The Sun

Our nearest star, the Sun is the center of the Solar System. Generating huge amounts of heat and light energy, it is the Solar System's power plant. The Sun is the source, directly or indirectly, of almost all of the energy we use.

Image at right: The Sun (courtesy of NASA).

Our Sun is just one of billions of stars. Studying the Sun helps us to learn more about stars in general. It is approximately 4.6 billion years old (middle-aged).

Scientists think that the Sun began when a huge, slowly spinning cloud of dust and gas collapsed under its own weight. Pulled together by its own gravity, this dense region began to shrink. As the cloud shrink the gas in the center became hotter and hotter, nucleur fusion began, and our star was born.

Features

: The Sun's layers (Original image from NASA)

Like all stars, the Sun is a huge ball of very hot gas composed of hydrogen and helium. It is a nearly perfect sphere. The Sun is extremely hot - about 10,000°F (or 5,5000°C or 5,800°K) at the surface and 27,000,000°F (or 14,999,982°C or 15,000255°K) at the core. The visible surface of the Sun is called the photosphere, which is mostly composed of hydrogen and helium.

The Sun's atmosphere has two transparent layers: the chromosphere and the corona. With solar telescopes and space-based instruments, scientists can study other features of the Sun like sunspots and prominences. Sunspots are dark, cool regions on the Sun's surface.

The Sun Cycle

: The Sun in ultraviolet light taken by SOHO spacecraft (Courtesy of SOHO/EIT consortium

Over time scientists have observed that our Sun has an 11-year cycle of sunspots. Scientists have known about sunspot activity for hundreds of years. Even Galileo made sketches of them.

Activity of the Sun's magnetic field, including sunspots, varies, hitting highs and lows of intensity every 11 years in a fairly steady cycle. Sunspots, flares and storms will rage, get gradually get quieter, then rise again in intensity a few years later. The last solar maximum period was in 2001 and the next peak of intensity is expected to happen around 2012 or 2013.

We are currently coming out of the solar minimum, which was the longest and quietest period in about a century. Scientists are still piecing together the puzzle of why this minimum was longer and quieter than what we have seen recently, but based on the past few decades of observations of solar minima and maxima, it seems that a “deep minimum” like we have just experienced tends to follow several cycles of intense solar maximum periods, with lots of sunspots, flares, and storms. This minimum was not necessarily unusual – the minima of 1901 and 1913 were even longer than the one we just experienced. Scientists need to figure out whether this minimum was “extreme” or just a “periodic correction.”

It is important to study the Sun intently. Solar weather can affect life on Earth. Bursts in activity can disrupt radio signals, electric power and global communication networks. There is more risk of this happening during a solar maximum, when activity is high.

Many scientists and organizations track solar weather patterns and analyze their impact on the Earth. For information on current conditions check out these links:

National Weather Service's Space Weather Prediction Center

Spaceweather.com

NASA's Soar Dynamics Observatory (SDO)

And for more information on the solar cycle, listen to these Adler podcasts:

Adler Night & Day- Episode 87- 5/11/10

Adler Night & Day- Episode 74- 11/10/09

Also, see Galileo's sketches of the Sun at the Galileo Project website.

Missions

: SOHO: The International Solar and Heliospheric Observatory (Courtesy of NASA)

The first American expedition to the Sun was the Pioneer 5 satellite launched by NASA in 1959. It took measurements of the Sun's diameter and solar wind and observed the surface activity of our star. Pioneer 5 was the first spacecraft of its kind to study sun phenomena.

The largest mission to investigate the Sun was SOHO, the Solar and Heliospheric Observatory, a joint effort between the United States and the European Space Agency. The spacecraft left Earth in 1995 and is still operating, sending back some of the best images ever taken of the Sun.

Learn more about missions to the Sun at NASA's Solar System Exploration.

Myths, Stories, and More

: Apollo 13th Patch featuring Apollo (Courtesy of NASA)

Throughout history, human beings have been fascinated by the Sun. The Romans called the golden orb Sol. Apollo replaced the Titan Helios as the sun god, but the Greeks continued to refer to both.

Our star has inspired mythological stories in cultures around the world, including the ancient Egyptians, the Aztecs of Mexico, and Native American tribes of North America and Canada. The Sun continues to be a dominant force in our lives today.

Discover more stories about the Sun at Windows to the Universe.

Earth Matters

: Size of Earth in relation to the Sun (Courtesy of NASA)

The Sun has a mass 332,900 times greater than the mass of Earth, and it is 109 times Earth's diameter. If the Sun were the size of a soccer ball, the Earth would be about the size of a pea!

Radiant energy from the Sun lights up our daytime sky and makes life possible on our planet, even though the Sun is 93 million miles away. It powers photosynthesis in green plants and drives the seasons, ocean currents, weather and climate.

Learn more about Sun-Earth connections from NASA's Sun-Earth Viewer.