Harry examines the spectra of stars like the Sun – the G type stars
A low resolution view of the spectrum of the Sun (sunlight broken up into its component colours with the help of a prism or a grating). Drawing by Harry Roberts with colour information from Philippe Rousselle
Two amazing G type stars dominate southern skies –the Sun, and Alpha Centauri (α Cen).
We started our tour of star colours (i.e. spectral types) with the two brightest in the night sky, Sirius (type A) and Canopus (type F). The third brightest star is Alpha Centauri A (the brighter of the well known double) a type G2 dwarf (dwarf here means ordinary main sequence stars of luminosity class V. They are not tiny stars, but they are less luminous than the giants and supergiants) while its companion B is a cooler K type star. So of the spectral type sequence OBAFGKM the three brightest stars are good examples of types AFG and K respectively. In this piece we look at stars of spectral type G – the solar type stars.
Why did we not begin with the hottest O type stars or coolest type M? In “her” infinite wisdom the deity has put us well out of harm’s way, with none of the hot types nearby (and truly, we wouldn’t want any) – there are no type O or B among the first magnitude stars nor the nearest stars – so we began with the brightest of the naked eye stars, type A, Sirius.
The sun, type G, has dominated life on our planet – and our vision has evolved peak sensitivity in the band where G stars emit most strongly, the yellow part of the spectrum. Although it always looks white to me, if we viewed the sun from four light years away it would be yellow like α Cen A. Both the sun and α Cen A are main sequence dwarf stars, as distinct from more common giant stars – and are rarities in the bright star list.
Star colour indicates the star’s surface temperature; in G type the range is 5000 to 6000ºK, and light from the sun approximates a black body of that temperature. Because we live so close to a G2 dwarf we know more about its spectrum than any other – with so much light there is no limit to the detail seen at high resolution; the diagram shows a low-resolution spectrum.
Remember, the dark lines in a stellar spectrum reveal different elements in the star’s atmosphere, and of 92 natural elements about 70 are found in the sun’s spectrum. In A type stars (e.g. Sirius) we saw very strong hydrogen lines but they were weaker in F stars, and weaker still in the solar type stars – though they are still there; Hα (Fraunhofer C band) being the “window” through which amateurs watch violent eruptions on our star. Hydrogen lines β, γ and δ are there too but ‘high res.’ is needed to see them.
Other features of type G are the calcium II lines in the violet (Fraunhofer H and K) that are weakest in the hottest type stars (O and B). Molecular bands are stronger in cooler stars and the G band is one of these, due to the CH molecule (see diagram) – it’s strongest in types K and M. Several bands in the solar spectrum are caused by Earth’s own atmosphere; note the oxygen bands (O2, A and B) in the far red. While a homemade solar spectroscope shows hundreds of thin lines in the solar spectrum, the Baader spectroscope shows very little detail in type G stars – the thin lines needing higher resolution.
G type stars are about 13% of all stars – and we’ll look at spectral type abundances in a future piece (Ed. willing) – but note that solar type stars are not so common. A glance at any shots of Milky Way star fields confirms this, with blue stars everywhere and only a few yellow ones; you’ve probably noticed too that deep red stars are rarer still [surely that is only because the extremely bright blue stars are easier to see than the fainter yellow and the even fainter red stars - ed] . These are types K and M that we’ll look at later; meanwhile binoculars (best on a tripod) are all you need to confirm spectral types in our galaxy.
Enjoy autumn skies – and the colours of stars!
Harry Roberts, frequent contributor to this blog and member of the Sydney City Skywatchers
