The Sun – A Typical Star

The Sun• The Sun is the largest object in the

solar system both in size and in mass

• Once worshiped as a god, it is now studied intently as the single, closest representative of an average star

• While just a 'simple collection of hot gas', it has amazing complexity and, of course, without it there would be no life on Earth

The Sun

We will NOT be looking at the Sun thru the 16"

Solar and Heliospheric Observatory

The SunPhysical Properties:

– Radius: 6.960 x 108 meter– Mass: 1.989 x 1030 kilogram– Luminosity: 3.847 x 1026 Watt– Average Density: 1.4 (Water = 1.0)– Rotation at equator: 24.7 days– Surface Temperature: 5800 ºK– Absolute Magnitude: 4.8– Spectral Class: G2– Luminosity Class: V

Solar Structure• Interior• Photosphere• Chromosphere• Transition Region• Corona• Solar Wind• Heliosphere

Solar Spectra

Solar Spectra

Solar Structure: Interior

• Core– Inner 25%– Energy Production

• Radiation Zone– Inner 53%

• Convection Zone– Outer 22 %

Solar Structure: Photosphere

The photosphere is the 'surface' of the Sun – where all the light appears to come from.

PhotosphereThe two most obvious things to note are

1. The 'sharp' edge of the Sun2. The dimming of the Sun's brightness as you look from the center to the edge

The seemingly sharp edge is caused because most of the light comes from the upper 200 Km (0.03%) of the solar disk

Limb Darkening

Limb darkening is caused by the opacity:

We can see only into a given distance; if we look at the center, then this relates to a greater depth (and therefore a higher temperature and brighter light)

Looking near the limb, we see a shallower depth, cooler temperatures and diminished light

Photospheric Features

• Sunspots• Faculae• Granulation• Supergranulation

Sunspots• They are dark markings on the

photosphere that can be easily seen.

• They measure a few tens of thousands of kilometers across.

• They are usually found in groups of several to a hundred spots, most of them small but often dominated by one or a couple large spots.

• They consist of two parts, a dark umbra and a lighter penumbra. Sunspots are areas where a concentrated magnetic field protrudes through the hot gases of the photosphere.

• The field inhibits convection from below, making sunspots about 2500 degrees K cooler than the surrounding area

• They are found in pairs, one with N magnetic polarity, the other with S polarity

Sunspots

Like the sunspot, squares A and B are exactly the same color of gray

Sunspot Group

A Bit Closer

Sunspot Cycle• The sunspot cycle is a periodic event

where the number of sunspots climbs from a minimum to a maximum in 11 years

• Magnetic Polarity flips and another 11 year cycle ensues

• Sometimes the minimum is little or none – Maunder minimum (1640 –1700 AD)

• “Little Ice Age”

Max Min

It seems that the Sun is not being very well behaved lately.

The current Solar maximum is very small - launching a debate between the Global Warming advocates and the ‘perhaps a new little ice age’ advocates.

SunspotsSpots start a cycle at about 30 degree latitude and move toward the equator as they form later in the cycle - giving rise to the Butterfly Diagram

This magnetogram of the Sun's disk was obtained by the MDI instrument on board SOHO on 27 August 2006.

The magnetogram shows the strength and sign of the magnetic field in the Sun's photosphere in greyscale:White

• strongly positive• outward directed magnetic

field• north (N)

Black• strongly negative• inward directed magnetic field• south (S)The sunspots in the active region just below the centre of the Sun's disk have

the reverse orientation compared to the sunspots of the current solar cycle. This region - no. 905, as assigned by the Space Environment Center - is located in the southern hemisphere with the white (N) spots leading and the dark (S) spots trailing with respect to the Sun's rotation.May be the start of the reversal process.

Solar Rotation

Babcock’s Magnetic Dynamo Theory

Faculae• Faculae are irregular

bright patches on the solar disk.

• Often, sunspots will be seen to be embedded in faculae, though free-standing faculae are also commonly seen as well.

• Faculae are most easily seen near the solar limb,

• They are about ten percent brighter than the bare photosphere.

Granulation• Granulation is the fine-

grain structure of the photosphere.

• Individual 'grains' are about 1000 km across.

• The granulation is constantly changing, usually over time scales of minutes or less.

• Each  'grain'  is a convective cell which consists of a bright, roughly polygonal area of hot rising gas, and a cooler edge channel of descending gas.

Supergranulation is a larger scale, about 30000 Km across, version of the same effect.

Granulation

Solar Structure: Chromosphere

• Plages• Spicules• Flares• Promenences and Filaments• Coronal Mass Ejections (CMEs)

Plages• Plages are bright emission

regions in the chromosphere which surround photospheric sunspots.

• They coincide with faculae in the photosphere beneath them.

• Plages are irregular in shape and variable in brightness, marking areas of nearly vertical emerging or reconnecting magnetic field lines.

Spicules• Spicules are small jet-

like eruptions. • They are usually seen

as dark streaks in hydrogen-alpha light except at the limb, where they are seen as emission features.

• They last only a few minutes and eject material into the corona at speeds up to 30 km/second.

Flares• Flares are the most violent

eruptions on the Sun. They occur in the chromosphere and corona above complex sunspot groups.

• During a flare temperature in the region rises to 5 million degrees Kelvin.

• Vast quantities of particles and radiation are released into space.

• The flare lasts no more than 20 minutes.

• Flares are the result of magnetic field stress and are seen most often where lines of opposite polarity conflict.

An X Flare – as seen by SOHO

Medium X-Ray Flare

Promenences and Filaments• Prominences are clouds of

material suspended above the chromosphere by magnetic field loops.

• When seen against the chromosphere, they are visible by absorption and are called filaments.

• When seen near the limb, they can be seen against the sky as emission features.

• They generally come in two broad classes: active and quiescent. – Quiescent prominences occur away

from active regions and last for many months.

– Active prominences are associated with sunspots and flares. They have violent motions, change fast and last for only a few hours.

• Coronal Mass Ejections occur when the tops of the prominences snap and eject material into space

The Solar Limb

The Solar Limb

Solar Structure: Corona

• Scattering of light from the photosphere by electrons

• The ionized gas is due to the temperature of 2,000,000 K

• Most of the light is in the X-ray range

Corona• Coronal holes are

large regions of  exceptionally low density and temperature in the corona.

• They last for several rotations of the Sun

• They are the source of the strongest solar winds

The corona in X-ray

Solar Structure: Solar Wind

• The flow of coronal gas into space is called the Solar Wind.

• It moves at a speed of 450 Km/s which means it moves past the Earth in about 4 days.

• A denser stream (at 700 Km/s) is emitted from a coronal hole

Coronal Mass Ejections

• The boundary between the region of space dominated by the Sun’s magnetic field and interstellar space is called the Heliopause– This seems to occur at about 100 AU

• The region within is known as the heliosphere

Solar Structure: Heliosphere

Where does the energy come from?

The intensity of sunlight on the Earth is the Solar Constant = 1400 Watts/meter2

Therefore the Solar Luminosity is 1400 Watts/meter2 times the area of a sphere at 1 AU

= 4 x 1026 Watts

Source of EnergyIdeas in the past have included:

– Combustion• Burning Hydrogen in oxygen produces about 107 Joules/Kg; at

the Sun’s energy output this would last about 50 billion seconds to consume all the mass of the Sun, or about 2000 years

– Meteor Impact• Objects falling into the sun impact at about 600 Km/sec• To match the energy output, about 1/10 of the Earth’s mass would

have to fall in each year (We don’t see this much!)• The additional mass would increase the gravitational force and cause

the Earth’s year to shorten by about 1 minute/century. (This is easily measured, but doesn’t happen)

– Gravitational Collapse• If the sun gets compressed under its own weight, the energy output

would imply that each 1 meter reduction would be about 10 days worth of energy

• This 1 Kilometer reduction every 50 years would be unnoticable• If the sun began as a large gas cloud, it would have reached its present

size in about 20 million years• Geological records indicate 3-4 billion years

Source of EnergyNuclear Fusion

Conversion of Hydrogen to Helium is a much more efficient process than combustion

There are two main processes:– The Proton-Proton Reaction– The Carbon-Nitrogen-Oxygen

Reaction

T < 10,000,000 KAt less than 10 million degrees, there is not enough energy to force the protons together against the repulsive electromagentic forces

Proton-Proton ReactionT > 10,000,000 K

Proton-Proton ReactionT > 10,000,000 K

Carbon-Nitrogen-Oxygen Reaction

T1/2 =124 s

Carbon-Nitrogen-Oxygen CycleT > 16,000,000 K

Diffusion of Energy

Prof. Gerhard H. JirkaInstitute for HydromechanicsUniversity of Karlsruhe

Diffusion of Energy

120,000 170,000 220,000

Energy produced in 1 sec in the Sun’s core

Energy reaches the Sun’s surface in 170,000 yrs but is spread out over 100,000 yrs

Brightness is insensitive to change

Solar Neutrinos• Produced in nuclear reactions,

these particles are so weakly interacting that the Sun is transparent

• Every second 1016 neutrinos pass through your body

• Problem: Where’s the neutrinos?

Super Kamokande