Star 40 Eridani is a triple system located not so far away – about 5 parsecs (that is about 16 light years). In cosmic scale it is just a neighbourhood. Its main component is K type star of main sequence. These kind of stars are pretty common in the Universe, and they are also pretty long living objects. This star apparent brightness is 4.4mag, real size is similar to the size of our Sun, but it is significantly colder.

About 70 arc seconds away from main component A there is a second component – B. This one made many astronomers scratch their heads about a hundred years ago. In those days spectroscopy started to play major role as a scientific technique, and first star classification systems were created.

Williamina Fleming and Annie Jump Cannon worked on the Henry Draper (HD) catalogue at the Harvard College Observatory. When Williamina Fleming obtained first spectra of 40 Eri B, she classified this star to A type, because it showed hydrogen as its most abundant element. This is usually a consequence of the high temperature, so the metals are mostly ionised. However as luminosity depends on temperature, this type of star is also very bright. 40 Eri B star was then too faint for typical A star. This suggested the existence of another class of stars that have low luminosity and spectra dominated by hydrogen

As you may already guessed these are white dwarfs. But over a hundred years ago it took astronomers a while to figure it out. 40 Eridani B white dwarf has apparent brightness 9.5mag, so it is an easy target to visual observations. White dwarfs are stars that already ended the job of synthesis new elements, and there is no force that prevents gravity to compress them to a small and extremely dense objects. A teaspoon of such material would weight more less the same as a car at the Earth. White dwarf spectrum contains widened hydrogen absorption lines. But these lines are not created by the high temperature (like in the A type stars), but by strong gravity, that pulls all the heavier elements to the core. So they can be both faint and show the hydrogen presence in the same time.

Here is the Eri 40 B spectrum that I recorded 23.01.2020 (brown graph). I also put there spectrum of A5V star (gray graph) for comparison. Both are pretty similar, although the white dwarf spectrum emission lines are significantly wider. And it is not a result of lower spectrum resolution, but they really are that wide.

40 Eridani B white dwarf spectrum
40 Eridani B white dwarf spectrum with superimposed A type star spectrum for comparison

Actually, this spectrum is not quite interesting 🙂 The story behind is much more disturbing.

As you may notice in the spectrum legend, the reference A5V spectrum star comes from Henry Draper catalog (HD023194). There are over 225,000 objects in this catalog, and 5,000 new spectra were added to this catalog each month.

Clear skies!

40 Eri B technical data:
Date:       2020-01-23
Location:   Nieborowice, Poland
Telescope:  Meade ACF 10"
Instrument: LowSpec spectroscope, 600 lpmm
Camera:     QHY163M, gain 100
Mount:      SW EQ6
Guiding:    ASI290MM
Exposure:   4x180s
Conditions: seeing good, transparency average

Cover photo by WikiImages from Pixabay