Here is the transcript for my podcast about how to see
the twin stars Castor and Pollux, the 9–12 Geminorum Cluster, and the Eskimo Nebula .
Check out the Astronomy.com's interactive star chart to see an accurate map of your sky. It'll help you locate some of this week's key targets. Astronomy magazine subscribers have access to a slew of cool functions with StarDome PLUS.
Each week, I highlight three different night-sky targets for you to see:
- One object you can find with your naked eyes or through binoculars
- One object to find with a small telescope
- One deep-sky object to find with an 8-inch or larger telescope for you avid astronomers
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A twin bill
This week’s naked-eye object is a pair of stars — Castor and Pollux — called the Twin Stars. They mark the heads of the Gemini twins, Castor and Polydeuces.
The two stars are bright. Castor (Alpha [α] Geminorum) shines at magnitude 1.59. Pollux (Beta [β] Geminorum) is even brighter, at magnitude 1.15.
To find the two stars, use Orion’s belt. Extend a line northeastward (up, based on Orion’s sky orientation) perpendicular to the belt. Nearly one-quarter of the sky later, you’ll arrive at Castor and Pollux.
But which is which? Even advanced skywatchers sometimes get confused. Well, when you’re looking at them, Castor is the most westerly. If you can identify a few other bright stars, telling Castor from Pollux becomes a bit easier. Castor lies on the same side of Gemini as Capella, the Alpha star in Auriga the Charioteer, and Pollux is on the same side as Procyon, Canis Minor’s Alpha star.
What’s in a name?
This week’s small telescope target is the unusually named 9–12 (pronounced “9 through 12”) Geminorum Cluster. The name has nothing to do with any date. Rather, it refers to numbers given to four stars by the first British Astronomer Royal, John Flamsteed (1646–1719). During his surveys of the constellations, Flamsteed assigned numbers to the stars in each constellation by increasing right ascension. His entire list contains 2,554 stars.
Although most astronomers no longer refer to the majority of stars by their Flamsteed numbers, a few famous exceptions exist: 61 Cygni was one of the first stars to have its proper motion determined; 51 Pegasi, in 1995, became the first Sun-like star found to have a planet orbiting it; and 47 Tucanae, also known as NGC 104, is the sky’s second-brightest globular cluster.
So, the four stars in the 9–12 Geminorum Cluster are 9, 10, 11, and 12 Geminorum. Their magnitudes are 6.3, 6.6, 6.9, and 7.0, respectively. This small cluster, which has a total listed magnitude of 5.7, also carries the designation Collinder 89. It lies about 1.5° northwest of magnitude 2.9 Mu (μ) Geminorum. The 9–12 Geminorum Cluster measures 1° across and makes a nice binocular target.
A fine planetary
This week’s deep-sky object makes it a clean sweep for Gemini. It’s a planetary nebula called the Eskimo Nebula (NGC 2392). This object resembles a face surrounded by a fur parka.
To find the Eskimo Nebula, point your telescope 2.4° east-southeast of magnitude 3.5 Delta (δ) Geminorum. The planetary glows at magnitude 9.1, which means you can spot it through just about any scope.
Seeing detail in this object, especially through telescopes with apertures of 10 inches or larger, is easy. Use as high a magnification as sky conditions warrant. You’ll easily spot the planetary’s 10th-magnitude central star.
NGC 2392 has a double-shell appearance. The inner shell appears bright, with a mottled texture. The outer shell looks fainter than the inner one, and a dark ring separates the two. The outer shell also dims with increasing distance from the central star.
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Previous episode: Musca Borealis the Northern Fly, open cluster Collinder 464, and spiral galaxy NGC 2403
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