Astronomers have found many awe-inspiring objects in the cosmos, but a pair of astronomers 20 years ago found something just plain confusing. They spotted a straight line of ionized gas in the direction of Ursa Major, which was never really explained to everyone’s satisfaction. A new analysis of this line involving astronomers from all over the world has revealed it is actually part of a much larger structure, an arc that covers a huge portion of the sky and suggests a supernova went off very near to Earth just a few hundred thousand years ago.
The original observation by astronomers Peter McCullough and Robert Benjamin was certainly unusual — after all, there aren’t many things in space forming a perfectly straight line. Several years ago, astronomers from the Netherlands, France, and Croatia joined Benjamin to analyze the mysterious line in the sky. Using data from the GALEX ultraviolet space telescope, they found that line is actually a small section of a massive arc.
The original segment was about 2 degrees long, but the newly discovered ark stretches 30 degrees across the sky. Extending the arc produces a circle with a diameter of 60 degrees, completely encircling the Big Dipper. It covers 6 percent of the entire sky.
According to the team, the “Ursa Major Arc” is composed of compressed interstellar gas that is visible in the ultraviolet part of the spectrum. Now that we know the overall structure is a circle and that the arc is very thin compared with its length, the team believes this is the shockwave from an ancient supernova blast. This hypothesis jives with what we know about this part of the sky. It’s one of several “windows” with very little interstellar gas, which makes it ideal for observing distant galaxies. The supernova that created the Ursa Major Arc shockwave may also have cleared dust and gas from that part of the sky.
We don’t have enough data to determine exactly where the supernova occurred just yet. However, it has to be close (astronomically speaking) considering how much of the sky it covers. It might have been a mere 600 light-years away. That’s not close enough to threaten Earth, but it would have been extremely bright for any of our early hominid ancestors who were looking skyward about 700,000 years ago. The team hopes to narrow down the supernova’s location and age with additional observations.
Top image: NASA/JPL/Caltech
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