Last month this feature was dedicated to the myth of Andromeda, and we learned of the fate of her boastful mother who was doomed to forever spend half her life upside down in the night sky. But Andromeda didn’t only change Cassiopeia’s perception of the world, she also changed our own, for within her constellation lies an object that transformed our understanding of the universe.

To find it, it is easiest to locate the distinctive W of Cassiopeia in the northeast sky and then follow the arrow created by the deeper ‘V’ of the constellation to find the faint smudge of light that lies between the mother and her daughter in the heavens. This indistinct object is the Andromeda Galaxy which has been observed by humans since records began, yet we had no idea of its significance to astronomy until the 1920s.

From the days of ancient Greece up until that point, it had been referred to as The Great Nebula and presumed to be a cloud of dust and stars within our own galaxy, the Milky Way. Messier himself observed it, and classified it as M31 in his seminal 1771 catalogue of astronomical objects. Its status remained uncontested until 1920, when astronomers Harlow Shapely and Heber Curtis entered into The Great Debate about the nature of the Milky Way and the dimensions of the universe, with Curtis arguing The Great Nebula was not a nebula at all, but another galaxy in its own right. It was a claim that rocked the astronomical world and beyond, for if it were true then it would shatter the long accepted understanding that the Milky Way was the only galaxy in the universe.

It was Edwin Hubble who finally settled the debate. In 1925, he identified Cepheid variable stars in astronomical photos of M31, and used them to prove that the object was indeed beyond the reaches of our own galaxy. Cepheid stars are important distance indicators in astronomy because their chemical composition causes them to fluctuate in both temperature and diameter. As these changes occur, the stars pulsate radially, causing regular and observable changes in their brightness. Because there is a direct relationship between a Cepheid variable’s luminosity and its pulsation period, they can be used to reliably ascertain the distance of both galactic and extragalactic objects.

Portion-of-the-Carina-Nebula-by-NASAThese stars enabled Hubble to determine the actual distance of The Great Nebula from Earth, and in doing so show conclusively that it was not a cluster of gas and stars within the Milky Way, but a separate galaxy in its own right. So it was renamed the Andromeda Galaxy, and subsequent studies have ascertained that it lies about 2.5 million light years away from Earth. Despite this great distance, it is still one of the closest galaxies to us in the universe, and its magnitude makes it visible to the naked eye on dark nights. With binoculars it will appear as a more distinct oval in the sky, and with a telescope you should be able to make out its bright nucleus and spiral arms stretching out into space.

Although the Andromeda Galaxy is bigger than the Milky Way, comprising around 1 trillion stars to our 400 billion, both are classed as large and are high in mass. And, as the laws of science dictate, this means that they are attracting one another, in this instance at a rate of about 110km/s. While this isn’t a cause of concern for us here on Earth at the moment, it is estimated that in approximately 3.75 billion years they will collide to form one super galaxy.

Colliding galaxies are common in galaxy evolution, and are not quite as dramatic as they sound. Far from crashing together and smashing each other to smithereens, the sparse distribution of matter in each galaxy means there is rarely any impact, so the event is more like a merger than a collision, with the two galaxies experiencing gravitational interaction as they pass through and then fall back into one another until they eventually stabilise and combine.

When this happens with the Andromeda Galaxy and the Milky Way, the repercussions felt here on Earth will be highly dependent on our position within the new super galaxy. If we are cast out to the peripheries there’ll be far fewer stars in the night sky, but if we find ourselves in the middle there will be millions more. While the latter sounds beautiful in theory, in reality it would have a major impact on life on our planet because as each of these stars supernova they will omit vast quantities of radiation that will make life on Earth incredibly hard to sustain. So we just have to hope that, when the day comes, we find ourselves in a lovely little spot in the suburbs.

With thanks as always to Lee Pullen for sharing his time and expertise. If you’d like to find out more about what you can see in the skies over the next few months, then book in for the Autumn Stargazing show (2D or 3D) at the Planetarium – and prepare yourself for a breath-taking journey through the cosmos. Tel: 0117 915 1000 or visit: www.at-bristol.org.uk.

Hubble’s law

As well as proving the existence of a galaxy beyond our own, Edwin Hubble (1889–1953) demonstrated the continuous expansion of the universe. Hubble’s law states that the distance between galaxies, or clusters of galaxies, is continuously increasing and that this can be observed by the changes of light wavelengths between them. As two objects move apart the wavelengths between them lengthen, and in doing so shift toward the red part of the spectrum. These ‘red shifts’ demonstrate how other galaxies are moving away from our own and so show, he concluded, that the universe is expanding.

Look out for…

Between twilight on 16 October and dawn on 17 October, five planets will be visible in the night sky. Early on in the evening, look for Saturn just below the crescent moon, then head to bed when it follows the Sun below the horizon so that you can rise again a couple of hours before sunrise to see Venus, Jupiter and Mars in conjunction in the sky. Then head to the kitchen and brew yourself a cuppa ready to enjoy as morning begins to break, when you can draw a line from Venus and through Jupiter to spot Mercury near the horizon.