Andromeda Galaxy
ANDROMEDA
THE ONE-TRILLION-STAR GALAXY
The constellation of Andromeda is home to our giant neighbouring spiral galaxy; here’s our in-depth guide to its secrets
Words by Giles Sparrow
Source: Wikipedia Commons © David (Deddy) Dayag
THE MOST DISTANT LIGHT IN THE SKY
The Andromeda Galaxy is the most distant object we can see with the naked eye. Even on the darkest night, all the individual stars we can see are part of our home galaxy, the Milky Way. Just three naked-eye objects lie beyond its boundaries – two small satellite galaxies, the Large and Small Magellanic Cloud, visible only from the far-southern skies, and an oval patch of light in the northern constellation of Andromeda. Catalogued as Messier 31, it’s easy to spot on moonless nights, and is best seen in the mid-evening sky around October to December.
The constellation of Andromeda comprises two diverging chains of stars, linked at their base by the bright star Alpheratz on the northeastern corner of the distinctive Square of Pegasus. An imagined line between the second stars of each chain – the bright Mirach, or Beta Andromedae, and the fainter Mu Andromedae – leads northwards to Messier 31, appearing as a fuzzy ‘star’ of moderate brightness. Binoculars or telescopes with low magnification will reveal the fuzzy oval of its nucleus, and in dark skies you may also detect the faint glow of its outlying regions. Messier 31 appears six times wider than a full Moon.
“It’s best seen in the mid-evening sky around October to December”
DEBATED DISTANCE
The starry nature of Messier 31 and many of the sky’s other fuzzy celestial objects were first recognised around the turn of the 20th century. Andromeda was among the finest so-called ‘spiral nebulae’, but the true nature and location of these objects remained uncertain. Around 1912, US astronomer Vesto Slipher discovered that spiral nebulae were moving at speeds much faster than any known star, and were therefore probably independent of the Milky Way, but the argument was only settled through the work of Edwin Hubble and Milton Humason, unveiled in 1924. They identified Cepheid variables – stars with regular pulsations in a period linked to their luminosity – in several spiral nebulae. Hubble was then able to work out the distance to these stars based on their apparent brightness as seen from Earth. From measurements of a dozen Cepheids in Messier 31, he put its distance at 930,000 light years - far beyond the bounds of the Milky Way. Later this was refined to more than double this distance, and today several different methods converge to agree the Andromeda Galaxy is actually some 2.5 million light years from Earth.
WHERE IS IT?
Our neighbour Andromeda is a barred spiral like the Milky Way