The brightness of any star that’s in the prime phase of life is controlled by the star’s mass: Heavy stars are brighter than lightweight stars. But it’s not a simple one-to-one kind of relationship. A star that’s twice the mass of the Sun isn’t twice as bright – it’s more than 15 times as bright.

That’s because gravity squeezes the core of a heavier star more tightly. That increases the core’s temperature, which revs up the rate of nuclear reactions. That produces more energy, which makes its way to the surface and shines out into space.

Regulus illustrates the point. The heart of the lion consists of four stars, three of which are in the prime of life.

The star we see as Regulus – Regulus A – is a little more than four times the mass of the Sun, yet it radiates about 340 times more energy. Much of that energy is in the ultraviolet, which we can’t see. But even at visible wavelengths, it’s about 150 times the Sun’s brightness.

Regulus A has a couple of distant companions. Regulus B is about 80 percent the mass of the Sun, but only a third of the Sun’s total brightness. And Regulus C is even more dramatic: a third of the Sun’s mass, but just two percent its brightness – a cool, faint ember in the heart of the lion.

Look for Regulus standing close above the Moon as they climb into good view around 1:30 or 2 in the morning. The star will be a little farther from the Moon at dawn.

Script by Damond Benningfield