T LYR (T Lyrae). When one comes across a star like T Lyrae, it begs to be included in spite of being below naked-eye visibility. In Lyra a couple degrees south-southwest of Vega and at best just short of seventh magnitude, T Lyrae is a deep red carbon star that is quite visible with binoculars. Among the rarer gems of the sky, T Lyr has been referred to as "the Jewel in the Harp." It's nearly five magnitudes (a factor of 100) brighter at visual wavelengths than it is in the blue part of the spectrum. Carbon stars are highly evolved red giants with dead carbon cores in which carbon, made fresh from helium fusion, has been brought upward by convection from deep in the stellar interior to the surface. All carbon stars are cool, their spectra rich in carbon molecules (absorption by which in part causes the color) as well as in heavier elements made by neutron capture. There is a distinct evolutionary sequence going from giants in which oxygen dominates carbon (class M, like Mira) to those in which the elements are in roughly equal proportion (class S: Chi Cygni) to those in which carbon dominates (class C). Among the well-known carbon stars are 19 Piscium, Y Canum Venaticorum, and Hind's Crimson Star (R Leporis). None is visually bright. All vary to some degree, while some (like R Lep) are hugely-varying Mira (long-period) variables. Carbon stars go way back to their discovery in the middle of the nineteenth century by Father Angelo Secchi, one of the founders of spectral classification. Carbon stars are the source of much if not most of the carbon in the Universe.

T Lyr The red carbon star T Lyrae glows at the center of the picture, its color contrasting strongly with those of other stars. North is to the left. The bright star to the lower left is Vega, Alpha Lyrae. EpsilonLyrae (slightly elongated horizontally because of its duplicity) is to the lower left of Vega, Zeta Lyrae to the lower right. Photo by Jim Kaler.

More specifically, T Lyr is a spectrally advanced (C6.5 )carbon-star giant and irregular variable (hence the Roman-letter variable star name) that changes between roughly magnitudes 7.5 and 9.8. The distance is not well known. Parallax measures by the Hipparcos satellite place it at a distance of 2400 light years, but the uncertainties are such that it could likely fall anywhere between 1730 to 3600 light years away. The stellar "surfaces" of these giant stars are highly diffuse, and the temperature depends on where in the spectrum and during what part of the variability cycle one observes. Values for T Lyr range from 2400 to 3200 Kelvin, while the luminosity (most of which is in the infrared) is, from two sources, around 15,000 Suns. That combined with an average temperature of 2600 Kelvin yields a radius of 600 solar radii, 2.8 Astronomical Units, almost twice the size of the orbit of Mars. If the star is closer to the upper distance limit, the radius could be considerably larger. The original mass is not known, but is probably at least triple that of the Sun. Mass loss, now at a rate near a millionth of a solar mass per year (100 million times that of the Sun), has however reduced the mass, indeed will strip the star of its outer envelope, first creating a surrounding carbon-rich shroud, a "planetary nebula" ionized by the exposed core, then a fairly massive white dwarf. Moreover, T Lyrae is a "J star" (from a carbon-star sequence invented by Philip Keenan, 1908-2000) in which the abundance of the stable isotope carbon 13 is oddly enriched by a large factor to be close to that of normal carbon 12 for reasons not understood, adding to the mystery of the red star of the Lyre. (Thanks to Rachelle Leger, who suggested this star.)

Written byJim Kaler 6/19/15. Return to STARS.