1.05 Main sequence stars
Main sequence stability
After hydrogen burning commences the star settles into a stable state. The force of gravity tending
to compress the star is balanced by the outward pressure from within and the contraction
of the star is halted. This stabilises the density and temperature within the core and therefore
the rate at which nuclear reactions take place. The outward pressure from the star is produced both
by the motion of the particles in the hot gas and by the radiation from the nuclear reactions in the
core travelling outwards. In lower mass stars the pressure from the hot gases is mainly responsible
for supporting the star, in more massive stars it is the radiation pressure that is the main factor.
The net result is that main sequence stars "burn" at a fairly constant temperature and luminosity
and maintain a constant size during their main sequence lifetime. (However as the hydrogen burning
gradually increases the helium content in the core it causes the core to contract slowly causing
a gradual increase in the luminosity of the star over its main sequence lifetime.) (The core
temperature of the star at this stage is not high enough for the fusion of helium nuclei to take
place. Therefore the helium content does not contribute to the energy production which is
supporting the core.)
Mass limits for main sequence stars
There is an upper and lower limit to the mass of a star. Stars which are more massive than planets
but less than about 0.05 solar masses never produce sufficient core temperatures to trigger nuclear
reactions, these objects are called Brown Dwarfs. For stars of mass greater than about 50 solar
masses the radiation pressure from nuclear reactions within the core would be so great that the
star would be blown apart. The mass of a stars is usually expressed in terms of solar masses.
1 solar mass is equal to the mass of the sun.
Solar mass and rate of fusion
The evolution of a star depends on the star's mass.
The mass determines the rate at which nuclear reactions occur within the core. Consider two
neighbouring stars of different masses the smaller of which is in a stable condition. The force of
gravity compressing the more massive star will obviously be much greater than that experienced by
its companion. Therefore the outward pressure required to stabilise this star would also need to
be much greater. For the more massive star contraction continues further until the core temperature
produces reactions at a rate sufficient to generate the pressure required to support the stars greater
mass. ie the core will be at a greater density and temperature and therefore the rate at
which nuclear reactions occur will be greater. The net result is that more massive stars burn at
higher temperatures and consume nuclear fuel at a much greater rate than less massive stars. The
relationship between the mass of the star and the rate at which nuclear reactions occur in the
core is not proportional. For an increase in mass the corresponding increase in the rate of
reactions is greater, much greater for very massive stars.
To put this into perspective, the sun has a main sequence lifetime of 10 billion years! ( it will
take a total of 10 billion years to consume all of the hydrogen in its core). Sirius has a mass
twice that of the sun but only has a main sequence lifetime of about 2.5 billion years, a quarter
that of the sun! Naos is a star with a mass of about 40 times that of the sun and it has a main
sequence lifetime of a mere 1 million years!
Main sequence characteristics of the sun
The sun has a surface temperature of 5800 - 6000 K, its core temperature is about 15,000,000 K, it
has a diameter of 1,400,000 km and its mass is about 2 x 1030kg (2 followed by 30 zeros!)
In the following sections we will discuss the evolution of stars of different masses.