A little extra weight around a star’s middle shortens its lifespan. The leading light of Aries, for example, is about a billion years younger than the Sun. Yet it’s already past the end of its “normal” lifetime — a point the Sun won’t reach until at least five billion years from now.

Hamal is low in the east at nightfall, far to the left of brilliant orange Mars. The star climbs high across the sky during the night, and is low in the west at first light.

Hamal is about half again as massive as the Sun. And that’s the key to its lifespan. The gravity of a heavier star squeezes its core more tightly, making it much hotter. That’s like slamming down the accelerator of your car — it makes the star “guzzle” the fuel in its core much faster.

Hamal has already finished the original hydrogen fuel in the core. Now, it probably is burning the hydrogen in a thin layer around the now-quiet core. The radiation from that shell of gas pushes outward on the surrounding layers, causing them to puff up. So Hamal is about 15 times wider than the Sun, and about 90 times brighter. As the outer layers expand, they also get cooler, so Hamal is redder than the Sun.

Before long, Hamal will cast those layers into space, surrounding itself with a colorful “bubble” of gas. The bubble will shine for thousands of years. As the bubble fades, though, only the star’s dead core will remain — shining feebly for tens of billions of years.

Script by Damond Benningfield

Support McDonald Observatory