Sol Lucet Omnibus - ESO

A protoplanetary disc
is seen as a green oval.
Radiation from the hot
star is heating up the
disc, causing matter to
dissipate, like steam
evaporating from the
surface of boiling water.
A strong stellar wind is
propelling the material
away from the disc.
The material is glowing
because it is gaining
energy from the radiation
from the hot star.
blades of a large turbine wheel,
turning the wheel. The next step
is to turn this energy into the
electricity that powers our homes.
To do this we’ll add a couple of
ingredients: a shaft with magnets
and some copper coils.
The turbine turns the shaft, rotates
the magnets and produces an
electric current in the copper coils.
This effect was first discovered in
1831 by Michael Faraday when
he measured how electric current
changed in a loop of wire it swept
through a magnetic field. It’s quite
simple and really easy to reproduce
at home. This is how we transform
gravitational energy into electric
power on Earth. The amount of
power generated depends on the
amount of water flowing on the
turbine; more water means more
electricity.
So what has a hydroelectric power
plant got to do with the creation
of a star? First you have to imagine
the disc where planets are built as
a gigantic reservoir and the growing
young sun as a gigantic magnetised
turbine. Let’s put some numbers
on this analogy: this stellar turbine
has a radius of some 100 000 km
and it is accommodating a matter
flow of about 2000 tons per second
coming in with a velocity of about 1
million kilometres per hour! In one
year, the growing star consumes
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about a billionth of the mass of our
Sun. The growing sun’s magnetic
field is about a thousand times
stronger than the field that moves
a needle in a compass. So these
young suns behave as gigantic
magnets onto which matter falls.
So far so good, but where is the
copper coil? And where does the
electrical energy go? The answer is
that there are no wires, instead the
currents are controlled by gravity
and the electromagnetic fields they
themselves generate. No wonder
this problem needed so many clever
people to understand it.
Young stars burn bright
The astronomical observations
of young stars are helping a lot
to understand the process. Today
we know that about 10% of the
gravitational energy is transformed
into the ejection of beams of gas
aligned with the rotation axis of the
stellar turbine. These beams move
at velocities comparable to those
of the matter falling onto the star.
They carry a significant fraction of
the mass of the disc reservoir away
from the young forming star. We
call them protostellar jets and they
make some of the most beautiful
astronomical images.
Close to the star, the environment
is very harsh. Gigantic eruptions
suddenly release a hundredth of
the solar luminosity as X-ray and
ultraviolet radiation are produced.
This action takes place within a
region that may extend as far out as
the orbit of Mercury. Modern space
experiments (such as the SOHO
satellite) show splendid pictures
of the Sun’s corona. The corona
has a temperature of ten million
degrees and radiates strongly in
hard X-rays. In the newborn Sun, a
similar but much larger structure
irradiated the disc; the corona could
reach Mercury’s orbit thus acting as
enormous heating panels irradiating
the disc, all the way out to the
Earth’s orbit. This is the reason why
neither ices nor gases like hydrogen
and helium can survive within the
inner region of the discs and why
terrestrial planets are solid and built
of very robust molecules such as
silicates or carbonates.
There is however, another intriguing
point to this story. We all know that
excess ultraviolet radiation harms
our bodies. The reason is that our
molecules, the molecules of life, are
extremely sensitive to ultraviolet
radiation. They react to it, and in the
presence of ultraviolet radiation the
organic chemistry is accelerated.
Ultraviolet radiation is bad for us,
but it is of great importance to
understand how life grew on our
planet, or perhaps even in other
forming solar systems in the Milky
Way.
Exciting times
We are just beginning to grasp
our origins but it is clear that
understanding this powerful engine
that controls the birth of planets
like ours and the appearance
of large organic molecules, and
maybe life, is of extraordinary
importance. Major clues are
hidden in this knowledge. Exciting
times are waiting ahead with the
new ultraviolet telescopes that
will allow us to peer into these
fascinating engines.
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Huge clouds of
interstellar gas and
dust, like the famous
Orion Nebula, are the
birthplaces of stars and
planets.