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Binocular Universe: Out of the Frying Pan and into the...
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Binocular Universe: Out of the Frying Pan and into the...
Created by Nicolas Louis de Lacaille in 1756 from stars originally belonging to Eridanus, the faint constellation Fornax, the Furnace, appears to the naked eye as a nearly starless void southeast of the bright star Diphda (Beta Ceti).
Fornax is littered with galaxies, with most belonging to the Fornax Galaxy Cluster. The Fornax Cluster lies at a distance of 62 million light-years (19 megaparsecs). It contains more than 50 galactic members, making it is the second richest galaxy cluster within 100 million light-years of the Milky Way, surpassed only by the significantly richer Coma-Virgo Cluster.
Unfortunately, all but two are too dim to be considered binocular objects. There is, however, a nice stellar asterism awaiting our arrival, along with a surprising planetary nebula. How many can you spot? You'll need a good eye and a clear southern view. I also recommend supporting your binoculars on a tripod or other mount. That will make the task a little easier.
Above: Autumn star map
from Star Watch by Phil Harrington.
Above: Finder chart for this month's Binocular Universe.
Chart adapted from Touring the Universe through Binoculars Atlas
The constellation's "brightest" star, Alpha Fornacis, shines only a bit above 4th magnitude. Only 46 light years away, Alpha Fornacis is a binary system that pairs a yellow-white class F8 subgiant star with a class G7 dwarf. The primary sun, Alpha For A, shines at magnitude 3.9, while Alpha For B is magnitude 6.5. While this places them well within the grasp of binoculars, they are too close to one another to be resolvable. The higher magnifications afforded by telescopes, however, will resolve each separated by about 4 arc-seconds. That gap is widening slowly, until a maximum separation of some 7 arc-seconds is reached in 2082.
The constellation's rogue planetary nebula, NGC 1360, is usually ignored by northern hemisphere observers, in part because it lies in such an empty area of the sky. For those who persevere, NGC 1360, nicknamed the Robin's Egg Nebula, rewards them with a faint, grayish disk of light. Most observers see it as noticeably elliptical, with photographs indicating an apparent diameter of more than 6' of arc. Given dark skies and 60-mm or larger binoculars, you should also notice the 11th-magnitude central star. Averted vision should help if you're having trouble making it out.
Fornax also holds a nice asterism for all binoculars, large and small alike. I refer to it as Harrington 2 in my book Touring the Universe Through Binoculars. The group is collected around the stellar triangle of Chi-1, Chi-2 and Chi-3 Fornacis, and includes a number of other, fainter stars in their immediate vicinity. Chi-1, the westernmost member of the Chi triangle, is an easy optical binary, with the 6th-magnitude secondary star found to the primary's south. Actually, this asterism appears more like a diamond thanks to an additional 6th magnitude star to the Chi triplets' east. In all, nine stars from 5th- to 9th-magnitude and scattered across about 0.5 degree compose Harrington 2. Adding to the scene is a semicircle of seven 9th-magnitude stars immediately north of Chi-2.
We can use this asterism as a starting point to explore the two brightest members of the Fornax Galaxy Cluster. First up, we have NGC 1316, the brightest member of the Fornax cluster. Confusion over its exact classification has always reigned over this 10th-magnitude galaxy. Threads of dark nebulosity lacing though this strange giant elliptical galaxy are likely the result of NGC 1316 having merged with a disrupted spiral galaxy. Long exposure photographs also reveal loops of material swirling out from NGC 1316 and engulfing NGC 1317, a faint nearby system that lies beyond the grasp of our binoculars. Some theorize that NGC 1317 will eventually be absorbed into its neighbor. This phenomenon of a larger galaxy devouring a smaller object is appropriately called galactic cannibalism, and results in the strong emission of radio noise detected from NGC 1316. Binoculars show NGC 1316 as a fairly large disk of light highlighted with a brighter central core. Sadly, the features that make this galaxy so unique are not detectable in binoculars due to their subtle nature.
To find NGC 1316, center on Chi-1, the westernmost member of the Chi triangle, and shift your attention just 1.5° to its southwest. It lies within an equilateral triangle of 8th-magnitude stars. Look for a subtle glow, slightly oval and without any real central concentration.
Next, we come to NGC 1365, one of most impressive examples of a barred spiral found south of the celestial equator. You'll find it hidden about a degree east of the Chi Triangle. Overall, NGC 1365 measures 8'x3' and shines at 10th magnitude. Look for a diffuse, oval patch growing steadily brighter toward its center. The galaxy's characteristic central bar, seen clearly only in photographs, is believed to span some 45,000 light years.
Finally, let's head just south of the Fornax border and wade into Eridanus, the River. Flowing across more than 50° our sky, Eridanus follows a narrow path through late autumn's southern sky. One of the oldest constellations, Eridanus has been associated with many waterways including the Nile of ancient Egypt.
There, you'll find the galaxy NGC 1291 about 4° southeast of Theta Eridani and 4° nearly due south of our new friend, NGC 1316. A slender triangle of 8th- and 9th-magnitude stars lying immediately to its west-southwest helps point the way. Through binoculars, this barred spiral looks like a soft, slightly oval glow punctuated by a brighter core. Smaller apertures may have a bit of trouble revealing this 9th-magnitude galaxy at first, but 70-mm and larger giants should show it plainly given a clear view in its direction.
These and other targets within this month’s Binocular Universe are listed in the table below.
Next month, we trek back north to explore the late autumn/early winter skies. Until then, remember my mantra: Two eyes are better than one.
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Looking for more targets to enjoy through your binoculars? Read Phil's article Tour 10 Fall Binocular Treats in the August 2015 issue of Astronomy magazine.