An Autumn Highlight - The Andromeda Galaxy

~ Dr. Mike Inglis

One of the highlights of the autumn sky is the Andromeda Galaxy, or Messier 31 [M31] as it is more commonly known. The most famous galaxy in the sky is probably also the most often visited one, and is always a first observing object for the beginner. It is visible to the naked eye, even on those nights when the conditions are far from perfect. Many naked-eye observers claim to have seen the galaxy spread over at least 2.5° of sky, but this depends on the transparency, or clearness, of the sky.

Sky map showing the position of M31 (the Andromeda Galaxy) relative to the constellation Andromeda.

Visible to the naked eye even under moderate conditions, this object was known as the "little cloud" to the Persian astronomer Abd-al-Rahman Al-Sufi, who described and depicted it in 964 AD in his Book of Fixed Stars: It must have been observed by and commonly known to Persian astronomers at Isfahan as early as 905 AD, or earlier. The famous author R.H. Allen (1899/1963) reports that it was also appeared on a Dutch starmap of 1500. Charles Messier, who catalogued it on August 3, 1764, was obviously unaware of these early reports, and ascribed its discovery to Simon Marius, who was the first to give a telescopic description in 1612, but (according to R.H. Allen) didn't claim its discovery. Unaware of both Al Sufi's and Marius' discovery, Giovanni Batista Hodierna independently rediscovered this object before 1654. Edmond Halley, however, in his 1716 treat of "Nebulae", accounts the discovery of this "nebula" to the French astronomer Bullialdus (Ismail Bouillaud), who observed it in 1661; but Bullialdus mentions that it had been seen 150 years earlier (in the early 1500s) by some anonymous astronomer.

It was wrongly believed that the "Great Andromeda Nebula" was one of the nearest nebulae. William Herschel believed, wrongly of course, that its distance would "not exceed 2000 times the distance of Sirius" (17,000 light years); nevertheless, he viewed it at the nearest "island universe" like our Milky Way which he assumed to be a disk of 850 times the distance of Sirius in diameter, and of a thickness of 155 times that distance.

In binoculars it presents a splendid view, with the galactic halo easily seen along with the bright nucleus. Large binoculars may even show one or two dust lanes. Using averted vision and with a very dark sky, several amateur astronomers have reported that the galaxy can be traced to about 3° of sky in telescopes of aperture 10 cm. In larger telescopes a wealth of detail becomes visible. With an aperture of about 20 cm, a star-like nucleus is apparent, cocooned within several elliptical haloes. Another striking feature are the dust lanes, especially the one running along its north-western edge. Many observers are often disappointed with what they see when observing M31, as the photographs seen in books actually belie what is seen at the eyepiece. M31 is so big that any telescope cannot really encompass all there is to show.

Patience when observing this wonderful galaxy will reward you with a lot of surprises. Spend several nights observing, and try to choose a dark night in a country location [such as Montauk!]. This really is a spectacular galaxy. It contains about around 400 million stars with a diameter of 130,000 light years and is among the largest galaxies known. It is the largest member of the Local Group., a group of galaxies that are gravitationally bound, which includes our own Milky Way. In older texts it is often referred to as the Great Nebula in Andromeda.

The Leonids

LATE EVERY AUTUMN around November 18th, in the still, silent hours before dawn, dedicated meteor watchers have long kept a vigil. In the cold, early morning darkness the sky glimmers with a preview of early spring constellations. Leading them up in the east is Leo. Its familiar Sickle asterism, a backwards question mark, seems especially full of meaning these nights, for on the cutting edge of the Sickle's curved hook is the emanation point of the greatest of all showers.

The Leonid meteors at their peak are capable of producing the grandest display of celestial fireworks known to man. For about an hour or so the heavens seem to burst open and pour forth a deluge of shooting stars. The spectator stands agape, his mind numbed, his eyes dazzled, and his soul thrilled by the sight of shooting stars appearing by the hundreds . . . perhaps even thousands per hour. Every year, as noted above, a scattered few Leonids can be seen. Normally that is all there is to the Leonid meteor shower. But every third of a century, for four or five Novembers in a row, there is a chance that the Leonids may fall by the dozens, the hundreds, or even the thousands per hour. Just what will happen in those years, no one can say. It may be nothing, or it may be everything.

The Leonids historically are active between November 14th and 21st. The source of the Leonids was first identified in 1866 by the Italian astronomer, Giovanni Schiaparelli (of Martian canali fame). Other experts in celestial mechanics, notably Urbain Le Verrier and Theodor von Oppolzer, independently spotted a striking resemblance of the orbit of the Leonids to that of the then-newly discovered Periodic Comet Tempel-Tuttle. This comet appears to orbit the Sun at intervals of roughly 33 years. Today, even though not all prominent meteor showers have been matched to known comets, the relationship is clear: meteor streams are the debris of crumbling comets.

The Leonids are the swiftest of all shower meteors, at 44 miles per second -- almost the highest theoretical speed or meteors belonging to the solar system -- because their orbit runs nearly head-on to the Earth's. No Leonid has been known to escape entire vaporization and reach the ground as a meteorite. Many are bright, some are fireballs; greenish or bluish; about two out of three leave trains, which can sometimes persist for five minutes or more! They first appear at altitudes of between 96 and 61 miles up. the Leonids apparently originate from the snow-and-dust surface of the Tempel-Tuttle Comet and likely have the consistancy of cigar ash).

As it nears the Sun every 33 years, the icy nucleus of Comet Tempel-Tuttle ejects a flurry of small particles, which spread out along its orbit over time. Earth crosses this stream of comet crumbs every November, creating a "shower" and rarely a "storm" of meteors in our atmosphere.