The Sun
Within the universe, the Sun is an ordinary star-not too spectacular. However, to you it's important. The Sun is the center of the solar system, and it makes life possible on Earth. More than 99 percent of all the matter in the solar system is in the Sun.
Notice the different layers of the Sun as you read about them. Like other stars, the Sun is an enormous ball of gas that produces energy by fusing hydrogen into helium in its core. This energy travels outward through the radiation zone and the convection zone. In the convection zone, gases
circulate in giant swirls. Finally, energy passes into the Sun's atmosphere.
Notice the different layers of the Sun as you read about them. Like other stars, the Sun is an enormous ball of gas that produces energy by fusing hydrogen into helium in its core. This energy travels outward through the radiation zone and the convection zone. In the convection zone, gases
circulate in giant swirls. Finally, energy passes into the Sun's atmosphere.
The Sun's Atmosphere
The lowest layer of the Sun's atmosphere and the layer from which light is given off is the photosphere. The photosphere often is called the surface fo the Sun, although the surface is not a smmooth feature. Temeratures there are about 6,000 K. Above the photosphere is the chromosphere. This layer extends upward about 2,000 km above the photosphere. A transition zone occurs between 2,000 km and 10,000 km above the photosphere. Above the transition zone is the corona. This is the largest layer of the Sun's atmosphere and extends millions of kilometers into space. Temperatures in the corona are as high as 2 million K. Charged particles continually escape from the corona and move through space as solar wind.
Surface Features of the Sun
From the viewpoint that you observe the Sun, its surface appears to be a smooth layer. But the Sun's surface has many features, including sunspots, prominences, flares, and CMEs.
Sunspots
Areas of the Sun's surface appear dark because they are cooler than surrounding areas are called sunspots. Ever since Galileo Galilei viewed sunspots with a telescope, scientists have been studying them. Because scientists could observe the movement of individual sunspots they concluded that the Sun rotates. However, the Sun doesn't rotate as a solid body, as Earth does. It rotates faster at its equator than at its poles. Sunspots at the equator take about 25 days to complete one rotation. Near the poles, they take about 33 days.
Sunspots aren't permanent features on the Sun. They appear and disappear over a period of several days, weeks, or months. The number of sunspots increases and decreases in a fairly regular pattern called the sunspot, or solar activity, cycle. Times when many sunspots occur are called sunspot maximums. Sunspot maximums occur about every 10 to 11 years. Periods of sunspot minimum occur in between.
Prominences and Flares
Sunspots are related to several features on the Sun's surface. The intense magnetic fields associated with sunspots might cause prominences, which are huge, arching columns of gas. Some prominences blast material from the Sun into space at speeds ranging from 600 km/s to more than 1,000 km/s.
Gases near a sunspot sometimes brighten suddenly, shooting outward at high speed. These violent eruptions are called solar flares.
CMEs
During a sunspot maximum, like the one that occurred in 2000, brilliant coronal mass ejectons (CMEs) are emitted from the Sun. When a CME is released in the directions of Earth, it appears as a halo around the Sun.
CMEs present little danger to life on Earth, but the highly charged solar wind material, along with ultraviolet light and X rays from solar flares, can reach Earth and cause disruption of radio signals. High-energy particles contained in CMEs are carried past Earth's magnetic field. This sets up electrical currents that flow toward the poles. The currents of electricity ionize gas in Earth's atmosphere. The ionized gases produce the light of an aurora. Power distribution equipment also can be affected.
Sunspots
Areas of the Sun's surface appear dark because they are cooler than surrounding areas are called sunspots. Ever since Galileo Galilei viewed sunspots with a telescope, scientists have been studying them. Because scientists could observe the movement of individual sunspots they concluded that the Sun rotates. However, the Sun doesn't rotate as a solid body, as Earth does. It rotates faster at its equator than at its poles. Sunspots at the equator take about 25 days to complete one rotation. Near the poles, they take about 33 days.
Sunspots aren't permanent features on the Sun. They appear and disappear over a period of several days, weeks, or months. The number of sunspots increases and decreases in a fairly regular pattern called the sunspot, or solar activity, cycle. Times when many sunspots occur are called sunspot maximums. Sunspot maximums occur about every 10 to 11 years. Periods of sunspot minimum occur in between.
Prominences and Flares
Sunspots are related to several features on the Sun's surface. The intense magnetic fields associated with sunspots might cause prominences, which are huge, arching columns of gas. Some prominences blast material from the Sun into space at speeds ranging from 600 km/s to more than 1,000 km/s.
Gases near a sunspot sometimes brighten suddenly, shooting outward at high speed. These violent eruptions are called solar flares.
CMEs
During a sunspot maximum, like the one that occurred in 2000, brilliant coronal mass ejectons (CMEs) are emitted from the Sun. When a CME is released in the directions of Earth, it appears as a halo around the Sun.
CMEs present little danger to life on Earth, but the highly charged solar wind material, along with ultraviolet light and X rays from solar flares, can reach Earth and cause disruption of radio signals. High-energy particles contained in CMEs are carried past Earth's magnetic field. This sets up electrical currents that flow toward the poles. The currents of electricity ionize gas in Earth's atmosphere. The ionized gases produce the light of an aurora. Power distribution equipment also can be affected.
The Sun - An Average Star
The Sun is a middle-aged star. Its absolute magnitude is typical, and it shines with a yellow light. Although the Sun is an average star, it is somewhat unusual in one way. Most stars are part of a system in which two or more stars orbit each other. When two stars orbit each other, they make up a binary system.
In some cases, astronomers can detect bingary systems because one star occasionlly eclipses the other. Algol, in the constellation Perseus, is an example of this. the total amount of light from the star system becomes dim and then bright again on a regular cycle. In other cases, three stars orbit around each other, forming a triple star system. The closest star system to the Sun-the Alpha Centauri system, including Proxima Centauri-is a triple star.
Stars also can move through space together as a cluster. In a star cluster, many stars are relatively close to one another, so their gravitational attraction to each other is strong. Most star clusters are far from the solar system, and each appears as a fuzzy patch in the night sky. The double cluster in the northern part of the constellation Perseus. On a dark night in autumn, you cna see the double cluster with binoculars, but you can't see its individual stars. The Pleiades star cluster can be seen in the constellation of Taurus in the winter sky. On a clear, dark night, you might be able to see seven of the stars in this cluster.
In some cases, astronomers can detect bingary systems because one star occasionlly eclipses the other. Algol, in the constellation Perseus, is an example of this. the total amount of light from the star system becomes dim and then bright again on a regular cycle. In other cases, three stars orbit around each other, forming a triple star system. The closest star system to the Sun-the Alpha Centauri system, including Proxima Centauri-is a triple star.
Stars also can move through space together as a cluster. In a star cluster, many stars are relatively close to one another, so their gravitational attraction to each other is strong. Most star clusters are far from the solar system, and each appears as a fuzzy patch in the night sky. The double cluster in the northern part of the constellation Perseus. On a dark night in autumn, you cna see the double cluster with binoculars, but you can't see its individual stars. The Pleiades star cluster can be seen in the constellation of Taurus in the winter sky. On a clear, dark night, you might be able to see seven of the stars in this cluster.