Credit: SOHO (ESA + NASA)

The X-ray Sun. This X-ray image of the Sun, taken by the SOHO satellite, shows numerous active regions in the Sun's atmosphere. The hottest and most active regions appear white, and the darker areas indicate cooler temperatures. The wispy feature in the lower left portion of the disk is a solar prominence, a huge cloud of relatively cool plasma suspended in the Sun's hot thin corona.

The Sun

(Links | Questions)

When we think of stars, we usually neglect the fact that the nearest one is right in our own backyard: the Sun! The Sun is not only the dominant object in the sky during the day, but it is the source of virtually all of the light and the heat that fuels life on Earth. In addition, the Sun provides an excellent opportunity for astronomers to deepen their understanding of stellar phenomena.

Fundamentals: the Sun as a Star

The Sun is by far the largest object in our solar system, containing more than 99% of the latter's total mass. Observations of other stars indicate that the Sun is fairly "normal": it has a mass, a luminosity and a temperature that is somewhere in the middle-to-low end of the observed spectrum. It is also one of about 100 billion similar objects in the Milky Way.

Its attributes are hard to fathom by Earth standards, with a mass of 2 x 10³⁰ kg, an atmospheric temperature of 5500 ^oC and a luminosity of 4x10²⁰ megawatts. This luminosity stems from hydrogen to helium fusion reactions that occur in the central regions, a characteristic of main sequence stars.

The Sun is mainly composed of hydrogen and helium (~75% and ~25% by mass, respectively), with traces of heavier elements synthesized by past generations of stars in the solar neighborhood. These heavier elements are the main constituents of the inner terrestrial planets in the solar system; the jovian planets have compostions almost identical to the Sun itself.

The proximity of the Sun to the Earth allows scientists to study phenomena in the solar atmosphere that are too small or too faint to be observed in even the nearest star to our own.

Sunspots and the Solar Cycle

Some of the most fascinating and complex features observed are sunspots. First seen by Galileo in 1613, they appear as small dark spots on the solar disk. This darkness stems from their cooler temperatures of about 3700 ^oC relative to the rest of the atmosphere. An individual sunspot generally lasts about a month, the imbalance in temperature being tempered by strong magnetic fields.

Observations show that the number of sunspots as well as their location on the solar disk varies semi-periodically in an 11-year solar cycle. At the start of the cycle, sunspots form about 30 degrees away from the solar equator. Midway through the cycle the number of sunspots observed is maximum, and they are typically about 15 degrees away from the equator. The cycle ends with lower numbers of sunspots very near the equator. The cyclic nature of the positions and numbers of sunspots on the solar disk is well demonstrated by solar butterfly diagrams. The polarity of the sunspots in each hemisphere also reverses with each new cycle; therefore, the Sun is said to have a 22-year cycle if this alternating polarity is taken into account. The sunspot cycle appears to be inextricably linked to the Sun's magnetic field, and may be a result of the magnetic dynamo hypothesized to explain its major features.

The Active Sun

The energy output from the Sun is not quite constant, but varies with the solar cycle. The least amount of energy is output at times of little sunspot activity. During this solar minimum, the luminosity of the Sun is stable and quite uniform across the disk. Solar maximum, on the other hand, occurs at times of maximal sunspot activity. The surface of the maximal Sun is violent and unpredictable; sudden bursts of radiation from solar flares or prominences may even disrupt communications here on Earth.

The Ask an Astronomer team's favorite links about the Sun:

The Sun: Wikipedia article, with lots of great information.
The Nine Planets Sun Page: Lots of basic information about the Sun and links to other good Sun sites
TRACE Solar Observations: Spectacular movies and images of the sun from the TRACE spacecraft.
A Virtual Tour of the Sun: An interactive tour of our star!
U.S. Naval Observatory Data Services: Easy to use web forms which provide data on the position of the Sun, rise and set times, etc.

Previously asked questions about the Sun:

General questions:

Is it possible to measure the temperature of the Sun? (Beginner)
Do we know everything about the Sun? (Beginner)
If we invented special suits could we ever go to the sun? (Beginner)
What are the names of the earth, moon, sun, and solar system? (Beginner)
How does the Sun compare to other stars? (Beginner)
Does the Sun orbit the Earth as well as the Earth orbiting the Sun? (Beginner)
How can the Sun have such a strong gravitational field if it's made of gasses? (Beginner)
What would happen to us if the sun went out for an hour? (Beginner)
Is there sound coming from the Sun? (Beginner)
Can the sun rise from the west on Mars? (Beginner)
Are there solar tides? (Beginner)
Does the Sun have a scientific name? (Intermediate)
How do you measure the distance between Earth and the Sun? (Intermediate)
When did people discover that the Sun is a star? (Intermediate)
Does the Sun have an iron surface? (Intermediate)
How long would it take the Earth to fall into the Sun? (Intermediate)
How are galaxies and solar systems similar? (Advanced)
Is the distance from the Earth to the Sun changing? (Advanced)
When the Sun converts mass to energy, do the orbits of the planets change? (Advanced)

Birth, death and evolution of the Sun:

What will happen to life on Earth when the Sun becomes a red giant? (Beginner)
Will the Sun become a black hole one day? (Beginner)
Is the Sun expanding? Will it ever explode? (Beginner)
How big was the Sun when it first "fired up"? (Intermediate)
Was the Sun made in a supernova? (Intermediate)
Will the sun go supernova in six years and destroy Earth (as seen on Yahoo)? (Intermediate)
Where is the supernova remnant that led to our solar system? (Intermediate)
What is the evidence supporting the nebula theory of Solar System formation? (Intermediate)
How do you calculate the lifetime of the Sun? (Advanced)

Solar cycle and sunspots:

What's that dark spot on the Sun? (Beginner)
What is the solar cycle? (Beginner)
How many sunspots can the Sun have at once? (Beginner)
Do the Sun's or Earth's magnetic field reverse? (Intermediate)
How do solar flares affect airline pilots? (Intermediate)
What are some of the effects of solar flares on Earth? (Intermediate)
How do we know that sunspots are associated with the Sun's magnetic field? (Intermediate)
Does the Sun rotate? (Intermediate)
How are sunspots formed? (Intermediate)

Observing the Sun:

How long does it take for the Sun's light to reach us? (Beginner)
Why are the Moon and Sun sometimes orange or red? (Beginner)
Why does the Sun's apparent path through the sky change throughout the year? (Beginner)
How do we measure the size of the Moon and of the Sun? (Beginner)
Which constellation does our Sun belong to? (Beginner)
Is the Sun always up for exactly 12 hours at the equator? (Beginner)
Why can we see the sun's image before sunrise and after sunset? (Beginner)
Which way does the sun travel in the southern hemisphere? (Beginner)
Is the green flash real? (Beginner)
How can I calculate the position or path of the Sun for a given time and location? (Intermediate)
Why does the Sun appear larger on the horizon than overhead? (Intermediate)
How do sunrise and sunset times change with altitude? (Intermediate)
Why doesn't the length of each day change much around the solstices? (Intermediate)
How do astronomers observe the Sun's interior? (Intermediate)
Why does the location of sunrise change? (Intermediate)
How is the time of sunrise calculated? (Intermediate)
How much can the location of sunset differ from due West? (Intermediate)
What's going to happen on December 21st 2012? (Intermediate)
How does the location of sunrise and sunset change throughout the year? (Advanced)
Why doesn't the earliest sunset occur on the shortest day of the year? (Advanced)

Composition:

Is it true that the Sun burns off a billion tons of gas every 5 seconds? (Beginner)
What elements make up the Sun? (Beginner)
Does the Sun have any heavy elements? (Beginner)
How do we know what other planets (and stars, galaxies, etc.) are made of? (Intermediate)
Can superheavy elements (such as Z=116 or 118) be formed in a supernova? Can we observe them? (Advanced)

Interior:

How many Earths fit into the Sun? (Beginner)
How hot is each one of the layers of the sun? (Beginner)
How do astronomers observe the Sun's interior? (Intermediate)
How do we know that nuclear fusion is still going on in the Sun? (Intermediate)
Does the Sun have an iron surface? (Intermediate)
Why will fusion wait until hydrogen is gone before starting to fuse heavier atoms? (Advanced)

The Sun in the Milky Way:

Does the Sun move around the Milky Way? (Intermediate)
How often does the Sun pass through a spiral arm in the Milky Way? (Intermediate)

How to ask a question:

If you have a question about the Sun which isn't answered above, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

URL: http://curious.astro.cornell.edu/sun.php
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Last modified: October 18, 2007 12:56:06 PM

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