Overdue Eruption

The star system U Scorpii ran a little late. It was expected to erupt in 2020. But it took some extra time — it didn’t pop off until last month.

U Scorpii is a binary system. One of its members is the corpse of a once normal star, known as a white dwarf. The other is in the prime of life. They’re so close together that they orbit each other once every 1.2 days.

At such close range, the white dwarf pulls gas off the surface of its companion. The gas forms a disk around the white dwarf. It spirals inward, piling up on the surface of the white dwarf.

But that can go on for only so long. In the case of U Scorpii, that’s about 10 years. At the end of that period, the new layer of gas is so hot that it triggers a nuclear explosion — the surface becomes a giant H-bomb — an eruption known as a nova. In just a few hours, that makes the system about 10,000 times brighter. It also blows away the disk around the white dwarf, although it begins to re-form within days.

U Scorpii’s first outburst was seen in 1863. Until June, the most recent was in January of 2010. Astronomers had expected it to erupt again as early as two years ago. But it came a little late — making the system flare thousands of times brighter than average.

U Scorpii is in Scorpius, which is low in the south at nightfall. The star is above Antares, the scorpion’s heart. It’s faded dramatically since its outburst, so it’s much too faint to see without a telescope.

Script by Damond Benningfield