are needed to promote its success and its increasing complexity? Astronomy has also shown that our universe has time—lots of it here, too. Life has had a very large, but not infinite, time in which to become established, to grow, and to develop. The time kept by the clocks of orbiting planets marches smoothly onward, but the time marked by living organisms is roiled by turmoil and occasionally punctuated by catastrophic astronomical events. How does the enormous span of time, punctuated by drastic events, affect the origin and evolution of life?

Scientists also know from astrophysical study that the galactic environment has provided crucial ingredients for life. The big bang provided hydrogen for the eventual formation of water, in addition to helium and a smattering of light elements, such as lithium, that are of little consequence for life. All the elements needed for life as we know it—oxygen to complete the water molecule, indispensable carbon, phosphorus for nucleic acids and key metabolites, metal ions and transition elements to serve as catalysts in the chemistry of life and more—came from generations of stars that formed from the dilute interstellar gas. These stars evolved and forged heavier elements from the primordial hydrogen and helium and then expelled these critical elements back into space, sometimes with a gentle push and other times with a catastrophic explosion, to form the next generation of stars and planets, which were becoming ever more hospitable to the origin and evolution of life. In addition to playing a crucial role in the chemistry of life, the elements forged in stars provided natural radioactivity, which is, in part, responsible for the tectonic activity that shapes the Earth and which is one unavoidable source of mutation, the driving force of genetic evolution.

The galaxy that hosts all these processes has its own structure and composition, which will affect the conditions for the development and flourishing of life on a planet. It has denser regions near the center that tend to have more supernova explosions and greater concentrations of life-giving heavy elements. In the vicinity of the Sun, the Galaxy winds spiral arms that are the site of ongoing star formation. Far from the galactic center the abundance of heavy elements may be too dilute to support the growth of planets. The explosions of stars and other processes push around the interstellar gas, creating pockets of dense gas where new stars can form, along with large volumes of more dilute gas. The shock waves from supernova explosions create the bath of cosmic ray particles that suffuses the Galaxy.

Life may have formed in a warm tidal pool or in the seas of Hadean Earth, when massive bolide impacts were the rule. Astronomy has taught us, however, that complex molecules had already formed in the interstellar medium. Scientists know that complex chemistry transpires in interstellar space: some through gas-phase chemistry, some through catalysis on the surface of grains of various composition, some in quiescent environments, and some in environments ablaze with the intense ultraviolet light of clusters of massive stars. The limits of that complex interstellar chemistry are not yet known. Nor do we know the relevance of these interstellar processes to the chemical environment on the surface of a planet or to the origin of life.

Astronomy has yielded a rapidly expanding knowledge of bodies that are possible hosts for life—not only the planets and moons of our solar system but also, possibly, further afield, as extrasolar planets are discovered orbiting other stars. How do all these planets come to be as they are? What is their geology, what are their tectonic regimes? The full range of environments that might harbor at least microbial life is yet to be explored.

The astronomical context of life includes a bath of photons of electromagnetic radiation and energetic particles that affect life. Optical photons are an important source of energy. The radiation from Earth’s central star provides the bulk of the free energy necessary for the maintenance of its biosphere, through the intricate mechanisms of photosynthesis. Ultraviolet (UV) and visible light are likely to have played a role on prebiotic Earth through photochemical reactions. The total luminosity from host stars defines the classic habitable zone, the region around a star where a planet will be at the right surface temperature



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