Red 'Hyper giants and Carbon Stars'

Herschel's Garnet Star Mu Cephei Spectral type M21a - Credit: The COAST Robotic Telescope - BVR filters - The Open University - telescope. org. Image Credit: Pip Stakkert

 Mu Cephei  is also known as Herschel's Garnet Star and is a red supergiant or hypergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. Since 1943, the spectrum of this star has served as the M2 Ia standard by which other stars are classified.

Mu Cephei is visually nearly 100,000 times brighter than the Sun, with an absolute visual magnitude of −7.6. It is also one of the largest known stars with an estimated radius over 1,000 times that of the sun, and were it placed in the Sun's position its stellar radius would reach to between the orbit of Jupiter and Saturn.- Credit Wikipedia

Hind's Crimson Star - R Leporis Spectral Type C7,6e. The COAST Robotic Telescope - BVR filters - The Open University - telescope. org. Image Credit: Pip Stakkert

R Leporis (R Lep), sometimes called Hind's Crimson Star, is a well-known variable star in the constellation Lepus, near its border with Eridanus. It is designated "R"

It is a carbon star which appears distinctly red. It is named after famous British astronomer J. R. Hind, who observed it in 1845. Its apparent magnitude varies from +5.5 to +11.7 with a period of 418–441 days; recent measurements give a period of 427.07 days. There may be a secondary period of 40 years.  It has been estimated to be around 1,350 light-years distant, shining with a luminosity approximately 6,689 times that of the Sun and with a surface temperature of 2,980 K.[8]

R Leporis has often been reported as an intense smoky red colour, although this is not pronounced when the star is near its maximum brightness. It is reddest when it is dimmest, which occurs every 14.5 months. The red coloration may be caused by carbon in the star's outer atmosphere filtering out the blue part of its visible light spectrum.- Credit Wikipedia

" Mixed in with ordinary GK and M stars (covering a temperature range from 4600 to 3100 degrees K) are Carbon or C stars. They are cool giant stars that are over abundant in carbon relative to oxygen. Under the old Harvard stellar classification these were divided into the hotter 'R' stars with spectral bands of (C2) and bands of cyanogen (CN) and cooler 'N' stars exhibiting (C2), (CN) and (CH) with very little (TiO) evident.

Both the 'Garnet Star' and the 'Crimson Star' are variable stars.  Indeed, most low temperature stars exhibit some form of variability in their luminosity and energy output. The 'Garnet Star' is a 'Cepheid' variable, one of an important group of variable stars used in estimating distances to remote star systems. The relationship between the period of pulsation for variable stars and their luminosity was discovered by the modest and admirable Miss Henrietta Swan Leavitt and was used by Edwin Hubble to first establish the distance to The Andromeda Galaxy M31 . This was the first proof that the Universe went on outside the confines of the Milky Way.

In 1912 Miss Leavitt, working at Harvard, found that the brighter the median apparent magnitude (and so luminosity since the stars are at the same distance) the longer the period of the Cepheid variable star.  Harlow Shapley realised the importance of the period- luminosity relationship and attempted to find the 'zero-point' in order that a knowledge of the period of a Cepheid would immediately determine its luminosity. This calibration was difficult to perform because of the relative scarcity of Cepheids within range of distance determinations to allow trigonometric parallax observations. Instead, Shapley depended upon the relatively inaccurate method of statistical parallax. His zero point was then used to find the distances to many other galaxies. As time has passed more accurate  data has become available which has enabled more reliable distances to some 20 or more stars which serve as calibrators for the period-luminosity relationship in the Milky Way"

- Archie Mendes - visiting theoretical astronomer at the Jodrell Plank Observatory - Reydon University - 'School of Computer Modelling and Difficult Sums' - author of "Quantity Surveying and Standard Methods of Measurement in Curved Space Time" or "When the Socks come off!" – Greek National