Mighty Orion the hunter has a mighty resting spot for his tired feet: Cursa, the second-brightest star of Eridanus, the river. The star’s name comes from a longer Arabic phrase meaning “footstool of the central one” – Orion himself. As night falls, the star stands above Orion’s foot: Rigel, the hunter’s brightest star.

Cursa is about 90 light-years away. It’s easy to see from that distance because it’s a giant. It’s several times the size and mass of the Sun, and 45 times the Sun’s brightness. Its classification as a “giant” tells us much more than just its size, though. It also tells us about its stage in life.

A giant star has puffed up as a result of changes deep in its heart. It’s burned through the hydrogen in its core to make helium, so it’s moved into a new phase. In the case of Cursa, it’s fusing hydrogen in a thin shell around the core. The shell is quite hot, so it produces a lot of radiation. That pushes on the surrounding layers of gas, causing the star to expand. And that makes it brighter.

Today, the surface of Cursa is thousands of degrees hotter than the Sun’s. At that temperature, the star shines almost pure white. As it continues to change, though, Cursa may get even bigger and brighter. But its surface will get cooler. So a bigger Cursa will shine redder – an angry-looking footstool for the hunter.

Tomorrow: from giant to supergiant.

Script by Damond Benningfield