24 Variable star formation in the universe

Mohd Altaf Sofi

Learning Outcomes

After studying this module, you shall be able to

• Learn a few basic processes which show how variable stars can be visualized and what are the different processes involved in the formation of these cosmic objects.
• Know that there are many types of variable stars which can be categorised on the basis of their light emission.
• Learn the concept involved in the formation of the variable stars.
• Know that (i) the observation of the variable stars is strictly dependent on the total output of the light received from them, (ii) the possibility of explaining existence of the variable stars on the basis of luminosity variation, (iii) the nature of different types of variable stars.

1. INTRODUCTION 

Star formation has been and still continues to be an important aspect for explaining the evolution of the universe. The stability of the newborn stars depends on their gravitational behavior and their initial mass content. Those of the stars having large masses compared to our sun have small chances of survival. Whereas those stars having masses comparable to the mass of the sun have more or less sun like life cycle. A star during its active phase never cools off due to the continuous nuclear reactions taking place in its core. These nuclear reactions change the lighter elements into the heavier ones resulting in the light output in the form of luminosity which we observe here on the surface of the earth. Hence the star distribution in the observable universe becomes possible on the basis of the relationship between luminosity and the spectral types. This relationship helps in developing a diagram popularly known as Hertzsprung-Russel diagram and it helps in tracing the stellar evolution in the most vivid manner (Fig.08.1). Many of the observable stellar systems in the universe show variation in their luminosity with the passage of time. Such stellar distributions are said to consist of variable stars.

If ρ is taken as the average density of a real star then the application of equation can be extended to all variable stars. In 1914 H. Shapley’s investigation into the nature of variable stars proved that these stars undergo astroseismological radial pulsations thus rejecting thereby the binary hypothesis. Shapley’s idea coupled with equations (13) and (14) by Ritter were later confirmed by Sir Eddington. However, in view of the overwhelming abundance  of  hydrogen  in  the  stellar  material,  if  ?be  taken  equal  to  5⁄3,  the  computed periods with Eddington’s formula fall short of the observed periods subject to the condition that normal main sequence stellar density is adopted. This led Eddington to the conclusion that density in Cepheids must be very much less compared to that of the main sequence stars. The most satisfactory explanation about the true nature of the pulsation that properly takes into account the observed phenomena of Cepheid variation came from Martin Schwarzchild in 1938. He suggested that it is only the interior of the star that pulsates and sends compressional waves to the photospheric layers of the star. According to this model the maximum brightness occurs when the speed of the compressional waves is highest. This observation compares well with the light curve and velocity curve relations. Thus modified pulsation hypothesis ultimately emerged as the only acceptable explanation which is in harmony with the observed Cepheid phenomena. Pulsation theory and spectroscopic analysis of variable stellar systems have been extremely helpful in understanding the Cepheid phenomena but the physical cause of pulsations is still not properly understood. One of the possible physical causes for the generation of pulses has been attributed to the creation of the critical Hydrogen/ Helium ionisation zones around the star envelop that initiates and maintains the mechanical oscillations. This theory was put forward by a Russian astrophysicist S. A. Zhevaskin and is yet to be confirmed by more independent surveys.

4.  SUMMARY

The main aim of this lecture has been to define a variable star in terms of its apparent changes in the brightness with the passage of time. We have remained focussed with the intrinsic pulsating variables in which changes of luminosity are mostly regular and periodic. The categorisation of the variable stars has been done on the basis of the length of their periods, nature of variation of their light and their spectral characteristics.

Suggested Reading

1. Astrophysicsby L.H. Aller, Ronald Press NY USA
2. Astrophysics by Biadyanath Basu, Prentice Hall of India
3. In Darkness Born by M. Cohen, CUP, Cambridge