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Red star YY Geminorum in the Castor 6-star system seen at X-ray wavelengths [Credit: ESA] Catalog of Red Stars - First edition, May 2004
- by A. Ahad

Compiled: circa late 1980s, posted: May 5th, 2004 (with later revisions)

Copyright © 2004-2012 Abdul Ahad. All rights reserved.

The night sky's reddest stars ranked in diminishing order of redness (colour index):

Star designation: 2000.0 R.A. & Dec: Colour Index (B-V): Mag range & Periodicity: Spectrum: My Observation Notes:
V Hya 10 51.6 -21 15 +4.5 6.5-13, ~18 years, SR Carbon This deep red star is undergoing major changes, as it approaches the end of its life. Its average brightness has diminished by one whole magnitude since records began in 1913. Between July 1993 and May 1994, the star suffered a deep fade and remained below mag 13 for 10 consecutive months. [See my analysis:AAVSO historical data]
U Cam 03 41.8 +62 39 +4.1 7.7-8.9, 400 days, SR Carbon
V Aql 19 04.4 -05 41 +3.9 6.6-8.4, 353 days, SR Carbon Seen and charted on 25 June 2004. Outstandingly red and easy to find close to the beak of Aquila - the celestial Eagle. At low power it appears in the same field as the orange 4th magnitude star 12 Aquilae.
SS Vir 12 25.2 +00 46 +3.9 7.0-9.7, 355 days, Mira Carbon
T Lyr 18 32.3 +37 00 +3.7 7.8-9.6, Irregular Carbon First seen on 25 June 2004. Appeared faint at around 8th magnitude but easily spotted in the wide field eyepiece as a genuinely colourful 'ruby in a field of diamonds'...
W Ori 05 05.4 +01 11 +3.5 5.5-8.0, Irregular Carbon
X Cnc 08 55.4 +17 14 +3.4 6.0-7.5 Carbon Imaged on 27 April 2010. Based on AAVSO estimates going back to the 1920s, the brightness of X Cancri varies from mag 6.0 to 7.5, in a semi-regular fashion though it sits around 6.7 on average.
AX Cyg 19 57.2 +44 16 +3.4 7.5-9.0, Irregular Carbon
RY Dra 12 56.4 +66 00 +3.3 6.0-8.0, 173 days, SR Carbon Observed 18 May 2004. Located in an impressive field allowing good colour contrasts with 8 Draconis (spectrum A5, white) and 9 Draconis (spectrum K2, yellow/orange) both of which flanked RY Draconis on either side in the same field of view at 36x on my 8-inch.
RS Cyg 20 13.4 +38 44 +3.3 6.5-9.5 Carbon Makes a beautiful triple star with unrelated blue and orange companions nearby in the same low power field.
WZ Cas 00 01.3 +60 21 +3.1 6.3-8.8 Carbon Forms a striking colour contrasted double (see finder chart) with an 8.3 mag white star, 58" away in the same low power field.
R Leporis 04 59.6 -14 48 +2.7 6 - 10.5, SR? Carbon Also known as astronomer Russell Hind's "Crimson Star!"
U Hya 10 37.5 -13 23 +2.7 4.7 - 6.2, Irregular Carbon
Y CVn 12 45.1 +45 26 +2.5 5.5-10?, 157 days, SR Carbon Studied in detail 17 May 2004. Deep red colour easily more pronounced than Garnet Star. The shade of red looks 'cooler' than a hot, ruby red and more like burgundy. Colour intensity remained invariant across 36x, 100x & 300x magnifications on my 8-inch f/5 Newtonian.
TX Psc 23 46.4 +03 29 +2.5 5 - 6 Carbon
Herschel's "Garnet star" 21 43.5 +58 47 +2.35 3.6-5.1 M2 Ia The reddest naked eye star in the whole night sky. However, it's not clear precisely what shade of red William Herschel had in mind in his description...since not all garnets are necessarily red! Here's a page on the Garnet Star.
HD 20797 03 24.7 +64 35 +2.08 5.2 var M0 II
"Ruby star" - CE Tauri 05 32.2 +18 36 +2.07 4.3-4.8 M2 Ib First spotted inadvertently in a casual binocular sweep in April 2004, colour intensity was an exceptional ruby red. Hence my decision to call this my very own "Ruby star"!
Psi1 Aur 06 24.9 +49 17 +1.97 4.9 var K5-M Observed 12 November 2004, deep orange. Rather isolated and faint in 8x30 binoculars; field not offering any direct colour comparisons.
BE Cam 03 49.5 +65 32 +1.88 4.4 var M1 III
Betelgeuse 05 55.2 +07 24 +1.85 0.5 var M2 Ib
Omicron1 Ori 04 52.5 +14 15 +1.84 4.7 var M3 Sv
Antares 16 29.4 -26 26 +1.83 1.0 var M1 Ib

Background to my Catalog of Red Stars

Since my earliest childhood days of stargazing, noticing the small variations in star colours, both intrinsically from one star to the next, as well as the jewel-like reds, greens and violets produced by atmospheric scintillation, has always been a top feature in my observing. I originally compiled about half of the above list of exceptionally red stars back in the late 1980s from a number of different sources, notably the "Yale Bright Star Catalog" entries listed in the Astronomical Almanac. The criteria for selecting entries in my red stars catalog are:-
  • the star must be visible in the British Isles,
  • it must reach a brightness of at least around mag. 8 at maximum and be observable across its full range in small telescopes, and
  • have a colour index of +1.8 or higher, as defined in the UBVRI Johnson photometric system.
My Red Star Catalog is therefore intended to be neither exclusive nor exhaustive in its coverage.

Definition of Colour Index (B-V)

Star colours are measured using a photometric system of 'colour index', denoted B-V.
The blue magnitude, B, is the brightness of a star as observed photoelectrically through a blue filter. The visual magnitude, V, is its brightness measured at visual wavelengths of light, which is of course the part of the electromagnetic spectrum to which our eyes are most sensitive. The difference B-V is therefore a measure of the colour of a star. There is a close relationship between B-V and the spectral type, but some of the stars are reddened by interstellar dust. The probable error of a value of B-V is thought to be 0.02 mag at most.
Stars that have a higher positive colour index value are redder, whereas those that have a lower negative B-V are bluer in their apparent hue.

NOTE: Colour indices stated above for each star were derived from their B and V magnitudes. In the case of carbon stars, the actual shade of colour will vary in line with variations in the star's intrinsic brightness. It is generally accepted that a carbon star will appear marginally redder when it is at the faintest end of its cycle, though this type of colour variations are not observed in the red supergiants.
Likewise, the indicated magnitude range for both carbon stars and pulsating red supergiants is somewhat variable and very approximate. These can be confirmed by consulting the AAVSO database, which is a fantastic resource pooling together ongoing observations from a global community of variable star observers.

My fascinations with the "Ruby Star" (CE Tauri / 119 Tauri)

The second reddest star in the entire night sky (ranked in terms of its colour index (B-V) in the Johnson UBVRI photometric system) to shine consistently at a naked eye brightness well above the 5th magnitude is the pulsating red supergiant CE Tauri or 119 Tauri. Only Mu Cephei (the "Garnet Star") is slightly redder in that brightness range, as first acknowledged by William Herschel back in the 18th century.

I first spotted this star totally inadvertently during a casual binocular sweep back in April 2004, when I instantly took a serious liking to its exceptionally ruby red colour. I then carried out an extensive search of all the online sources and found no prior references to say anyone else had either noted its exceptional gemstone-like colour or made any remarks, comparisons or colour rankings of this relatively *bright* star to Mu Cephei.

It never featured in a single red star listing anywhere, and I became the FIRST to identify CE Tauri as the *second* reddest star in the entire night sky to shine consistently at a naked eye brightness well above the 5th magnitude. Hence my decision to call this the "Ruby Star" as discussed here :) :) !

The night sky's reddest stars plotted on magnitude vs colour index [Image: Abdul Ahad]
Above: The night sky's reddest stars plotted on apparent magnitude vs colour index - click to see a larger image. [Abdul Ahad]

The Ruby Star is on view in the evening skies between October and April each year, and very easy to locate close to the lower horn of Taurus, as shown in the photograph below. Read my in-depth analysis of this star here.

My Ruby Star in Taurus! [Sky photographic credit: © T. Credner & S. Kohle, AlltheSky.com]
[Above image credit: © T. Credner & S. Kohle, AlltheSky.com]

The Bluest Stars in the Sky

By definition, if the reddest stars in the night sky represent the heaven's sparkling garnets and rubies, then the bluest stars - on the opposite end of the spectrum - must represent the night sky's sapphires.

The very hot, bluish O-class star Zeta Puppis in the southern skies has one of the highest negative colour indices (B-V) at -0.28 and should appear attractively blue in the eyepiece. Visible in the northern hemisphere, the 4th magnitude variable star S Monocerotis is amongst the bluest, with a color index of -0.25. The brightest of all the blue stars in the night sky is Spica, with a colour index of -0.23. A good look at Spica through an instrument of any aperture never fails to make a totally mesmerising experience on spring evenings.

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Research reference on the Ruby Star

Star Colours - The Johnson UBVRI Photometric System

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Copyright © 2004-2012 Abdul Ahad. All rights reserved.

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