Saturday, November 7, 2015

Lyra - R Lyrae and other red giants

"RLyr" by Kevin Heider / NASA, ESA. Credit: A. Fujii -
Cropped from "Wide-field view of the Summer Triangle (ground-based image)" at heic0720c.
Licensed under Public Domain via Commons

R Lyrae is an easy-to-spot variable north of the main asterism. Also known as 13 Lyrae, it is a 4th-magnitude red giant. It is semiregular variable that varies by several tenths of a magnitude. The star is approximately 350 light years away from Earth.

The star is of the spectral type M5III, meaning it has a surface temperature of under 3,500 kelvins. It is much larger and brighter, yet cooler, than our Sun. It is a semiregular pulsating star. Its periodicity is complex, with several different periods of varying lengths, most notably one of 46 days and one of 64 days.

It is unusual in that it is a red star with a high proper motion (greater than 50 milliarcseconds a year).

In the near-infrared J band, it is brighter than Vega.
From Wikipedia

Four other red giants in Lyra constellation
  • Kappa Lyrae, a typical red giant around 238 light years distant  
  • Theta Lyrae,  4th magnitude
  • Lambda Lyrae, 5th magnitude  1100 ly distant. Orange bright giant star of the spectral type K2.5II. It is much brighter and larger, yet cooler, than our Sun. The star is about 6.3 solar masses and has a diameter over a hundred times that of the sun's.
  • HD 173780  (Henry Draper Catalogue). Orange giant. 5th magintude 250 ly distant
  • XY Lyrae red bright giant  just north of Vega that varies between 6th and 7th magnitudes over a period of 120 days

Red giant
the life of a Sun-like star, from its birth on the left side of the frame
to its evolution into a red giant on the right after billions of years
"The life cycle of a Sun-like star (annotated)" by ESO/M. Kornmesser
Licensed under CC BY 4.0 via Commons
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, from 5,000 K and lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

The most common red giants are 
  • stars nearing the end of the so-called red-giant-branch (RGB) but are still fusing hydrogen into helium in a shell surrounding a degenerate helium core. 
Other red giants are:
  • the red clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process
  • the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and sometimes with a hydrogen burning shell just beyond that.

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