Moon and Antares 
A big bonfire burns hotter than a small campfire. And the same thing applies to the stars — sort of. The core of a big, heavy star is many millions of degrees hotter than the core of a small, lightweight star. But at the surface, it can be just the opposite — a big star can be cooler than a small star.
For a prime example, look to the lower right of the Moon at first light tomorrow for Antares, the bright orange heart of Scorpius, the scorpion.
Antares is one of the biggest, heaviest stars in our part of the galaxy. It’s about 15 times as massive as the Sun, and hundreds of times wider.
Antares is so heavy that gravity squeezes its core tightly. That heats the core to hundreds of millions of degrees — far hotter than the core of the Sun. That allows the star to “burn” successively heavier elements, creating new elements as it does so.
Because the core is so hot, it produces intense radiation, which exerts pressure on the layers of gas around the core. That causes Antares’s outer layers to puff up like a giant balloon. As those layers puff up, they get cooler. So Antares’s surface is several thousand degrees cooler than the surface of the much smaller Sun. And since a star’s surface temperature determines its color, Antares glows reddish-orange.
Even though it’s cool, Antares is tens of thousands of times brighter than the Sun. That makes it easily visible in our sky even though it’s 550 light-years away.
Script by Damond Benningfield, Copyright 2012