The production of cosmic Helium
Hydrogen and helium are
the most abundant elements - together they account for
about 98% of the mass of all the atoms in the Universe today.
The remaining 2% consists of all the other elements
put together - for example, all the iron, calcium, nitrogen
and carbon present in our bodies, all the silicon in the rocks beneath
our feet and the oxygen in the air we breath.
It wasn't always so. Astronomers
can easily identify stars which are very young, and those
which are very old. In the oldest stars known, the amount
of these heavier-than-helium elements is very small, much less
than the 2% found in the youngest stars. In really old stars,
hydrogen and helium together account for some 99.99% of their composition.
What happened in the meantime is
that those elements upon which our existance depends were
created in the central furnaces of successive generations of
stars; supernovae and hot winds blew these elements back into
the surrounding gas clouds which float between the stars; and new
stars were born which were enriched with these heavier elements.
Most stars shine by fusing Hydrogen
atoms into Helium atoms, as the Sun does; other stars,
having used up their Hydrogen supply, convert helium into carbon,
or carbon into heavier elements still, and release energy in
that way. As stars are born, grow old and die, the amount of helium
and the amount of the other heavier elements has been slowly increasing
in the Universe.
Measuring the amount of helium and
heavier elements tells us much about the stars: the number
which have been born and have died; the processes which cause
them to shine; and how they enrich the Universe with the elements
they have created.
Now, researchers in Finland, Australia
and the USA have made an accurate measurement of the production
rate of Helium in the universe. The team (
of Tuorla Observatory;
of the University of Pennsylvania;
of the University of Delaware and
of Swinburne University of Technology) have
used data from the
European Space Agency
satellite and so-called
K dwarf stars.
are cooler and fainter than the Sun and are essentially stellar
fossils. They have changed very little of their initial supply of
hydrogen into helium during their long lives; in other words the
hydrogen, helium and heavy elements we see in them today is they same
as when they were born. We can follow the production of helium and
heavy metals with a set of these stars.
satellite measures very accurately
the real energy output of these stars. The research team
have used computer calculations to predict how brightly such
stars should shine depending on how much hydrogen, helium and heavier
elements they contain. Measuring the amount of heavier elements
using telescopes can be done very easily -- it is the amount of
helium in stars which has been very difficult to measure. Now, the
comparison of the model computations with the real stars reveals,
indirectly, the amount of helium they contain.
The team have found that over the
billions of years since the Universe was born, stars have
produced just about exactly twice as much helium as everything
else. Stars are primarily helium factories!
The research appeared in the March 7th, 2003 issue of the
The Goldilocks effect.
The plot shows nearby stars measured
with the Hipparcos satellite, arranged from hot to cool (right
to left) and very bright to very dim (up to down). "Hot stars"
are too young, to be useful : they tell us little about the history
of Helium production. The "Evolved stars" have been producing
lots of Helium --- making them of little use as fossil records
of the Helium in the Universe at the time such stars were born. Very
cool stars (lower right) have very complex spectra making the prediction
of their brightness still too difficult to be reliable. The
K dwarfs utilised
in this study are neither too cool, nor too hot, but "just right".
European Space Agency
measured distances and spatial motions of 125,000 nearby stars
with a huge increase in precision compared to a previous 100
years of astronomical observations done with telescopes on the
Production of Helium.
The plot shows how the luminosity
of K dwarfs increases as the amount of Helium and heaver elements
increases. The data points represent real K dwarf stars observed
with the Hipparcos satellite. Three computer calculations are
shown by the dark blue, light blue and green lines: they represent
different amounts of Helium produced in stars relative to the amount
of heavier metals. The data indicate that the amount of Helium
produced has been twice the amount of all the heavier than Helium
elements combined (purple line).