What is a Star?
A star is a luminous globe of glowing gas
producing its own heat and light by nuclear
reactions in the star's core.
Stars vary in size, mass, brightness,
temperature and colour. The smallest mass
possible for a star is about 1/10 that of the
Sun, while the brightest stars have masses
more than 60 times that of the Sun.
Stars are born from nebulae and consist
mostly of Hydrogen and helium gas. Surface
temperatures range from 3.000 K to above
60.000 K, and the corresponding colours
range from red (the coolest stars) to yellow
(like the Sun, which has a surface temperature
of about 6.000 K), then to white and (the
hottest ones) blue.
The brightness is measured in magnitude, the
brighter the star the lower the magnitude.
There are two ways to measure the brightness
of a star, apparent magnitude is the brightness
seen from Earth, and absolute magnitude
which is the brightness of a star seen from a
standard distance of 32.6 light years (10
parsecs).
If brightness and surface temperature are
considered, stars can be plotted on a graph
called the Hertzsprung-Russell Diagram, a
graph of great utility for the understanding of
the life cycle of the stars.
Birth of a star
Stellar evolution begins with the gravitational
collapse of part of a nebula cloud. A nebula is
a cloud of gas (mainly Hydrogen and helium)
and dust. The collapsing gas and dust
condenses into a rotating sphere of superhot
gas known as a protostar.
If the the core temperature of the protostar
will eventually reach 10 million K, initiating
the nuclear fusion reaction and allowing
Hydrogen to fuse to helium and release
energy, a star is born.
Stable state
The onset of nuclear fusion leads relatively
quickly to a equilibrium in which energy
released by the core exerts a "radiation
pressure" balancing the weight of the star's
matter, preventing further gravitational
collapse. The star thus evolves rapidly to a
stable state, beginning the main-sequence
phase of its evolution at a specific point of the
Hertzsprung–Russell diagram, depending
upon the mass of the star.
Small, low-mass red dwarfs fuse Hydrogen
slowly and will remain on the main sequence
for hundreds of billions of years or longer,
whereas massive blue giants will leave the
main sequence after just a few million years.
A mid-sized star like the
Sun will remain on the main sequence for
about 10 billion years. The Sun is thought to
be in the middle of its lifespan; thus, it is
currently on the main sequence.
Maturity of a star
Eventually the core exhausts its supply of
Hydrogen and the star begins to evolve off of
the main sequence. What happens next
depends upon the star's mass.
Red Giant
This is a large bright star with a cool surface.
It is formed during the later stages of the
evolution of a star like the Sun, as it runs out
of Hydrogen fuel at its centre. Without the
outward pressure generated by the fusion of
Hydrogen to counteract the force of gravity
the core contracts until it becomes hot enough
(100.000.000 K) for Helium fusion to happen
and Carbon to be produced. The star then
expand greatly, Red giants have diameter's
about 100 times that of the Sun.
Red SuperGiant.
This is a star that has diameters up to 1.000
times that of the Sun and luminosities often
1.000.000 times greater than the Sun. The
core of a red supergiant can reach the
temperature (billions of degrees) at which all
the atoms with atomic number bigger then
Carbon (Z=6) but not bigger then Iron (Z=26)
can be produced by nuclear fusion reactions.
White dwarf
White dwarfs are shrunken remains, the last
stage in the life cycle of all the stars with a
mass smaller than eight times that of the Sun
and whose nuclear energy supplies have been
used up. White dwarfs consist of degenerate
matter with a very high density due to
gravitational effects, one spoonful of a white
dwarf has a mass of a thousand kilograms.
White dwarfs have approximately the
diameter of the Earth, and will cool to "black
dwarf" over several billion years.
Planetary nebula
Planetary Nebulae are the outer layers of a
star that are lost when the star collapses from
a red giant or a red supergiant to a white
dwarf.
Supernova (of Type II)
This is the explosive death of a star more than
eight times as massive as our Sun. So big is
the star's mass that, when all the nuclear
fusion process in the red supergiant stage
comes to an end, it will suffer a gravitational
collapse so intense that it leads to a big
explosion. The energy liberated by the
explosion is immense, resulting in the
supernova emitting the light of thousands
million suns in a brief period of time.
Production of all elements from Iron (Z=26)
to Uranium (Z=92) occurs within seconds in a
supernova explosion, thanks to the energy
released.
We are made of star dust.
The Iron in our blood, the Oxygen we
breathe, the Calcium in our bones, all the
atoms of which we are made of (with the
exception of Hydrogen, which together with
Helium was produced in the very first few
minutes after the Big Bang) were produced
within the stars billions of years ago, just like
every other element on our planet. Therefore
we can say that we are literally children of the
stars. When a supernova explodes, the atoms
that have been produced are scattered in the
interstellar space, and from this "dust"new
stars and planets are born.
Stellar remnants: Neutron stars
These stars are composed mainly of neutrons
and are produced from the remnants of the
core when a supernova explodes. Neutron
stars are very dense, having a mass between
1,44 and 3 times the Sun but a diameter of
only few kilometres. A spoonful of a neutron
star has a mass of 10.000.000.000 kg. If the
mass of the remnant of the core is any greater,
its gravity will be so strong that it will shrink
further to become a black hole.
Black holes.
The gravitational pull in a black hole is so
great that nothing can escape from it, not even
light. The black hole is a "singularity" in the
spacetime continuum.