SCIENCE7 - Our All-Powerful Sun

 

I’ve written about the history and science of the universe and its countless galaxies of stars, the Earth, and the Moon.  It’s time I wrote about the Sun.

 

After a short introduction, I’m going to review the Sun’s history and science in terms of how and when our solar system was formed; the Sun’s star classification; early human observations of the Sun; our exploration efforts to date; the Sun’s physical characteristics - including its composition, structure, how it work, and solar activity; the Sun’s location and motion within the Milky Way galaxy, and finally, the life cycle of the Sun.

My principal sources for this article include “Our Sun,” solarsystem.nasa.gov; “Earth’s sun:  Facts about the sun’s age, size and history,” space.com; “Sun - astronomy,” britannica.com; “The Sun,” universetoday.com; “Life Cycles of Stars,” imagine.gsfc.nasa.gov; “Sun,” Wikipedia; and numerous other online sources.

The Sun has been called by many names.  The Latin word for Sun is “sol,” which is the main adjective for all things Sun-related:  solar.  Helios, the Sun god in ancient Greek mythology, lends his name to many Sun-related terms as well, such as heliosphere and helioseismology.

The Sun is one of more than 100 billion stars in the Milky Way galaxy.  It’s an average sized star at the center of our Solar System, with eight planets revolving around it, about 93 million miles from Earth.  The Sun is a nearly perfect sphere of hot plasma (a soup of ionized atoms), heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light, ultraviolet light, and infrared radiation. 

Life as we know it could not exist on Earth without the Sun.  The Sun warms our seas, stirs our atmosphere, generates our weather patterns, and gives energy to the growing green plants that provide the food and oxygen for life on Earth.  Without the Sun's heat and light, the Earth would be a lifeless ball of ice-coated rock.

 

In this closeup photo of the Sun, the small black dot on the upper left of the Sun's disk is the planet Venus passing in front of the Sun.

Formation of the Solar System

Prevailing theories say that our universe was created from a single point with infinite density by the “Big Bang” explosion, roughly 13.8 billion years ago.  After an initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms.  Giant clouds of these earliest elements later coalesced through gravity to form galaxies (gravity-bound systems of stars).  Our Sun is a star in the Milky Way galaxy.

The Sun formed about 4.60 billion years ago out of a giant, spinning cloud of gas and dust called the solar nebula.  As the nebula collapsed under its own gravity, it spun faster and flattened into a disk. Most of the nebula's material was pulled toward the center to form our Sun, which accounts for 99.8% of our solar system’s mass.  As the solar nebula collapsed, it also began to heat up with the increasing pressure.  Gravity and pressure within the core of the cloud generated a lot of heat as it accumulated more matter from the surrounding disk, eventually triggering nuclear fusion to power the sun (see below).

Much of the remaining material in the solar nebula formed the planets, planet moons, asteroids, and comets that now orbit the Sun.  (Earth formed about 4.54 billion years ago.)  The rest of the leftover gas and dust was blown away by the young Sun's early solar wind, the continuous flow of charged particles from the Sun.

The Sun rotates on its axis.  Its spin has a tilt of 7.25 degrees with respect to the plane of the planets’ orbits.  Since the Sun is not solid, different parts rotate at different rates.  At the equator, the Sun spins around once about every 25 Earth days, but at its poles, the Sun rotates once on its axis every 36 Earth days.

 

Our solar system was formed from a disk-shaped cloud of gas and dust called the solar nebula.

Our Sun and its eight planets.  Sizes and orbital distances are not to scale.

Star Classification of the Sun

Simplifying a complex (to me anyway) classification system:  In astronomy, stars are classified according how hot they are and how bright they are.  Most stars are currently classified under the Morgan-Keenan system using the letters O, B, A, F, G, K, and M, a sequence from the hottest to the coolest, that correlates to a star’s color:  blue, white, yellow, orange, and red.  Within each stellar type, stars are placed into subclasses based on surface temperature (from 0 to 9, with 0 being hottest and 9 being coolest).  A brightness class is added using Roman numerals to distinguish a star’s size and how they generate light.  The letter K is used to identify so-called main sequence stars that fuse hydrogen atoms to form helium atoms in their cores. 

About 90% of the stars in the universe, including the Sun, are main sequence stars.  These stars can range from about a tenth of the mass of the Sun to up to 200 times as massive.  Our Sun is classified as a yellow dwarf star, G2V, indicating a main sequence star with a surface temperature of around 10,000 degrees Fahrenheit (F).  For a more complete explanation of star classification, see https://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml.

Early Observations of the Sun

The Sun has been an object of veneration throughout Earth’s prehistory and ancient human history.  Most cultures believed it to be supernatural in nature, or a deity, one who’s presence was intimately tied to time, the seasons, and the cycle of life.  Worship of the Sun was central to civilizations such the ancient Egyptians, Sumerians, Inca, Aztecs and Mayans, as well as numerous cultures in Europe, West Asia, and Africa.  Ancient cultures often modified natural rock formations or built stone monuments to mark the motions of the Sun and Moon, charting the seasons, creating calendars and monitoring eclipses.   

Over time, ancient astronomers began to develop a scientific understanding of the Sun, based on ongoing observations of its movements.  Many believed the Sun revolved around the Earth, with the ancient Greek scholar Ptolemy formalizing this "geocentric" model in 150 B.C.  Then, in 1543, Nicolaus Copernicus described a heliocentric (Sun-centered) model of the solar system, and in 1610, Galileo Galilei's discovery of Jupiter's moons confirmed that not all heavenly bodies circled Earth.

Exploration of the Sun

Much of the following was adapted from “History of Observing the Sun” at space.com. 

To learn more about how the Sun and other stars work, after preliminary observations using rockets, scientists began studying the Sun from Earth orbit.  NASA (National Aeronautics and Space Administration) launched a series of eight orbiting observatories known as the Orbiting Solar Observatory between 1962 and 1971, analyzed the Sun at ultraviolet and X-ray wavelengths, and photographed the super-hot gaseous envelope (corona) of the Sun.

In 1990, NASA and the European Space Agency (ESA) launched the Ulysses probe to make the first observations of the Sun’s polar regions.  In 2004, NASA's Genesis spacecraft returned samples of the continuous flow of charged particles from the Sun. (solar wind) to Earth for study.  In 2007, NASA's Solar Terrestrial Relations Observatory mission returned the first three-dimensional images of the Sun.

ESA’s Solar and Heliospheric Observatory, launched in 1995 and still active, has been one of the most important solar missions to date.  Designed to study the solar wind, as well as the Sun's outer layers and interior structure, it has imaged the structure of sunspots (see below) under the surface, measured the acceleration of the solar wind, discovered coronal waves and solar tornadoes, found more than 4,000 comets, and revolutionized our ability to forecast space weather.

The Solar Dynamics Observatory, launched in 2010, returned never-before-seen details of material streaming outward and away from sunspots, as well as extreme close-ups of activity on the Sun's surface, and the first high-resolution measurements of solar flares (see below) in a broad range of extreme ultraviolet wavelengths.

The newest additions to the Sun-observing fleet are NASA's Parker Solar Probe, launched in 2018, and the ESA/NASA Solar Orbiter, launched in 2020.  Both spacecraft orbit the Sun closer than any spacecraft before, taking complementary measurements of the environment in the vicinity of the star.  The measurements the Parker Solar Probe makes are helping scientists learn more about how energy flows through the Sun, the structure of the solar wind, and how energetic particles are accelerated and transported. 

The Solar Orbiter is equipped with high-tech cameras and telescopes that take images of the Sun's surface from the closest distance ever.  After the Solar Orbiter completes a few close passes, mission controllers will start elevating its orbit out of the ecliptic plane in which planets orbit, to enable the spacecraft's cameras to take the first ever close-up images of the Sun's poles.  Mapping the activity in the polar regions will help scientists better understand the Sun's magnetic field.

 

The ESA/NASA Solar Orbiter and NASA's Parker Solar Probe currently study the Sun in unprecedented detail from closer distance than any spacecraft before.

Physical Characteristics of the Sun

The Sun’s diameter is about 864,000 miles, or 109 times that of Earth.  It would take 1.3 million Earths to fill the Sun's volume.  Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.

Structure and Fusion.  Referring to the figure below, the Sun and its atmosphere are divided into several zones and layers.  The interior regions include the core, the radiative zone, and the convective zone.  Moving outward:  the visible surface or photosphere is next, then the chromosphere, followed by the transition region, and then the corona - the Sun’s expansive outer atmosphere.

Structural diagram of the Sun.

 

The core extends from the Sun's center to about a quarter of the way to its surface - a thickness of about 86,000 miles.  The core is the hottest part of the Sun.  Nuclear reactions here - where hydrogen is fused to form helium - power the Sun’s heat and light.  Temperatures top 27 million degrees F.  The fusion process releases energy, and the core gradually becomes enriched in helium.

Energy from the core is carried outward by radiation.  This radiation bounces around the radiative zone, taking about 170,000 years to get from the core to the top of the convective zone.  (The Tachocline is the boundary region between the radiative and convective zones.)  Moving outward, in the convective zone, the temperature drops below 3.5 million degrees F.  Here, large bubbles of hot plasma move upward toward the photosphere.  The Sun is cool and diffuse enough for convection to occur and this becomes the primary means of outward heat transfer.

The Sun doesn’t have a solid surface like Earth and the other rocky planets and moons.  The part of the Sun commonly called its surface is the photosphere.  This is the layer that emits the most visible light.  It’s what we see from Earth with our eyes. 

The photosphere is actually the first layer of the solar atmosphere.  It's about 250 miles thick, with temperatures reaching about 10,000 degrees F.  That's much cooler than the blazing core, but it's still hot enough to make carbon - like diamonds and graphite - not just melt, but boil.  Most of the Sun's radiation escapes outward from the photosphere into space.

Above the photosphere, are the chromosphere, the transition zone, and the corona - additional parts of the Sun’s atmosphere.  The transition zone is simply the thin layer where the chromosphere rapidly heats and becomes the corona.

In one of the Sun’s biggest mysteries, the corona is much hotter than the layers immediately below it.  The source of coronal heating is a major unsolved puzzle in the study of the Sun.

Magnetosphere.  The Sun generates magnetic fields that extend out into space that pervades our solar system.  The field is carried through the solar system by the solar wind - a stream of electrically charged gas blowing outward from the Sun in all directions.  The vast bubble of space dominated by the Sun’s magnetic field is called the heliosphere.  Thus, Earth exists inside the Sun’s atmosphere.  Outside the heliosphere is interstellar space.

Solar Activity.  The Sun’s atmosphere is where we see features such as sunspots, coronal holes, and solar flares.  Sunspots look like dark holes in the Sun, but they are actually areas that are slightly cooler than the surrounding photosphere.  Sunspots are created where bits of the Sun's magnetic field poke out from the interior into the Sun's atmosphere.  Lasting from days to months, sunspots range in size from 1,000 to 100,000 miles in diameter

Visible light from these top regions of the Sun is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the Moon covers the photosphere, the chromosphere looks like a fine, red rim around the Sun, while the corona forms a beautiful white crown with plasma streamers narrowing outward, forming shapes that look like flower petals.

The Sun goes through phases of high and low activity, which make up the solar cycle.  Approximately every 11 years, the Sun’s geographic poles change their magnetic polarity - that is, the north and south magnetic poles swap.  During this cycle, the Sun's photosphere, chromosphere, and corona change from quiet and calm to violently active.

The height of the Sun’s activity cycle, known as solar maximum, is a time of greatly increased solar storm activity.  Sunspots (as many as 250), eruptions called solar flares, and coronal mass ejections are common at solar maximum.  Solar flares are intense bursts of radiation coming from the release of magnetic energy associated with sunspots, and are the most violent eruptions in the solar system.  Coronal mass ejections are significant releases of plasma and accompanying magnetic field from the Sun's corona into the solar wind.  They are less violent than solar flares, but involve extraordinary amounts of matter - a single ejection can spout roughly 20 billion tons of matter into space.  Coronal mass ejections are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established.

 

Photo of a giant solar flare taken in 1973 from the Skylab space station.

Solar activity can release huge amounts of energy and particles, some of which impact us here on Earth.  Much like weather on Earth, conditions in space - known as space weather - are always changing with the Sun’s activity.  "Space weather" can interfere with satellites, GPS, and radio communications.  It also can cripple power grids, and corrode pipelines that carry oil and gas.

The Sun’s Location and Motion

The Sun is one of more than 100 billion stars in the Milky Way galaxy.  It’s located in a spiral arm of the Milky Way, about 26,000 light years from the center of the galaxy.  (A light year is the distance that light travels in one Earth year at a speed 186,000 miles per second:  5.88 trillion miles.  For additional perspective - the Earth orbits the Sun at an average distance of about 93 million miles; that’s 0.0000158 lightyear.  It takes sunlight 8.33 minutes to reach Earth.)

The Sun orbits the center of the Milky Way, bringing with it the planets, asteroids, comets, and other objects in our solar system, moving with an average velocity of 450,000 miles per hour.  But even at this speed, it takes about 230 million years for the Sun to make one complete trip around the Milky Way.  The Sun’s orbit is believed to be elliptical, but perturbations, due to the non-uniform distribution of stars, cause the Sun’s orbit to oscillate up and down relative to the galactic plane approximately 2.7 times per orbit.  Thus, the Sun’s orbit is an unclosed curve; see the figure below. 

 

This artist's concept of the disk-shaped Milky Way galaxy shows the Sun's orbital path around the galaxy.  The galaxy's center is the bright white spot, where hot massive stars crowd together.  

Note:  There are 81 visible stars within 40 lightyears of Earth. Proxima Centauri is the closest star to Earth (after the Sun) at a distance of 4.25 lightyears.

Evolution of the Sun

The Sun has been shining for 4.6 billion years and is presently nearly halfway through its life cycle as a yellow dwarf star.  The Sun has enough energy to burn as it does now for around 5 billion more years.  The life cycle of the Sun is shown in the figure below.

 

The life cycle of the Sun.

As the sun burns, hydrogen is converted to helium in the core by nuclear fusion.  The helium remains there, where it absorbs radiation more readily than hydrogen.  This raises the central temperature and increases the brightness.  It is estimated that the Sun has become 30% brighter in the last 4.6 billion years, and is currently increasing in brightness at a rate of about 10% every billion years.  The increase in solar brightness can be expected to continue until the hydrogen in the core is depleted, leaving a helium-filled core.

Gravitational forces will then compress the core and allow the rest of the Sun to expand.  At this point, the Sun will grow so big that it absorbs the inner planets of the solar system, including Mercury and Venus, and possibly Earth (currently orbiting the Sun at distances of 36, 67, and 93 million miles, respectively) - completely destroying these planets.  Long before this happens however, the increasing heat from the Sun will be so intense that liquid water won’t be able to exist on the Earth’s surface.  Earth will be scorched and uninhabitable.   

As the Sun expands, it will become a red giant star and burn that way for another billion years.  Once the Sun runs out of helium, it will shrink, begin ejecting ionized gas called a planetary nebula, and become a small, dense, dim white dwarf star, in which no fusion occurs, and luminosity comes from emission of residual thermal energy. 

Slowly, the white dwarf Sun will fade, and will eventually enter its final phase as a cold, dark black dwarf star.