Heel and Toe of a Twin!

There’s a lot going on in this image taken over the past couple of weeks. The bright stars here are the heel (on the left) and the toe (on the right) of Castor, the rightmost of Gemini’s Twins (his bro is Pollux). The Heel star is called Tejat and the Toe star is Propus. You can see the Gemini Twins striding high across the zenith these midwinter nights, above and the the left of Orion the Hunter. They represent twin sons of Zeus.

But oh what’s all that wild-looking red stuff in Castor’s foot?!

The weird claw-like cloud is the remnant of a supernova that exploded in this part of our galactic neighborhood around 30,000 years ago – about the time of the earliest paleolithic cave artists of western Europe. Supernovae are absolutely spectacular stellar blasts and there hasn’t been one in our galaxy for over 400 years. They briefly outshine the rest of the trillion stars in our galaxy put together, cast deep shadows at night and are often visible during the day.

Unlike most supernovae, this one took place in a dense cloud of cold molecular gas and dust, and the expanding blast debris continues to interact with that pre-existing gunk. To the bottom-right of this image, the blast wave flies fast and fierce, streaming into the void as it carries newly-forged oxygen, carbon, silicon, and iron off into the galactic disk. But along its upper left margin (in the ball of Castor’s foot) the blast wave is slowed and compressed as it slams into dense dust and molecular gas. The converging material piles into itself at supersonic speed and forms billowing shock fronts on top of shock fronts that ionize the gas and cause it to fluoresce in that lovely characteristic rose colored plasma.

Beyond the shock front of expanding supernova blast, the molecular cloud shades its inner recesses from heat and radiation so that new stars can collapse out of the cold material. A cluster of these newborn stars is visible at the upper center of the image, shining brightly through the murk. Mineral dust scatters some of the bluest starlight in an ethereal reflection nebula that’s blue like our sky, for the same physical reasons.

Invisible ultraviolet rays from those freshly minted furnaces ionizes hydrogen in the cloud, lighting it gently with an electromagnetic buzz of fluorescence. This dim glow is a faint counterpoint to the violently expanding blast wave sweeping back through Castor’s foot.

These clouds are about 5000 light years distant, so we see the drama unfold just as writing was being developed in the city-states of Mesopotamia. The heel and toe stars are superimposed foreground stars in our own neighborhood. Tejat and Propus are just 230 and 350 light years distant. Both are red giant stars swollen with old age to around the diameter of the orbit of Venus around our Sun.

This is a stack of 551 x 60s color photos for a total of 9 hours of exposure taken over many nights with a 5-inch refractor at a focal length of 488 mm. I captured the data in KStars (ubuntu linux) and processed the images in PixInsight on a mac.