Solar Activity, Sunspots, and Stellar Snapshots

Posted on Feb 27, 2025 in Geology

Last Friday’s Lecture (February 7th): Religion Class Assignment

The Sun’s nuclear fusion is achieved by a chain reaction, confirmed by detecting neutrinos.
A site “thermostat” maintains a constant core temperature.
Solar radiation is created as gamma rays. It takes a million years to reach the surface after many scatterings.
Convection carries solar energy for the final third of its journey to the surface, causing “granulation.”
The Sun’s “surface” = photosphere = thin gas.
Above the photosphere are: 1) chromosphere, 2) corona (faint, thinner). That gas is much hotter than the photosphere.
Heating of the chromosphere and corona is caused by motions from the photosphere carried by magnetic fields.

The gas atoms in the hot corona move at high speed (high kinetic energy).
The Sun’s gravity cannot hold the hot gas.
As a result, the corona evaporates from the Sun = Solar wind.
At the Earth, the solar wind is still hot (100,000 K), flowing at the rapid speed of 400 km/s, distorting the Earth’s magnetic field.

(Did you say that we can see the Northern Lights from our bedroom?)

The Sun is constant or “quiet” due to the balance of gas pressure and gravity: fusion in the core, convection, and temperature of the photosphere, chromosphere, and corona.
But, the Sun is also highly variable = “Active.”
Sunspots are one example of activity – They exist only for a few hours to a few weeks.

The number of sunspots varies with time.
The cycle time is about 11 years, but it is variable by plus or minus a few years (could be 9-13 years).
The cycle has been observed since the 1600s, but the spots have vanished for decades.
The Sun is actually brighter with more sunspots than with fewer.
Indirect evidence indicates the cycle has gone on for thousands of years or longer.

Sunspots also form at different locations on the Sun during the 11-year cycle:
- At the beginning of the cycle: the few spots form and live = 30 degrees N and S of the equator.
- At the middle of the cycle: the many spots form and live = 15 degrees N and S of the equator.
- At the end of the cycle: the few spots form and live = 5 degrees N and S of the equator.

Sunspots emit less light than the adjacent photosphere, so they appear darker.
Sunspots emit less light because they are cooler (4000 K) than the photosphere.
Sunspots are cooler because of powerful magnetic fields: 100x stronger than the surrounding gas.
The magnetic field stops convection, reducing energy reaching the surface, thus reducing the temperature.

(If sunspots emit less light, ultimately from reduced temperature, how is the Sun brighter with more sunspots than with fewer?)

In the other hemisphere, solar sunspot pairs all have the opposite magnetic orientation.
Example: left spot = N, right spot = S.
Each hemisphere switches its magnetic orientation each sunspot cycle.

The Sun’s basic properties (temperature, radius, luminosity, etc.) are constant, which is good for life on Earth.
The Sun varies in many ways, which impacts life on Earth.
Many things we observe on the Sun are still poorly understood and are being studied intently.

We know the Sun’s basic properties (Ch. 10).
How would the Sun be different if it had a different age, diameter, mass, composition, or temperature?
Astronomers cannot experiment; this is where the snapshot approach comes in.
“Snapshot” = Observe many stars as they are now: some are young, some are old, some are hot, some are cool, some are more massive, and some are less massive.
Though we cannot do experiments, by looking at lots of stars, we can make comparisons and use inductive reasoning to compare certain attributes to our Sun.