Astronomy Picture of the Day

The Sun

• Core temperature - 15 million K

• Surface temperature - 6000 K

• 99.9% of all of the matter in the solar system

• Entirely gaseous

• Supplies the energy essential for life on Earth

• All normal matter was created inside a star similar to the sun!

The Sun

The Sun in X-rays over several years

The Sun is a star: a shining ball of gas powered by nuclear fusion.

Mass of Sun = 330,000 MEarth

Radius of Sun = 109 REarth

Luminosity of Sun = 4 x 1026 Watts(Equivalent to 100 billion 1-megaton explosions every second!)

Why doesn't the sun blow itself apart?

Hydrostatic Equilibrium

Internal heat and pressure directed outward Gravity pulling mass inward Two forces balance When all hydrogen transformed to helium, Sun will begin to collapse

Let's start with the core, where the Sun's energy is generated.

The Interior Structure of the Sun

• Core - nuclear fusion• Radiation zone - completely

ionized => no photon absorption

• Convection zone - less ionization => photon absorption

• Photosphere - radiation, the part of the sun that we see

• How do we get this info?

What Powers the Sun?

Nuclear Fusion: An event where the nuclei of two atoms join together.

Need high temperatures. Why? To overcome electric repulsion.

Energy is produced. (A small amount of mass = a lot of energy)

E = mc2. Sum of mass and energy always conserved in reactions. Mass is just “frozen” energy.

Why High Temperatures?

To overcome electric repulsion High temp => fast atomic motion (Molecular Motion Demo)

Nuclear forces - very short range (10-15m)

100 times EM force

1039 times gravity(“Fusion” Demo)

How does energy get from core to surface?

core

"radiation zone":

photons slowly drift outwards

"convection zone"

"surface" or photosphere: photons escape into space.

photon path

Sunspots (evidence for 11 year cycle)

Roughly Earth-sized

Last ~2 months

Usually in pairs

Follow motion of sunspots => can determine rotation rate(Sunspot Demo)

Magnetic Fields

Rotating Earth generates a magnetic field

- Like bar magnet aligned with Earth’s axis

Influences paths of electrically charged particles

AuroraeCharged particles from solar wind spiral towards polesCollide with and excite atmospheric moleculesEmission spectra!

The Sun's Rotation

The Sun exhibits Differential Rotation just like the Jovian planets. ~25-36 days

What is this? (animation)

What might this mean for the magnetic field of the Sun?

Sun’s Magnetic FieldRotating sun also generates a magnetic fieldDifferential rotation => magnetic field distortion

Loops extending beyond photosphere can form

SunspotsThey are darker because they are cooler (4500 K vs. 5800 K).

Related to loops in the Sun's magnetic field.

radiation from hot gas flowing along magnetic field loop of Sun.

Solar Flares

Violent ejection of large amount of particles from Sun surface.

(animation)

Above the photosphere, there is the chromosphere, transition zone, and...

The Corona

Best viewed during eclipses.

Solar wind => Evaporation

1 million tons/s lost. But Sun has lost only 0.1% of its mass from solar wind in its 4.6 billion year lifetime.

Apparent Brightness vs. Luminosity

Luminosity – Intrinsic power output

Apparent Brightness – Perceived intensity when viewed from some distance

Does an object emitting light appear brighter or dimmer as it moves away from you?

Apparent Brightness and Distance

• Calculate luminosity, measure apparent brightness => figure out distance!