without rapid changes in structure or appearance, gradual changes occur while a star consumes its central hydrogen. As hydrogen is depleted, main sequence stars increase in both size and luminosity. Because main sequence stars of a given mass vary in luminosity as they age, the main sequence appears as a band rather than a line in the H-R diagram. For the Sun, the increases during its main sequence lifetime are about 40% in radius and 100% in luminosity. The Sun’s temperature will remain nearly constant throughout its main sequence career. The doubling of the Sun’s luminosity during the 10 billion years it spends in the main sequence is a small increase compared with the luminosity changes that precede and follow the main sequence phase of its life. However, even this increase in solar energy output has had and will have consequences for the other bodies in the solar system.