The Sun: Our Star

The Sun is an ordinary star and shines the same way other stars of its type do.

The bright part normally seen is called the photosphere, which is about 500 km deep.

It is an almost perfect black body with a temperature of 5800 K.

Assigned Reading

• Chapter 8.

Why is the Sun important for life?

Provides virtually all of the energy necessary for life on Earth: food, weather, etc.

What is the Sun?

• The Sun is a star, a failry common and typical star• Its spectral type is G2• A star is a ball of gas held in equilibrium against

its own self-gravity by the thermal pressure and outflow of energy from cener to the surface

• This energy comes from nuclear fusion reactions at the center of the Sun

Balance in the stars

ThermalPressure

GravitationalContraction

Pressure and Temperature of a Gas

This balance between weight and pressure is called hydrostatic equilibrium.

The Sun's core, for example, has a temperature of about 16 million K.

How does a star hold itself?

What is the Sun made of

It is made of hot gas; there is no solid material in the sun!

The gas is mostly hydrogen (~80%) and helium(~20%), as is in the whole universe

Why does the Sun have such a sharp edge?

An optical illusion

Structure of the Sun

The Atmosphere of the SunAlthough the sun appears to have sharp edge, its “surface” actually has complicated structure.

Its atmosphere, too, is complexIt consists of:

Photosphere: 5,800 K, continuum + abs. Chromosphere: 4,500-500,000 K, emiss.+abs.

Corona: 1-2 106 K, continuum+emiss.+abs.

Corona (1-2,000,000 K)

(4500-500,000 K) (5,800 K)

The light we see originates from the thin photosphere

Each square millimeter of the Photosphere emits the same Energy as a ~60 Watt light bulb

The Sun surface (photosphere) would cool off very quickly if Energy were not produced continuously from below

spicules

What would you expect to see if you looked at the Sun’s

photosphere now?

• The photosphere: the region where the black body radiation that we see is made

• Thin gas: 3,400 times thinner than air• It is the transition region from a black-body emitting

plasma to a transparent gas• Its upper layer produces the absorption spectrum• Limb darkening: proof that the Photosphere has a

thickness and that energy comes from below• How does the photosphere look like if we close up

on it?

The limb is slightly fainter

Granulation of the Photosphere

Each Granule is about the size of Texas and lasts for only 10-20 minutes before fading away!

Granulation of the Photosphere

Each Granule is about the size of Texas and lasts for only 10-20 minutes before fading away!

Granules: convective cells

• The granules are just densely packed convective cells

• Convective cells are very similar to thunderstorms

Sunspots

Sunspots, about 1000 K cooler than the rest of the Sun's photosphere, appear as dark spots.

Sunspots come and go with time. Big sunspots can live for several weeks.

ChromosphereReddish in color, which is the origin of its name (chromos meaning ``color'')

2000-3000 km thick

Faint relative to the photosphere

From 4,500 up to 500,000K: hotter than the photosphere and much less dense.

Absorption + Emission-line spectrum

Corona- the outermost layer- T~1 million K-made up of very diffuse(but extremely hot gas)- coronal emission is dominated by X-rays

X-ray emission from the Sun's corona

The corona is so hot that it emits X-ray radiation

Rotation period: about a month

The middle rotates faster than the north or south.

Corona has an emission line spectrum and a continuum spectrum (dust scattering of photospheric black body light). Some of the continuum spectrum has no absorption lines, because of the dilution of the absorption lines by very hot electrons

The heating of the corona

The heating of the corona has puzzled scientists for a long time.

The corona is heated by energy outflowing from the sun's interior not as heat but as magnetic energy.

The gas is heated by the motion through it of long lines of magnetic force, which act like whips. The gas is being “whipped”.

How can heat go from cooler regions to hotter ones?

It doesn’t! Radiant heat penetrates the corona nearly Undisturbed

Corona heated by something else: whipping by magnetic force lines

UltraViolet (UV) image of the Sun.

It shows regions of the Photosphere above Sunspots are more energetic than elsewhere.

It also shows magnetic link between Sunspots extending into the Cromosphere and Corona

Solar Prominence

Composed of hot gas trapped in magnetic fields extending from one sunspot to another.

Prominence

Solar Flare

A solar flare is a violet outburst that lasts in an hour or less. It radiates X-ray, ultraviolet, and visible radiation, plus streams of high-energy protons and electrons.

A large flare can be a billion times more energetic than a large hydrogen bomb.

Solar Wind

Sun loosing mass, but only a small fraction: 10-14 Mo/yrMass loss will intensify later, and will much larger (~1/3)

Auroras: the Northern and Southern Lights

May 11, 2002• Auroras seen as far south as New England

• Caused by a gust of solar wind

• These powerful gusts are guided by Earth’s

magnetic field and can excite gases in the upper atmosphere, causing the air there to glow

The key point here is that nearly all “solar weather” is a result of changes in the magnetic fields that penetrate the Photosphere.