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
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.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
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 compositions 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.
- When the Sun converts mass to energy, do the orbits of the planets change? (Advanced)
- How long does it take for the Sun's light to reach us? (Beginner)
- Is it possible to measure the temperature of the Sun? (Beginner)
- Do we know everything about the Sun? (Beginner)
- Do asteroids hit the sun? (Beginner)
- If we invented special suits could we ever go to the sun? (Beginner)
- How does the Sun compare to other stars? (Beginner)
- How can the Sun have such a strong gravitational field if it's made of gases? (Beginner)
- Is there sound coming from the Sun? (Beginner)
- Does the Sun have a scientific name? (Intermediate)
- When did people discover that the Sun is a star? (Intermediate)
- 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)
- 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)
- Can the theory of formation of the solar system explain all the observables? (Intermediate)
- !How do I make a scale model of the Solar System? (Intermediate)
- What is the size of the Solar System? (Intermediate)
- What are the names of the earth, moon, sun, and solar system? (Beginner)
- What is the evidence supporting the nebula theory of Solar System formation? (Intermediate)
- Where is the supernova remnant that led to our solar system? (Intermediate)
- How are galaxies and solar systems similar? (Advanced)
- Are the planes of solar systems aligned with the plane of the Galaxy? (Intermediate)
- Why does the Sun's apparent path through the sky change throughout the year? (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)
- Why does the Sun appear larger on the horizon than overhead? (Intermediate)
- How do astronomers observe the Sun's interior? (Intermediate)
- Why does the location of sunrise change? (Intermediate)
- Why can we have solar eclipses? (Beginner)
- How long does a solar eclipse last? (Beginner)
- Will I be able to see the shadow of the Moon streaking towards me in a total solar eclipse? (Beginner)
- Is there a simple formula to calculate solar eclipse timings from any position on Earth? (Intermediate)
- Is it a coincidence that we can have total solar eclipses? Are there other planets which also have them? (Intermediate)
- Will we ever stop having solar eclipses because of the moon's motion away from the Earth? (Intermediate)
- How does night and day work? (Beginner)
- Is the Sun always up for exactly 12 hours at the equator? (Beginner)
- Why is a day divided into 24 hours? (Intermediate)
- How can I calculate the position or path of the Sun for a given time and location? (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 is the time of sunrise calculated? (Intermediate)
- How much can the location of sunset differ from due West? (Intermediate)
- Why is twilight short near the equator? (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)
- What causes seasons? (Beginner)
- Why does the Earth tilt back and forward once a year? (Beginner)
- Will the seasons change due to precession? (Intermediate)
- What are the "dog days of summer"? (Intermediate)
- What is the significance of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle? (Beginner)
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.
- Solar and Heliospheric Observatory: Beautiful images and videos from the SOHO spacecraft
- Spaceweather: News and space weather reports
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