The Sun!!
The sun contains most of the mass in the solar system. All we can see is the top of a dense cloud deck.
The home page for the sun at LANL and SEDS.
Most of the planets orbit the sun above its equator. The Ulysses mission used the gravity of Jupiter to hurl the spacecraft above the sun's poles. Spartan 201 is a NASA Space Shuttle instrument designed to study the sun.
A solar eclipse happens when the moon passes in front of the sun. Pages describing this are found at the Solar Data Analysis Center.
The Northern Lights are caused by the sun's solar wind moving past the Earth's magnetic field.