The theory of stellar structure and evolution is elegant and impressively powerful. The brilliance of stars on a clear night at a place not yet affected by light pollution is amazing. The uneven distribution of stars over the sky, their range in brightness and possibly the recognition that they have different shades of color make stars fascinating objects.
Almost all the light of galaxies comes from their stars. The theory of stellar structure and evolution is one of the most exact of the astrophysical sciences. It is inextricably involved in many of the topics needed to understand the role which high energy astrophysical processes play in the origin and evolution of stars and galaxies, providing for example, evidence on their chemical abundances, the ages of the systems and so on.
Stellar evolution refers to the changes that take place in stars as they age, the life cycle of stars. These changes cannot be observed directly, because they take place over millions or billions of years.
What is stellar evolution?
The changes that take place in stars as they age, the life cycle of stars.
These changes cannot be observed directly because they take place over millions or billions of years. Astronomers construct a theory of stellar evolution that is consistent with the laws of physics. Then they check their theory by observing real stars shining in the sky.
The life stages of a star is like our sun.
- Protostar, gravitational contraction of cloud of gas and dust. (a)
- Stable main sequence star, shining by nuclear fusion (converting hydrogen to helium. (b)
- Evolution to red giant when helium core forms. (c)
- Red giant shining by helium fusion. (d)
- Variable star formation of carbon core. (e)
- Planetary nebula enriched hydrogen envelope ejected into space. (f)
- White dwarf, mass packed into star about the size of earth. (g)
- Dead black dwarf in space. (h)
Modern theories have been enormously successful in describing the properties of stars. Stellar evolution is an active research field and it is worthwhile to keep in mind the uncertainties in the model calculations.
One of the main goals in nuclear astrophysics is to better understand the inner workings of stars. A chart showing the main evolutionary phases for single stars of various initial masses is shown in the diagram. The age indicator logic chart for stellar populations are shown below.
This will be helpful for the subsequent studies. The stellar masses and time increase are shown.The age resolution is greatest for stellar evolutionary phases corresponding to the more quickly evolving high mass stars - From Grebel (1998).
Stellar evolution is a mature field that is now encountering a stiff set of challenges from rapid improvement in astronomical data. Understanding of stellar evolution is also one of the foundations for inferring the evolution of star clusters and galaxies, through age estimates and element production by nucleo synthesis. One dimensional simulations have been exploited to the limit; three dimensional simulations including rotation and magnetic fields are beginning to be feasible.
To simulate stars accurately requires a predictive theory of stellar evolution. The elemental yields from such stars are especially sensitive to mixing because there are almost an elements but H and He, so that any newly synthesized element is important. Three dimensional simulations of turbulence, rotation, binary interaction, and conversion combined with three dimensional simulation of thermonuclear burning are not currently feasible, but they are needed.
The combination of large scale simulations with sub grid modeling and direct numerical simulation of subsections of the star is needed.