.
Light exhibits all wave phenomena and has all the measurable wave characteristics (as a
BUT, light does not require any physical matter for its transfer. Light can exist and travel
through the vacuum of space.
Colors are determined by wavelength of light
Wavelength is the distance between successive crests (or troughs) in an electromagnetic
Similar to the distance between the crests in ocean waves.
How fast successive crests pas by a given point.
Frequency has units of Hz (hertz) and is symbol v. 1 Hz = 1 cycle/sec.
Long wavelength light has a low frequency, and short wavelength light has a high
Lambda x v = c
„c‟ is the speed of light. White Light
Light from the Sun arrives with nearly all wavelengths, and we perceive this mixture of
colors as white.
Is all the colors
The Electromagnetic Spectrum I
There is much more to light than just visible light.
Radio waves ave very long wavelengths, as much as a meter and more.
Microwaves are at the upper end of the radio part of the spectrum.
Infrared wavelengths are longer in wavelength than visible light.
The Electromagnetic Spectrum II
Ultraviolet waves are shorter in wavelength than visible waves.
X-rays come mostly from stellar sources in nature, and can penetrate many materials, like
skin, muscle and bone.
Gamma rays have the shortest wavelengths.
Energy Carried by Photons
A photon carries energy with it that is related to its wavelength or frequency:
E = h x c = h x v
Long wavelength (low frequency) photons carry less energy than short wavelength (high
frequency) ones. This is why UV waves give us a sunburn, and X-rays let us look through skin and muscles.
The Nature of Matter
An atom has a nucleus at its center containing protons and neutrons.
Outside of the nucleus, electrons move in “clouds” called orbitals.
Orbitals are quantized - exist only at very specific energies.
Lowest energy orbital is called the ground state.
To move an electron from one orbital to the next higher one, a specific amount of energy
must be added. Likewise, a specific amount of energy must be released for an electron to
move to a lower orbital.
The are called electronic transitions.
The Chemical Elements
The number of protons (atomic number) in a nucleus determines what element a
Atom that is neutrally charged has a number of electrons equal to the number of protons.
Electron orbitals are different for each element, and the energy differences between the
orbitals are unique as well.
Means that if we can detect the energy emitted or absorbed by an atom during an
electronic transition, we can tell what element the atom belongs to, even from millions of
light years away!
If a photon of exactly the right energy (equal to the energy difference between orbitals) strikes an electron, that electron will absorb the photon and move into the higher orbital.
The atom is now in an excited state.
If the photon energy doesn‟t match any of the orbital - energy differences it can not be
absorbed – it will pass through. We say the element is transparent to those frequencies or
If the electron gains enough energy to leave the atom entirely, we say the atom is now
ionized, or is an ion.
If an electron drops from one orbital to a lower one, it must emit a photon with the same
amount of energy as the orbital-energy difference.
•This is called emission.
Imagine that we have hot hydrogen gas. Collisions among the hydrogen atoms cause
electrons to jump up to higher orbitals, or energy levels.
Electrons can jump back to lower levels, and emit a photon of energy h x f.
If the electron falls from orbital 3 to orbital 2, the emitted photon will have a wavelength
of 656 nm.
If the electron falls from orbital 4 to orbital 2, the emitted photon will have a wavelength
of 486 nm.
We can monitor the light emitted, and measure the amount of light of each wavelength
we see. Seeing Spectra
Seeing the Sun‟s spectrum is not difficult. A narrow slit only lets a little light pass.
Either a grating or a prism splits the light into its component colors.
If we look closely at the spectrum, we can see dark lines. These correspond to
wavelengths of light that were absorbed.
Emission Spectrum of Hydrogen
This spectrum is unique to hydrogen.
Different Atom, Different Spectrum!
Every element has its own spectrum.
Spectrum is like a chemical fingerprint!
What if we had a cloud of cool hydrogen gas between us and a star?
Photons of energies that correspond to the electronic transitions in hydrogen will be
absorbed by electrons in the gas.
The light from those photons is effectively removed from the spectrum.
The spectrum will have dark lines where the missing light would be.
This is an absorption spectrum.
Also unique for each element.
Types of Spectra - Summary If the source emits light that is continuous, and all colors are present we say that this is a
If the molecules in the gas are well-separated and moving rapidly (have a high
temperature), the atoms will emit characteristic frequencies of light. This is an emission-
If the molecules of gas are well-separated, but cool, they will absorb light of a
characteristic frequency as it passes through. This is an absorption line spectrum.
It is useful to think of temperature in a slightly different way than we are accustomed to.
Temperature is a measure of the motion of atoms in an object.
Objects with low temperatures have atoms that are not moving much.
Objects with high temperatures have atoms that are moving around very rapidly.
The Blackbody Spectrum
As an object (piece of iron for example, or the gas in a star) is heated, the atoms in it start
to move faster and faster.
When they collide, they emit photons with energy proportional to how hard they hit.
Collide lightly - produce long-wavelength radiation.
Collide very hard - produce short-wavelength radiation.
Most are somewhere in between.
Results of More Collisions
Additional collisions mean that more photons are emitted, so the object gets brighter. Additional hard collisions means that more photons of higher energy are emitted, so the
object appears to shift in color from red, to orange, to yellow, and so on.
Of course we have physical laws to describe these effects.
Hotter bodies emit more strongly at shorter wavelengths. The hotter it is, the shorter the
Lets us estimate the temperature of stars easily and fairly accurately.
Th luminosity of a hot body rises rapidly with temperature.
If we know an object‟s temperature (T), we can calculate how much energy the object is
emitting using the SB law.
Distance on Light
Light from a distant source seems very dim
Light is spreading out as it travels from its source to its destination.
The farther from the source you are, the dimmer the light seems.
The objects brightness, or amount of light received from source, decreases with increased
distance. The relationship is mathematical.
Brightness = Total Light Output
This is an inverse-square law - the brightness decreases as the square of the distance (d)
from the source. Doppler Shift in Sound
As a car approaches, the sound from a car seems to have a higher pitch - this is due to
As the car passes, the sound shifts to lower pitch due to the longer wavelengths.
Police radar guns work on the same principle. The waves reflected off the car will be
shifted by an amount that corresponds to the car‟s speed.
Doppler Shift in Light
If an object emitting light is moving toward you, the light you see will be shifted to
shorter wavelengths - toward the blue end of the spectrum. The light is blue-shifted.
If an object is moving away from you, the light will be red-shifted.
If we detect a wavelength shift of Δλ away from the expected wavelength λ, the radial
(line-of-sight) velocity of the object is:
Doppler Shift in Space
When we look at most every object in deep space,we see that the light from those objects
That means that most everything is moving away from us (and everything else too).
Therefore the universe is EXPANDING!
Components of the Solar System
Vast majority of the Solar System‟s mass resides in the Sun.
The rocky inner planets (Mercury, Venus, Earth and Mars) are called the terrestrial planets.
The gaseous outer planets (Jupiter, Saturn, Uranus and Neptune) are the Jovian planets.
An asteroid belt lies between the inner and outer planets.
The outer most icy planet, Pluto, is in a class called Trans-Neptunian Objects (TNO). It‟s
a dwarf planet.
The Kuiper Belt
Outside the orbit of Neptune lies the Kuiper Belt.
Located about 40 AU from the Sun.
Home of TNO‟s
Hundreds of objects smaller and larger than Pluto have been found here.
How to Be a Planet
Once upon a time, be a wanderer in the night sky.
When Ceres and Vesta (asteroids) were discovered, we had 10 planets. Pluto made 11.
Ceres and Vesta were demoted to asteroids.
Be massive enough that your own gravity pulls you into a spheroid shape.
Be the dominant mass in your orbital neighborhood.
Pluto makes the cut in the first category but not the second.
Meet the first criterion, you can be a dwarf planet.
The Oort Cloud
The Solar System is surrounded by a cloud of cometary bodies. Located about 50,000 AU from the Sun, out to perhaps a light-year.
Gravitational influences from passing stars occasionally send comets into the Solar
Possibly containing more than a trillion comet-like bodies.
Rotation and Revolution in the Solar System
Because of the conservation of angular momentum, all planets revolve around the Sun in
the same direction and in more or less the same plane.
Mercury‟s orbit is tipped by 7 degrees.
Pluto‟s is tipped by 17 degrees.
Most of the planets rotate in the same direction.
Counterclockwise as viewed from above.
Venus rotates clockwise as viewed from above.
Uranus and Pluto‟s rotational axes are tipped significantly.
Composition of the Solar System Objects
Spectrum analysis shows us the Sun is 71% hydrogen, 27% helium, 2% everything else.
Jovian planets have similar composition to our Sun although so cold that the matter is
liquefied or frozen. Also, frozen methane, ammonia, and water.
Inner planets are rocky, silicon oxide, iron, aluminum, etc.
Spectroscopy tells us surface composition.
Calculating a Planet‟s Density
Calculate the planet‟s mass (M) by observing its satellite‟s orbital distance (d) and period (P).
Uses Newtons modified form of Kepler‟s 3 law:
Average Density tells us a lot
Inner planets have high average densities (~5 kg/liter)
Mostly rock and iron
Outer planets have lower densities (~1 kg/liter)
Gasses, ices and other volatile gasses
The Role of Mass and Radius
Mass and size of a planet help determine its environment.
Small planets cooled quickly, leading to dead worlds with little seismic activity.
Small planets also have trouble holding an atmosphere. (low gravity).
Larger planets hold on to their heat, and have active interiors and surfaces.
Mars is right in the middle, not too large, and not too small.
Once had water and an active surface.
Now is cold and dead.
The Role of Water and Biological Processes
The presence or absence of water helps determine the nature of the atmosphere.
Water acts as a sink for carbon dioxide, removing it from the atmosphere.
Water helps lock CO2 into rocks as well. Too much CO2 can lead to a runaway greenhouse effect (as with Venus).
Too little CO2 can lead to cooling (as on Mars).
Biological activity impacts the environment, too.
Animals remove oxygen from the atmosphere (and get carbon from plants), and release
CO2 (and methane).
Plants remove CO2 from the atmosphere, and with sunlight and water, convert it into our
food, and release oxygen.
Burning (wood, fossil fuels) releases CO2 into the air.
The Role of Sunlight
A planet‟s distance form the Sun determines how much sunlight it receives.
Venus receives ¼ of the energy per square meter that Mercury does.
Planets in eccentric orbits receive varying amounts of sunlight.
The axial tilt of a planet determines its seasons.
Sunlight warms a planet, but the atmosphere has an impact, too
Venus‟s atmosphere warms the surface to 750 K, but it would be very warm even without
Mercury is closer to the Sun, but still cooler than Venus.
The Moon is cooer than the Earth, even though they are at the same distance from the
Sunlight also determines the makeup fo the planets.
Inner planets are rocky. (iron)
Outer planets are gaseous. The Outer Planets
Far from the Sun, temperatures are cold enough that water vapor can condense into ice.
Beyond the frost line, planets are primarily composed of hydrogen and various ice.
The low temperatures allowed the outer planets to capture hydrogen and helium gas, and
to grow to immense sizes.
High Rotational Speed
The outer planets rotate much faster than the terrestrial planets. These high rotational
Make the outer planets much wider at the equator.
Create dramatic Coriolis Effects.
Create strong magnetic fields.
All the gas giants have a ring system
Some are easy to see like Saturn‟s
Others are faint and not visible form Earth.
The gas giants have many more moons
Due to rotation of planet (or moon, or sun).
Due to differential rotation speed at different latitudes. E.g. Higher latitudes are moving
at slower speed.
Causes counterclockwise rotation of weather systems in Northern Hemisphere (incl. Tornadoes, hurricanes)
Coriolis Forces effect aircraft and long-range projectiles (missiles, cannon shells.)
An insignificant effect on a small system like the water in your tub or toilet. Any
“common” demo of such is faked.
Rapid rotation gives rise to strong Coriolis forces, and very high winds.
Measured max wind speeds of 500 km/hr at Jupiter, and faster at Saturn.
Bands of clouds move in opposite directions, creating very large wind shears, vortexes,
The Great Red Spot
On Jupiter, these wind shears give rise to enormous vortices, or storms, seen as white,
brown or red ovals in its clouds.
The Great Red Spot on Jupiter is one such vortex
Has lasted for at least 300 years
Storms on Saturn
Has turbulent storms as well as Jupiter.
Higher wind speeds than Jupiter
Storms are deeper in its atmosphere.
The liquid metallic hydrogen in Jupiter and Saturn can carry electrical currents, similar to the liquid core of the Earth.
These currents generate very large magnetic fields.
Jupiter‟s is 20,000 times as strong as Earth‟s, and if it were visible, would appear larger
than the full Moon in our sky.
Saturn‟s field is 500 times as strong as Earth‟s.
Both Jupiter and Saturn experience auroras (like the Northern or Southern Lights on
Discovery of Uranus
1782 discovered by W. Herschel
Originally thought to be a comet
Discovery of Neptune
Uranus was not following its calculated orbit.
Another planet, further out, must be effecting its orbit.
Scientists calculated where the unseen planet should be. (1840‟s)
Astronomers searched in this location and found Neptune.
Others (Galileo) had seen Neptune but didn‟t realize they were seeing a (new) planet.
As of 2011, Neptune has made ONE orbit since it was discovered in 1846.
The Atmospheres of Uranus and Neptune
The atmospheres of both planets are rich in hydrogen and helium.
Both have larger amounts of methane, giving hem their blue color.
Methane crystals scatter blue light, and methane gas absorbs red light. Their interiors are believed to be ordinary water mixed with methane and ammonia,
surrounds a core of rock and iron-rich material.
Rotational speeds create similar atmospheric conditions - storms, winds - as on Jupiter
Uranus: 1.3 kg/liter
Neptune: 1.6 kg/liter
Both planets are very cold
Uranus‟s Axial Tilt
Uranus is tipped almost 90 degrees to the ecliptic plane.
Possible that a collision early in its history tipped the axis, and broke out material that
formed its moons.
This inclination means that for one part of Uranus‟ orbit, one hemisphere is in
uninterrupted daylight, while the other hemisphere is in darkness.
Odd Magnetic Fields
Both Uranus and Neptune have strong magnetic fields.
Uranus: 47x Earth
Neptune: 25x Earth
Possibly generated by currents in the liquid water in their interiors.
Not centered on the center of the planet and tipped in odd directions. Earth‟s Magnetic Field
Earth‟s magnetic north pole and the “north pole” (i.e. North end of axis) are not in the
Earth‟s magnetic north (and south) pole aren‟t fixed but change over time.
The poles have “flipped” throughout history.
Last one,780,000 years ago.
We may be “due” for a flip again.
Results not likely to be catastrophic but could be interesting if so..
The Galilean Satellites
The four largest moons of Jupiter are called the Galilean satellites, in honor of their
Visible through even a small telescope or binoculars.
Their positions change rapidly; orbital periods ranging from 2 to 17 days. One can see
relative motion in even one evening‟s viewing.
The Galilean satellites would be considered planets if they orbited the Sun independent of
Temperatures rang from 110-130 K.
The Origin of Planetary Rings
If a body held together only by gravity gets too close to a planet, tidal forces pull it apart.
This distance is called the Roche Limit. Solid bodies are safe, (chunks of rock or ice, or even the space station) as they are held
together by forces other than gravity.
The fragments of the broken-up satellite go into orbit around the planet, f