“While a star is on the main sequence, it makes only helium. A star leaves the main sequence when it exhausts the supply of available hydrogen in its core. The post-main sequence fate of the star, and the range of elements produced, depends on the mass of the star. The lowest mass M stars, roughly below .2 Solar masses, are well mixed and can always bring hydrogen from outer layers to the core to sustain fusion. Such stars become gradually more luminous over trillions of years as they convert their hydrogen into helium, until they exhaust their fuel and fade away as white dwarf stars. White dwarf stars are no longer producing energy by fusion. Because of contraction, their surface temperatures are high (hence their bluish-white colour), but they have low luminosity because they are very small. White dwarfs are a common form of stellar remnant. All white dwarfs eventually cool down and go dark, but this process takes many times the current age of the Universe.” Stellar EvolutionWhite DwarfMain Sequence Book:Planetary Systems: A Very Short Introduction Source: Planetary Systems: A Very Short Introduction
“Although the first stars of the Universe could not have formed planetary systems, the process did not take long to get underway. Because massive stars are short-lived, the first billion years of the Universe already had time for 1,000 generations of production of the heavier elements. Observations show that the early universe was already a quite dusty place. Although massive stars do not live long enough to host planetary systems where life is likely to emerge, they are essential to producing the elements that lower-mass systems use to build habitable worlds. The Milky Way galaxy, our home, formed not long after the Big Bang, and has been building its stock of heavy elements ever since. Most of this galactic chemical evolution remains internal to the galaxy, although galaxies do sometimes collide and exchange material. Over the past thirteen billion years of nucleosynthesis in the Milky Way, there has been ample time for thorough mixing across the galaxy. Thus, our Solar System incorporates ingredients from a mix of myriad expired stars, most of which have been processed multiple times through short-lived stars. Every breath you take includes oxygen atoms from thousands of different stars that have lived and died in our galaxy over the past thirteen billion years.” Stellar EvolutionNucleosynthesis Book:Planetary Systems: A Very Short Introduction Source: Planetary Systems: A Very Short Introduction
“It is triste to contemplate the winding down of the Universe into a cold, dark, lonely place, but we are a young species in a young Universe, with vast reaches of time before us. It is certainly true that there are countless worlds out there that could support life as we know it, and probably countless more that could support life as we don’t know it. It may be that the Universe is teeming with life waiting to make our acquaintance. Or, we may well be the first ones in our galaxy to make the leap to sentience. The vast distance between stars poses a severe barrier to individuals or even societies making the journey. Protoplasm is just too fragile and short-lived a medium to be up to the task of such voyaging. However, at a tenth the speed of light, the whole galaxy can be traversed in a million years. That’s a long time for protoplasm, but it is not a stretch to think of the data that makes us what we are—embodied perhaps in silicon or some other sturdy information-bearing material and reconstituted at destination—spreading throughout the galaxy, hopping from planet to planet along the way like Pacific Islanders in their canoes. If life—or complex life—is rare, it may well be our destiny to seed the Universe with an expanding wave of consciousness. But it is to be hoped that we will leave abundant worlds alone to develop their own destinies. There are worlds enough, and time.” Intelligent LifeExoplanetsFermi ParadoxDirected Panspermia Book:Planetary Systems: A Very Short Introduction Source: Planetary Systems: A Very Short Introduction