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Next US Total Solar Eclipse (Apr 8, 2024)
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Our Sun Is a Star

sunset
Our Sun Is a Star

 

Our Sun is a star, like the hundreds that you see at night, only much, much closer. The Sun is a huge ball of hot, churning, unpredictable supercharged gasses called plasma.

 

Held together by gravity, the Sun produces the light and heat that make life on our planet possible. The light from our Sun is surprisingly steady considering that the Sun itself is always changing. The Sun’s plasma is in constant movement, generating areas of powerful magnetic forces called magnetic fields. These fields cause the plasma to tangle, stretch, and twist, producing dramatic results such as solar prominences, flares, and coronal mass ejections. 

coronal mass ejection

The surface of the Sun is in constant motion. You’ll be able to see some of this dramatic solar activity during the next eclipse season because the Sun enters its most active time between 2023 and 2025. (NASA/LMSAL)

The Sun is like an onion: it has layers.

Although the Sun doesn’t have a solid surface like Earth, it does have six layers, each one with a different density of plasma. There are three inner layers, where sunlight and energy are generated and circulated: the core, where energy is generated by nuclear reactions; the radiative zone, where energy travels outward by radiation; and the convection zone, where hot currents circulate the Sun’s energy to the surface. There are also three outer layers, where there is a lot of dramatic activity: the photosphere, the “visible” surface of the Sun; the chromosphere, a turbulent area of great activity; and the corona, the wispy, outer atmosphere.

Normally, the photosphere is the only visible layer, but during a total eclipse, the chromosphere and the corona become visible.

diagram showing the layers of the sun

 

The Photosphere

The photosphere is what you see when you look up at the Sun. (Always use “eclipse glasses” to do this!) It’s the visible surface of the Sun, not a solid surface like on planets. When you see images from a telescope that are called “white light images,” you are looking at the photosphere.

Strong magnetic fields are repeatedly formed and dissolved here, releasing energy and stirring up the plasma in the next layer, the chromosphere.

photosphere of the sun

LEFT: The Sun photographed in white light (true color)   

RIGHT: An image of the Sun using a white light filter on a telescope. You can see a similar image if you look at the Sun using eclipse glasses. If you look closely, you may be able to see dark sunspots. (See below to learn more about sunspots)

 

The Chromosphere

Sandwiched between the visible photosphere and the corona is the gorgeous chromosphere. Wild magnetic fields formed in the photosphere extend out through the chromosphere, creating eruptions and storms, prominences and flares.

This layer can usually only be observed by using a telescope with a special filter (called H-alpha) which filters out the brilliant light coming from the photosphere. The exception is during a total solar eclipse, when the moon covers the photosphere and the edges of the chromosphere can be glimpsed like a crimson-pink rim around the moon.

chromosphere

LEFT: The chromosphere, visible during an eclipse, appears dark pink or crimson to the naked eye.

RIGHT: When you see an H-alpha image of the Sun, like this one, you are looking at the chromosphere. This filter reveals details of the turbulent motion of the Sun. The H-alpha filter makes the chromosphere appear yellow, instead of the bright crimson pink that we see with the naked eye.

 

The Corona

The corona is the outermost layer of the Sun. Like the chromosphere, the corona can’t be seen with the naked eye except during a total eclipse; the light from the photosphere overpowers it. The corona’s plasma is much less dense than in other layers, which gives it the beautiful wispiness that we see during an eclipse.

comparison of images from total solar eclipses showing how the corona is different

Images of the corona during total solar eclipses in 2006 and 2017. The corona looks different every time depending on how active the Sun is. During times of less activity, the corona tends to have a simple structure radiating from the equator. During times of greater activity, the corona tends to be more complex, radiating out haphazardly from all parts of the Sun. (Fred Espenak)

 

The corona is far away from the Sun’s surface, yet it is hundreds of times hotter for reasons that we don’t understand (usually things get cooler as you move away from a heat source). To help solve the mystery, NASA sent the Parker Solar Probe spacecraft to investigate. On April 28, 2021, it passed through the corona several times, becoming the first artificial object to “touch the Sun.” Scientists are eagerly awaiting more data, but early analysis leads them to believe that the extra heat has something to do with the interaction of magnetic fields.

The corona extends far out into space, and from it comes the solar wind, a constant stream of particles that travels through our solar system and beyond. These particles crash into Earth’s atmosphere, creating beautiful auroras—but they can also cause some damage on our planet. 

Special filters enhance the different temperatures in the corona during total solar eclipses

Special filters enable scientists to measure different temperatures in the corona during total solar eclipses, such as this one seen in Mitchell, Oregon, on August 21, 2017. The red light indicates areas that are only half as hot as those in green. (Miloslav Druckmüller)

 

Visible Surface Features on the Sun (That You Can See During an Eclipse)

 
Sunspots

Sunspots appear as dark spots on the surface of the Sun. They are places of great magnetic activity, which make them cooler than the areas around them and give them a darker appearance. An average sunspot is about the same size as Earth, and can last from a day to many weeks.

NASA_ SDO-HMI-solar-max-2014

Sunspots filmed near solar maximum in 2014. The small dark regions are the sunspots: areas that are cooler than the Sun’s surface but very active. (NASA’s Scientific Visualization Studio)

large field-of-view image of sunspots

This large field-of-view image of sunspots in Active Region 10030 was observed on July 15, 2002. (The Royal Swedish Academy of Sciences/The Institute for Solar Physics)

 

The number of sunspots at any given time varies in a cyclical way—what we call the sunspot cycle or solar cycle. The length of the cycle is eleven years on average, although it has varied from nine years to as long as fourteen years. Over the course of each cycle, the Sun transitions from relatively calm to active and stormy, and then quiet again. The quiet periods are referred to as times of solar minimum. When sunspot activity peaks with dozens of sunspots, it’s called solar maximum. Each cycle is counted from one solar minimum to the next. The last solar minimum was in 2019, which launched our current Solar Cycle 25.

The next solar maximum is predicted to occur between 2023 and 2025. That means there should be plenty of activity on the Sun during the upcoming eclipses!

sun with no sunspots next to sun with several sunspots

Images from NASA’s Solar Dynamics Observatory highlight the appearance of the Sun at solar minimum (left, December 2019) versus solar maximum (right, April 2014). (NASA)

NASA_SDO-images

The Sun during solar minimum and maximum. (NASA)

 

Prominences

A solar prominence is a giant loop of plasma extending out from the photosphere, through the chromosphere, and into the corona. It’s a stunning phenomenon, glowing brightly against the black background of space. As with other solar phenomena, prominences are shaped by the Sun’s complex magnetic field and are in constant motion, producing striking loops, twists, and arches. Prominences are enormous, but they keep each side of their loops firmly anchored to the Sun. They usually form over the course of a day, but they can last for several months.

solar eruptive prominence on the sun with Earth superimposed for a sense of scale

A solar eruptive prominence as seen in extreme UV light on March 30, 2010 with Earth superimposed for a sense of scale. (NASA/SDO)

 

During a total solar eclipse, when the moon completely covers the Sun, you will be able to see prominences with your naked eye—they appear crimson pink along the edges of the Sun. But remember that prominences happen all the time! Usually hidden from our view by the overwhelming light of the photosphere, they can normally only be seen with a sophisticated telescope. The easiest way to see them is online through a space telescope, like NASA’s Solar Dynamics Observatory, which posts images of the Sun every day.

Visible prominences during an eclipse

Visible prominences during an eclipse. (Luc Viatour )

 

Solar Flares

A solar flare is an explosion on the surface of the Sun. They last in length from minutes to hours. Flares occur near sunspots, where magnetic fields build up an enormous amount of tension and explode in order to realign (called magnetic reconnection). The explosion is a giant burst of energy that travels the speed of light in all directions, taking eight minutes to reach Earth. Flares can damage communication systems on Earth, as they did on July 6, 2012, when a flare caused a radio blackout. The number of flares depends on the Sun’s 11-year cycle, ranging from several per day during solar maximum to one per week during solar minimum.

NASA_moderate-solar-flare

A moderate flare was emitted by the Sun on July 19, 2012. (NASA/GSFC/SDO)

 

Coronal Mass Ejections

Occasionally there is a sudden and violent release of energy and plasma from the Sun’s corona, resulting in a gigantic explosion called a coronal mass ejection (CME). Caused by the twisting and realigning of magnetic forces, a CME is like a cannonball of a billion tons of matter hurled through space in a single direction. Traveling over a million miles per hour, the hot plasma takes up to three days to reach Earth.

If the CME is pointing toward Earth, it can cause damage to satellites and our power grid. CMEs also create the beautiful auroras we see on Earth.

CME2

This video shows the ejection from a variety of viewpoints as captured by NASA's Solar Dynamics Observatory (SDO), NASA's Solar Terrestrial Relations Observatory (STEREO), and the joint ESA/NASA Solar Heliospheric Observatory (SOHO).

 

 

Fun Facts About the Sun
  • It is a G-type star, or “yellow dwarf.”
  • It is estimated to be 4.6 billion years old.
  • Its gravity is what holds the solar system together.
  • It contains 99.86% of all the mass in the solar system.
  • It takes about eight minutes for the light from the Sun to reach Earth, so when you look up at the Sun (with proper eye protection!), you are seeing what the Sun looked like eight minutes ago.
  • The Sun rotates counterclockwise every 26 days.
  • Our Sun is about 93 million miles away. The next closest star is about 4.25 light-years away or 25.5 trillion miles away (one light-year is about six trillion miles—or a six followed by 12 zeros!).