Download Stars with mass less than 0.5 solar masses

Stellar
Evolution
The stellar evolution involves two opposite forces:
on one side, the star’s mass produces the force of
gravity, which leads to a contraction, on the other
side the nuclear forces in the core produce
expansion.
BIRTH OF A STAR
Stars were born from enourmous cloud of gas and dust.
The explosion of a near supernova or a collision
between two nebulas marks the beginning of the force
of gravity: a protostar is forming.
These objects aren’t real stars, because their inner
temperature isn’t enough to prime nuclear fusion.
After a long process of condensation, if the mass of
the protostar is less than 1/10 of solar mass, the
protostar isn’t able to reach the temperature which
permits the nuclear fusion and it starts to cool, very
slowly. Jupiter, in particolar, but the Earth too, are
examples of protostars which continue to be…
protostars, or, as we say, planets.
When the mass of a protostar is more than 1/10 of
solar mass, temperature in the core reach 10 millions
kelvin: nuclear fusion starts.
Than stars live a period of instability: contractions and
expansions try to sorpass each other, till they come to
a balance. Here is the star.
Astronomers use to classify stars on the basis of temperature and magnitude,
in what they call the H-R Diagram (h-R from hertzsprung-Russel)
Here we see the H-R diagram.
In abscissa the surface temperature of the star, in kelvin, and the
corrispondent spectral class are riported. As you can see, from low
to high temperature, they are O, B, A, F, G ,K, M. Here is a trick to
remember them: they are the initials of this sentence: Oh Be A
Fine Girl, Kiss Me.
In the y-axis, the absolute magnitude of the stars (not to be
confused with the apparent magnitude) is reported, with, on the
other side of the diagram, the luminosity compared to the Sun.
So, the hottest, brightest stars are at the top left while the coolest,
faintest stars are at the bottom right. The diagonal band of stars
running from the upper left to lower right is known as the Main
Sequence and includes those stars which are converting hydrongen
into helium in their cores under stable conditions (90% of all stars
known). Red Giants or red Supergiants represent the second step
of the life of a star, as we’ll see. Then, on most occasions, white
dwarf are the lastest period of a common star’s life.
Maturity
Stars live the 90% of their life in the main
sequence, placed according to their mass. At a
certain point in their life the hydrogen in the core
finishes. As a conseguence, the nuclear fusion
finishes too, and so the star starts to contract
under the pressure of its own mass.
Then, the destiny of the star depends on its mass:
• Stars with mass less than 0.5 solar masses: in this
case the core’s temperature doesn’t reach
sufficient values to prime the nuclear fusion of
helium. This star is going to die in a white dwarf.
These are little stars, very hot initially, which cool
slowly till they swich off completely, in black
dwarf.
If a white dwarf is part of a bynar system, for
example with a red giant, the first one can steal
some of the red giant’s mass and prime the
fusion of hydrogen in the external layers. This
cause a a big explosion which can be seen from
the Earth. These stars are called Novae.
• Stars with mass more than 0.5 solar masses: in this
case, the contraction provokes the core’s
temperature of 100 million kelvin, which is enough to
prime the helium fusion. In the region around the
core, instead, temperature is more than 10 million
kelvin, and so here the hydrogen fusion starts,
causing the expansion of external layers: the Red
Giant.
The red giant’s life is short,
because the helium is less
than hydrogen and because
the energy produced by
helium is less than that
produced by hydrogen.
The subsequent phases of the life of a star depends,
once again, on its mass:
• Stars with mass less than
1,44 solar masses: this star
can’t have the nuclear fusion
of carbon, and lives a period
of instability, in which it
expells the external layers,
made of carbon and oxygen,
and it becomes a white
dwarf. What we see is a
Planetary Nebula.
• Stars with mass more than 1,44 solar masses: the
star starts the process of nuclear fusion which
leads to the formation of heavier and heavier
elements. The star results as a composition of
layers of different density, the lighter ones on the
top and the heavier ones in the core.
When the reactions get to iron, things change: the
fusion of iron doesn’t trasform mass in Energy,
but Energy in mass. Because of that, the star
collapses into itself, and esplodes violently: it’s a
Supernova, that can be visible from the Earth
during the daylight.
Heaviest elements
If the heaviest elements producted by stars is
iron, how about all the others?
In this phase, part of the espelled matter creates
a bow wave that produce condensation among
elements which forms new heavier elements. It’s
the only way to produce the heavy elements we
can find in nature. That’s why we are called sons
of stars.
Then the future of the core of the star depends again
to its mass.
• Core with mass less than 3-4 solar masses: it
become a neutron star. It’s a little star in which all
the protons and electrons have lost their
individuality, and have fused into neutrons. The
neutron star has a strong magnetic field and quick
rotation. For that reason this type of objects are
also known as Pulsar.
• Core with mass more than 4 solar masses: the
contraction goes on till unimaginable density and,
at the end, the star becomes a
Black Hole, one of
the most mysterious
objects in the
universe.