Introduction to Astronomy

• Announcements

The Sun, Our StarThe Sun, Our Star

Size & Structure

Energy Generation

Magnetic Activity

The Solar Cycle

Size & StructureSize & Structure

Basic properties of the Sun…

• DSun ~ 100 Dearth

– A million Earths could fit inside the Sun

• MSun ~ 300,000 MEarth

– I weigh 225 lbs on Earth.– On the Sun, I would weigh ~ 3 tons!

• Distance to Earth = 150 million km = 93 million miles = 1 AU– Driving at 75 miles/hour, you would reach the

Sun in 141½ years!

• Surface Temperature ~ 6000 K– Measured from color & Wien’s Law

• Core Temperature ~ 15 million K ~ 27 million °F– from modeling/calculations & indirect

measurements

• Composition– 71% Hydrogen– 27% Helium– 2% vaporized heavy elements

• Oxygen, Silicon, Calcium, Iron, Strontium, Titanium, etc…

• Radiant Energy ~ 4 x 1026 Watts– Compare this to your 60 Watt household light

bulbs

Solar Structure

• Photosphere (top-most, only directly visible layer)

• Convective Zone (pot of boiling water)

• Radiative Zone (direct energy radiation)

• Core (powerhouse)

Core

• Fusion here to produce energy

• More on this in a minute…

Radiative Zone

• Energy (photons of light) produced in core must travel outward

• In this zone, energy moved by direct radiation– Campfire analogy: energy from fire travels

outward by RADIATION

• You can feel the heat from the fire w/o putting your hands directly into the flames

• Incredibly dense, so photons can travel only ~ 1 inch before they are absorbed by atoms in the gas– Re-radiated, but then can only travel another

inch before re-absorbed• “Random Walk”

– Photons re-radiated in random directions

• These photons take an average of 16 million years to reach the surface!

““Today’s sunshine was born in the Sun’s core before we even existed as Today’s sunshine was born in the Sun’s core before we even existed as a species!”a species!”

The distance traveledis much greater thanthe net displacement

Convective Zone

• Further from core, so gas is much cooler…

– Lower temps mean gas atoms can “hang on” to more of their electrons

– This causes much more absorption at the base

– Therefore, heat piles up at the base, starts the gas “boiling”

Photosphere

Convective Zone

Radiative Zone

Core

Not to scale

• “convective currents”– Hot gas near base is less dense, so it starts to

float to surface– Gets to surface, cools down, becomes more

dense, sinks back below– Continue ad infinitum…

• Granulation– Convection cells– Move at ~ 1 km/sec

Cooler here

Hotter here

Photosphere

• Name = “light sphere” (visible surface)• Has density similar to liquid water• Below this surface, gas density increases rapidly

– The Sun is gaseous throughout

– No solid core• Temperatures too high to form molecular bonds required to

have solids and/or liquids• Despite incredibly huge densities

• Solar atmosphere– Everything above the visible surface

(photosphere)– Very low density gases

– Chromosphere– Corona

Chromosphere

• Thin layer

• Typical temperature ~ 25,000 K

• Name = “color sphere”– Most emission is red light from Hydrogen

• Spicules– Vertical jets of hot gas– Formed from shock waves rising through

photosphere (from convection)

Mostly vertical…

Due to many overlappingspherical shock waves

Corona

• Typical temperature 1 – 10 million K

• Such low density, we look right through it without noticing it– Only visible during solar eclipse – Or with special coronagraphic telescopes

• Coronal Streamers (X-Ray)

• Coronal Holes (source of solar wind)

chromosphereStreamers

Corona in visiblelight

Coronal Holes

Corona in X-Rays

Energy GenerationEnergy Generation

What makes the Sun shine? How?

• Hydrostatic Equilibrium

– Outward pressure balances inward force of gravity

– Ex: tire pressure• Air pressure in tire balances weight of car• Let air out = no pressure = flat tire = weight of car

not balanced = car sinks down

– Ex: floating• Buoyancy forces balance your weight in pool

• In a gas:– Pressure increase comes from squeezing

atoms closer together...(density increase)

– …or making them move faster…(temperature increase)

• Increase density– More atoms in a given volume = more

frequent collisions

• Increase temperature– Atoms move faster = harder, more frequent

collisions

• Therefore, PRESSURE = DENSITY x TEMPERATURE x A CONSTANT

• So to balance Sun’s enormous gravity, need high densities and high temperatures in the interior…

Powering the Sun

• Early theories1. Sun made of coal

• if true, Sun would only shine for few thousand years

2. Sun NOT in H.E., but gravitationally compressing & heating gases

• If true, Sun would only shine for 10 million years• If true, Sun would be shrinking enough to observe

over the last 100 years or so…

• Bogus.

• New theory (late 1920s – early 1930s)

– Nuclear fusion in Sun’s core (heavy elements could be forged by smashing light elements together)

– E = mc2

• 1 gram of matter “contains” energy equal to that released by small nuclear device!

• In Sun’s core, have incredibly high temperatures– Atoms move incredibly fast

– Normally, 2 protons moving toward each other would repel

• Recall opposite charges attract, like charges repel

– But, if moving fast enough, protons (nuclei) are driven extremely close together

• Strong force overcomes electrical repulsion• Protons bound together to form heavy nucleus

“Collision” of like-charged particles

• Proton-Proton Chain (PP Chain)

– 11HH + 11HH 22HH + e+ + neutrino + extra energyextra energy– 11HH + 22HH 33HeHe + photon + extra energyextra energy– 33HeHe + 33HeHe 44HeHe + 11HH + 11HH + extra energyextra energy

– Note that a little bit of energy is liberated at each step of this process…

– Note also that the final step regenerates the reactants needed for the initial step!

• 4 protons (1H) needed to produce each 4He

• Mass difference:

– 4 x mass of proton > mass of 4He

– “extra” mass converted to extra energy (E=mc2)

• 0.048 x 10-27 kg = 4.3 x 10-12 J of energy– Pretty small, but…– ~ 30 MeV (1/40 eV = room temperature)

• Many, many, many of these reactions happen every second!– Total rate equivalent to 100 billion 1-megaton

nuclear devices exploding per second!– Hiroshima Bomb ~ 15 kilotons = 0.015

megatons

• The PP-Chain is the dominant fusion reaction in stars that are roughly the same size/age as our sun…

• Other reactions are possible (triple-α process, CNO-cycle)– These occur mostly in heavier, older stars

Introduction to Astronomy

• Announcements– HW #5 due monday

The Sun’s Core

• Two “experiments” allow us to figure out what’s going on in the core

– Neutrino observation

– Helioseismology

Neutrinos

• Generated in PP chain– “counting” them gives info about the rate of

fusion in the core• Counting is difficult: neutrinos have NO electric

charge and a vanishingly small mass• VERY weakly-interacting

• Recall photons take ~ 16 million years to reach surface

– Neutrinos take ~ 2 seconds!

• ~ 1 trillion solar neutrinos pass through your body every second!

– Could pass through a light-year of lead without “hitting” a single atom!

– So how do we detect them, if they don’t interact?

– Can only detect a very small fraction

• Neutrino detectors– Huge containers of “HEAVY WATER”

• D2O: each Hydrogen atom in H2O also contains a neutron

– Containers buried deep below mountains to filter out cosmic rays & other high-energy particles

• “outer space” particles• Constantly raining down on Earth

– In fact, may be cause of lightning

• When neutrinos interact with a neutron in the heavy water, it changes the neutron into a proton and an electron– the electron zooms off, passing through the

heavy water– When it does, it emits a faint flash of light

(Cerenkov radiation)– Tank of heavy water lined with photodetectors

that can detect this flash.– VERY RARE: only expect one or two flashes

per year!

Sudbury NeutrinoDetector

Located about 1.5miles undergroundnear Sudbury,Ontario, Canada

Contains 1000 tonsof heavy water in~ 20 foot sphere

Helioseismology

• Just like earthquake seismic waves gives info on Earth’s interior

• Looking at the sound waves produced by the solar convective zone gives info on density & temperature of solar interior

– Analogy: hearing further on a cold day• Low temps mean slow atom motion means

pressure waves travel farther before they die out

• As the sound waves travel throughout the Sun, they venture into regions with different temperatures, pressures, and densities

• This changes their speeds, and hence the ways in which they overlap…

Vibration patterns onthe Sun…

Red = surface movingoutward

Blue = surface movingInward

Gray = nodal lines(no movement)

• The true vibration pattern is a superposition of many different vibrations

Magnetic ActivityMagnetic Activity

The most interesting phenomena on the Sun!

Tutorial: Magnetic Fields• Ferromagnetism

– Most familiar…kitchen magnets, compass needles

• Induced magnetism– Produced by moving

charges– Current-carrying wire

deflects compass needle

• Currents create magnetic fields

• A field is just an easy way of describing the effects of long-range forces– i.e. the forces that drive the pistons in an

engine are mechanical and thermodynamic in nature

• Require contact between the various components

– Gravity is a long-range force• “gravitational field”• Interaction between two massive bodies that are

not necessarily in contact

• Similarly, the magnetic field describes the interaction of charges (protons, electrons, ions) that are not necessarily in contact

• A bar magnet produces a magnetic field at every point in space– Charges interact with the bar magnet by the

forces that the field exerts on them

Sunspots

• Large, dark areas in the photosphere– Typically >> the size of the Earth!– Dark because they are cooler than

surrounding areas (3000-4000K)– Regions of STRONG magnetic fields

• Charged particles (p+ & e-) are forced to spiral around field lines

– Therefore, heat is rising more slowly than in the surrounding areas

– So appears cooler…

Meanwhile, particles outside the magnetic field can zoom straight up and out, much faster than those spiraling around the field lines…

…therefore, they transport heat faster, and areas outside sunspots are much brighter

Umbra

Penumbra

Subsurface magnetic fieldlines organized in “bundles” called flux tubes, held together by gas pressure pushing in on them…

Above surface (photosphere), fieldlines spread out due to drop in gas pressure

Umbra

Penumbra

Visible(“white-light”)

Calciumemission

So how do we know that Sunspots are related to magnetic fields?

The Zeeman Effect

In the presence ofa magnetic field,absorption linessplit into multipledistinct lines

Yet anotherpiece of informationthat an object’s spectrum tells us.

The magnetic fields in and around sunspots act like north and south poles…

X-ray image of “coronal loops”

Sunspots here

Prominences & Flares

• Prominences– Reduced heat flow to a small region inside

flux tubes cools the gas there• Therefore, the pressure is smaller than outside this

region• Gas is well-confined by this high-pressure “bottle”

– gas can flow through, sometimes rising into the corona, sometimes raining down to photosphere

• Flares– Brief, bright eruptions of hot gas, usually at or

near sunspots• VERY bright in X-rays, UV• Not so bright in visible radiation (generally)

– Caused by twisting motions of sunspots?• KINK INSTABILITY

– Highly twisted magnetic fields violently re-adjust themselves into a new, lower-energy configuration

– Drastically heats surrounding gases in the process

CCD bakeout(like over-exposingfilm)

“Halo” flare: “it’s coming right for us!”

Flaring prominence

Magnetic (“arcade”) loopsseen edge-on

Notice all the different orientationsof the loops…this weaves what wecall the “magnetic carpet” of the Sun

• Solar Wind– Steady mass loss due to high temperatures in

corona• Protons & electrons move fast enough to escape

Sun’s gravity– Recall atom speed depends on temperature

• Travel outward into solar system

The Parker Spiral

The Solar CycleThe Solar Cycle

Periodic changes in the Sun’s magnetic activity

About 6 years between opposite ends of this full-disc X-ray montage

Solar Maximum

Solar Minimum

• Number of sunspots & frequency of flares changes over an interval of ~ 11 years

• Cause is hotly-debated– Main theory is “α-Ω Dynamo”– Combination of Sun’s rotation and the

convection in the convective zone

Amplitude (max. number of sunspots) is wildly variable, butperiod (# years between successive peaks/troughs) is very

steady (~ 11 years)

• Differential Rotation (“The Ω Effect”)

– Sun rotates faster at equator, slower at poles

– Magnetic fields are “stretched” along direction of rotation

– Makes magnetic fields stronger (like stretching a rubber band increases its tension)

Differential Rotation

• Convection (“The α Effect”)– When magnetic field is strong enough, the

gas it contains becomes buoyant (starts to float)

– Convection then drags it up toward surface– SUNSPOT PAIRS

• Whole process still not well-understood– Helioseismology can’t probe ALL the way

down, so lack of data

northsouth

Sunspot Rules

• Joy’s Law– Tilt of bipolar

sunspot pairs

• Hale’s Law– Sunspot polarity reversal

Hemispheric rule:Hemispheric rule:

If N magnetic pole leadsin Northern Hemisphere,then S magnetic pole leadsin Southern Hemisphere

Reversal rule:Reversal rule:Leading polarities reverse from one solar cycle to the next…

Links Between Sun & Earth Climate

• Sun heats corona

• Corona drives solar wind

• Solar wind alters Earth’s upper atmosphere

• Jet streams may shift (not conclusive)– May alter rainfall and storm patterns

• Maunder Minimum– 70 year period of little or no sunspots– Corresponds to ~ 70 year period of

abnormally-cold winters in Europe, rapid glacier advances, longer periods of frozen-over rivers

• “Little Ice Age”

A couple more movies to tie it all together…

Zoom-in on the Sun

Hinode Ca II Sunspot & Two-ribbon Flare

NEXT TIMENEXT TIME

• Measuring the Properties of Stars