The Sun

The Sun - the nearest star

mass is around 2 x 10^30 kg - about 330,000 Earth's in mass

150 million km from Earth (approx) - about 8.3 light minutes

about 28 g's in surface gravity

Density is around 1/4 Earth's

1.3 million times the volume of Earth

12,000 times the surface area of the Earth

escape velocity - about 618 km/s (around 55x Earth's)

angular size - about 0.5 arc minutes

moving around 220 km/sec around the center of the Milky Way

Sun’s composition is typical of a star
 ~94% H
 ~5.9% He

Radiation from the photosphere peaks in the middle of
the visible spectrum - We’ve evolved over time to have our eyesight most
sensitive there

About 10,000 km (~1.5% of solar radius) thick, directly above photosphere, is the jagged, spiky layer called the chromosphere (seen pinkish during an eclipse)

Above this layer is the corona, extending tens of millions of kms into space - solar wind; We’re bathed in this solar wind

Photosphere - the part we see

Surface temp about 5800 K

We can resolve details to about 700 km across

Granulation at surface, but current telescope resolution isn’t quite enough to distinguish details
Photosphere oscillates up and down, which tells us about interior (temperature, density, rate of rotation of interior)

Chromosphere

Look at sun through an H-filter to see this layer

Temp is between7000 K and 15,000 K - higher than photosphere

Composed of small spicules - jets of gas rising and falling, looking like blades of grass some 7000 km tall, 700 km across, lasting 5 to 15 minutes

Corona

Very  irregular in shape - streamers away from sun

Too faint to see, except during eclipses

Temp near 2,000,000 K at lowest levels

Emits mainly x-rays

There are a few “cooler” holes - out of which solar flows to Earth

Corona is not uniform

Sunspots

11-year cycle (first noted in 1850)

Dark center - umbra

Surrounded by penumbra

Regions of high magnetic field strength (1000’s of times stronger than that of Earth)

Discovered by Galileo in 1610 by projection of solar image

There was an instance between 1645 and 1715 where there were NO sunspots - the Maunder minimum - activity is less regular than might be thought, though the evidence is a bit sketchy; perhaps Earth’s
climate affected observing