The Big, They Die Young!

The goal of program G32 is to understand the evolution of some of the most massive stars known, which go by the name of Wolf-Rayet stars. Although such stars are few in number, they are important in the generation of the chemical elements, and they play a key role in the life-cycle of stars.

There are about a hundred billion stars in the Milky Way Galaxy. One of these stars is the Sun, which gives our Earth the light and warmth that it needs to sustain life. However, even though the time scales are long by human standards, the Galaxy is a dynamic, changing place, with new stars being born and old stars evolving and eventually dying. New stars are born out of collapsing clouds of gas and dust that float between the stars. Initially, these clouds are cold, but as they contract, they heat up. The temperature at the core of this "protostar" eventually becomes so high that nuclear fusion reactions set in. Hydrogen atoms are fused into Helium atoms, releasing energy which makes the newborn star shine. Once born, the life-stories of the stars are different depending on how much material, they started out with at their births.

Light-weight stars like our Sun live a long and quiet life. After 5 billion years, the Sun still generates light and heat from fusing Hydrogen into Helium. Later in its life, it will become a red giant star, fusing Helium into Carbon. The higher temperatures in the core will cause the Sun to puff up to 100 or more times its current size, engulfing the Earth; but that will not happen in another 5 billion years! Finally, the nuclear reactions in the Sun will stop and it will become what we call a white dwarf star. Without a power source at its center, the Sun will slowly cool and grow dimmer and dimmer. There are a lot of stars like our Sun in the Galaxy.

Massive stars, such as the Wolf-Rayet stars, are very rare (one in 10 million) and differ from our Sun in several important ways. These heavy-weight stars live relatively short but very intense lives. They have very strong stellar winds, about 10 billion times stronger than the solar wind. The solar wind can sometimes be noticed on Earth when it creates the northern lights or disrupts radio communications, but it has very little direct effect on the sun's evolution. The winds from massive stars are so substantial that matter from the stars is carried away by the wind; they are kind of evaporating as we watch.

Massive stars burn much hotter than that the Sun and are strong sources of UV radiation. They are capable of fusing heavier and heavier elements, elements such as Carbon, Nitrogen, Oxygen and so on up to Iron. If you have ever wondered where all the chemical elements came from, the answer is that nearly all of them were made inside of massive stars. The very material in our bodies was once inside of a massive star. We are all made of stardust!

The strong stellar winds of massive stars carry away so much material that they peel away layer after layer from the star like the different layers of an onion. Different chemical abundances become exposed at the surface of the star, allowing us to study the material that was once inside the star. At the ends of their lives, massive stars explode as supernovas. Thus when massive stars die, they scatter their ashes back out into the Galaxy. This material mixes with gas and dust in interstellar space and provides the building materials out of which new and different stars, the next generation of stars, may form.

Massive stars are a fundamental link in the life-cycle of the stars. In our investigation, we will use two of Astro's telescopes, HUT and WUPPE, to investigate the stellar winds and the chemical properties of selected Wolf-Rayet stars. These stars are in a phase when their stellar winds are strongest, and they are probably at the verge of blowing up as supernovas.

Two Wolf-Rayet stars were observed during the Astro-1 mission. These observations confirmed that our basic ideas are correct. With Astro-2, we hope to be able to test some of our understanding of Wolf-Rayet stars and their importance for the life-cycle of the stars more thoroughly.

Regina Schulte-Ladbeck