Finding the Andromeda Galaxy – 2015-11-05

As night sky objects go, i think the Andromeda Galaxy has the most engaging story. And it’s a relatively modern story considering the ancient folklore associated with the constellations and the heavens in general.

In 1920 there was a raging disagreement among astronomers and scientist in general about the size of  universe. The generally accepted view at the time was that our Milky Way galaxy was somewhere between 7,000ly and 30,000 light years in diameter. And the Milky Way was the entire extent of the universe. The fuzzy dense objects visible in telescopes (that we now know are distant galaxies) were described as nebula – “a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch”. And these nebula were thought to be part of the Milky Way.

To jump forward a bit, the Milky Way is currently estimated to be between 100,000ly and 180,000ly in diameter and about 2,000ly thick. The “observable universe” is thought to be somewhere around 93 billion light years across.

The discussions about the size of the universe culminated in what has become to be known as “the great debate” between Harlow Shapley and Heber Curtis. Shapley argued that the Milky Way was the entire extent of the universe and that nebula such as Andromeda were contained within the Milky Way. Although he maintained that the main part of the Milky Way could not be more that 30,000ly across, he did propose that nebula such as Andromeda and globular clusters could form at the distant edges making the diameter 300,000ly. Therefore also fixing the upper size of the universe at 300,000ly. Curtis argued that the Andromeda nebula was in fact at a great distance and an “island universe” (a term coined by Immanual Kant). He accepted the thinking of the day that the Milky Way was no more than 30,000 ly in diameter but proposed that the Andromeda nebula was 500,000ly away and other such nebula as far as 100mly. (Current estimates put Andromeda at 2.5 million light years.)

Well before the debate reached this pinnacle, Henrietta Leavitt, a deaf astronomer working at the Harvard College Observatory, discovered a relationship between the absolute luminosity of a particular type of variable star and its period. In short, she found a way to measure, with certainty, stellar distances using the period of Cepheid Variables.

A variable star changes brightness over a predictable time frame – the period. In 1908, Leavitt noticed a relationship between Cepheid Variable periods and their absolute luminosity and published the results of her initial observations. Then in 1912 after analyzing more stars, she confirmed her observations. Essentially, she found that the actual or absolute brightness of a Cepheid Variable can be calculated from its period. And knowing the absolute brightness, its apparent brightness can be used as a measure of its distance – the further away it is, the dimmer it will appear.

As a side note, Shapley took over as director of the Harvard observatory in 1921 and promoted Leavitt to head of Stellar Photometry.

In 1919, the 100″ Hooker telescope – the largest telescope at the time – was installed on Mt Washington. At about the same time, Edwin Hubble started working at the observatory. Between 1922 and 1923, Hubble used the Hooker telescope to photograph the Andromeda “nebula” and was able to identify Cepheid variables. Aware of Leavitt’s observations, he was then able to calculate the distance to Andromeda. His initial estimate put it at 930,000ly – not quite the actual 2.5mly – but sufficient to declare that the Andromeda Nebula was indeed a separate galaxy. Shapley was still unconvinced as were many of the astronomers of the day. But by 1925 it was clear that the results were inescapable and the universe got a whole lot bigger.

Hubble went on to measure the distances to a number of other “nebula”; confirming that they too were separate galaxies. Then in 1929 Hubble published another ground breaking result – the universe was expanding! He discovered that the further a galaxy was away, the faster it was receding from us. (He used redshift to calculate relative velocity.) And if the universe was expanding, they it must have been smaller in the past. In 1931 Georges Lemaître, a Belgian cosmologist and Catholic priest proposed that the universe must have started out as a single point – later to be coined the Big Bang. As early as 1922, Alexander Friedmann had produced a solution to Einstein’s General Relativity field equations that showed space must be expanding.

So as late as 1920, the universe and the size of the Milky Way were thought to be in the range of 7,000 to 30,000 ly across. Then with a series of observations from 1908 (Leavitt) to 1924 (Hubble) the size of universe expanded to millions and billions of lights years.

The background is interesting, but what makes it even more engaging is that the Andromeda Galaxy can actually be seen unaided! At a distance of 2.5 millions light-years, it’s the furthest object that can be seen without a telescope or binoculars and only requires a moderately dark sky to see it.

By November it’s high in the sky facing south in the evening and appears as a faint, but distinct oval smudge. The darker the skies, the more obvious it is.


The chart below provides some tips on how to find it. Pegasus is due south in the November evening sky and is the large square (or diamond) shape as wide as your hand is stretched from thumb to baby finger. The Andromeda constellation is the “v” shaped arrangement of stars to the east and they share the star Alpheratz. Step 1: Start with Alpheratz in the upper left corner of Pegasus. Count three stars down to the left to Mirach (with Alpheratz being star No 1). Step 2: Then count three stars up (Mirach as No 1). The second set of 3 stars are faint so may not be obvious at first. Step 3: M31 will be 4° to the right of the 3rd star (about 2 finger widths) and elongated as shown.