Harry breaks light into its component colours and discusses the spectrum of the two brightest stars in the sky – Sirius and Canopus

Synthetic spectra of stars such as Sirius – spectral type A – and Canopus – spectral type F. Courtesy Harry Roberts

Stars appear as pinpoints of light even in the largest telescopes. Understanding their nature seems like a hopeless task, but astronomers obtain a great amount of information by breaking their light into their component colours through a device called a spectroscope. They have classified the spectra of stars into a range of classes called OBAFGKM (the famous mnemonic is Oh Be A Fine Girl/Guy Kiss Me). The classes basically indicate the outside temperature of the stars with O stars being the hottest and M stars the coolest. Here Harry discusses the spectra of the two brightest stars in the sky – Sirius and Canopus – that are prominent in the Australian summer and autumn sky:

Colour is a guide to spectral type for stars F to M – the progression from white through yellow to orange and red is clearly visible. But stars of types O to F appear pure white, and there are, it seems, no green stars.

The two brightest stars in the night sky are Sirius and Canopus, with Sirius seemingly brighter at visual magnitude (Mv) –1.46 and Canopus at Mv –0.72 the fainter star. Yet Canopus is actually 1400 times brighter than the sun while Sirius is only 50 times brighter; but Canopus is 1200 light years (Ly) away compared to Sirius’s 9 Ly, so the latter seems brighter in the night sky. And Canopus is massive at 8Msolar compared to Sirius 2 Msolar.

Sirius (spectral type A) is the hotter of the two however: its surface temperature is 10,000K, compared to Canopus’ (type F) at 7,500K. All agree that Sirius is pure white in colour, and cooler Canopus should seem yellowish. Compare them in binoculars; can you see a colour difference (it works with wines)?

Spectroscopes can! Their starlight when dispersed in a spectrum shows real differences between the two brightest stars (see diagram above). Dark (absorption) bands in the spectrum are due to chemical abundances in their upper atmospheres – the bands wavelength (in Nm in the diagram) indicates the elements present.

Sirius and other type A stars have the strongest hydrogen bands of any type; and they’re all there, wide and dark – hydrogen alpha, beta, gamma, delta, crowding together at the violet end of the spectrum. Thin bands due to ionised “metals” are also found in high dispersion spectra of type A.

Canopus (like Procyon amongst the brighter type F stars) has much narrower (i.e. fainter) hydrogen bands, see drawing above), the most obvious difference when compared to type A – and easily seen in a small spectroscope. I use a Baader Planetarium Spectroscope [this seems to have been discontinued. You could investigate this "star analyser" instead - Nick].

Summer and autumn – it’s a great time for comparing star spectral types, even with binoculars – and more fun if you have a spectroscope!

Harry Roberts is an active member of Sydney City Skywatchers and a frequent contributor to this blog