Chris Flynn : Tuorla Observatory

  disk mass
  black holes
  white dwarfs
  stellar halo
  K dwarfs
  local group

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 ( Chris Flynn of Tuorla Observatory; Raul Jimenez of the University of Pennsylvania; James MacDonald of the University of Delaware and Brad Gibson of Swinburne University of Technology) have used data from the European Space Agency ' s Hipparcos satellite and so-called K dwarf stars. These 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. 
The Hipparcos 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 journal Science .


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 's Hipparcos
satellite has 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 Earth.

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).

For further information:  

Chris Flynn
Tuorla Observatory
Väisäläntie 20
FIN-21500 Piikkiö

Tel: +358 (0)2 2744244
Fax: +358 (0)2 2433767


Research supported by the ANTARES program of the Academy of Finland