This is a small exercise in splitting double stars by means of three separate disciplines:

- Visual split, viewed through either a Celestron C8 8" SCT at F/10 or a 60 mm Unitron achromatic refractor at F/15.
- Photographic split, captured through a Celestron C8 at F/10. Just 20 minutes of total exposure for each target.
- Spectroscopic split, using a slit-less spectroscope consisting of a Star Analyser 100 diffraction grating and a 3.8 degree prism mounted on a Celestron C8 at F/5.

Six relatively bright double stars were chosen on the basis of the simple fact that they were accessible for view in the April evening sky. The images and spectra for the six objects are presented in separate posts:

- Mizar (ζ Ursae Majoris) - https://www.astrobin.com/69gv2x/
- κ Boötis - https://www.astrobin.com/dnqbcf/
- Cor Caroli (α Canum Venaticorum) - https://www.astrobin.com/huompf/
- Castor (α Geminorum) - https://www.astrobin.com/4ikigx/
- Algieba (γ Leonis) - https://www.astrobin.com/zdxhri/
- 38 Geminorum - https://www.astrobin.com/jd1ktj/

All spectra shown are corrected for instrument response.

Part 1: Mizar with neighbouring stars Alcor and Sidus Ludoviciana

Mizar was the first telescopic binary to be discovered (in the 17th century) and Mizar A the first spectroscopic binary to be discovered (1890). The angular separation between Mizar A and B is 14.4" and, thus, an easy telescopic split. The luminosity difference between the two components is ~4.5-fold. Neighbouring star Alcor is speculated to be gravitationally bound to Mizar A/B. All three stars are spectroscopic binaries and, hence, the Mizar-Alcor system may be an example of a sextuple star system. In between Mizar and Alcor is the fainter companion star Sidus Ludoviciana, which is, however, 3-4 times further distant from earth than Mizar and Alcor and, hence, merely a line-of-sight companion.

The spectra of the four stars are quite similar, all displaying early to mid A classification with deep, broad hydrogen Balmer lines, implying a relatively early stage in stellar evolution. In this respect, Sidus Ludoviciana is a curious case. The SIMBAD Astronomical Database assigns no luminosity class to Sidus Ludoviciana, and the Wikipedia entry refers to Sidus Ludoviciana as a "giant" star, implying that it has evolved beyond the main sequence. This, however, contradicts its measured distance of 92 (± 0.2) parsecs and apparent magnitude of 7.58, which (given negligible interstellar extinction, as seems to be the case) would give it an absolute magnitude of 2.7. For an A5 class star this would correspond to luminosity class V, i.e., a non-evolved dwarf star of the main sequence, as also suggested by the broad Balmer lines in the spectrum.

Part 2: κ Boötis
The two components A and B of Kappa Boo are, similar to Mizar, an easy telescopic split with an angular separation of 13.5" and a difference in luminosity of ~7.5-fold.

The two spectra are fairly similar, the primary being slightly hotter and more evolved at spectral class A7IV than the secondary at F2V. Compared with Mizar, the two κ Boo spectra are somewhat later, which manifests itself in the emergence of additional (mostly faint, but significant) metallic absorption lines, such as those for neutral calcium at 3934 and 3968 Å as well as the magnesium triplet at ~5175 Å and even a hint of molecular CH at ~4300 Å.

Part 3: Cor Caroli
Cor Caroli is another easy telescopic split, with the two components A and B displaying an angular separation of 19.6". The primary star is ~12-fold more luminous than the secondary.

The two spectra are well separated both physically and in terms of spectral class, the primary being an A0 class main sequence star and the secondary an F2 class main sequence star. Both spectra are dominated by broad hydrogen balmer absorption lines, with additional metallic lines (most noticeably neutral calcium at 3934 Å) emerging in the secondary star.

Part 4: Castor
Castor is a fairly close visual binary with an angular separation between the A and B components of (presently) 6" and a ~3-fold difference in luminosity between the primary and secondary. Both the A and the B star are spectroscopic binaries, albeit with the secondary of each pair being much smaller and fainter than the primary and, hence, not contributing much to the spectra.

Castor A/B is a fairly easy split visually but slightly more challenging spectrocopically due to the small physical separation between the spectra. However, given the similar class of the two stellar components, both being early A-class subgiants, it probably makes little difference at this limited resolution if there is a little spill-over between the spectra. Both spectra are dominated by deep, broad hydrogen Balmer lines as expected for the class.

Part 5: Algieba
Algieba is a fairly tricky object with an angular separation of only ~4.7", requiring a rather high-power telescope for the visual split. The difference in luminosity between the two components is 3-fold. In terms of class, both are yellow-orange giants of similar spectral classification, late G to early K.

The two spectra are very similar in the blue end, but start to diverge in the red end. This is probably an artifact due to spill-over of intensity from the brighter to the fainter component. In terms of absorption lines the spectra are, as expected, very similar with a multitude of metallic absorptions lines characteristic for the G and K class stars.

Part 6: 38 Geminorum
38 Gem proved by far the most tricky of the six targets. With an angular separation of only 7.4" and a more than 50-fold difference in luminosity, visual and photographic separation were challenging and spectroscopic separation unfeasible with the equipment at hand. Hence, the spectrum shown is a blend of the two components, with the main contributor being the much brighter primary main sequence star of late A / early F class. The spectrum is dominated by broad hydrogen Balmer lines.