Chris Forder's Star

posted: 961 days ago, on Wednesday, 2015 Mar 04 at 22:26
tags: astronomy, astrophotography, Walter.

Beta Crucis, the 2nd brightest star of the famous Southern Cross, is an easy stepping-stone to finding the über-famous Jewel Box (NGC 4755), just 1° to the south-east. But less than 3 arcminutes from beta Cru lies a real gem: a beautiful deep red star (see this earlier item, too).

Known variously as DY Cru, TYC 8659-1394-1, Espin-Birmingham 365 (EsB 365), Birmingham 291, Schjellerup 153, CCCS 2031, Hen 183, and so on, it has more recently been given the nick-name Ruby Crucis, although I think of it as Chris Forder's Star.

The star's striking red colour was first noticed by John Herschel while visiting the Cape in the 1830s. It was taken up in Danish astronomer Hans Schjellerup's catalogue of red stars (Schjellerup 1866), which was the first comprehensive listing of these interesting and attractive objects. Referring to Herschel's observations, Schjellerup annotated the star's entry with: "Cape obs.: most intense blood red. In field with beta Crucis. Several obs." (Birmingham 1879).

Meanwhile, the 'patron saint of British amateur astronomy', Rev. Thomas Webb, was working on a revision of his already-famous book Webb's Celestial Objects for Common Telescopes with the help of a young observer, Thomas Espin. (Espin would go on to update the sixth edition of Celestial Objects in 1917.) Espin was a very active observer, discovering and measuring 2,575 double stars as well as building his own spectroscope, with which he examined stellar spectra. Based on this work, he compiled a catalogue of 3,800 red stars.

Over in Ireland, John Birmingham has begun his own researches into red stars, and on Webb's suggestion he revised and updated Schjellerup's famous red star catalogue. Birmingham wrote: "The Red Stars must be considered as a class of heavenly bodies particularly worthy of attention; for not alone, as compared with the other stars, do they seem to differ most widely in constitution from our own sun, but they show a peculiar inclination to periodic change, while some of the most noted Variables are found amongst them."

The link between variability and colour is not purely coincidental. Today we know that DY Cru is a carbon star, i.e. an older star with a carbon-rich atmosphere. It's advanced age suggests it is variable, which is indeed the case: DY Cru is a semi-regular variable, pulsating erratically between 8.4 - 9.9 V with a period around 9 months (Otero 2001).

Its carbon-rich atmosphere gives it a striking red colour. Its visual (yellow-green filtered) brightness is V=8.86, but through a blue filter, it is a feeble B=14.66, giving an amazing colour index B-V = +5.8! Compare this with values for some well-known stars: Betelgeuse +1.85, Antares +1.83, gamma Crucis +1.6, and Aldebaran +1.54.

My photo above, taken with a Canon EOS 60D and a 4.7-inch f/8.3 refractor, Walter, nicely shows the striking colour of Chris Forder's Star, which lies west (to the right of) beta Crucis.

Striking as this star is, there's an even more impressive gem in the southern sky. Some 20° from Crux, near to and southward from the western tip of the Southern Triangle, lies the bright ruby-coloured X TrA (HD 134453, HR 5644, SAO 253062). To find it, simply draw a line from beta to gamma TrA (the short edge) and continue southward for just 1.5°. At magnitude V=5.8, X TrA is easy to see in binoculars, and with B-V = +3.6, it is also extra-colourful. Enjoy!

Find out more

AAVSO light curve for DY Cru

DY Cru in SIMBAD

Brian Skiff's list of very red stars

References

Birmingham, J. (1879) The Red Stars: Observations and Catalogue The Transactions of the Royal Irish Academy. Vol. 26, pp. 249-354 [search for it on jstor.org]

Schjellerup, H.K.F.K. (1866) Catalog der rothen, isolirten Sterne, welche bis zum Jahre 1866 bekannt gevorden sind. AN No. 1591. [1866AN.....67...97S]

Otero, Sebastian (2001) [vsnet 2088] Tentative periods for BO Mus, RX Lep and EsB 365. [vsnet-chat link]

nothing more to see. please move along.