The Sun is a sedate, middle-aged star. It steadily “fuses” the hydrogen in its core to make helium, which releases the energy that makes the Sun shine. And the Sun changes very little — its size and brightness are always almost exactly the same.

A star in the big dog is much different — it pulses in and out like a beating heart.

Mirzam represents a paw of Canis Major, the big dog. As night falls, it’s close to the right of Sirius, the brightest star in the night sky, in the southeast.

Mirzam is much bigger and heavier than the Sun, and thousands of times brighter. And it’s only about 12 million years old, compared to billions of years for the Sun.

Even so, Mirzam is nearing the end of its life. It’s just about used up the hydrogen in its core, so it’s getting ready to fuse the helium to make other elements.

A layer around the core contains a lot of iron. The iron absorbs energy from the core. When enough energy builds up, that layer pushes outward. When the layer cools, it falls back. That process carries out to the star’s surface, causing it to pulse in and out.

There are actually many different pulsations, all working in different ways. The pulses last different amounts of time, for example, and they move at different speeds. Combined, they cause Mirzam’s diameter to vary by thousands of miles. That changes the star’s brightness, too, but not enough to see with the eye alone. So the dog’s paw looks steady, even though it pulses once every few hours.

Script by Damond Benningfield

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