Messier 31, the Andromeda Galaxy, needs no introduction. It’s a large bright spiral galaxy 7.7° northwest of Mirach (Beta (β) Andromedae).
Visible to the naked eye, a small scope reveals that the famous elongated glow is just the core of the galaxy, an ellipse approximately 30×10 arcminutes in size. The fainter spiral arms sit outside the core. A low power shows the galaxy off best.
Take our tour of the Andromeda Galaxy and discover its most intriguing sights. These targets can be seen with a refractor under 4 inches or a reflector/SCT under 6 inches.
Start your journey into M31 by looking for the dark dust lane that runs along the northwest edge of the core. Look for the faint glow of the spiral arms beyond this lane.
This glow continues toward another dark lane located further out. Notice also how the central region fades inwards towards a star-like core
Measure how good your skies and eyes are by observing how far M31’s core extends from the galaxy’s centre. One measure for this is satellite galaxy M32 which sits 24 arcminutes south of M31’s star-like core. This mag. +8.1 elliptical appears like a large fuzzy star at low powers. M31’s elliptical core should extend, along its main axis beyond M32. If it reaches the 7th magnitude star HIP 3293, 13 arcminutes southwest of M32, you’re doing well.
M32 itself appears non-circular, an oval glow measuring 3×2 arcminutes. Like M31, the core of M32 appears almost stellar in nature, but larger apertures will reveal it as an extended region about 10-15 arcseconds across. M32 is an elliptical dwarf galaxy with a mass equal to around 3 billion Suns.
Like M32, M110 is another gravitationally bound satellite galaxy of M32 and another elliptical galaxy. It appears fainter and more elongated than M32, M32 being classed as type E2 while M110 is type E6p. M110 sits 35 arcminutes northwest of the centre of M31, 1.5x the apparent distance of M32 from the centre of M31’s core.
The surface brightness of this mag. +8.5 galaxy is lower than M32’s and it can be lost due to light pollution. It has an apparent size about 10×3 arcminutes, appearing like a north-south aligned streak.
We return to the main galaxy for our next object, NGC 206, a bright star cloud. When observing the Andromeda Galaxy through a scope, it is obvious that the main galaxy is dominated by its bright core. And it takes patience to see anything other than the core. The dark dust lanes mentioned previously are obvious candidates but there are other parts of the galaxy to see.
The trick to finding NGC 206 is to use the obvious visible components – the centre of M31 and M32. The star cloud lies at one vertex of a squat isosceles triangle formed using M32 and M31’s star-like core, M32 being the apex of the triangle. NGC 206 is truly a part of M31.
M32 and M110 are often cited as M31’s satellite galaxies, but they’re not the only ones. NGC 185 is another example. To see it you’ll need to move into Cassiopeia and head 7° north of M31 to arrive at mag. +4.5 Omicron (ο) Cassiopeiae. Mag. +9.2 NGC 185 sits 1° to Omicron’s west.
This lesser-known satellite of M31 is a moderately bright dwarf spheroidal galaxy. A 150mm scope will reveal it as about 4 arcminutes across, a bit elongated in an east-west direction. It appears 25% larger in a 250mm scope, with a more circular appearance
NGC 147, is a tricky object, even with large instruments. Head west from NGC 185 for 1°, nudging a little north. This is another dwarf spheroidal galaxy and another M31 satellite.
A 300mm instrument will show it as a faint smudge, 3×2 arcminutes in size, appearing to brighten as you head into the centre towards a stellar nucleus. Like M31, it’s around 2.5 million lightyears from us.
This guide originally appeared in the December 2019 issue of BBC Sky at Night Magazine. Pete Lawrence is an experienced astronomer and a co-presenter of The Sky at Night.