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X-ray image of the Sun
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

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:

Previously asked questions about the Sun:

General questions:

Birth, death and evolution of the Sun:

Solar cycle and sunspots:

Observing the Sun:



The Sun in the Milky Way:

How to ask a question:

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