In Greek mythology, Orion and Scorpius were mortal enemies. So the gods placed them on opposite sides of the sky. When one is rising, the other is dropping from view.

The constellations have a lot in common. And so do their best-known stars – Betelgeuse in Orion, and Antares in Scorpius. Both stars are red supergiants. Both are many times more massive than the Sun, and a hundred thousand or more times brighter. And each is destined to explode as a supernova – probably within the next million years.

Betelgeuse and Antares also are hundreds of times wider than the Sun. So even though they’re hundreds of light-years away, they’re big enough for astronomers to see them as disks instead of just pinpoints of light. In fact, they’re among the first stars to have their size measured directly.

Antares was measured 100 years ago. Astronomers used a special device attached to the 100-inch telescope at Mount Wilson in California – the largest telescope in the world at the time. It gave them a diameter of more than 400 times the size of the Sun. As technology has improved, astronomers have found that Antares is probably about 50 percent larger than that. So if Antares took the Sun’s place, it would engulf the four innermost planets – including Earth.

Betelgeuse is in the east at nightfall. Antares is on the other side of the sky – low in the south-southeast at dawn. Tomorrow, it’s close to the lower left of the Moon.

Script by Damond Benningfield