It’s one thing to theorize about an exploding star the size of our sun, it’s another to look up in the sky and watch one getting ready to blow.

Astronomers are now doing this.

On Feb. 12, a star known as RS Ophiuchi, some 8,000 trillion miles away, erupted in an explosion so bright it could be seen on Earth without a telescope. It was the star’s sixth attention-getting blowout since 1898.

Using satellites and ground-based telescopes, observers from the Harvard-Smithsonian Center for Astrophysics (CfA) and their colleagues from other institutions caught the eruption near its maximum brightness. They measured high-energy X-rays, low-energy radio waves, and heat coming from the outburst.

“Studying such emissions from RS Ophiuchi and stars like it will give us more confidence in measuring distances in the universe, one of the most important topics in astronomy and cosmology,” says Jennifer Sokoloski, a researcher at CfA in Cambridge, Mass.

Certain kinds of supernovae act like measuring posts in the vast emptiness of space. When you look at the brightness of a star, one of the big problems is determining whether it is a faint star that is fairly close, or a bright one very far away. There is no way to distinguish apparent brightness from intrinsic or actual brightness. That type of riddle can be solved with supernovae that all burn with about the same celestial wattage.

“These explosions always have about the same brightness,” Sokoloski explains. “Therefore, the supernovae act as ‘standard candles’ in the sense that their apparent brightness indicates how far away they really are.”

The super flashes also shed light on how fast the universe is expanding, which in turn provides clues to how old the universe is and how long it will live. Say you have a standard candle glowing in a distant galaxy. By measuring shifts in its light you can tell how fast it is moving away from us. Getting such information from supernovae at a different distance from Earth tells you how fast everything is expanding.

“Using this technique, astronomers have been surprised to find that the expansion of the universe is actually accelerating,” Sokoloski notes. This finding runs counter to the old idea that gravity will eventually slow the expansion and start a contraction that will cause the universe to eventually end in a colossal crunch. Acceleration, on the other hand, forecasts that the universe will go on expanding forever.