The life and death of stars

Interview with Prof. Beatriz Barbuy by Susan Schneegans

We see them sparkling in the night sky but how much do we really know about the stars above us? Beatriz Barbuy has studied different stellar populations in the Milky way using modern telescopes which allow us to penetrate the secrets of the universe more deeply than ever before.

These include the Hubble Space Telescope and the very Large Telescope of the European Southern Observatory in the Chilean desert.

Most of the stars visible to the naked eye are 10–100 light-years away. The nearest star to earth after the sun is Alpha Centauri. As Alpha Centauri is 4.3 light-years away, we see it as it was four years ago. With modern telescopes, astronomers are able to travel billions of years back in time; they can observe distant galaxies as they were just a few billion years after the Big Bang.

Full Professor at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo in Brazil, Beatriz Barbuy was awarded one of this year’s five L’OREAL UNESCO prizes for her work on the life of stars from the birth of the Universe to the present time. In the following interview, she explains how to determine the approximate age of a star, among other secrets.

What do we know about the formation of galaxies?

The Big Bang occurred 13.7 billion years ago. Three minutes after the Big Bang, hydrogen and helium formed and, 400 000 years later, the nuclei of hydrogen and helium and their electrons recombined into neutral atoms. Only from this period onwards did the structures of the Universe begin converging locally by the force of gravity, even as the Universe continued to expand. Massive stars seem to have been the first objects to form. The most distant such star, which exploded 13 billion years ago, was detected recently by the Swift satellite. Later on, small galaxies formed and began merging with each other to form bigger galaxies. Today, we can create computer models of how spiral and elliptical galaxies formed. Observing stars within galaxies also tells us a lot about how galaxies form.

How is a star born and why does it die?

Even though the Universe is expanding, stars are still able to form, since gravity is the dominant force within local groups of galaxies and within galaxies themselves. Stars are formed essentially by the gravitational contraction of gas clouds. A star is born when it starts igniting hydrogen then transforms this hydrogen into helium, in a process called nucleosynthesis. By this stage, it is a so-called dwarf star, like our own Sun. Massive stars will go through several phases, burning first hydrogen then helium then carbon then neon then oxygen then silicon. As they go through these different stages of nucleosynthesis, they are effectively in a state of permanent nuclear fusion. This powers the star for the duration of its lifetime and accounts for the extreme heat and light it projects. After the formation of the iron group elements, the core of the star will keep contracting for a few million years until it ends up either as a compact neutron star or as a black hole, depending on the star’s mass. The rest of the star will be ejected in a supernova explosion. After the explosion, only the neutron star or black hole will remain.

Neutron stars are hard to see; they are very hot and the densest objects known to us. They are only about 16 km in diameter, yet are more massive than our Sun! Low mass stars like our Sun will not go through most of these phases, nor will they explode. Rather, they are born when hydrogen begins burning at the core. Later, helium starts burning, mostly in the shell around the nucleus. Low mass stars will go through a phase of being a planetary nebula, by ejecting layers of gas that are illuminated by the star itself, before ending up as a white dwarf. Both neutron stars and white dwarfs will remain as compact remnants of their star, in principle forever.

 

Full interview in :
::  A World of Science, Vol. 7, N° 4, October-December 2009 page 14-16 [.pdf]

• Source:A World of Science, Vol. 7, N°4 (October-December 2009)
• 17-10-2009

• © NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration - The Brightest of Stars