The Sun is a huge, glowing ball at the center of our solar system. The sun provides light, heat, and other energy to Earth. The sun is made up entirely of gas. Most of it is a type of gas that is sensitive to magnetism. This sensitivity makes this type of gas so special that scientists sometimes give it a special name: plasma. The planets and their moons, dwarf planets, tens of thousands of asteroids, and trillions of comets revolve around the sun. The sun and all these objects are in the solar system. Earth travels around the sun at an average distance of about 92,960,000 miles (149,600,000 kilometers) from it.
An Extreme Ultraviolet Imaging Telescope (EIT) image of the Sun and a huge, handle-shaped prominence, taken on September 14,1999, in the 304 angstrom wavelength. Prominences are huge clouds of relatively cool dense plasma suspended in the Sun's hot, thin corona. At times, they can erupt, escaping the Sun's atmosphere. Image credit: NASA/European Space Agency
The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers), approximately 109 times Earth's radius. The following example may help you picture the relative sizes of the sun and Earth and the distance between them: Suppose the radius of Earth were the width of an ordinary paper clip. The radius of the sun would be roughly the height of a desk, and the sun would be about 100 paces from Earth.
The part of the sun that we see has a temperature of about 5500 degrees C (10,000 degrees F). Astronomers measure star temperatures in a metric unit called the Kelvin (abbreviated K). One Kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales begin at different points. The Kelvin scale stars at absolute zero, which is -273.15 degrees C (-459.67 degrees F). Thus, the temperature of the solar surface is about 5800 K. Temperatures in the Sun's core reach over 15,000,000 K (27,000,000 degrees F).
The energy of the sun comes from nuclear fusion reactions that occur deep inside the sun's core. In a fusion reaction, two atomic nuclei join together, creating a new nucleus. Fusion produces energy by converting nuclear matter into energy.
The sun, like Earth, is magnetic. Scientists describe the magnetism of an object in terms of a magnetic field. This is a region that includes all the space occupied by the object and much of the surrounding space. Physicists define a magnetic field as the region in which a magnetic force could be detected—as with a compass. Physicists describe how magnetic an object is in terms of field strength. This is a measure of the force that the field would exert on a magnetic object, such as a compass needle. The typical strength of the sun's field is only about twice that of Earth's field.
But the sun's magnetic field becomes highly concentrated in small regions, with strengths up to 3,000 times as great as the typical strength. These regions shape solar matter to create a variety of features on the sun's surface and in its atmosphere, the part that we can see. These features range from relatively cool, dark structures known as sunspots to spectacular eruptions called flares and coronal mass ejections.
Flares are the most violent eruptions in the solar system. Coronal mass ejections, though less violent than flares, involve a tremendous mass (amount of matter). A single ejection can spew approximately 20 billion tons (18 billion metric tons) of matter into space. A cube of lead 3/4 mile (1.2 kilometers) on a side would have about the same mass.
The sun was born about 4.6 billion years ago. It has enough nuclear fuel to remain much as it is for another 5 billion years. Then it will grow to become a type of star called a red giant. Later in the sun's life, it will cast off its outer layers. The remaining core will collapse to become an object called a white dwarf, and will slowly fade. The sun will enter its final phase as a faint, cool object sometimes called a black dwarf.
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