Thursday, June 18, 2009

Stars of Other Worlds: 55 Cancri

Every once in a while, Acts of Minor Treason takes a jaunt through subspace and hyperspace and across cosmic strings and collapsars to some strange new world, to set up a telescope and look up at the stars from a new angle.

Though I was a star nerd during my youth - I learned the Greek alphabet from Terence Dickinson's Nightwatch, and pored over its starcharts religiously even though I never mustered the gumption to actually take the telescope out for a spin - it wasn't until the 21st century that I first heard of 55 Cancri. Also known as Rho1 Cancri, 55 Cancri is a binary star in the constellation Cancer, approximately forty-one light-years away. The larger of the two stars is a G8V yellow dwarf, a bit dimmer than our own sun, and it's accompanied by a much dimmer red dwarf. From Earth, 55 Cancri A has an apparent magnitude of 5.95, just barely visible to the naked eye under ideal conditions.

The stars themselves aren't too remarkable. Binaries are, after all, one of the most common stellar arrangements in the galaxy. The 55 Cancri planetary system, on the other hand, is something else entirely. It's the largest known system beyond the solar system, with at least five planets detected in orbit of 55 Cancri A, all thought to be gas giants based on their mass. One of them, 55 Cancri f, orbits its sun in 260 days and spends its orbit entirely within the habitable zone - close enough to the sun that a hypothetical moon of the planet could sustain liquid water, which might enable an ecosphere similar to Earth's.

So, hypothetically, if there are thinking beings on a moon of 55 Cancri f - whether they're humans or otherwise - what would they see?

There'll always be the stars. Earth and 55 Cancri are close enough to share bright stars in common - Canopus is effectively just as bright when viewed from either system, and Rigel and Betelgeuse in Orion are a bit dimmer here than there. Discounting 55 Cancri B, when observing from a moon of 55 Cancri f Pollux would be the brightest star in the sky, a bit dimmer than Sirius appears on Earth.

Still, even though Sol and 55 Cancri occupy the same galactic neighborhood, perspective changes fast with only a handful of parsecs. While snooping around in its sky I found two constellations that struck my interest, constellations I've named the Broken Plough and Atalanta.

The Broken Plough

Aside from Orion, the Big Dipper is probably the single most recognizable star pattern in Earth's skies. It's known by many names by many cultures, and in the British Isles it's also called the Plough. As many of its stars move together in a cluster called the Ursa Major Moving Group, it's no surprise that from 55 Cancri they're still together in the sky. Even the pattern we're familiar with on Earth is mostly intact - if a bit distorted.

Viewed from Earth, the two brightest Big Dipper stars are Dubhe and Alioth, which share an apparent magnitude of 1.8. Although Dubhe doesn't figure into the Broken Plough as I've laid it out, Alioth is its keystone and even brighter than we see it, with an apparent magnitude of 1.22 from 55 Cancri. Alkaid, at the western end, is still brighter than any of the Big Dipper stars from Earth, and Thuban - the brightest star in Draco, and the northern polestar in ancient Egypt's heyday - at the eastern edge is only slightly dimmer than Megrez seems to us, and again is somewhat brighter than it appears from Earth.


In Greek mythology, Orion was a masterful hunter who was killed either by Artemis (who, presumably, did not like competition of Orion's caliber) or a giant scorpion and subsequently elevated to the heavens in the form of the constellation we know today. He's one of the many larger-than-life heroes that populate the myths - taken together, they're a real sausagefest, if you ask me.

Orion the constellation has such mythological significance because it's so damned obvious. Owing to its position in the sky it's visible in every part of the world, and it's anchored by some of the brightest stars in the sky. Bright patterns like that definitely attract attention.

Should there ever be humans looking up from a moon of 55 Cancri f, they might not take long to notice a particular arrangement of stars that reminds them of what they left behind.

In Greek mythology, Atalanta was also a hunter, and one of the few non-goddess females in that canon. The correspondence isn't exact, but haunting nevertheless. Antares, at her right foot, is Atalanta's brightest star, with Graffias (Beta Scorpii) her head while Zubenelgenubi and Sigma Scorpii form her belt. Here, Atalanta comprises stars from no less than three Earth constellations - while the majority are in Scorpius from Earth's perspective, Zubenelgenubi is the brightest star in Libra, and Atalanta's left foot is formed by Gamma Virginis, in Virgo.

This correspondence is particularly interesting because none of the familiar stars of Orion are involved in it. Still, the human brain is the most sophisticated finder of patterns in the static that we have, and it's no suprise that on distant worlds we might look up to see something that, while not the same, remains vaguely familiar - enough to put one at ease, remembering home.

What of home, by the by? Forty-one light-years is hardly enough to make a star like ours invisible. Sol's above the limits of vision from 55 Cancri, but not by much - it would have an apparent magnitude of 5.32, best viewed with binoculars.

As previously, all glory goes to Celestia for making these views possible without a hell of a lot of painful mathematics on my end.

Further Stars of Other Worlds

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