The Sun

A Model Star

Factoids to Start:• Official name: Sol (as in Solar System)

• 1.5 X 1011 meters distant (93 million miles, 8.3 light minutes)

• Size: 6.96 X 108 meters (432,000 miles) in radius; about 54 times the Earth's radius

• Mass: 2 X 1030kg; about 1 million Earths

• 74% H, 24% He, 2% all other materials

• A Variable Star: output +/- 0.07%

Present age: ~4.567 billion years old

• How do we know this?– Stellar models (another slide show for another day)– Luminosity rate

• t = M/L

– Chondrite meteorites• Unchanged since they formed 1 million years (or less) after the Sun

– Time to condense

• Radiometric dating/half lives/proportions– K-Ar– Sm-Nd– U-Th-Pb

• Three methods return very similar results

But factoids say so little!• Let's use the Sun as a sample, a basis to compare to

all the other stars

• Many say the Sun is an average star, but you can't take that meaning too seriously– It’s just not an unusual star

• There are many, many smaller, cooler stars.

• There are some stars that are much, much brighter, much, much bigger, and much, much more massive than the Sun

Stages of the *Sun’s Life

• Globule• Protostar• Main Sequence• Hydrogen Core Exhaustion• Red Giant• Helium Flash• Horizontal Branch• Asymptotic Giant

– AKA Double shell burning• Planetary Nebula• White Dwarf

*Other stars have different later-life experiences!

A Star's Beginnings

• What the Sun has in common with all stars is its origins, a large dark, cold cloud of interstellar gas and dust

• This is a so-called Bok globule, named after UAz astronomer Bart Bok

Qualities of a star-forming region• Must be cold: too warm, and the speed of the atoms

and molecules overwhelms the forces trying to compress them

• Therefore, what we see in visible light is a blocking of light, at least in the very early stages

• Must be dense enough for gravity to pull atoms and molecules together– Still not very dense by Earth standards!

• Must not have any great rotation, or it will fly apart

• One Bok globule can produce a single to maybe a dozen stars, and there can be many Boks in a nebula

• The stars form all at one time– Once bigger stars form they inhibit the formation of

new stars, and may affect smaller stars' evolution as well

• The population of stars formed from a nebula is called a cluster

• The stars are composed of the same stuff that made up the nebula: H, He, and small amounts of other materials

• Let's just think about the Sun, for now, a glimmer in the nebula's eye

• Over a period of hundreds of thousands to a million years, the region of the nebula that will become the Sun slowly does three related things:– It contracts

– It gets hotter

– It spins faster

Why those three things?

• It contracts because of gravity

• It heats up because of thermodynamic pressure

• It spins faster due to conservation of angular momentum

• Let’s look at these concepts one at a time

Gravity

• Isaac Newton developed a simple working definition for gravity 300 years ago– Einstein refined it 100 years ago, but we can

use Isaac’s for now: it is much simpler

• “Every mass in the Universe attracts every other mass with a force proportional to the product of their mass and inversely proportional to the square of the distance between their centers”

Simple, he said?

• Yes, it truly is:– Bigger masses attract each other more

– A small change in distance means a big change in the gravitational attraction

• So each little bit of the globule attracts every other little bit, which pulls them closer, which further increases the attraction, and so on.

Thermodynamic Pressure

• As the globule gets smaller, each particle is more likely to bang into another one

• The consequence of all this banging increases the pressure, much like people squeezing into the exits after a concert

• As pressure increases so does the temperature, like squeezing air in a bicycle pump

• Ultimately, the release of gravitational energy from the contraction provides the energy to heat up the gases

Conservation of Angular Momentum

• Momentum is a concept from Newton’s First Law of Motion:– An object in constant in constant

motion in a straight line tends to stay in constant motion (including spinning) in a straight line (or spinning in a circle) unless acted upon by an external force

But why was the globule spinning in the first place?

• It spins for roughly the same reason a hurricane spins

• Part of the globule is closer to the center of a spinning galaxy

• In a hurricane, part of the cloud mass is closer to the equator which has a faster speed than northern latitudes

This part of the globule (hurricane) is closer…

…than this part to the galactic center.

Getting from Here to Here

Stellar Eggs, and the “Yolk”

• The globule can be up to a light year across– Very cold ~100K, 106

particles/cm3

• After it contracts it is about the size of the Solar System– 10,000 years have passed– 1012 particles/cm3

– 105K core– IR

• It becomes a protostar when it is about the size of Mercury’s orbit– 100,000 years after starting– 106K core

• (surface 4000K)

– Visible light

Ready to Ignite

• Material above the rotation plane falls in

• Disk material moving too fast will condense into planets, asteroids, and cometary mass (ice)

• When the core temperature rises to 107K fusion begins

Nuclear Fusion:

• E = mc2 keeps the Sun going

• 5 X 106 tons of H ‘burned’ every second

• Yields 4 X 1026 W!– The Solar Constant

• Remember, +/- 0.07%

– Irradiance: ~750W/m2

• Requires the proper pressure and temperature– 1.5 X 107 K– 1.3 X 109 ATM

p+-p+ chain

Interior View

• We divide the Sun’s interior into three broad categories:– Core (where fusion

happens)– Radiative Zone– Convective Zone

• The surface of the Sun is called the Photosphere

In Balance

• The balance between the crushing force of gravity and the explosive force of fusion is called Hydrostatic Equilibrium– Hydro for fluid– Static for not moving– Equilibrium for balance

Energy Flow

• Understand this and you’ll understand much about not just the Sun but all stars

• Thermodynamic Law states that heat flows from a high level to a low level– Don’t mistake heat for temperature!

• Heat is a form of energy• Temperature is the level of the heat, not how much there is

• The outward flow of energy establishes hydrostatic equilibrium

• The names radiative and convective zones give a clue about how heat flows in the Sun

Surface Features

• Granules

• Spicules

• Sunspots

• Prominences

• Flares

Helioseismology

• Pulsations in the Sun cause it to ring with hundreds of modes of vibration

• Three prominent modes are shown

Granules

• The tops of convection rolls

• The yellow areas are the hotter upwelling plasma

• The orange regions are the cooler material starting to sink

• There are some sunspots in this picture

Sunspots

Early Records

• As early as 28 BC, Chinese Astronomers recorded dark patches on the Sun

• Galileo was the first to see sunspots through a telescope– Much consternation over an

‘imperfect’ Sun

• German Astronomer Johann Hevelius made this map in 1644

• Maunder Minimum– 1645-1715– Virtually no sunspots seen

Cause

• Convection currents carry huge amounts of charge

• Electric current in a loop makes a looping magnetic field

• The sunspots are evidence of that field

Rotational Influence

• The Sun is not a solid but instead is a sphere of fluid plasma

• Different latitudes rotate at different speeds

• The magnetic field lines stretch, pinch off, break the surface, and dissipate – and the cycle starts again

Cycles• 11 or 22 year cycle, depending on how you count

– 11 peak to peak

– 22 for polarity change

Long Term Changes

Recent Data

An aside: correlation vs causation

• In the case of causation, one parameter is the cause of another– Smoking can cause lung disease, not the reverse

• In the case of correlation, two or more parameters appear to track but are actually under the influence of an external, untracked variable– Gamblers often smoke, but gambling doesn’t cause lung disease

• One must examine graphs carefully to determine if the data depicted are just correlations or are actually causation. In the latter case, which is the cause and which is the effect is critical.

Climatic Effects

14C concentrations

• "The solar cycle may be going into a hiatus," Frank Hill, associate director of the National Solar Observatory's Solar Synoptic Network, said in a news briefing today (June 14, 2011). "This is highly unusual and unexpected," Hill said. "But the fact that three completely different views of the sun point in the same direction is a powerful indicator that the sunspot cycle may be going into hibernation.“

• “…[T]he recent findings indicate that the activity in the next 11-year solar cycle, Cycle 25, could be greatly reduced. In fact, some scientists are questioning whether this drop in activity could lead to a second Maunder Minimum, which was a 70-year period from 1645 to 1715 when the sun showed virtually no sunspots…”

Space.com article 6/14/2011, Denise Chow, author

Frost Fairs

• Popular from 17th until the early 19th century

• Thames froze every winter from 14th to 19th century

• Coldest during the Maunder Minimum

Spicules

• Spikes of superheated plasma– 5 minute cycle– 100,000 on the surface

at any one time

• Caused by standing waves at the solar surface – Remember

helioseismology?

Solar Prominences

Plasma caught in sunspot magnetic field loop

Solar Flares• When the magnetic loop twists and breaks, the

plasma is released into space

Flare Power

• A magnetic field can only store a limited amount of energy

• When the energy density in the field exceeds confinement it is suddenly and violently released

• The energy released is equivalent to millions of 100-megaton hydrogen bombs exploding simultaneously

Chromosphere

• The lower atmosphere of the Sun

• Low density hot gas

• ~ 8000 miles thick– Visible only during eclipse

• Varies from 4000K at the photosphere interface to 10,000K at the coronal interface

Corona

• Extremely hot outer atmosphere

• Up to 1 million K, heated by magnetic fields

• Very rarified

• Merges into the solar wind, 300-1000kps

Corona visible during eclipse

Solar Wind

Coronal Mass Ejection (Earth-Sun distance not to scale)

The Sun Does Affect the Earth

• Coronal Mass Ejections, Flares, radiation—all affect modern life

• Satellites– Radiation damage

– Atmospheric drag

• Power Grids– Induced currents

• Aurorae

Changes

• The output of the Sun changes on long time scales as well as short ones

• The Sun is about 25% more luminous than when its fusion engine first turned on– Due to convective mixing– The ambient temperature of the Earth has not changed

25% because the Earth also changes• The Sun will continue to heat up as it evolves but

will remain essentially the same for another 5 billion years

Timeline

Main Sequence

ZAMS

Late Age and Death of the Sun

• Hydrogen Core Exhaustion• Red Giant• Helium Flash• Horizontal Branch• Asymptotic Branch• Planetary Nebula• White Dwarf• Black Dwarf?

Hydrogen Core Exhaustion

• Eventually (~5 Gyr from now) there will not be enough hydrogen in the Sun’s core to maintain hydrostatic equilibrium

• Gravity wins temporarily, core collapses– Temperature increases– Radiant pressure pushes solar envelope outward– Surface cools– Outer layers expand, keeping luminosity more or less constant

• The Sun will expand, filling half the sky– One side of the Earth will see sunset as the other sees

sunrise!– At noon the Sun will fill the sky

Red Giant Phase

• About 1 Gyr has passed since HCE began• As the helium rich core contracts it heats up

enough for an inner shell of hydrogen to start fusing

• Surface temperature stays the same• Further expansion

– Expands out to Venus

• Luminosity increases

Helium Flash

• The contracting helium rich core grows hotter, reaching 108K

• Density up to 106 g/cc• Gas can no longer contract• Increased pressure without possibility of

expansion further heats the gas• Helium ignites throughout the core more or less

simultaneously

Horizontal Branch

• On the Horizontal Branch

• Fusion continues– He inside

– H in shell

• Very inefficient process

• HB lasts for 100Myr

Asymptotic Giant Branch

• Eventually, the He in the core is exhausted– C, O left– Not hot enough for C,

O to fuse

• Double Shell burning• More expansion• Outer cooling• Increased luminosity

Planetary Nebula• First seen by William

Herschel in 1784• Not planets!

– It fit in with solar system formation theories of the day

• See the naked C-O core?• Super energetic photons

from the interior disassociate molecules in the ejecta, causing unusual spectral lines– Misinterpreted in the 19th

century as a new element!

Mass Ejection

• Outer layers of the Sun will be blown off by the superwind created by the double shell burning in about 10,000 years

• The very hot (but not enough for further fusion) core is revealed

White Dwarf

• Naked, the C-O core cools and becomes less luminous

• Shrinks down to roughly the size of the Earth

• No more fusion• Surface temperature due to

gravity: 10,000K• Hugely dense: 1 tbls = 10

tons!

Black Dwarf

• The theoretical conclusion to the Sun’s existence

• The state after the white dwarf cools to ambient temperature (3K)

• No black dwarfs have been seen because:– They are black– It would take longer than the age of the

Universe to make one!

Remember:

• This is how our Sun will expire

• Different stars will end up differently, depending largely on their initial mass and how much mass they can shed

The End!