Astrophysics
A
primary objective of our ISAC facility will be the study
of subatomic reactions occurring within stars during the final stages
of their life cycle. Currently, reactions using beams of stable nuclei
have revealed a large variety of phenomena involving strong and
electroweak interactions, and have allowed scientists to create many
new nuclei at or near the limit of particle stability. Soon the
accelerated radioactive beams of unstable nuclei, produced in the
ISAC facility, will provide a major new tool for studying the formation
of elements. In nature, elements with nuclei heavier than lithium are
formed in the intense heat and pressure of stellar interiors and
are then ejected into space in explosive events such as supernovae.
[ Below: the red giant Betelgeuse ]
How does a star become a red giant?
A star's primary source of energy, during its lifetime, is the
fusion of hydrogen occurring in its core. As the hydrogen
is used up, the helium which is produced fills up the core.
But the temperature is not high enough for helium
fusion to occur, so core energy production slows down, its outward
pressure decreases, and the gravitational forces cause the core to
contract. As the core contracts the atoms bunch closer together causing
an increase in density and temperature. When the core temperature is high
enough, helium fusion begins. At the same time as the helium core
is contracting and heating up, an outer hydrogen shell expands and begins
fusing to form more helium. It is this expansion and fusion reaction in
the hydrogen shell which pushes the star's envelope out into space.
The surface of the now giant star is so far away from
the hot core that it cools down and turns red (hence the name red giant).
Will our Sun go supernova?
No. In single, [1]
low-mass stars, such as our Sun, the helium fuses to form
a carbon core, but does not have enough gravitational force to
contract this core (thus making it hot enough) to
begin a carbon-fusion reaction. Before our Sun dies (in about 5 billion years)
it will become a red giant but it will not go supernova.
Instead it will eject its envelope leaving a small,
white core which, because of the heat, will continue to emit light
for billions of years. During this stage it will be classed as a white dwarf.
([1]
Binary star systems are another matter and will not be discussed.)
How does a supernova occur?
Large-mass stars (over 8 times the mass of the Sun)
have enough gravitational pressure to continue beyond a carbon
core until eventually a dense iron core forms. At this point the
outer surface has expanded several times and the star may now be
classified as a 'super giant'.
When the core is mainly iron, the star is unable to produce sufficient
energy and pressure (through the fusion of iron) to counter the
intense gravitational forces from within. At this point the core
very quickly collapses, leaving an unsupported inner shell. This
shell also collapses, striking the collapsed core to produce an intense
shock wave. The extreme force of the shock wave causes the star to explode
and a supernova occurs. If the mass of the collapsed core is 3 or
more solar masses, then a black hole is formed. Otherwise a neutron
star is born.
How is a neutron star formed?
When the iron core collapses,
the gravity is intense enough to compress the electrons into the
protons to form a dense core of tightly packed neutrons. (How dense? A
teaspoon of this material would weigh 50 billion tons on Earth!)
Similar to the formation of white dwarfs though, if the core is not
massive enough to generate sufficient gravity, the next stage
- a black hole - is not reached and it
remains a neutron star (sometimes observed as a pulsar -
a star that appears to blink on and off).
To form a black hole requires an iron core equivalent to 3 or more
solar masses - such that after the core collapses, and the
supernova occurs, there remains enough of the core to form a mass so
magnetically strong that any light it creates is withheld inside
the core's gravitational field.
In the supernova explosion, material generated during the lifetime
of the star is thrown out into space at enormous speeds and eventually
is incorporated into new stars and planets. At the temperatures (hundreds
of millions of degrees) involved in stellar explosions, many different
types of unstable nuclei are involved in the resulting nucleosynthesis
(the formation of new atomic nuclei).
To study these reactions, it is essential that we simulate
a similar environment. The
new ISAC facility will produce intense beams of unstable nuclei, with energies
(temperatures) relevant to hot astrophysical environments. Using
these beams researchers will study the chains of
reactions - leading to heavier elements - involved in these
later stages of stellar evolution to test the current theories of the
weak interaction, and to gain insight on the standard Solar Model.
|
