![]() ![]() These are extremely rare events to observe, because they only produce a moderate amount of energy. The heating speeds up the nuclear reactions, which provide more heating, and which accelerates the nuclear reactions even more.The technical jargon term for these red nova events are "intermediate luminosity optical transits," or ILOTs. (More massive stars also ignite helium but more gradually and not with a flash.) As soon as the temperature at the center of the star becomes high enough to start the triple-alpha process, the extra energy released is transmitted quickly through the entire helium core, producing very rapid heating. When the triple-alpha process begins in low-mass (about 0.8 to 2.0 solar masses) stars, calculations show that the entire core is ignited in a quick burst of fusion called a helium flash. This process is called the triple-alpha process, so named because physicists call the nucleus of the helium atom an alpha particle. Once it reaches a temperature of 100 million K (but not before such point), three helium atoms can begin to fuse to form a single carbon nucleus. As time goes on, the temperature in the core can rise to much hotter values than it had in its main-sequence days. The core, having no source of energy to oppose the inward pull of gravity, is shrinking and growing hotter. Remember that red giants start out with a helium core where no energy generation is taking place, surrounded by a shell where hydrogen is undergoing fusion. ![]() By the way, we carefully used the term initial masses of stars because, as we will see, stars can lose quite a bit of mass in the process of aging and dying. We will see what happens to more massive stars in the next section.) Because there are much more low-mass stars than high-mass stars in the Milky Way, the vast majority of stars-including our Sun-follow the scenario we are about to relate. (Such mass may not seem too low, but stars with masses less than this all behave in a fairly similar fashion. Let’s begin by considering stars with composition like that of the Sun and whose initial masses are comparatively low-no more than about twice the mass of our Sun. Instead, we will focus only on the key stages in the evolution of single stars and show how the evolution of high-mass stars differs from that of low-mass stars (such as our Sun). We will (perhaps to your heartfelt relief) not delve into all the possible ways aging stars can behave and the strange things that happen when a star is orbited by a second star in a binary system. But after this point, the life stories of stars of different masses diverge, with a wider range of possible behavior according to their masses, their compositions, and the presence of any nearby companion stars.īecause we have written this book for students taking their first astronomy course, we will recount a simplified version of what happens to stars as they move toward the final stages in their lives. The “life story” we have related so far applies to almost all stars: each starts as a contracting protostar, then lives most of its life as a stable main-sequence star, and eventually moves off the main sequence toward the red-giant region.Īs we have seen, the pace at which each star goes through these stages depends on its mass, with more massive stars evolving more quickly. Discuss the creation of new chemical elements during the late stages of stellar evolution.Define “planetary nebulae” and discuss their origin.Explain what happens in a star’s core when all of the hydrogen has been used up.\)īy the end of this section, you will be able to:
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