AST101 Sun and its Neighbors

Solar System

• consists of at least one star and includes any objects orbiting it (planets, asteroids, colts, etc) or any objects orbiting those objects

Star

• ball of high energy plasma driven by nuclear fusion

• produces its own energy

Sun

• main source of energy in the solar system

• produces energy by fusing hydrogen into helium in its core.

Sun spots

• very magnetically active areas on the sun

• dark because it prevents energy from reaching the sun

What is a planet?

• orbits a star

• mostly round

• massive enough that it has cleared its neighborhood of massive objects

Inner planets/inner solar system

• terrestrial: small and rocky

• mercury, venus, earth, mars

• have thin or no atmosphere

• few or no moons

Outer planets/solar system

• jovian: large and gassy

• more distance

• jupiter, saturn, uranus, neptune

• massive systems of moons

• main of light gasses

What are moons?

• object orbiting a planet

• the largest moons are bigger than planets

• the smallest moons are <1km wide irregular objects

Our moon

• rocky and heavily cratered

• not geologically active

• little atmosphere

Dwarf planets

• objects massive enough to become approximately spherical

• not massive enough to clear their orbit

• ceres, pluto, eris, haumea, makemake

• more elliptical orbits than planets, not always in the same plane

The Kuiper Belt

• region of countless icy bodies in the outer solar system

• contains most known dwarf planets

Comets

• small bodies in the outer solar systems

• rich in ices

• elliptical orbits

The celestial sphere

• the sky appears as a dome above our heads

• north pole of the earth points at the north celestial pole - same with south

• projection of the earth’s equator on the sky

Right ascension

• analogous to longitude lines on the Earth

• sky version of longitude

Declination

• analogous to latitude lines on the Earth

• sky version of latitude

Day-to-day sky motions

• solar system objects and stars rise in the east and set in the west because of earth’s rotation

• counter-clockwise in the north and clockwise in the south

• circumpolar stars never set: altitude of the pole = your latitude

Ecliptic

• the path travelled by the sun and planets

• all planets orbit the sun in roughly the same plane

• 23.5 degrees from the celestial equator

Axial tilt

• the angle between the ecliptic and equator is caused by this- Earth doesn’t spin in line with the plane of the solar systems

• this causes seasons

Seasons

• when the northern hemisphere towards the sun: more direct sunlight, longer days, summer

• when the northern hemisphere is tilted away: less direct sunlight, shorter days, winter

• earth points in the same direction throughout the year

• causes by the tilt of our rotational axis

New moon

• the moon is between the earth and sun

• unlit side faces earth

• solar eclipses can happen

• rises and sets with sun

moon phases

Waxing crescent

• visible soon after sunset

• happens a few days after new moon

• sets a few hours after the sun

First quarter

• at its highest point at sunset

• happens 1 week after new moon

• right half of the moon is lit

• sets at midnight

waxing gibbous

• at its highest point around 9pm

• happens 10ish days after new moon

• sets before sunrise

• moon ~3/4 lit on right side

Full moon

• rises around sunset and sets around sunrise

• the only time lunar eclipses can happen

• happens two weeks after new moon

• visible almost all moon

Waning gibbous

• happens 17-18 days after new moon

• sets in late morning after sunrise

• moon ~3/4 lit on left rise

Third quarter

• at its highest point at sunrise

• happens one week before new moon

• sets at noon

• left side of moon is lit

Waning crescent

• rises right before the sun

• happens a few days before the new moon

• sets before sun

Tidal locking

• the moon is tidally locked to the earth

• same face is always pointing toward the earth

• not rare in the solar system

Eclipses

• occur when one object blocks the light of the sun from reaching another object

• lunar and solar eclipses happen when the moon cross the ecliptic, aligning the earth, moon, sun

Solar eclipse

• can happen during new moon

• moon blocks the sun, cases shadow on the earth

Total eclipse

• entire surface of the sun is blocked out

• dim corona of the sun is visible

Lunar eclipse

• often called blood moon

• moon is covered by earth’s shadow

• moon is lit by light filtered through the atmosphere all sunsets on earth projected on the moon

Geocentric model

• the earth is at the center of the universe

• everything revolves around it

• planets move in circles within circles

Retrograde motion

• The planets don't circle the earth consistently, they occasionally reverse, this is retrograde motion

• issue with geocentrism

• opposite to prograde: the direction in which the sun moves

The heliocentric model

• sun is at the center of the universe

• the planets orbit the sun in circles

• the moon orbits the earth

• predicts retrograde motion when earth overtakes outer planets in their orbits or the reverse for inner planets

Kepler and Brahe

• Brahe used naked eye instruments to track positions of the stars and planets

• Kepler collaborated and inherited Brahe’s data after his death made the three laws of planetary motion

Kepler’s First Law

• all planets move in ellipses with the sun at one focus

• sun is not at the center but rather at one focus of the ellipse

• e = 0: perfectly circular

• e>1: not in a bound orbit

Semimajor axis (A)

• the major axis is the longest distance across the ellipse so semi major is half of that

• usually represents the average distance from planet to sun

Semi-Minor Axis (B)

• the minor axis is the shorter width of the ellipse so its half of that

• tells you how “wide” the ellipse is

Circle vs Ellipse

• a circle: major axis = minor axis

• an ellipse: major axis = longer minor axis =shorter

Foci

• ellipse has two focus points

• sun sits at one

Keplers second law

• a planet moves faster in the part of its orbit nearer the sun and slower when farther from the sun, sweeping out equal areas in equal times

Keplers third law

• more distant planets orbit the sun at slower average speeds, obeying the relationship

Issac Newton

• developed a unified physical framework that could explain planetary motions

• explained tides, shape of the earth, and speed of sound

Newtonian physics: speed

• The rate at which an object is moving. This does not consider direction – e.g. the car is moving at 45 km/h

Newtonian physics: velocity

• The speed at which an object is moving in a specific direction – e.g., the car is moving at a rate of 45 km/h due north

Newtonian physics: acceleration

• The rate of change of velocity – e.g. The car accelerated from 0 to 45 km/h moving north over 9 seconds - a change in direction is acceleration

Newtons first law

• the law of inertia: an object in motion remains in motion unless acted upon by an outside force

• centripetal force: force is needed to keep an object moving in a curved path

  • if the force making the object move in a curved path disappears, the object will continue with velocity it had when that force disappeared

Newton’s second law

• acceleration is proportional to force and inversely proportional to mass

• F = ma or a=F/m

• more force means more acceleration, higher mass means less acceleration

Newtons third law

• for each force, there is an equal and opposite reaction force

• the force with which the gas leaves the rocket engines downwards is the same force that accelerates the rocket upwards

Momentum

• defined as p=mv

• m = mass

• v= velocity

• always conserved in a closed system

Momentum astronaut question: Let’s say our astronaut pushes away at 10 km/h. If the spacecraft weighs 100 times as much, how fast does it move away?

• If initial momentum is zero, then they must have equal and opposite momentum after 

• Astronaut: 1*10 km/h = spacecraft: 100*x km/h Spacecraft moves away at 0.1 km/h

Angular momentum

• L=mvr

• angular momentum is momentum times distance from center of rotation

• if a skater pulls their arms in, r decreases, so v must increase to conserve L

Universal law of gravitation

• all objects in the universe experience and produce gravitational force

• force increases with mass of each object

• force decreases with distance squared

• bigger masses stronger gravity/ larger distance weaker gravityt

Gravitational acceleration

• combines newtons 2nd law and gravity

• gravity causes the same acceleration for all objects at the same location

• does not depend on the falling objects mass (ignoring air resistance)

Escape velocity

• bigger object, higher escape velocity, surface further from center of mass, lower escape velocity

Tides

• force of gravity is highest on the side toward the sun: near side of the earth drawn to it - gravity weakest on far side

• two bulges: two high tides per day - water moves easier than rock

• since earth rotates the tidal bulges lag - result is earth’s orbit is slowing down

Spring/neap tides

• the sun can cause tides

• if the sun and moon are aligned (full or new), forces add - spring

• at first or third quarter moon, forces of sun and moon opposed each other so tides are smaller - neap

Tidal locking of the moon

• the lag and pressure now has the moon only showing one face toward the moon through tidal locking

• picture holding a pen while spinning in a chair

Kinetic energy

• energy of motion/momentum

• objects have this energy through their motion - faster = more energy

• hot particles have more

Potential energy

• stored energy/energy from gravity

• objects higher up have more potential gravity

Nuclear energy - mass-energy equivalence

• the nuclei of atoms store energy, when you combine or split them you release some of the energy

• E=mc²

• mass can be converted into energy without violating conservation of enerfy

• nuclear reactions release an enormous amount of energy

Nuclear fission

• splitting the atom

• nuclear reactors use this as energy source

Nuclear fusion

• combining atoms

• extra mass is released as energy during fusion

Nuclear fusion in the sun

• fusing hydrogen nuclei together to create helium releasing enormous amounts of energy

• the sun squeezes hydrogen together that fuses into helium the tiny bit of mass converts into energy and energy moves outward

Conditions for nuclear fusion in the sun

• the suns core is extremely hot and dense, gravity crushes the core inward creating conditions for fusion

• gravity pulls inward and pressure from hot fusion gas pushes outward creating balance: hydrostatic equilibrium

The solar thermostat/Hydrostatic equilibrium

1. equilibrium state

2. density increases

3. fusion rate increases

4. temperature increases

5. pressure increases

6. core expands

7. density decreases

8. equilibrium restored

Sun’s core

• nuclear fusion is active

• dense

• 15 mill degrees

Sun convective zone

• outermost layer of solar interior

• hundreds of thousands degrees C

• less dense air at the top of the convective zone

• energy moves outward through convection

Sun photosphere

• the surface of the sun we see

• point where visible light can freely escape the sun

Sun chromosphere

• extends 2000 km above the photosphere

• normally only visible through solar eclipse

• prominences: gas help up by magnetic activity

• filaments: prominence seen in front of the suns surface

Sun corona

• ultra hot gas above the photo and chromosphere

• a million degrees but low density

• only visible during eclipse

Light

• the transport of energy through electromagnetic waves

Waves

• oscillations that propagate outwards and carry away energy e.g. ripples in water, sound waves

Propagation of light

• when you move a charge the electric field changes

• the change propagates at the speed of light

• a changing electric field induces a magnetic field

Light waves

• light moves at a constant rate

• crest and troughs - shorter wavelength makes the crest and troughs of the wave pass more often

• distance=velocity*time

• can be described in terms of wavelength or frequency

Visible light

• light we can see

• λ ~ 400 - 700nm

• tiny portion of the EM spectrum

Ultraviolet

• light with wavelengths too short for us to see

• λ 10-400 nm

• sources: the sun, black lights, tanning lamps

X-ray

• even higher energy and shorter wavelength than UV

• used for medical imaging

• Chandra space telescope

Gamma rays

• even higher energy and shorter wavelength than UV

• λ<10nm

• produced during fusion and fission

• emitted during supernovae, neutron star mergers, high energy events

Infrared

• light with wavelength too long for us to see

• λ 700nm-1mm

• sources: heat from most objects in this room including you

Microwaves

• even longer wavelengths than infrared

• λ 1mm-1m

• sources: gas in space, residual heat from the formation of the universe

Radio

• longer wavelengths than microwaves

• λ> 1m

• telecommunications, radio galaxies, gas emission

Emission

• an object produces its own light

Reflection

• light bounces off an object and does so at a consistent angle

• reflection lets us see a complete image of objects in the reflecting surface

Scattering

• light hits an object and is reflected in many different directions

• because the angles are not consistent, scattered light does not form an image

Absorption

• light from one object hits another object and is captured by that object

• objects that absorb the most light are dark

Transmission

• light from one object travels through another object

• light interacts differently in different wavelengths

Light through a prism

• some colors are bent more than others passing through glass so white light splits into a rainbow

Thermal radiation

• moving particles or charges produces light

• idealized form of this is blackbody radiation: object is in equilibrium, it only absorbs energy and re-emits it as thermal radiation

Kelvin Scale

• same as celsius scale but at 0K objects have no thermal enerfy

• 0 K is also called absolute zero

Blackbody radiation

• for a hotter blackbody more energy at all wavelengths

• peak intensity at shorter wavelengths

• sun isn’t a perfect one but its close

The Bohr Atom Model

• a model where electrons orbit the nucleus in fixed energy levels

Nucleus

• positively charged nucleus containing protons and neutrons

Electrons

• orbiting the nucleus in specific energy levels

Quantized energy

• electrons can only exist at certain energy levels, not between them

Emission lines

• extra light at the same wavelengths

• common in hot low density gas

Absorption lines

• dark lines that appear in a spectrum when atoms absorb specific wavelengths of light.

• A hot object produces a continuous spectrum
  (like the Sun)

• Light passes through cooler gas

• The gas absorbs specific wavelengths

• Dark gaps appear in the spectrum