Week 6The Sun

Stellar Evolution:

Low Mass Stars

White Dwarfs

Reading:

Chapter 9, Sections 1, 4, 5a (10 pages)

Chapter 10, Sections 3, 5 (6 pages)

Chapter 12, Sections 1-3 (10 pages)

The Sun

93,000,000 miles from Earth

• Photosphere – visible “surface” of the sun

• Chromosphere – layer just above photosphere. Sort of the “lower” atmosphere.

• Corona – above the chromosphere. Sort of a hot “upper” atmosphere.

The Sun’s Outer Layers

The visible top layer of the convection zone is granulated, with areas of upwelling material surrounded by areas of sinking material:

The Corona• The Sun’s hot “upper” atmosphere

• Extremely hot (~1-2 million K) – opposite of what you would expect

• Very low density, highly ionized gas

• Some coronal gas is moving so fast that it escapes the pull of gravity of the Sun and escapes as the solar wind.

Solar corona can be seen during eclipse if both photosphere and chromosphere are blocked:

The Solar Corona

Sun Size Tutorial

The Sun is not static.

Quiet Sun vs. Active Sun

Active Sun

Quiet Sun

Sunspots

Dark “blotches” on the “surface” (photosphere) of the Sun

Umbra

Penumbra

Sunspots come and go, typically in a few days.

Sunspots are linked by pairs of magnetic field lines:

Sunspots

The rotation of the Sun drags magnetic field lines around with it, causing kinks

Sunspots and Magnetic Fields

The Sunspot Cycle

The number of sunspots varies with time.

11 year cycle from one max to the next.

Last max 2000

Coming out of minimum right now (2008)

The Sunspot Cycle

Sunspots “live” closer to the equator later in the cycle; farther from the equator early in the cycle.

Sunspot location (latitude) vs. time

The Sun has an 11-year sunspot cycle, during which sunspot numbers rise, fall, and then rise again:

The Solar Cycle

Sunspots and Magnetic Fields

• Sunspots are caused by the Sun’s magnetic field

• Sunspots occur where the Sun’s magnetic field breaks through the “surface” of the Sun

• The magnetic field prevent hot, ionized material from rising

• Dark sunspots result

The Solar Cycle

The Sun’s magnetic field is responsible for the sunspot cycle.

This 11 year cycle is just half of a 22 year solar cycle.

The north magnetic pole becomes the south magnetic pole after 11 years.

After 11 more, it is back to being the north magnetic pole again.

Prominences and Filaments

Prominences are filaments seen edge on.

Solar Flares

Solar flares – eruptions in the Sun’s atmosphere that cause energetic particles to escape from the Sun.

Not the same as the solar wind, which is continuous

Probably related to filaments/prominences. Flares simply escape instead of falling back down.

A solar flare is a large explosion on Sun’s surface, emitting a similar amount of energy to a prominence, but in seconds or minutes rather than days or weeks:

Solar Flares

Solar Flares Movie

X-ray Movie of the Corona

A coronal mass ejection emits charged particles that can affect the Earth:

Coronal Mass Ejection

CME

3-day Solar-Geophysical Forecast issued Oct 1 at 22:00 UTC

Solar Activity Forecast: Solar activity is expected to remain at moderate to high levels. Isolated M-class flares are probable. The is also a chance for a major flare from Region 9632 (S21W73) as it begins to exit the disk.

Geophysical Activity Forecast: The geomagnetic field activity is expected to be at unsettled to minor storm levels for the next two days due to CME effects. Mostly unsettled conditions are expected on the final day of forecast. The greater than 10 MeV proton event is expected to continue through most of the forecast period.

Solar Forecast

Universe Video

Ch 3. – Sunspots and magnetic fields Skip to 16:55 FlaresCh 4 – CMEs – 27:50Ch 5 – 34:11 11 year cycle 35:40Ch 6 – start to 38:48

How Do Stars Shine?Stars shine by nuclear fusion: the process of extracting energy from the fusion of lighter elements into heavier elements.

The main fusion process is the conversion of 4 Hydrogen atoms into 1 Helium atom.

However, one Helium atom is slightly less massive than the 4 Hydrogen atoms combined.

Nuclear FusionThe “extra” mass is converted into energy according to Einstein’s famous formula:

E = mc2

The amount of energy created per Helium atom is small, but there is a LOT of Hydrogen in stars.

Fusing Hydrogen into Helium requires temperatures greater than 15 million degrees Kelvin.

The pressure at the core of Main Sequence stars causes the core to heat up to over this temperature.

Hydrostatic Equilibrium

Fusion keeps stars from collapsing under their own weight. Pressure from the outflowing hot gas balances the pressure of gravity.

This process is called hydrostatic equilibrium

TMBG