Main Sequence
Low mass
stars spend billions of years fusing hydrogen to helium in their cores
via the proton-proton chain. They usually have a convection zone,
and the activity of the convection zone determines if the star has
activity similar to the sunspot cycle on our Sun. Some small stars
have very deep convection zones. Some of these stars also rotate
very quickly which twists their magnetics fields. When these field
lines line up, the result can be a flare of radiation including X
rays.
Over its
lifetime, a low mass star consumes its core hydrogen and converts it
into helium. The core shrinks and heats up gradually and the star
gradually becomes more luminous. Eventually nuclear fusion exhausts
all the hydrogen in the star's core.
A star's
lifetime is proportional to its mass divided by its luminosity t ∝
M/L. A star's luminosity is roughly proportional to the 3.5 power of
its mass so L ∝
M3.5.
Substituting t
∝
1/M2.5
where t is the Sun's main
sequence lifetime, a star with a mass 4 times the Sun's would have a
lifetime of 1/42.5
or 1/32 solar lifetimes.
Red Giant
When hydrogen fusion can no longer happen in the core, gravity
begins to collapse the core again. The star's outer layers expand
while the core is shrinking and as the expansion continues, the
luminosity begins to increase. For a star with the mass of the sun,
this expansion takes about a billion years and the star's radius
increases 100 times, and its luminosity increases even more. The star is called a red giant. A
hydrogen burning shell forms around the helium core, and she shell
contributes more and more helium to the core over time.
Eventually the
core becomes hotter and denser and reaches a temperature of 100
million K, and helium nuclei begin to fuse into carbon. The helium
fusion then heats the core rapidly even more and a helium flash takes place. This causes the core to expand, which lowers
the temperature of the core and reduces the total energy output from
what it was during the red giant phase. The outer layers then
contract and the star's temperature increases a bit.
After about
100 million years, the star fuses all its core helium into carbon.
Then a helium fusion shell forms around this core, and the hydrogen
fusion shell remains around that. It then becomes a red giant again
and remains like this for a few million years with its outer layers
continuing to expand.
Planetary Nebula
Eventually
gravity can no longer contain the outer layers of the red giant and
the star ejects these layers into space. The remaining carbon core
is still very hot and emits ultraviolet radiation that ionizes the
gas in the expanding shell and makes it glow brightly. This glowing
gas is called a planetary nebula, but has nothing to do with
planets. Planetary nebulae are relatively common and astronomers
estimate that there are between 20,000 and 50,000 in our galaxy.
Planetary nebulae often have elongated shapes. One theory is that
the star first ejects a doughnut shaped cloud of gas and dust from
its equator, then ejects gas from the entire surface. The doughnut
blocks some of this ejection and it is channeled in two opposite
directions. As the core cools, the glowing gas fades and disperses
and the nebula disappears within a million years or so.
White Dwarf
The cooling
carbon core is all that is left. At first its surface temperature is
around 100,000 K and emits ultraviolet radiation which ionizes the
gas in the nebula and makes it glow. The cooling core is called a
white dwarf, and eventually can no longer be seen and is then called
a black dwarf. The matter in a white dwarf is very dense about 109
kg/m3, which is a million times denser than water. A teaspoonful of
white dwarf matter if brought to Earth would weigh about 5 tons!
