Every million years, the sun—and the solar system it carries with it—makes one orbit around the Milky Way's center. Though we can't feel it, the sun traces its orbit at an average velocity of , miles an hour. The sun formed more than 4. As it did, the cloud spun and flattened into a disk, with our sun forming at its center. The disk's outskirts later accreted into our solar system, including Earth and the other planets.
Scientists have even managed to see these planet-birthing disks around our sun's distant young cousins. Our home star is a yellow dwarf, a medium-size variety that's fairly common in our galaxy.
On Earth, the sun can take on warmer hues, especially at sunrise or sunset, because our planet's atmosphere scatters blue and green light the most. At about , miles 1. If it was a hollow ball, more than a million Earths could fit inside it. But the sun isn't hollow: It's filled with scorching gases and soups of electrically charged particles called plasma. The sun's surface temperature is about 10, degrees Fahrenheit 5, degrees Celsius , and it's 27 million degrees Fahrenheit Deep in the sun's core, nuclear fusion converts hydrogen to helium, which generates energy.
Particles of light called photons carry this energy through a spherical shell called the radiative zone to the top layer of the solar interior, the convection zone. There, hot plasmas rise and fall like the ooze in a lava lamp, which transfers energy to the sun's surface, called the photosphere. It can take , years for a photon to complete its journey out of the sun, but once it exits, it zips through space at more than , miles a second.
Solar photons reach Earth about eight minutes after they're freed from the sun's interior, crossing an average of 93 million miles to get here—a distance defined as one astronomical unit AU. Out beyond the sun's photosphere lies the atmosphere, which consists of the chromosphere and the solar corona. The chromosphere looks like a reddish glow fringing the sun, while the corona's huge white tendrils extend millions of miles long.
The chromosphere and corona also emit visible light, but on Earth's surface, they can be seen only during a total solar eclipse, when the moon passes between Earth and the sun. The corona runs far hotter than the photosphere, hitting temperatures of more than a million degrees Fahrenheit.
How the corona gets so hot remains a scientific mystery, which is partly why NASA launched its Parker Solar Probe , the fastest spacecraft ever built, and the first ever sent into the corona. In addition to light, the sun radiates heat and a steady stream of charged particles known as the solar wind. The wind blows about miles kilometers a second throughout the solar system , extending the sun's magnetic field out more than 10 billion miles.
Beyond that distance, the solar wind gives way to the colder, dense material that drifts in between stars , forming a boundary called the heliopause. So far, just two spacecraft—Voyager 1 and Voyager 2 —have crossed this cosmic threshold, which defines the start of interstellar space. Related: What is the sun made of? Sunspots are relatively cool, dark features on the sun's surface that are often roughly circular.
They emerge where dense bundles of magnetic field lines from the sun's interior break through the surface. The number of sunspots varies as solar magnetic activity does — the change in this number, from a minimum of none to a maximum of roughly sunspots or clusters of sunspots and then back to a minimum, is known as the solar cycle , and averages about 11 years long.
At the end of a cycle, the magnetic field rapidly reverses its polarity. Related: Largest sunspot in 24 years wows scientists, but also mystifies. 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.
Many believed the sun revolved around the Earth, with the ancient Greek scholar Ptolemy formalizing this "geocentric" model in B. Then, in , Nicolaus Copernicus described a heliocentric sun-centered model of the solar system, and in , Galileo Galilei 's discovery of Jupiter's moons confirmed that not all heavenly bodies circled Earth.
To learn more about how the sun and other stars work, after early observations using rockets, scientists began studying the sun from Earth orbit. Seven of them were successful, and analyzed the sun at ultraviolet and X-ray wavelengths and photographed the super-hot corona, among other achievements.
The Solar and Heliospheric Observatory SOHO , which last year celebrated 25 years in space, 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 below the surface, measured the acceleration of the solar wind, discovered coronal waves and solar tornadoes, found more than 1, comets, and revolutionized our ability to forecast space weather.
The Solar Dynamics Observatory SDO , launched in , has 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 in a broad range of extreme ultraviolet wavelengths.
Both of these spacecraft orbit the sun closer than any spacecraft before, taking complementary measurements of the environment in the vicinity of the star. During its close passes, the Parker Solar Probe dives into the sun's outer atmosphere, the corona, having to withstand temperatures hotter than one million degrees Fahrenheit.
At its nearest, the Parker Solar Probe will fly merely 4 million miles 6. The measurements it 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. While Solar Orbiter doesn't fly as close as the Parker Solar Probe, it is equipped with high-tech cameras and telescopes that take images of the sun's surface from the closest distance ever. It was not technically possible for the Parker Solar Probe to carry a camera that would look directly at the sun's surface.
During its first perihelion, the point in its elliptical orbit closest to the sun, the spacecraft approached the sun to about half the distance from earth. The images acquired during the first perihelion, released in June last year, were the closest images of the sun ever taken and revealed previously unseen features on the star's surface — miniature flares dubbed the campfires. After 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, which drives the year solar cycle. Radiative and convective zones got their names from the way the energy is being transferred through each zone, i.
In between the two zones is tachocline, a thin [but very important] interface area. Finally, the outermost part of the Sun is the atmosphere. Our photon loses a lot of energy to these collisions, becoming first X-ray and then UV photon. Through this continuous random bouncing back and forth no wonder that mathematically this problem is known as random walk , up to a million years after it was born, our photon finally enters the convective zone.
Our photon hitches a ride in one of these bubbles up to the visible surface. The sunlight we see is years and 8. It is ancient! But not to the photons themselves. You see, according to Einstein, the closer to the speed of light you travel , the more the time dilates i.
Ultimately, for a photon that travels…well… at the light speed, the fastest speed there is, there is no time and no distance. In other words, photons have no age and they do not experience time. To them, entering your eye happens instantaneously after their birth — no thousands of years of bouncing, no huge distance from the Sun traveled!
Did you enjoy this post? Here are some other cool posts about the Sun. Why white dwarfs crystallize and how the Sun will turn into a giant diamond.
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