Wednesday, 26 August 2009

10 Facts about the Sun

1. The sun is actually a star, the closest star to Earth
The sun is an average star, meaning its size, age, and temperature fall in about the middle of the ranges of these properties for all stars. While some in our galaxy are nearly as old as the universe, about 15 billion years, our sun is a 2nd-generation star, only 4.6 billion years old. Some of its material came from former stars.

* Velocity Relative to Near Stars: 19.7 km/s
* Spectral Type: G2 V
* Synodic Period: 27.2753 days
* Solar Constant (Total Solar Irradiance): 1.365 - 1.369 kW/m2

2. The sun is by far the largest object in the solar system
The sun contains over 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

* Equatorial Radius: 695,500 km
* Equatorial Circumference: 4,379,000 km
* Volume: 1,142,200,000,000,000,000 km3
* Mass: 1,989,000,000,000,000,000,000,000,000,000 kg
* Density: 1.409 g/cm3
* Surface Area: 6,087,799,000,000 km2

3. We have always known the sun
Unlike many other objects in our solar system, the sun has been known to humans since the dawn of time. There is no discovery date or discoverer.

4. Since its creation, the sun has used up about half of the hydrogen in its core
Over the next 5 billion years or so, it will grow steadily brighter as more helium accumulates in its core. As the supply of hydrogen dwindles, the Sun's core must keep producing enough pressure to keep the Sun from collapsing in on itself. The only way it can do this is to increase its temperature. Eventually it will run out of hydrogen fuel. At that point, it will go through a radical change which will most likely result in the complete destruction of the planet Earth.

5. The Greeks named the sun Helios but the Romans used the name Sol, which is still in use today. Because of the important role the sun plays in our lives, it has been studied, perhaps, more than any other object in the universe, outside out own planet Earth. Our Sun has inspired mythology in almost all cultures, including ancient Egyptians, Aztecs, Native Americans, and Chinese.

. Ulysses was the first spacecraft to study our Sun's poles
Launched aboard the Space Shuttle Discovery and sent towards Jupiter with powerful booster rockets. After studying Jupiter for 17 days, Ulysses used the giant planet's gravity to hurl it into an orbit out of the Ecliptic Plane, where planets orbit our Sun.

The other primary Solar mission is SOHO. The international Solar and Heliospheric Observatory (SOHO) has been keeping a steady watch on the Sun since April 1996.
7. The sun's strong gravitational pull holds Earth and the other planets in place
It keeps the planets orbiting inside the solar system:

* Equatorial Surface Gravity: 274.0 m/s2
* Escape Velocity: 2,223,720 km/h

8. The sun is made up of distinctive areas
In addition to the energy-producing solar core, the interior has two distinct regions: a radiative zone and a convective zone. From the edge of the core outward, first through the radiative zone and then through the convective zone, the temperature decreases from 8 million to 7,000 K. It takes a few hundred thousand years for photons to escape from the dense core and reach the surface.

9. How does the sun's "surface" and "atmosphere" compare to planets?
The "surface," known as the photosphere, is just the visible 500-km-thick layer from which most of the Sun's radiation and light finally escape, and it is the place where sunspots are found. Above the photosphere lies the chromosphere ("sphere of color") that may be seen briefly during total solar eclipses as a reddish rim, caused by hot hydrogen atoms, around the Sun. Temperature steadily increases with altitude up to 50,000 K, while density drops to 100,000 times less than in the photosphere.

10. An unsolved mystery of the sun involves the corona ("crown")
Above the chromosphere lies the corona ("crown"), extending outward from the Sun in the form of the "solar wind" to the edge of the solar system. The corona is extremely hot - millions of degrees kelvin. Since it is physically impossible to transfer thermal energy from the cooler surface of the Sun to the much hotter corona, the source of coronal heating has been a scientific mystery for more than 60 years.

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