Alpha Cygni star
Alpha Cygni stars are pulsation-variable stars . The prototype of these stars is Deneb (α Cygni). It shows fluctuations in the apparent brightness from +1.21 to +1.29 mag. Some of the brightest stars in the night sky belong to this massive class of stars .
properties
spectrum
The Alpha Cygni stars are supergiants of the spectral class late B or A.
Pulsations
The alpha Cygni stars show periods between 10 and 100 days in the light curves . The amplitude of the brightness fluctuations is in the range from 0.01 to 0.1 mag and is multiperiodic in every intensely observed Alpha Cygni star. The changes in brightness are accompanied by a change in the pattern of the absorption lines .
Cause of the pulsations
The period values are too long for radial pulsations . Instead, the following causes are discussed for the reasons for variability:
- The long periods on the order of months in Alpha Cygni stars could be stimulated by the epsilon mechanism , while the short periods, ranging in length from days to weeks, are controlled by the conventional kappa mechanism . The epsilon mechanism is based on a fluctuation in the production of energy by nuclear reactions . In the Alpha Cygni stars, a shell with active hydrogen burns lies in a zone with energy transport by radiation, which could be sensitive to instability.
- In stars with over 40 solar masses , non-radial pulsations can occur and lead to strange modes . These occur in zones of increased opacity , such as the second ionization level of helium, and are excited in stars not far from the Eddington limit . However, the calculated amplitude of the strange modes is considerably larger than observed for Alpha Cygni stars.
Occurrence in star catalogs
The General Catalog of Variable Stars currently lists around 60 stars with the abbreviation ACYG , which means that a little over 0.1% of all stars in this catalog belong to the class of Alpha Cygni stars. A similar number is believed to belong to the Alpha Cygni stars.
development
The Alpha Cygni stars are very massive with at least 10 solar masses . They are so massive that they will explode as a core collapse supernova in the course of their further development . During the evolution of massive stars there are two phases when they cross the instability strip in the Hertzsprung-Russell diagram :
- Once shortly after the main sequence phase, the stars migrate rapidly through the instability streak and then develop into a red supergiant , losing a lot of mass at this stage.
- Due to the strong loss of mass, the helium-burning stars in the HR diagram move to the left again and come back into the area of the instability strip for supergiants .
However, the Alpha-Cygni variability only appears to occur during the second phase, since, according to simulation calculations, the loss of the outer envelope created the conditions in which pulsations can be excited. However, these theoretical calculations depend heavily on assumptions that are difficult to model about the internal mixing and the strength of the stellar wind .
Examples
Individual evidence
- ↑ Ehsan Moravveji, Andres Moya, and Edward F. Guinan: ASTEROSEISMOLOGY OF THE NEARBY SN-II PROGENITOR RIGEL PART II. EPSILON - MECHANISM TRIGGERING GRAVITY-MODE PULSATIONS? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1202.1836 .
- ^ Aerts, C., Christensen-Dalsgaard, J., Kurtz, DW: Asteroseismology . Springer Verlag, New York 2010, ISBN 978-1-4020-5178-4 .
- ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 12, 2019 .
- ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
- ^ Hideyuki Saio, Cyril Georgy, Georges Meynet: Evolution of blue supergiants and αCygni variables; Puzzling CNO surface abundances . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.2474v1 .