thesis - IRS, The Infrared Spectrograph

156 English SummaryFigure 1–. Our Sun (left) is just one of the hundred billions of stars that form the Milky Way, a spiralgalaxy that might look like the external galaxy NGC 628 (right). Photo credits: Sun → SOHO/EIT(ESA & NASA). NGC 628 → Gemini Observatory-GMOS Team.The lifeThere are several similarities between the life of a star and our own. From the moment weare born, our body starts to change. We grow, get bigger, the colour of our hair might change,and eventually we will even lose hair in a dramatic way. These changes are different for eachindividual. There are people taller than others, some are blond some are brownish, somepeople lose more hair than others etc... In the case of stars it is the mass which governs theirevolution (aging). The thermonuclear reactions play the role of the heart in our lives andkeep the star shining (living). The difference is that thermonuclear reactions can keep starsshining for millions and millions of years. For instance, the Sun is 5 billion years old andwill live another 5 billion more!! The life time of the stars correlates with their mass, highermasses mean shorter life (they age faster). In the course of their evolution (or of their life)the stars will experiment physical changes. They will increase in size. Our Sun will becomeso big that it will engulf the orbit of the Earth. They will then become brighter, colder andwill even start to lose mass as they become older. At some moment they will die, but beforedoing so they will give us the most magnificent spectacle of their glorious life (see cover andback-cover figures).The endThe way stars end their lives depends on their original mass. Stars with masses ∼8 timeslarger than the mass of the Sun 7 will suffer a sudden violent explosion that can cause thedisruption of the star. This explosion is what we call Super Nova. Stars with masses similar tothat of our Sun and up to 8 solar masses start to pulsate in the last stages of evolution. At somemoment these pulsations become unstable and the star ejects part of its envelope. The ejectedgas is illuminated by the hot central star giving origin to what we call the Planetary Nebulaphase (see Fig. 2). A few thousand years after the ejection of the envelope the thermonuclearreactions (the heart) stop. The star cools down and becomes what astronomers call a white7 The mass of the stars is often measured in solar masses. The mass of the Sun is 1.989 × 10 30 kg = 1M ⊙ .