Stars, star, stars: this month it’s all about spotting clusters and pairs
1. Melotte 20
Recommended equipment 10×50
This stunningly beautiful cluster extends nearly 4° southeast from mag. +1.8 Mirfak (Alpha (α) Persei), with its hot young blue-white stars sparkling like diamonds on black velvet. It is only about 60 million years old. It’s known as the Alpha Persei moving cluster because all the stars share a similar proper motion (motion relative to the celestial sphere) of around 33 milliarcseconds per year.
2. Algol
Recommended equipment 10×50
If you want to dip your toe into variable star observing, Algol (Beta (β) Persei), is a good place to start, being at a convenient altitude on autumn evenings. It is an eclipsing binary star, meaning that it dips in brightness when the fainter member of the system passes in front of the brighter one. With Algol, this happens every 2.85 days, when its magnitude falls from +2.1 to +3.4 for about 10 hours.
3. M34
Recommended equipment 10×50
If you pan across from Algol in the direction of mag. +2.1 Almach (Gamma (γ) Andromedae) you should find M34 approximately one field of view from Algol. You are seeking a fuzzy patch about 0.5˚ across. You should be able to resolve at least a dozen stars, the brightest of which form a distorted ‘H’ shape. You are looking at starlight that left this 220-million-year-old cluster some 1,400 years ago.
4. 10, 11 & 14 Arietis
Recommended equipment 10×50
Here we have three pairs of stars in the same field of view. The components of 10 Arietis are magnitudes +5.7 and +7.1, separated by 9.5 acminutes, and appear slightly different shades of yellow, whereas those of 11 Arietis are +6.0 and +7.3, 7.7 arcminutes apart, and are much brighter white. The yellow-white components of 14 Arietis, which is actually a triple star, are +5.0 and +7.9, and much closer at 108 arcseconds.
5. 30 Arietis
Recommended equipment 15×17
30 Arietis is a binary pair that, for some people, creates a curious optical illusion. The components, 38 arcseconds apart, shine at magnitudes +6.4 and +7.0 and are both of spectral class F5. This means that they should both appear a very pale yellowy-white but, for some reason, possibly an afterimage effect, some people report the fainter of two as being blue- or lilac-tinged. What do you see?
6. Pleiades
Recommended equipment 10×50
Although you can see the blue-white stars of the Pleiades (M45), with your naked eye even in suburban skies, you really need binoculars to get the best out of it. Compare the view as this gorgeous open cluster rises and more and more stars become visible. It’s hardly surprising that they have inspired poets from Sappho to A E Housman.