The Sun is a main-sequence star, one of over 100 billion stars in the Milky Way Galaxy. It
takes the Sun over 200 million years to complete one orbit of the galaxy. It is located at
present close to the Sagittarius-Carina spiral arm, in what is called the Orion spur.
The orbital trajectory is not planar, but undulating, the Sun passing through the principal
plane of the galaxy once every
years. The planets of the Solar System revolve about the Sun in a plane normal to the Sun's
circum-galactic trajectory. Thermonuclear reactions in the Sun's core convert hydrogen into
helium, so that the helium fraction is progressing increasing through time. The reactions
produce the energy to make the Sun a yellow-orange main-sequence star of spectral type dG2
(d indicates dwarf). The central temperature is about 15 million K; the surface temperature
is about 6000 K.