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Deep-sky objects in Cancer

One of the faintest figures lying along the ecliptic, Cancer the Crab often confounds naked-eye constellation hunters in search of new conquests. Even without the extinguishing effect of light pollution, the Crab’s stars are just above the limit for most observers.

The Crab’s body is drawn from four stars in a trapezoid, while its legs extend north and south. The brightest of the bunch, Beta (β) Cancri, shines at only magnitude 3.5; the rest are fainter.

Binoculars prove helpful to identify the Crab’s stars. Aim about halfway between Castor and Pollux, in Gemini, and Regulus, in Leo. There, keep an eye out for the Crab’s trapezoidal body. The stars Asellus Borealis (Gamma [γ] Cancri) and Asellus Australis (Delta [δ] Cancri) bound the eastern side; Eta (η) and Theta (θ) Cancri form the western boundary.

Look closely at the colors of the four stars. See any differences? Slightly defocusing them will accentuate the effect. Delta, Theta, and Eta are all spectral type K orange giant stars, larger but cooler than our Sun. The fourth, Gamma, is a type A white subgiant, making it both larger and hotter than the Sun.

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