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Cluster M48, a magnitude 5.5 open star cluster in the constellation Hydra, was first discovered by comet-hunter Charles Messier in 1771 but, because Messier misstated its coordinates, it was "lost" until 1934, when German astronomer Oswalt Thomas demonstrated that the cluster Messier described was NGC 2458. The cluster is shown slightly right of center near the bottom of the chart, along with other nearby stars that we will use in this month's hunt:

• Pollux • • • •

• • Gamma

M44 •

Cancer. . . •

• ~ • ;. I • ·HYdra~ Procyon •• • Sextans

• • Monoceros • 0 ~\ • M48 . .

To find M48 using binoculars or a finderscope, start at Procyon, the bright mag 0.3 star at the SE corner of The Winter Hexagon. Procyon is the 8th brightest star in the sky, and quite close to us, at a distance of only 11~ light years. It was named Procyon, meaning "Before the Dog," because it rises just before Sirius, The Dog Star: that was important because Sirius heralded the annual flooding of the Nile River, which was crucial to the life of ancient Egypt. Just over 4° NW of Procyon is mag 2.9 blue dwarf Beta Canis Minoris, which forms a pretty lYz ° binocular group with a mag 4.3 orange giant and a mag 5.0 yellow giant. Using the distance from Beta to Procyon as one step, a second step along the same line will bring you to Zeta CMi (mag 5.1L and a third step reaches 28 Monocerotis (mag 4.7). 28 Mon forms a 2Yz° equilateral triangle with two other mag 4 stars, 27 Mon and Zeta Mon. Zeta in turn forms a triangle with a distinctive group of three stars just over 4° to its east (mag 3.9 C Hydrae and a mag 5.6 star to each side of it) and a mag 5.4 star 5° to its south. M48 is located inside that triangle, 3° SE of Zeta and 3Yz° SW of C Hya. If you have trouble finding the cluster, you can use the triangle of mag 6 stars lying between CHya and M48 as slightly-uneven stepping stones.

M48 is twice as wide as the full Moon. In binoculars and small scopes, it appears as a vaguely triangular patch of filmy haze with a few overlying stars. More stars emerge with increasing aperture and magnification, showing 50 stars or more in 6" scopes, including a nice arc of bright stars near center. Use low power to help the cluster stand out against the background field.

Next, we'll hunt down a relatively bright carbon star, X Cancri. Scan in binoculars or a finder, just over halfway from Regulus to Pollux and slightly south of a line between them, to find The Asses, mag 4.6 Gamma and mag 3.9 Delta Cancri. Starting at the brighter one, Delta, we'll scan roughly 2Yz° east and 1° south to find a distinctively-shaped "bent feathered arrow" of four mag 6-7 stars. The one marking the spot where the "feathers" join the "shaft" is our target.

A carbon star is a red giant whose temperature-mediated redness is amplified by the presence of carbon in its atmosphere: it has already converted its hydrogen to helium, and is now fusing helium into carbon. When the carbon is carried outward by convection currents, it coalesces into fine particles of soot in the star's outer layers. Just like particles in our own atmosphere, the particles of soot in the atmosphere of a carbon star scatter the star's light at the blue end of the spectrum, allowing only the red end of the spectrum to come through. Carbon stars are therefore the reddest of all stars, and some of them reach a remarkably deep red hue - but paradoxically, they are reddest when they are dimmest, and their color's intensity fades considerably as their magnitude brightens. The magnitude range of X Cancri is 5.7 to 7.5. Although it's not one of the reddest carbon stars, even at minimum it's reasonably easy to see in any telescope and most binoculars.

• .' ,. . ~...... ,. • . ;.oc... .*. • --- • • M44 I· . .• . . Delta • • . .:/=:"' ..' '~ ••• • .-- X Caneri zetaCMi / . ' , I . 28Mon • ~-~~------. • ·1 • . • • Rick Gering / NAA Public Outreach • • March 2019

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