The Pinwheel galaxy, M101, spins across the sky in this Hubble Space Telescope image. It is about 22 million light-years away, in Ursa Major, the great bear. Its striking spiral arms are outlined by hot young stars, some of which will explode as supernovae at the end of their short lives. [ESA/NASA]
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Not all galaxies are created equal. Many are featureless blobs of light. But others feature spectacular spiral arms that make them look like glowing celestial hurricanes. We live in one such galaxy, the Milky Way. And the constellation Ursa Major features another, known as M101.
Galactically speaking, M101 is a close neighbor — it’s about 22 million light-years away. It’s visible through a telescope near the handle of the Big Dipper, which is high in the north this evening. We see the galaxy face-on, so it’s one of the most beautiful spirals in all the night sky.
But the spiral arms of M101 are more than just beautiful. They also serve an important function: They squeeze interstellar clouds of gas and dust, causing them to collapse and give birth to new stars. Some of these newborn stars are quite bright, so they outline the galaxy’s spiral pattern.
These bright stars don’t live long, though — they quickly explode as supernovae. But M101’s most recent supernova, in 2011, came from a faint star that was no bigger than Earth. The star exploded when a “normal” companion star dumped gas onto its surface. Despite its diminutive nature, though, it was the brightest supernova ever seen in M101.
Stars will continue to explode in this spectacular galaxy. In fact, thousands of M101’s stars must have exploded over the past 22 million years — and their light is now racing toward Earth, ready to dazzle future skywatchers.
Script by Ken Croswell, Copyright 2013