61 Cygni is one of our closest stellar neighbors — just 11 light-years away. And it consists of two stars, not one.

Even so, it’s only barely visible to the unaided eye — a faint dot not far from the graceful outline of Cygnus, the swan. It’s so faint because its two stars are only about half as massive as the Sun. But what the stars lack in flash, they’ll more than make up for in longevity — both will live much longer than the Sun.

In fact, not a single star of that class has had time to die from old age.

A star’s lifespan is determined by its mass.

Heavier stars have more hydrogen to power their nuclear reactions, so you might expect them to live longer. But that’s not the case. A massive star’s gravity is especially strong, so it squeezes the star’s core more tightly, making it hotter. That revs up its nuclear reactions, so the star burns through its hydrogen in a hurry. In fact, the heaviest stars live only a few million years.

Lightweight stars aren’t squeezed as tightly, so they consume their fuel at a slower pace. A star the mass of the Sun will “burn” its hydrogen for about 10 billion years. And a star that’s only about three-quarters of the Sun’s mass will live several billion years longer. Since the universe is less than 14 billion years old, that means that not a single one of these stars has had time to burn out on its own. Only interactions with other stars could snuff out such feeble cosmic lights.

Script by Damond Benningfield

Support McDonald Observatory