The Sun

Young Astronomers Blog, Volume 29, Number 15.

If you travel far enough north (or south) and look up at the night sky, you might see some swirly patterns of light. These are the northern (or southern) lights, more correctly called Aurora Borealis (or Aurora Australis). Auroras start with the Sun.

The Sun is huge ball of plasma made up mostly of hydrogen with some helium. The Sun is huge with a diameter of 864,000 miles. It is so large that it contains over 99% of the mass in the Solar System. Over one million Earths could fit inside with room left over. The Sun is spinning (rotating). It completes one rotation every 25 days at its equator and one rotation every 36 days at its poles. The Sun is tilted just a little (7 ¼ degrees) to the plane of the Solar System.

The Sun is a star, thought to be around 4 ½ billion years old. Stars are so massive that their internal heat is sufficient to ignite nuclear fusion, which turns hydrogen into helium and creates huge amounts of energy. Stars exist because of a balance between the force of gravity trying to collapse the star with the pressure of the nuclear reactions pushing outward from the star’s core.

Our Sun is a yellow dwarf star, officially classified as a type G2 V star using the Morgan-Keenan system. This system of classification assigns the letters O B A F G K M (and a number) to stars based on their color and temperature. The classification is further enhanced by a letter corresponding to the stars’ luminosity. For more on this see the Harvard Computers.

The Sun can be found midway along the main sequence of the Hertzsprung-Russell (H-R) diagram. Larger hotter (bluer) stars are to the upper left and cooler smaller (reddish) stars are to the lower right. Stars remain in the main sequence for most of their lifetime. Once their hydrogen fuel is exhausted, stars such as the Sun and smaller will first expand into a red giant and move to the right in the H-R diagram. They will then collapse into a white dwarf and move toward the left middle of the H-R diagram. Larger stars will expand into red giants and eventually explode in what is called a supernova. The remnants of the explosion will then collapse into a neutron star or black hole depending on the size of the star. For more on this see Betelgeuse is Dimming.

The Sun has six distinct layers. (See NASA “Layers of the Sun”).

• The Core is at the center of the Sun (20%). Here the pressure is so great and the temperature so hot (27 million degrees Fahrenheit) that nuclear fusion takes place converting millions of tons of hydrogen into helium every second, while a small percentage is converted into energy (e = mc²).
• The Radiative zone surrounds the core (54%). Here, photons of light are absorbed and reemitted by subatomic particles. The energy from the core slowly moves through this zone losing energy over a one to two hundred-thousand-year journey.
• The Convection zone (26%). Streams of hot gas cycle through this zone as they are carried upward toward the surface and then cool and sink back down.
• The Photosphere (250 miles wide) is the Sun’s surface where the temperature drops to around 10,000^o Light and energy escape from this level eventually reaching the Earth.
• The Chromosphere (250 to 1,300 miles) is the first layer of the Sun’s atmosphere. Here the temperature rises to 14,000^o
• The Corona is the second layer of atmosphere, where temperatures reach over one million degrees F. The Corona is much less dense than the Sun’s surface. So, it isn’t visible against the bright glare from the surface. It can only be seen with special solar telescopes or during a total solar eclipse.

The core, where nuclear fusion process takes place, is the Sun’ engine. This process is a bit more complicated that just four hydrogen atoms combining to form helium. It is described as the proton-proton chain. It starts with four hydrogen atoms, goes through three steps, and ends with one helium and two hydrogen atoms.

1. Two sets of hydrogen (H¹) atoms fuse to create one atom of deuterium (H²) each. They also emit a positron, neutrino, and some gamma radiation. In this step, four protons are converted into two protons and two neutrons. 4H¹ -> 2H² + u + p + g.
2. Next, a hydrogen (H¹) atom fuses with each of the deuterium (H²) atoms to create two atoms of Helium-three (He³). They emit some more gamma radiation. The internal structure remains four protons and two neutrons. 2H¹ + 2H² -> 2He³ + g.
3. Finally, the two helium-three (He³) atoms fuse to create a single helium four (He⁴) atom and two hydrogen (H²) atoms. Again, the internal structure remains four protons and two neutrons. 2He³ -> 2H¹ + He⁴.

Electromagnetic radiation pours out of the Sun reaching the Earth in around eight minutes. It is this light and energy that drives much of the weather on the Earth and supports life on the planet. The Sun, however, emits more than just light. The surface and exterior of the Sun exhibits “weather” in a form quite different from the weather we experience here on the Earth. The Sun has a huge magnetic field, or more precisely multiple fields. Unlike the Earth’s magnetic field that lines up north and south, the Sun’s magnetic field is twisted and bent due to the nature of the Sun’s rotation.

The Sun produces a steady flow of particles called the Solar Wind. If you’ve ever observed a comet in the night sky, you might have noticed a gas tail that always extends away from the Sun. There is also a dust tail that is more in line with the comet’s movement. In the 1940s, Cuno Hoffmeister and Ludwig Biermann suggested that this gas tail was due to particles streaming from the Sun. It wasn’t clear what could cause this until 1958 when Eugene Parker explained that the high temperature of the corona was too much for the Sun’s gravity to hold on to all its atmosphere. Because of this, charged particles speed away from the Sun at up to one million miles per hour. At first, Parker’s theory wasn’t well accepted. However, Mariner II confirmed the existence of this stream of particles a few years later.

Sunspots are small dark (cooler) spots on the surface of the sun resulting from the interaction of surface plasma with the Sun’s magnetic field. Although relatively small and dark, they are larger than the diameter of the Earth and brighter than a full moon.

Sunspots typically follow an 11 year “Sunspot cycle”. These cycles have been tracked since around 1750. We just ended the 24^th cycle, which peaked around 2013. We are now beginning the 25^th cycle, which could peak in the 2023-26 timeframe. There is a longer twenty-two-year cycle as the Sun’s magnetic field flips poles every eleven years. This typically occurs in the middle of a sunspot cycle during the peak sunspot activity. Note that the terms sunspot cycle and solar cycle are often used interchangeable, and generally refer to the 11-year cycle in sunspots. Although the term solar cycle can also refer to the twenty-two-year cycling of the magnetic poles (+- to -+ to +-).

Sunspots are associated with huge disturbances of energy and ionized particles that extend above the surface.

• Solar Prominences are clouds of hot plasma that extend over the surface of the Sun carried by loops in the Sun’s magnetic field. Prominences are best viewed during a solar eclipse or through a solar telescope.

The Sun’s magnetic loops will twist, bend, and sometimes break. When this happens, immense explosions can occur.

• Solar Flares send bursts of electromagnetic radiation that fan out into the Solar System at the speed of light.
• Coronal Mass Ejections (CMEs) send charged particles (plasma) and magnetic fields into space in a specific direction. CMEs are slower than solar flares and can take up to four days to reach the Earth.

Most of the time, charged particles are harmlessly deflected by the Earth’s magnetic field toward the poles where they are observed as auroras. Auroras are caused as the charged particles interact with molecules in the Earth’s upper atmosphere. Although, usually driven by the solar wind, auroras can intensify as the results of a solar flare or CME.

The extent of the Sun’s influence extends out to almost 100 AU, far beyond the planets and dwarf planets. The two spacecraft, Voyager 1 and Voyage 2, have reached this point and are starting to move out into interstellar space.

• The Heliosphere is the bubble of plasma created by the Sun where the solar wind is greater than that of the interstellar medium.
• The Heliosheath is the area at the outer edge of the Heliosphere and is bounded by the Termination Shock (where the solar wind substantially slows down) and the Heliopause (where the solar wind meets interstellar space),
• Out beyond the Heliopause is the Bow Shock, which is a shock wave created as the Solar System moves through interstellar space.

Studying the edge of the Solar System is NASA’s Interstellar Boundary Explorer (IBEX). which was launched in 2008. Just recently a group of scientists from the Los Alamos National Laboratory completed the first 3D model of the heliosphere using data from IBEX.

Over the last half century, many spacecraft have been sent to observe the Sun. A few of the more notable missions are the:

• Solar and Heliospheric Observatory (SOHO) launched in 1995.
• Solar Dynamics Observatory (SDO) launched in 2010.
• Parker Solar Probe launched in 2016.
• Solar Orbiter launched in 2020.

You can view the sun (online and in safety) at NASA’s Solar and Heliospheric Observatory (SOHO) and Solar Dynamics Observatory (SDO). Here you can see the sun in many different wave lengths as well as checking out the latest solar weather.

For a more detailed look at the current space weather conditions, visit the NOAA Space Weather Prediction Center. These folks track the latest geomagnetic storms, solar radiation storms, and radio blackouts, each on a scale of 1 to 5 (minor to extreme).