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Our Star — The Sun

www.nasa.gov
Our solar system’s central star, the Sun, has inspired mythologi- as part of the Sun’s magnetic activity cycle. Also connected to Significant Dates
cal stories in cultures around the world, including those of the this cycle are bright solar flares and huge coronal mass ejections 150 A.D. — Greek scholar Claudius Ptolemy writes the
ancient Egyptians, the Aztecs of México, Native American tribes that blast off the Sun. Almagest, formalizing the Earth-centered model of the solar sys-
of North America and Canada, the Chinese, and many others. tem. The model was accepted until the 16th century.
The temperature of the photosphere is about 5,500 degrees
A number of ancient cultures built stone structures or modified 1543 — Nicolaus Copernicus publishes On the Revolutions of
Celsius (10,000 degrees Fahrenheit). Above the photosphere lie
natural rock formations to mark the motions of the Sun and the Celestial Spheres describing his heliocentric (Sun-centered)
the tenuous chromosphere and the corona (“crown”). Visible light
Moon — they charted the seasons, created calendars, and mon- model of the solar system.
from these top regions is usually too weak to be seen against the
itored solar and lunar eclipses. These architectural sites show 1610 — First observations of sunspots through a telescope by
brighter photosphere, but during total solar eclipses, when the
evidence of deliberate alignments to astronomical phenomena: Galileo Galilei and Thomas Harriot.
Moon covers the photosphere, the chromosphere can be seen
sunrises, moonrises, moonsets, even stars or planets. Many cul- 1645–1715 — Sunspot activity declines to almost zero, possibly
as a red rim around the Sun while the corona forms a beauti-
tures believed that the Earth was immovable and the Sun, other causing a “Little Ice Age” on Earth.
ful white crown with plasma streaming outward, forming the
planets, and stars revolved about it. Ancient Greek astronomers 1860 — Eclipse observers see a massive burst of material from
“points” of the crown.
and philosophers knew this “geocentric” concept from as early the Sun; it is the first recorded coronal mass ejection.
as the 6th century B.C. Above the photosphere, the temperature increases with altitude, 1994 — The Ulysses spacecraft makes the first observations of
reaching temperatures as high as 2 million degrees Celsius the Sun’s polar regions.
The Sun is the closest star to Earth, at a mean distance from
(3.5 million degrees Fahrenheit). The source of coronal heat- 2004 — NASA’s Genesis spacecraft returns samples of the solar
our planet of 149.60 million kilometers (92.96 million miles). This
ing has been a scientific mystery for more than 50 years. Likely wind to Earth for study.
distance is known as an astronomical unit (abbreviated AU), and
solutions have emerged from observations by the Solar and 2006 — Ulysses begins its third set of data-gathering passes
sets the scale for measuring distances all across the solar sys-
Heliospheric Observatory (SOHO) and the Transition Region over the north and south poles of the Sun.
tem. The Sun, a huge sphere of mostly ionized gas, supports life
and Coronal Explorer (TRACE) missions, which found patches 2007 — NASA’s double-spacecraft Solar Terrestrial Relations
on Earth. The connection and interactions between the Sun and
of magnetic field covering the entire solar surface. Scientists Observatory (STEREO) mission returns the first three-dimension-
Earth drive the seasons, ocean currents, weather, and climate.
now think that this magnetic “carpet” is probably a source of the al images of the Sun.
About one million Earths could fit inside the Sun. It is held to- corona’s intense heat. The corona cools rapidly, losing heat as 2009 — After more than 18 years, the Ulysses mission ends.
gether by gravitational attraction, producing immense pressure radiation and in the form of the solar wind — a stream of charged Ulysses was the first and only spacecraft to study the Sun at
and temperature at its core. The Sun has six regions — the core, particles that flows to the edge of the solar system. high solar latitudes.
the radiative zone, and the convective zone in the interior; the
visible surface (the photosphere); the chromosphere; and the Fast Facts About the Images
outermost region, the corona. Spectral Type of Star G2V 1 2 1 Two huge clouds
Age 4.6 billion years of plasma erupt from
At the core, the temperature is about 15 million degrees Celsius
Mean Distance to Earth 149.60 million km the chromosphere of
(about 27 million degrees Fahrenheit), which is sufficient to 3
(92.96 million mi) the Sun (SOHO image
sustain thermonuclear fusion. The energy produced in the core
(1 astronomical unit) taken in extreme ultra-
powers the Sun and produces essentially all the heat and light 4 5
Rotation Period at Equator 26.8 days violet light).
we receive on Earth. Energy from the core is carried outward by
radiation, which bounces around the radiative zone, taking about Rotation Period at Poles 36 days
2 Magnetic fields are believed to cause huge, super-hot
170,000 years to get from the core to the convective zone. The Equatorial Radius 695,500 km (432,200 mi)
coronal loops to tower above the Sun’s surface (TRACE image).
temperature drops below 2 million degrees Celsius (3.5 million Mass 1.989 × 1030 kg
Density 1.409 g/cm3 3 An illustration of a coronal mass ejection and interaction
degrees Fahrenheit) in the convective zone, where large bubbles
Composition 92.1% hydrogen, 7.8% helium, with Earth’s magnetic field (not to scale). The pressure from the
of hot plasma (a soup of ionized atoms) move upwards.
0.1% other elements Sun forces Earth’s magnetic field into a windsock shape.
The Sun’s “surface” — the photosphere — is a 500-kilometer- Surface Temperature (Photosphere) 5,500 deg C 4 A false-color image of the Sun’s corona taken in three
thick (300-mile-thick) region, from which most of the Sun’s (10,000 deg F) wavelengths emitted at different temperatures (SOHO image).
radiation escapes outward and is detected as the sunlight we Luminosity* 3.83 × 1033 ergs/sec
5 These large sunspots in the photosphere were associated
observe here on Earth about eight minutes after it leaves the
*Luminosity measures the total energy radiated by the Sun (or any with several powerful solar flares in 2003 (SOHO image).
Sun. Sunspots in the photosphere are areas with strong magnet-
ic fields that are cooler, and thus darker, than the surrounding re- star) per second at all wavelengths.
For More Information
gion. The number of sunspots goes up and down every 11 years
solarsystem.nasa.gov/sun

LG-2009-09-564-HQ — JPL 400-1344C 09/09