The sun Our solar system
The Sun is a typical star, little different from billions of others in our galaxy, the Milky Way. It dominates everything around it, accounting for 99.8 percent of the solar system’s mass. Compared with any of its planets, the Sun is immense. Earth would fit inside the Sun over one million times; even the biggest planet, Jupiter, is a thousandth of the Sun’s volume. Yet the Sun is by no means the biggest star; VY Canis Majoris, known as a hypergiant, could hold almost 3 billion Suns. Our star will not be around forever. Now approximately halfway through its life, in about 5 billion years it will turn into a red giant, swelling and surging out toward the planets. Mercury and Venus will be vaporized.
The Earth may experience a similar fate, but even if our planet is not engulfed, it will become a sweltering furnace under the Itense glare of a closer Sun. Eventually, the Sun will shake itself apart and puff its outer layers into space, leaving behind a ghostly cloud called a planetary nebula.
IT MAY SEEM LIKE AN UNCHANGING YELLOW BALL.
IN THE SKY, BUT THE SUN IS INCREDIBLY DYNAMIC.
A GIANT NUCLEAR FUSION REACTOR, IT FLOODS.
The SOLAR SYSTEM WITH ITS BRILLIANT ENERGY.
The Sun has no solid surface—it is made of gas, mostly hydrogen. Intense heat and pressure split the gas atoms into charged particles, forming an electrified state of matter known as plasma. Inside the Sun, density and temperature rise steadily toward the core, where the pressure is more than 100 billion times greater than atmospheric pressure on Earth’s surface. In this extreme environment, unique in the solar system, nuclear fusion occurs. Hydrogen nuclei are fused together to form helium nuclei, and a fraction of their mass is lost as energy, which percolates slowly to the Sun’s outer layers and then floods out into the blackness of space, eventually reaching Earth as light and warmth.
Coronal mass ejection
The most sizable and impressive
explosive events anywhere in the solar
system occur when the Sun throws out
a mighty eruption of plasma known as
a coronal mass ejection (CME). As the
name suggests, the plasma is spat out
from the Sun’s atmosphere (corona).
The sheer violence of the explosion can
accelerate solar particles toward the
speed of light. When CME material
reaches the Earth, it may trigger a
geomagnetic storm. In the ultraviolet
photograph on the left, a CME is seen
swelling out from the Sun’s corona like
a giant bubble.
As well as producing light in the visible part of the spectrum, the Sun emits wavelengths our eyes cannot see, from radio waves and infrared to ultraviolet radiation. By capturing these rays, solar observatories can image parts of the Sun that are normally invisible.
NASA’s space-based Solar Dynamics Observatory (SDO) produces new images of the Sun every second; those shown here were all taken in a single hour in April 2014. The first one shows what the human eye would see if a direct glance were possible—the Sun’s brilliant photosphere is reduced to a smooth yellow disk, with dark sunspots where magnetic disturbances have cooled the surface. For most of the images that follow, SDO used filters to select various wavelengths of ultraviolet light, revealing solar flares high in the Sun’s outer atmosphere above sunspot regions. The final two photographs are composites that combine several wavelengths.
THE SOLAR CYCLE
THE SUN IS A CHANGEABLE STAR, SOMETIMES CALM AND PEACEFUL, SOMETIMES ERUPTING WITH GREAT VIOLENCE. THESE CHANGES FOLLOW A CLEAR PATTERN, WITH A CYCLICAL RISE AND FALL OF SOLAR ACTIVITY EVERY 11 YEARS OR SO.
For the last four centuries, scientists have kept records of the Sun’s activity.
During the early 19th century, German apothecary-turned-astronomer Samuel Heinrich Schwabe spent 17 years trying to spot a planet that he believed existed closer to the Sun than Mercury. He failed to see the silhouette of a new planet against the Sun, but he did keep accurate records of sunspots. Looking back over his observations, he noticed that the number of sunspots varied in a regular way, and the idea of the solar cycle was born. Today’s orbiting and ground-based solar telescopes constantly scrutinize the Sun, revealing further details of this recurring pattern.
Once thought to be storms in the atmosphere of the Sun, we now know that sunspots are merely cooler regions of the solar surface. Typically lasting a few weeks, they are caused by intense, local magnetic activity and often appear in pairs. Records of sunspot observations date from the early 17th century, though sunspots were probably seen earlier. Scientists can trace sunspot activity further back by studying tree rings: carbon-14 levels in tree rings are lower during times of sunspot abundance, and greater when there are few sunspots.
WHEN SOMETHING AS CONSTANT AND UNERRING AS
THE SUN’S LIGHT IS SUDDENLY INTERRUPTED DURING THE DAY, WE CANNOT FAIL TO NOTICE. FOR A FEW MINUTES, IT SEEMS AS IF THE WORLD STANDS STILL.
History books are littered with tales of the Sun disappearing; today we call these events solar eclipses. Every so often, during
its steady crawl around Earth, the Moon occupies the exact same part of daytime sky as the Sun. Since the Moon is closer, its presence obscures our view of the Sun, causing an eclipse.
Total solar eclipses
During a total solar eclipse, the Sun is completely hidden by the Moon’s disk for a few minutes. A total solar eclipse is perhaps nature’s ultimate spectacle: the sky darkens, the temperature drops, and birds stop singing.
If the Moon orbited exactly on the line between the Sun and Earth, we would get an eclipse every month. However, because the Moon’s orbit is tilted by five degrees, eclipses happen only every 18 months or so. Each is visible from only a small part of Earth’s surface, where the Moon’s shadow falls.
STORY OF THE SUN
OVER THE CENTURIES, THE SUN’S PLACE IN OUR CULTUREHAS CHANGED DRAMATICALLY—SCIENCE ANDEXPERIMENTATION HAVE OVERSEEN ITS TRANSITIONFROM ALL-POWERFUL GOD TO HOT, GAS-FILLED STAR.
The Sun’s movements have been tracked for thousands ofyears, and were used by many ancient civilizations as thebasis for their calendars. However, the same people stillbelieved the Sun circled Earth; it was not until 1543 thatCopernicus suggested the Sun was at the center of thesolar system. Later, Newton’s theory of gravity allowed theSun’s enormous mass to be calculated, and Einstein’s workin the early 20th century explained how the Sun can shinefor billions of years without running out of fuel. Modernspacecraft allow us to study the Sun in intimate detail andpredict the storms that rage on its surface.