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Health & Fitness

Andromeda : The Milky Way's Sister

Early fall is a fantastic time for observing the night sky.  Night falls earlier, the air is becoming crisper, the humidity and haze diminishing, and increasingly interesting objects are coming into view.  It is also the time of year when we can begin observing the most distant object that can be seen with the unaided human eye – the Andromeda Galaxy.

Andromeda was known to ancient man, the earliest existing mention of it recorded by Persian astronomers in 905AD.  Al-Sufi, one of the great Persian astronomers who were key in saving the astronomy of the Greeks from being lost in the European Dark Ages, mentions Andromeda as “The Little Cloud” in his Book of Fixed Stars (964AD). 

The first observation of Andromeda through a telescope was recorded by Simon Marius in 1612 (following Galileo’s first use of a telescope in 1609). Marius described Andromeda as looking “like a candle seen through horn”, indicating a bright center, and a fading glowing elliptical halo.  This is much like what can be seen through a typical modern small telescope when aimed at Andromeda from suburban skies – a rather bright hazy cloud, elliptical in shape with a bright center.

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Charles Messier, a French astronomer most famous for his catalog of fuzzy objects that he did not wish to confuse with comets, catalogued Andromeda as a bright “nebula” (designated M31), along with two companions M32 and M110, which are the brightest of over 20 smaller satellite galaxies of Andromeda.  Astronomers throughout the 1800’s regarded the Andromeda Nebula as an enormous cloud of gas, possibly representing a solar system in formation, located relatively nearby to Earth.

This explanation began to be challenged in 1864, when the first spectroscopic observation taken of the nebula revealed a broad and continuous spectrum, much more like that of a cluster of stars than a gaseous nebula.  In 1885, a supernova occurred in Andromeda, and in 1887 the first long exposure photographs of Andromeda showed the spiral structure in the nebula.  That this was a vast collection of very remote stars became an obvious conclusion.

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In the first three decades of the 20th century, a great debate within the astronomical community over size, distance, and therefore, nature of the “spiral nebulae” raged.  The long-standing assumption that the Milky Way contained everything within the Universe was being threatened.  Were these objects collections of stars being formed from clouds of gas within the Milky Way, or “island universes” of their own located far outside our own “universe”?

In 1923, using the newly-built 100-inch (diameter) reflecting telescope on Mount Wilson near Los Angeles, at the time the largest telescope in the world, Edwin Hubble obtained photographic plates of Andromeda showing individual stars, putting to an end the term “nebula” attached with this object.  Furthermore, he was able to identify several variable stars (stars that change their brightness over time) within Andromeda, which matched the variability pattern of “Cepheid variables”.  Cepheid variable stars have the amazing property that the length of their period of variability is related to their average brightness, which allows a simple calculation of the distance to these stars once we know the period of variability.

When Hubble computed the distances to the Cepheids found in the Andromeda spiral, he found them to be approximately 900,000 light years distant (to be later revised), placing Andromeda well outside the Milky Way.  With the similarity in structure seen between Andromeda and hundreds of other known “spiral nebulae”, all dimmer, and therefore more distant, than Andromeda, the known size of the Universe expanded rapidly to hundreds of millions of light years, from a prior dimension of about 100,000 light years.

As the 20th century progressed and more modern astronomical techniques, and larger telescopes, came into use, the measured distance to Andromeda was revised to a currently-accepted value of 2.54 million light years. 

The Andromeda Galaxy is now known to be very similar in overall structure to our own Milky Way Galaxy, and its relatively close proximity to our galaxy allows us to use observations of Andromeda to gain an improved understanding of the Milky Way.  The presence of defined spiral arms outlined with dark dust lanes, seen only in long exposure photographs, are believed to be very similar to how the Milky Way would appear to an observer in the Andromeda galaxy. 

Andromeda is slightly less massive than the Milky Way, with about 1.2 trillion times the mass of the Sun, compared to 1.9 trillion for the Milky Way.  On the other hand, it appears to have significantly more stars than our own galaxy, with an estimated brightness of 2.6 trillion suns, versus about 2.1 trillion in the Milky Way.  All of these estimates have large uncertainties in them however, so exact comparisons cannot be made.

Andromeda is surrounded by both satellite galaxies, as mentioned earlier, and at least 400 globular clusters, which can be thought of as mini-galaxies containing several hundred thousand stars each.  There is evidence of prior collisions with other galaxies and Andromeda, including a collision in which the companion galaxy M32 passed through the disk of Andromeda over 200 million years ago, losing most of its stars in the process.

The central nucleus of the Andromeda Galaxy is extremely compact, with millions of stars packed into a space only a few 10’s of light years in diameter.  At the very center of the nucleus lies a supermassive black hole, with an estimated mass of 100 million suns.

Turning back now to the hunt for Andromeda in the evening sky, go outside and face East during October and November evenings when the moon is not in the sky (around Halloween will be ideal).  Using the attached star charts, first find the vast rectangle of stars that forms the body of the constellation Pegasus.  The rectangle is tilted as it rises in the East with the corners roughly falling north-south and east-west.  Follow the diagonal of the rectangle from south to north, and you will arrive at a pattern of three stars roughly in a line running vertically from east to west.  Just beyond the uppermost star of this pattern lies Andromeda.

Although it is visible to the naked eye (on a cloudless, moonless night), it will be easiest to find Andromeda first in a pair of binoculars.  Once you have located it (and in binoculars it will be very obvious once you see it), you can try to find it with your naked eye.  Stare in the region of space above that third star in the vertical line, moving your eyes around slightly to try to see it from the corners of your vision (which are much more sensitive to dim light than the center of your vision).  Once you have spotted it, continue to stare and try to see the full extent of “The Little Cloud”.  In reality, the Andromeda Galaxy covers a region of sky as large as 6 times the area of the full moon! 

The experience of finding Andromeda with your naked eye, and then realizing its true size and distance from us can be awe-inspiring.  And, if you have difficulty in finding it, you need only wait a few billion years for it to become much (much) easier to find.  Measurements of the velocity of Andromeda relative to the Milky Way show that it is on a collision course with our galaxy, due to reach us in about 4.5 billion years.

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