Download STELLAR EVOLUTION

Stellar Evolution
An Overview
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Stellar “evolution” describes the change in luminosity, surface
temperature, and size of stars with time. This evolution is studied using the
HR diagram.
Stars evolve because they consume their nuclear “fuel", forcing changes in
their internal structure, until they are no longer capable of continuing nuclear
fusion.
Stars remain on the main sequence for about 90% of their total lifetimes,
fusing hydrogen quietly in their cores. Their surface temperature and
luminosity change very little during this time.
Stars then evolve, progressively burning the “ash” of one fusion process in
the next fusion process, until they exhaust all fuel possibilities. The star then
ends its existence as a star. A portion of the star’s mass remains as a dead
star.
The main sequence lifetime of stars depends on the star’s initial mass (the
mass contained with the star when it formed).
 Solar-mass stars remain on the main sequence for about 10 billion years.
The Sun is currently about halfway through its main sequence phase.
 Massive stars live for shorter periods of time compared to the Sun,
burning their fuel at a higher rate, due to the increased gravitational force
produced by their higher mass. O stars last only from 3 to 15 million
years.
 Low-mass stars have much longer lifetimes compared to the Sun. An M
dwarf may live for hundreds of billions of years. All of the M dwarfs ever
formed in the universe are still around.
 The initial mass of a star also defines its fate.
 High-mass stars, those with an initial mass > 8 M, end their lives
violently in tremendous supernova explosions.
 Low mass stars, those with an initial mass < 8 M end their lives more
“gently”, by losing their outer layers. The core remains as a dead star.
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Star clusters provide natural laboratories to test understanding of stellar
evolution. All stars in the cluster are the same age; the evolution of each star
depends only on the stars’ individual masses.
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Stars in close binary systems may undergo very different evolutionary paths
compared to their evolution as single stars, due to interactions between the
stars in the system. Stars may exchange matter, combine into one star,
undergo repeated novae explosions, or disintegrate as a supernova.
Almost all evolutionary processes in stars take much longer than a human
lifetime; we cannot watch a star evolve from birth to death. A few exceptions
are supernova and nova explosions, which happen in hours and days.
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To compensate for our inability to watch stellar evolution, astronomers
explore this phenomenon in a variety of ways:
 Theoretical models of the internal structure of stars, derived using
computer-intensive calculations, are compared with the observed
properties of individual stars. The theoretical models are then continually
“tweaked” to match the data. These models incorporate the basic laws of
physics as we understand those laws.
 Computational models which predict the lifetimes and properties of
stars of all masses. Astronomers then match these predictions with
observed populations of stars, especially the stars found within clusters.
 Observations of the temperature, density, and motions of interstellar gas
and dust clouds, which inform us about the process of star formation.
 Observations of the matter ejected by stars from stellar winds, and more
violent explosions. The composition of this ejected matter provides
insight into the fusion process.
 Astrophysicists have been successful at understanding the basic precepts for
why stars evolve. This understanding of stellar evolution is perhaps the most
significant achievement of 20th century astrophysics.