Sky Roundup: September, 2008

September's Embers

This month sees the unofficial end of Summer and official start of Autumn. Labor Day weekend typically marks the end of our warmest months for most, while the autumnal equinox on September 22 ushers in the start of the Fall season and all of the memories that go with it. We look ahead to crisp, clear days and a decided chill in the clear, night air. 

This also starts the season of glowing embers. When once we raked leaves and burned them, the smoldering piles filled the air with a memorable Autumn scent. These days, there are still places for outdoor fires, but well away from cities.  In some rural areas, a Fall bonfire occasionally is lit. On farms or at stables, folks will typically pile up the past year’s old wood and debris and set it alight on a cool evening. For those who choose to find their warmth indoors, a frosty Fall evening may prompt some to open the chimney flue and enjoy a cozy crackling blaze in the fireplace. At the end of the evening, the flames die away, and both bonfire and fireplace glow red with embers.

Up in the night sky, we can enjoy September's embers glowing red from afar. Bright stars, such as Arcturus and Antares, are seen early in the evening, heading west to set for the season. After midnight, Aldebaran shines high in the east. And before dawn, Betelgeuse makes its appearance. All four of these stars show a slightly reddish color to the eye or with any optical aid. 

Arcturus, the brightest star in Boötes (the Herdsman), brightest in the northern celestial hemisphere and 3rd brightest in the entire sky, marks the foot of the Herdsman as he scrabbles down the side of the sky to set in the northwest. The name Arcturus translates from the Greek as "bear guard" or "bear chaser" since this star represents his left, or leading, foot as Boötes chases the Bears (namely, Ursa Major and Ursa Minor) from his pasture.

Antares, the brightest star in Scorpius (the Scorpion) and 16th brightest in the whole sky, shows the blazing heart of that crawly creature, at which nearby Sagittarius (the Archer) has aimed his bow's arrow. The arrow finds it mark and the maimed Scorpion crawls out of the sky, into the southwest, and below the horizon. As is fairly well known, the name Antares derives from a Greek word construction which loosely translates as "rival" or "opposite" of Mars. That is to say, the color of the brightest star in Scorpius resembles the appearance of the Red Planet. In the original Greek pantheon of deities, Ares was a god of war and is the equivalent of his Roman counterpart Mars. In September the Moon hides Antares, but the occultation cannot be seen in Ohio. 

Aldebaran, the brightest star in Taurus (the Bull) and 13th brightest in the entire sky, glows as the ruddy eye of the Bull. He rears up in the east to stare down the mighty warrior Orion wielding a heavy club. The name Aldebaran derives from Arabic and means “follower” since this bright star, as seen in the night sky, appears to follow the star cluster of the Pleiades across the heavens.

Betelguese, the brightest star in Orion (the Hunter) and 9th brightest in the whole sky, shines as the reddish shoulder of the warrior, armed against the onrush of the Bull. The name for this bright star was originally listed in Arabic as "hand of the central one." A transliteration error from Arabic to Latin during the Middle Ages misrepresented the star's name, and its meaning became "armpit of the central one." This mistaken meaning of the original Arabic name eventually led to the modern rendering as Betelgeuse.

Antares and Aldebaran are roughly the same apparent brightness, at just less than 1st-magnitude whereas Arcturus and Betelgeuse are both brighter and closer to zero magnitude. All are enormous red giant stars, many times larger, with diameters that extend beyond the orbits of our own terrestrial planets, and more luminous, owing to their huge surface area, than our own Sun. 

All four are thought to be late-stage stars, that is to say, in the autumn of their years, as they near the end of their life cycle. Their fusion furnaces have consumed most of their supply of hydrogen fuel and are fusing helium or heavier elements. At some point―no one knows for certain―these big red stars will either slough off their outer layers as their interiors collapse (like Arcturus and Aldebaran) or, if they're overly massive (like Antares and Betelgeuse), explode as supernovae. 

Two charts for this month show the night sky at mid-September. The first chart (with a view toward the west) shows the sky early in the evening (about 9:00 p.m.) and the relative locations of Arcturus and Antares, essentially leaving the sky for the passing season.  

Chart 1: Arcturus and Antares at mid-month, early evening toward the west, showing these two reddish stars to mark the passing of Summer.

The second chart (with a view toward the east) shows the sky well after midnight (around 3:00 a.m.) and the relative locations of Aldebaran and Betelgeuse, clearly entering the sky for the coming season.

Chart 2: Aldebaran and Betelgeuse at mid-month, early morning toward the east, showing these two reddish stars to herald the onset of Autumn.

Other embers, even larger than the red giants named so far, burn quietly in September's skies. Well known for its color is the aptly named Garnet Star, aka Mu Cephei, in the constellation of Cepheus (the King). A deep-red dazzler in a telescope, the Garnet Star is ponderously huge, which, if it were to occupy the place of our Sun in the Solar System, would reach beyond the orbit of Jupiter! Mu Cephei, also an old star like previous four, glows deep red not only from its fairly low surface temperature but from its fusion furnace fizzling. Staggeringly larger is another star in that same constellation, the slightly reddish ember shown as VV Cephei. It is among the largest known stars, whose width, if placed in our Solar System, would extend beyond Jupiter and near to the orbit of Saturn! 

Both of these huge stars in Cepheus are more massive than the Sun and likely will meet a catastrophic end as supernovae. Both glow at about 4th-magnitude and are fairly distinct to view in a good telescope.

One familiar red ember that glows and ebbs is the planet Mars. As we observe it from Earth, Mars changes brightness as it approaches and recedes from our position in our own orbit around the Sun. Mars joins with Venus and Mercury during the first half of the month as the trio appears to skim the treetops at sunset. To see this planetary threesome, one needs a flat horizon, such as a western view from the shore of Lake Erie, and a big binocular or a good telescope. A small hill or ridge on land may be sufficient to place one high enough for a look, but you'll need to peer into a sunset sky, still fairly bright, to catch a glimpse of the trio. Unfortunately, the Red Planet is dim in our early evening sky in September and disappears in the glare of the Sun by mid-month. It won't be favorably placed for viewing until it reappears in the morning sky by next Spring. 

We may not think of the Moon as warm like a glowing ember; more often we see it as cool and pale. But the Moon, seen on rising, takes on a golden carroty hue in September. Often referred to as the Harvest Moon, the rising Full Moon will appear huge and round and orange. This is an illusion that all of us see. To appear so large, the Moon is no closer than usual; rather, seeing the Moon appear to rise amid familiar objects, like trees and houses, simply makes it look big. By the way, the name Harvest Moon is given to that Full Moon seen closest to the autumnal equinox, the astronomical start of Fall. 

In the skies of September we watch the approach of Autumn. Redgold twilight fades to cool cobalt blue. One by one, by the dozen, by the hundreds, stars peek out from the deepening shadows and fleck the dome of the night sky. Among those myriad stars, many of the distant stellar fires glow red to light the night, heralding the change of seasons. It's a subtle ceremony worth revisiting every clear evening in Autumn.

Sky Roundup Almanac for Cleveland, September, 2008

September   6:  Bright star Antares less than 1° north of Moon; no occultation 
                        seen in Cleveland
September   7:  First Quarter
September 10:  Mercury at greatest elongation, visible in west at sunset
September 11:  Venus less than ½° north of Mars
September 12:  Neptune less than 1° south of Moon, occultation visible in North America
September 15:  Full Moon
September 22:  Last Quarter Moon; Autumnal Equinox, i.e., first day of Autumn
September 27:  Zodiacal light visible in east before sunrise for next two weeks
September 29:  New Moon
September 30:  Mercury just over 1° north of waxing crescent Moon
All celestial events corrected to approximate local Eastern Daylight Time.