Why is the Sun so bright?

The sun is actually a large ball of fire, really a sphere of active energy that is the largest thing in the solar system. It is not the biggest star in our galaxy, but it is the closest. A mere 93 million miles separate us from that. Now when you have that intense an event happening at that magnitude so close to us, you could easily see it if you're on the right side of the planet, and you could feel the effects all the time. On Mars the sun is not so bright as it is here, and on Pluto te sun is little more than the brightest star in the sky. On Mercury, of course, the sun is about 11 times brighter and more effective than it is here on Earth.

Because it has so many degrees.

Everyone so far has it part right even if Linus's answer should go back in the cracker until Christmas!

I think the main reason is that the weight of the Sun pushes down so hard on its core that the atoms in the core are crushed together. This makes hydrogen atoms join together with strong nuclear bonds to make helium atoms (a process called nuclear fusion). When strong bonds like this are formed, energy is released in the form of heat and light. When it happens to billions of atoms in the Sun's centre, the energy given off is enough to heat the core to millions of degrees Centigrade. This temperature radiates outwards from the core to the surface and keeps on radiating across the 93 million miles of space between the Sun and us and beyond.

This is borne out by a new field of astronomy called helioseismology. Just as scientists use earthquakes to measure what the Earth's interior is made of, so scentists have now observed similar tremors on the Sun's surface, showing astronomers that the core is the ball of hydrogen/helium mix that the above theory would suggest.