Harry observes the white dwarf star 40 Eridani B and considers if it really is degenerate
Sketch of 40 Eridani A & B, drawn by Harry Roberts
Degenerate! was an unflattering term common in the 1970’s – and when I came across it in Kaler’s Stars and their Spectra I realized the word had another usage.
When stars exhaust their nuclear fuel via the proton-proton reaction they have other fusion reactions available to keep burning – but many finally run out of juice. Such stars are termed degenerate – that is, they no longer have any way of generating energy.
Contraction accompanies the failing of fusion, and titanic explosions or the rapid shedding of matter can result – it depends on the star’s initial mass. Some stars simply fade away having shrunk from a million km diameter to only 10,000 km – or 1% of their initial size! Such stars are called white dwarfs, and their average density can increase from 1 to 100,000 (!) with a similarly huge rise in their magnetic fields – up to millions of gauss!
How abundant are white dwarfs? is a tricky question (for example, see Napiwotzki R. The galactic population of white dwarfs Journal of Physics; Conference Series 172 (2009)) with complex answers: broadly speaking they are very abundant. We learn from the University of Texas that Although abundant, as they cool, white dwarfs fade and become difficult to detect with telescopes.
For amateurs there are probably only two white dwarfs visible – Sirius B and 40 Eridanus B – the latter star being quite easy to see, while Sirius B is almost impossible. The Table of White Dwarfs above lists the thirteen brightest stars of the type – six of which are too far north for us to see. Of the remainder the best are Sirius B, 40 Eri B, Procyon B or W1346 in Cygnus – the latter being Mv 11.5 is a pretty faint star.
What about Procyon B? As the Mv10.9 companion to bright Procyon A (Mv 0.35) is currently only 2.5 “arc away from the primary it’s impossible for amateurs to see.
40 Eridani B is the best white dwarf for amateur ‘scopes and it’s part of a triple star system with members of contrasting spectral types and colours that is both beautiful and interesting to view. The sketch at the top shows my impression of the 40 Eri system. To the naked eye it is one of a wide pair of stars also named Omicron1 Eri and Omicron2 Eri (40 Eri being O1) The Omicron pair is easy to see just one hour (of RA) west of Rigel. In the ‘scope O1 is seen to be an orange type K1 star Mv 4.5 with companion B at PA 105º and 83”arc separation – a bluish star Mv9.7 – the easiest white dwarf visible. As the table shows this star’s diameter is only 1½ times Earth! The 40 Eri system is only 16Ly away – the main reason we can see this tiny white dwarf – there’s lots of white dwarfs; we just can’t see them!
We have considered 40 Eri A and B – there is also star 40Eri C, a fainter (Mv10.8) red dwarf in orbit about B, and now ~7”arc separation NW of B (period 248y). I have not yet seen this red dwarf, and as the group is too far west now, it’s on the agenda for next summer!
WIKI says 97% of all stars will (it’s believed) become white dwarfs eventually. Another site says that since the number of fossilized stars is almost as great as the number of living stars, white dwarfs are abundant in our galaxy. i.e. the number of white dwarfs is roughly the same as the number of all other stars, or ~50% of the total. But a recent paper suggests their mass totals only 10% of the mass of all stars in our galaxy.
Harry Roberts, a regular contributor to this blog and a member of the Sydney City Skywatchers.
