Stars

cosmic address (small → large) 

• earth → social system → milky way → local group → local supercluster → universe 

light year

• unit of distance, finite speed → looking farther = looking back in time

cosmic calendar

• compresses 13.8 billion years into 1 calendar year. humans appear on dec 31, last second 

spaceship earth 

• earth moves in space (rotation, orbit, suns motion in galaxy)

scale of universe

• huge distance, use scientific notation 

light pollution

limits ability to see faint celestial objects

ecliptic

• path the suns takes across the sky, tilted 23.5 relative to earths equator 

equinoxes

• sun directly above equator (equal day/night), sun rises due east and sets due west

solstices

• sun at most northern/southern point (longest/shortest day), sun rises/sets farthest north or south horizon 

sidereal day

• time for earth to rotate once relative to distance stars (23 h 56m) 

solar time

• time for earth to rotate relative to the sun (24h) difference due to earths orbital motion  

cause of seasons

• tilt of earths axis (23.5), not distance from sun

eclipses

• require alignment of earth, moon, sun. rare because moons orbit is tilted

earth → moon

• 384,000 km

earth → sun (1 AU)

• 150 million Km

light year

• 9.46 trillion Km

milky way galaxy

• 100,000 light year-years across 

local groups (galaxies)

• a few million light-years

observable universe

• 93 billion light-years in diameter 

constellation

• officially recognized region of the sky (Orion) 

asterism

• a pattern within or across constellations (big dipper)

light population 

• artificial lights brighten the night sky

• makes faint stars, galaxies, and nebulae invisible

visibility of objects

• some stars/constellations are only visible from certain latitudes

• ex: Southern cross only visible from southern hemisphere

seasonal visibility

• earth orbit changes which part of the sky in nighttime 

sidereal month 

• moons orbit relative to stars (27.3 days)

synodic month

• full cycle of moons phases (29.5 days)

northern hemisphere

• summer solstice is sun high long arc

• winter solstice is sun low short arc

southern hemisphere

• opposite pattern (their summer = our winter)

solar eclipse 

• moon between earth and sun

lunar eclipse

• earth between sun and moon 

types of eclipse

• lunar: total, partial, penumbral

• solar: total, partial, annular 

why not monthly 

• moons orbit is tilted 5 → doesn’t always align 

moon phases

• causes by relative positions of sun, earth, moon

• half of the moon is always lit, but we see changing portions

sequences 

• New → waxing crescent → first quarter → waxing gibbous → full → waning gibbous → laster quarter → waning crescent

polaris

• lies very close to earths current north celestial pole → appears fixed 

precession

• earths axis slowly wobbles (26,000 years) → pole star changes over millennia (vega will be next)

scientific method steps

1. observation

2. hypothesis 

3. prediction 

4. experiment/test

5. refine or reject hypothesis

ptolemaic (geocentric)

• earth-centered; planets move in circles on epicycles (small circles) around earth. retrograde motion explained by epicycles 

heliocentric (sun-centered)

• sun at center; planets orbit sun. retrograde motion happens naturally when earth overtakes another planet (like passing a slower car on a highway)

copernicus (543)

• proposed heliocentric model. wrongly kept circular orbits

tycho brahe

• made the most accurate naked-eye measurements of planetary positions. believed in hybrid earth-centered model but his data was crucial

galileo 

• used telescope → saw phases of venus (proved heliocentric), moons orbiting jupiter sunspots, craters on moon → challenged perfection of heavens 

kepler

• used brahe’s data to derive 3 laws of planetary motion 

keplers three laws 

• elliptical orbits

• equal areas in equal time 

• harmonic law

elliptical orbits

• planets orbit sun ellipses, sun at one focus

equal areas in equal time

• planets move faster when closer to sun (perihelion), slower when farther (aphelion)

• this explains planets speeding up in orbit 

harmonic law

• square of orbital period = cube of semi-major axis

• allows us to find distance to the sun

parallax method

• nearby stars shift relative to background stars as earth orbits sun

• by measuring angle of shift, distance can be calculated (trigonometry)

• larger shift = closer star 

newtons laws of motion

• inertia: objects stay in motion unless acted on 

• force = mass x acceleration (f=ma)

• action-reaction: equal and opposite forces 

law of gravitation

• if mass increases → force increases

• if distance increases → force decreases 

why don’t planets fall into the sun

• planets are in free-fall, but their sideways (tangential) motion keeps them moving around the sun instead of straight in 

• balance between gravity pulling inward and orbital velocity 

angular momentum 

• L = mvr (mass x velocity x distance) 

• if conserved: when radius decreases (ice skater pulling arms in), rotation rate increases 

• explains why collapsing clouds spin faster during star/planet formation 

types of orbits 

• circular (bound)

• elliptical (bound)

• parabolic (unbound, escape)

• hyperbolic (unbound, escape)

young double slit (1801)

• shined light through two slits → created an interference pattern

• proved wave nature of light 

photoelectric effect (Einstein 1905)

• light shining on metal ejects electrons, but only above a certain frequency 

• showed particle nature of light (photons)

• together: light has wave-particle duality 

refraction

• bending of light when it passes between materials of different densities (like air → water → glass)

• light changes speed when it enters a new medium 

• lenses (refracting telescopes), atmospheric distortion 

light as an electromagnetic wave

• light = oscillating electric field and magnetic filed, perpendicular to each other and to the direction of travel 

electromagnetic spectrum

• radio → microwave → infrared → visible -? ultraviolet → X-ray → gamma rays 

• long wavelength = low frequency = low energy (radio)

• short wavelength = high frequency = high energy (gamma rays)

EM waves that reach the ground

• visible light 

• radio waves 

refracting telescope

• uses lenses to bend (refract) light to a focus 

• chromatic aberration, heavy glass lenses 

reflecting telescope

• uses mirrors to focus light 

• easier to build large mirrors, no chromatic aberration 

• modern observatories = almost all reflectors 

light gathering power

• area (bigger mirror collects more light → see fainter objects) 

angular resolution

• 1/D (bigger mirror → sharper detail)

magnification 

• depends on eyepiece, not main mirror 

limitations of ground-based telescopes

• atmospheric turbulence (blurs images → “twinkling stars”)

• atmospheric absorption (blocks many wavelengths)

• weather and location dependence (need dark, dry, high-altitude sites)

blackbody

• an ideal object that absorbs all radiation and re-emits it based only on its temperature 

curve shape

• graph of intensity vs wavelength

as temperature increases

• peak shift to shorter wavelengths (bluer) → wien’s law

• overall intensity (area under curve) increases (hotter object is brighter) → stefan-boltzmann law

continuous spectrum

• smooth rainbow of colors, no breaks 

• produced by a hot, dense object ( hot solid, liquid, or dense gas like a star’s surface) 

emission spectrum

• bright lines at specific wavelengths on a dark background

• produced by a hot, low-density gas

absorption spectrum

• dark lines within a continuous spectrum 

• produced when cooler has is in front of hot, dense source 

why emission spectra matter in astronomy 

• each element has a unique fingerprint (specific emission lines) 

• lets astronomers determine the composition, temperature, motion (via doppler shift), and density of stars and galaxies 

nucleus 

• protons (+), neutrons (0)

electrons

• (-), orbit nucleus

ions

• atoms with unequal protons/electrons → charged 

isotopes

• same element (same protons), different number of neutrons

Bohr model and spectra

• electrons orbit nucleus in discrete energy levels

• emission spectrum: when electron falls to a lower level → photon released 

• absorption spectrum: when electron absorbs energy and jumps to a higher level → photon absorbed 

raw material for planet formation

• came from gas and dust left over from earlier generations of stars 

• elements heavier than hydrogen and helium were created in stars and spread into space by supernova explosions 

jeans instability

• when a gas cloud collapses under its own gravity 

• happens if gravity > internal pressure (from heat or turbulence)

• produced by cooling (less pressure), adding mass or external shock (like a nearby supernova) 

protoplanetary disk

• flattened, rotating disk of gas and dust around a new born star 

• site where planets, asteroids, and comets form through accretion 

transit method

• planet passes in front of star → brightness dips

• reveals planet size and sometimes atmosphere (via starlight filtering) 

radial velocity 

• planets gravity makes star “wobble”

• wobble shifts stars light (red/blue shift)

• reveals planets mass

astrometric method 

• measures stars actual position shifts on the sky 

• similar in idea to radial velocity but racks position, not spectrum 

gravitational lensing (microlensing)

• gravity bends light 

• if a star with a planet passes in front of a background star, its gravity focuses the light like a lens

• can reveal hidden exoplanets, even very fart away

• works best for planets not easily detected by other methods

hot jupiters

• giant gas planets close to their star (easier to detect)

• gas giants like jupiter but orbiting extremely close (day-long periods)

super-earths

• rocky planets larger than earth but smaller than neptune

• 2-10x earths mass, rocky or water-rich, no exact counterpart in our solar system 

habitable zone

• region around a star where liquid water can exist on a planets surface 

• determined by stars luminosity and temperature

• hotter, brighter star → habitable zone farther out

• cooler, dimmer stat → habitable zone closer in 

nice model

• explains how our solar system formed and evolved 

solar nebula

• a cloud of has and dust collapses 

planetesimals form

• small rock/icy bodies clump together via collisions

protoplanets form

• larger bodies grow by accretion and gravitational attraction 

gas giants migrate 

• jupiter and saturn shift positions; their gravity reshapes orbits of other bodies 

planetary instability

• gravitational scattering of leftover planetesimals causes impact across the solar system

clearing of the disk 

• remaining gas/dust blown away by solar wind; only stable planets, moons, asteroids, and comets remain

changes in planetary orbit

• jupiter and saturn migrated slightly inward/outward, changing orbital resonances 

• this scattering rearranged smaller bodies → asteroid belt thinned, kuper belt formed

• neptune moved outward, pushing icy bodies into the kuiper belt and Oort could

role of jupiter and saturn

• their strong gravity shaped the solar system

• prevented a planet from forming in the asteroid belt

• controlled delivery of water/volatiles to inner planets 

• migration triggered the late heavy bombardment, influencing terrestrial planet surface 

formation of the moon

• earths moon likely formed by a giant impact with a mars-sized body (theia)

• debris from the collision formed a disk → coalesced into the moon 

• different from other moons: most large moons formed from co-accretion around their planets or were captured 

• out moon is unusually large relative to its planet

kuiper belt

• region beyond neptune with icy bodies pluto, eris)

asteroid belt

• rocky bodies between mars and jupiter

Oort cloud

• distance spherical shell of icy bodies; source of long period commets