Tuesday, September 22, 2009

The fun-house porch: this asterism holds the Garnet Star, an open star cluster, and an emission nebula.(Secret Sky).

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Cepheus the King stands high in the northern night sky shortly after sunset. By mid-October, at 9 p.m., its famous house-shaped asterism appears upside down directly above the North Celestial Pole. The House and its greater surroundings are home to several celestial delights. But I'm going to bring you only to what I call the House's "front porch"--the region south of Zeta ([zeta]) and Alpha ([alpha]) Cephei. Here we'll find a few of the constellation's most exotic wonders in an area of sky less than 3[degrees] across.


Scarlet star

What could be better than to start with Mu ([mu]) Cephei, the reddest star visible to the unaided eye? You'll find this gem about 4 1/2[degrees] southeast of 2nd-magnitude Alpha or 3 1/2[degrees] west (and ever-so-slightly north) of 3rd-magnitude Zeta.

Mu, popularly known as Herschel's Garnet Star, shines with the creepy color of blood in small apertures and appears to stain the carpet of the Cepheus region of the Milky Way. In larger telescopes, or at high magnifications, its color can appear more pumpkin or squash. The color you see may also depend on the star's magnitude. Mu is a slow variable star whose brightness changes from magnitude 3.4 to 5.1 in a semiregular period of 800 to 1,000 days. Generally, the fainter Mu is, the redder it appears. Of course, color perception is highly subjective; everyone sees color differently. Atmospheric and physiological effects can further affect what we see.

But Mu is much more than a colorful curiosity. It is one of the largest stars visible to the unaided eye. This luminous red supergiant measures some 1.4 billion miles (2.3 billion kilometers) across at its equator. If it were to replace our Sun, its atmosphere would reach halfway between Jupiter and Saturn. The life expectancy of such an enormous star is only around a few million years. So keep watch because Mu Cephei could possibly become our next supernova!

Colorful couples

Less than 1 1/2[degrees] south of Mu is another large Cepheus wonder: open star cluster Trumpler 37. Through a telescope at low power, this 3.5-magnitude object is a rich cluster of stars with an X-shaped core and looping outer arms--like flower petals. We see its members against the backdrop of the Milky Way, so the field is a visual feast.

A 5.6-magnitude star lies close to the cluster's core. But look carefully at it through your telescope because here is the colorful triple system Struve 2816--one of the prettiest multiple-star systems in the heavens.

In his Cycle of Celestial Objects, Admiral William Henry Smyth (1788-1865) writes of it: "Through all anomalies, here are palpable gleams of a force which penetrates through creation; and wondrous is the inference! Those mighty orbs move through a space in a curve which it bewilders the mind to figure, obediently to a mysterious impulse which binds them in affinity with some vast central phenomenon."

Under steady atmospheric seeing at 72x and greater in my 5-inch f/5 refractor, the primary burns with a warm-topaz light. Its two nearly 8th-magnitude attendants--one 12" away, the other 20" (almost in a straight line with the primary)--share pale-aqua hues. James Mullaney, co-author of The Cambridge Double Star Atlas with Wil Tirion, says that through 5-inch and larger telescopes, the primary appears bluish-white, while its "nearly matched companions are blue-green and purple or violet."


Move your scope just 15' northeast, and you'll see double star Struve 2819. In my scope at 72x, both multiple-star systems fit comfortably in the same field of view! Struve 2819 is an uneven pair that consists of an eggshell-white primary (magnitude 7.5) and a white secondary (magnitude 8.5) 12.4" to the east-northeast. Mullaney says the primary is "pale yellowish" and the dimmer companion looks "white."

Mystery mist

To the unaided eyes, Trumpler 37 appears as a breath of light against the soft glow of the Cepheus Milky Way. You may have difficulty detecting the cluster, which requires a dark and crystal-clear sky. Even then, you'll need to ponder the view carefully. The cluster covers an area of 11/2[degrees] (the apparent diameter of 3 Full Moons), so the low-contrast glow makes a fun naked-eye challenge.

Curiously, the visual haze you see may not be just from the cluster. Some of it could be from IC 1396--the larger (nearly 3[degrees] by 2[degrees]) swath of emission nebulosity surrounding Trumpler 37. I have suspected seeing IC 1396 with my unaided eyes and binoculars under extremely dark Hawaiian skies and am wondering if you can do the same, especially with the aid of a deep-sky or ultra-high-contrast filter.

IC 1396 is one of the largest emission nebula complexes in the sky. Its northern limit extends to Mu Cephei, which makes the nebula and star appear intimately related. Actually, they're not. Mu Cephei lies at a distance of some 2,700 light-years from Earth, but the IC 1396 and Trumpler 37 complex is roughly 100 light-years closer.

The nebula's shell-like glow may appear more definite in a rich-field telescope at low power. Mullaney, for instance, says that he spied IC 1396 using a 3-inch f/5 refractor at 30x. Seeing it, he says, demanded a moonless night of better-than-average transparency: "Slow sweeping back and forth using averted vision definitely showed the nebulosity in the 11/2[degrees] field of view."

I've seen it in a 4-inch refractor at 23x. But others under dark skies using a variety of instruments have not had success, even with filters. So the visual sightings remain controversial.

By the way, if you have a rich-field telescope and can see IC 1396, I'd especially like to know if you can pick out any of the nebula's dusty cobwebs (the dark nebulae) that cover its face. The darkest and most prominent patch is Barnard 161. It's a small dark patch about 20' north of Struve 2819. It measures some 20' in extent, but the darkest part is only 3' across. Good luck. Send your reports to someara@interpac.net.

Source Citation:O'Meara, Stephen James. "The fun-house porch: this asterism holds the Garnet Star, an open star cluster, and an emission nebula.(Secret Sky)." Astronomy 37.10 (Oct 2009): 66. General OneFile. Gale. Alachua County Library District. 22 Sept. 2009

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