Until the early- to mid-twentieth century, scientists believed that stars generate energy by shrinking. As stars contracted, it was thought, they would get hotter and hotter, giving off light in the process. This could not be the primary way that stars shine, however. If it were, they would scarcely last a million years, rather than the billions of years in age that we know they are. We now know that stars are fueled by nuclear fusion. Each time fusion takes place, energy is released as a by-product. This energy, expelled into space, is what we see as starlight. The fusion process begins when two hydrogen nuclei smash together to form a particle called the deuteron (a combination of a positive proton and a neutral neutron). Deuterons readily combine with additional protons to form helium. Helium, in turn, can fuse together to form heavier elements, such as carbon. In a typical star, merger after merger takes place until significant quantities of heavy elements are built up.

We must distinguish, at this point, between two different stellar types: Population I and Population ll, the latter being much older than the former. These groups can also be distinguished by their locations. Our galaxy, the Milky Way, is shaped like a flat disk surrounding a central bulge. Whereas Population I stars are found mainly in the galactic disk, Population II stars mostly reside in the central bulge of the galaxy and in the halo surrounding this bulge.

Population II stars date to the early stages of the universe. Formed when the cosmos was filled with hydrogen and helium gases, they initially contained virtually no heavy elements. They shine until their fusible material is exhausted. When Population II stars die, their material is spread out into space. Some of this dust is eventually incorporated into newly formed Population I stars. Though Population I stars consist mostly of hydrogen and helium gas, they also contain heavy elements (heavier than helium), which comprise about 1 or 2 percent of their mass. These heavier materials are fused from the lighter elements that the stars have collected. Thus, Population I stars contain material that once belonged to stars from previous generations. The Sun is a good example of a Population I star.

What will happen when the Sun dies? In several billion years, our mother star will burn much brighter. It will expend more and more of its nuclear fuel, until little is left of its original hydrogen. Then, at some point in the far future, all nuclear reactions in the Sun’s center will cease.

Once the Sun passes into its "postnuclear" phase, it will separate effectively into two different regions: an inner zone and an outer zone. While no more hydrogen fuel will remain in the inner zone, there will be a small amount left in the outer zone. Rapidly, changes will begin to take place that will serve to tear the Sun apart. The inner zone, its nuclear fires no longer burning, will begin to collapse under the influence of its own weight and will contract into a tiny hot core, dense and dim. An opposite fate will await the outer region, a loosely held-together ball of gas. A shock wave caused by the inner zone's contraction will send ripples through the dying star, pushing the stellar exterior's material farther and farther outward. The outer envelope will then grow rapidly, increasing, in a short interval, hundreds of times in size. As it expands, it will cool down by thousands of degrees. Eventually, the Sun will become a red giant star, cool and bright. It will be so large that it will occupy the whole space that used to be the Earth's orbit and so brilliant that it would be able to be seen with the naked eye thousands of light-years away. It will exist that way for millions of years, gradually releasing the material of its outer envelope into space. Finally, nothing will be left of the gaseous exterior of the Sun; all that will remain will be the hot, white core. The Sun will have become a white dwarf star. The core will shrink, giving off the last of its energy, and the Sun will finally die.

直到20世纪中叶以前，科学家们一直都相信恒星是通过收缩产生能量的。人们认为，随着恒星收缩，它们会变得越来越热，在这个过程中它们会发光。然而，这可能不是恒星发光的主要方式。假如是这样的话，它们几乎不可能存在一百万年，而据我们所知，它们一直存在了几十亿年。我们现在知道恒星是从核聚变中获得能量的。每当核聚变发生时，能量就会作为副产品被释放。这些能量，被喷射到太空中，就成了我们所看到的星光。核聚变的过程开始时，两个氢原子碰撞在一起，形成了一个被称作氘核（正质子和中性中子的结合）的原子。氘核很容易与其他的质子结合形成氦。氦，又会融合在一起形成更重的元素，比如碳。在一个典型的行星上，合并融合的过程会不停地上演，直到形成大量的重元素。 就此，我们必须区分两种不同的恒星类型：星族I和星族II，后者比前者的年龄要更大。这些恒星群也可以根据它们所在的位置来区分。我们的星系，银河系，它的形状就像一个围绕着中央凸起的扁平圆盘。星族I的恒星主要位于银盘之上，而星族II的恒星大部分都位于银河系中央凸起的部分，以及凸起部分周围的光晕中。 星族II的恒星可以追溯到宇宙的早期阶段。这些恒星是在宇宙充满氢气和氦气的情况下形成的，它们最初几乎不含重元素。它们闪闪发光直到耗尽易熔物质。当星族II的恒星死亡的时候，它们所含的物质会分散到宇宙中。其中一些尘埃物质最终并入了最新形成的星族I的恒星中。虽然星族I的恒星的组成物质大部分是氢气和氦气，它们也含有重元素（比氦更重），这些重元素约占其质量的1或2%。这些较重的物质是由恒星所积聚的较轻的元素融合而成的。因此，星族I的恒星包含了属于以前恒星的物质。太阳就是一个很好的星族I恒星的例子。 当太阳死亡的时候会发生什么？在几十亿年中，我们的母星将燃烧变得更亮。它将消耗越来越多的核燃料，直到它原来所含的氢都所剩无几。然后，在遥远未来的某个时刻，太阳中心所有的核反应都将停止。 一旦太阳进入“核后”阶段，事实上它就会分为两个部分：内部和外部。虽然内部区域不再有氢燃料，但在外层区域仍会留下少量的氢燃料。很快，变化将开始发生，这些变化将会把太阳撕裂。在内部区域，太阳核心不再燃烧，在自身重力的影响下它将会开始崩塌，并收缩为一个微小而炽热的核心，密度很高，颜色昏暗。而外部区域的变化却截然相反，外部区域将变成一个由气体组成的松散的球状物。由内区收缩引起的冲击波会使垂死挣扎的恒星发出涟漪，将恒星表面的物质向外推得更远。恒星的外层会迅速成长，在很短的时间内，体积就会增加数百倍。当它膨胀时，它的温度会下降几千度。最终，太阳将变成一颗红巨星，又冷又亮。 它将变得如此之大，以至于它将占据曾经是地球轨道的整个空间，它是如此明亮以至于在数千光年以外就可以用肉眼看到它。红巨星将以这种方式存在数百万年，逐渐将外层外壳的物质释放到太空中。最终，太阳的气体外层将不再释放任何物质；剩下的将是炽热的、白色的核心。太阳将变成一颗白矮星。它的核心会收缩，释放出最后的能量，最终太阳会“死亡”。