This DRAFT document is an excerpt from Principles of Planetary Biology, by Tom E. Morris. Chapter 5 Astronomical Circumstances Author’s note (March 6, 2009): This DRAFT document was originally prepared in 1998 and has not been fully updated or finalized. It is presented here in rough draft form. Despite its unpolished condition, some might find its contents useful. 5.1 Introduction The stars that you and I see twinkling in the night sky are visible to us because they are ferociously ‘burning’ The initiation of life and the progress of biological hydrogen in unimaginably powerful internal nuclear evolution are probably extremely rare things in the fusion reactions. The result of those reactions is the Universe. First you need a planet upon which life can brilliant dots of light that decorate the night sky. take root. Then, a star provides the warmth needed to Astrophysicists call this burning stage the ‘main keep planetary water from freezing. But some planets sequence’ stage of a star’s life. The main sequence stage can be so close to the star that their reserves of water is the longest of several possible stages in a star’s will evaporate from the heat. While other worlds will overall life. Stars go through the following stages during orbit at such great distances that their icy coating will their lifetimes (see fig. 1, Life stages of a star): never be melted into seas of life by a warming star. The combinations of circumstances that planets may find 5.2.1 Protostar themselves in are, well, astronomical. There is nothing This is a large and massive body of gas that has we can do about that. But we can find a reasonable way aggregated into a collapsing ball, driven by gravity. It is to cope with such complexity so we can find the dim although it may emit some infrared radiation late in answers that concern us about a planet’s prospect for its formation. As the collapse continues the internal life and evolution. That is the purpose of this chapter. pressure and temperature becomes high enough to start The ultimate value of this chapter is the presentation of nuclear fusion reactions. When they begin, the star the three following methods: enters the next stage of its life, the main sequence stage. 1. An integrated method for predicting best case scenarios for biological evolution on planets orbiting 5.2.2 Main Sequence Stage stars of different masses For those of us who don’t study stars, this is the stage 2. An integrated method for predicting the with which we are most familiar. Our sun and the stars historical profile of a planet in relation to its star’s we see at night are in the main sequence stage. During habitable zone the main sequence stage, the star’s interior is occupied by violent nuclear fusion reactions. The electromagnetic 3. A method for assessing a planet’s risk of impact energy (mostly infrared radiation, visible light, from comets and asteroids ultraviolet radiation) that results from these reactions makes its way to the star surface, through the star’s To a planet, the most influential outsider is its primary atmosphere and out into space. The result is a star—the star it orbits. The star provides the planet with luminous body that radiates energy in all directions. radiant light and heat. And its powerful gravity keeps This is the energy that can illuminate and warm the the planet from drifting off and getting lost in space. As star’s orbiting planets. The vigor of the internal part of this exposé, we must consider these stellar reactions and the availability of hydrogen fuel limit the properties if we are to consider planets as candidates duration of the main sequence stage. Big stars tend to for life. consume their fuel quickly and have short main sequence stages. Small stars burn their fuel slowly and 5.2 The life history of a star have long main sequence stages. Once the fuel is Planetary biology theory ultimately wants to be able to burned, the star enters into the next stage of its life. estimate the maximum amount of time available for biological evolution for planets under different astronomical circumstances. For example, how much time would be available for a planet orbiting around a large star compared to a planet orbiting around a small star? It turns out that there is a dramatic difference because of the properties of stars. Copyright© 1999 by Tom E. Morris. http://www.planetarybiology.com 58 Chapter 5 This DRAFT document is an excerpt from Principles of Planetary Biology, by Tom E. Morris. 5.2.3 Giant Branch Stars whose masses differ greatly from that of the sun may have much different fates. For example, small stars Once its hydrogen fuel is mostly consumed, a medium­ do not go through the giant stage but simply collapse sized star like our own will expand outward to become a into white dwarves after the main sequence. The death red giant. During this stage (which will happen to our throes of large stars are the most dramatic of all. After sun in about 5 billion years), our sun will expand large stars finish their giant branch, they explode in outward to engulf Mercury, Venus and possibly Earth. what is called a supernova. This explosion can utterly Compared to the main sequence stage, the giant branch destroy the star or, for really massive stars, the is short­lived. aftermath of the supernova can leave behind an 5.2.4 Aftermath extremely massive and dense neutron star or even a black hole. At the end of the red giant stage, a medium­sized star will eject its outer shell and then collapse for one final For a more detailed description of stellar evolution, time becoming a white dwarf. White dwarves are what consult Iben (1967) and Sackmann et al. (1993). remains of the star’s core and shell after it has concluded all of its nuclear burning. With no internal nuclear force to buoy the star outward or generate heat, the star’s mass collapses and begins to cool. Brown Dwarf < 0.08 M (too small for main sequence) White Dwarf 0.08 to 0.26 M Core collapses and begins to cool White Dwarf Planetary Nebula 0.26 to 1.5 M Protostar Star casts off its outer shell. Core collapses and begins to cool. Interstellar Nebular Cloud Hydrogen Cloud 1.5 to 5 M Star explodes and is utterly destroyed Nebular Cloud Neutron Star or Black Hole 5 to 30 M Star explodes and leaves behind a massive core Main Sequence Stage Giant Stage (star shines brightly) (short­lived) Explosion and Aftermath M represents the mass of a starmeasured in solar masses, where our sun has a solar mass of 1M . A star twice as massive as our sun would have a solar mass of 2M . Figure 1. Life histories of stars. Copyright© 1999 by Tom E. Morris. http://www.planetarybiology.com Astronomical Circumstances 59 This DRAFT document is an excerpt from Principles of Planetary Biology, by Tom E. Morris. based on their brightness (magnitude), temperature, receive each moment, so the warmer the planet will get radius, luminosity, mix of colors (spectral class), and (all other things being equal). This being the case, mass. For our purposes, we are going to use mass as around all stars there is a distant zone in space where our basis of stellar comparison. the star’s radiation levels would cause the surface of an Earth­like planet to be not too cold for liquid water, and The term, mass, is used by physicists to describe the not too hot for liquid water. On Earth, water exists in the property of an object that gives the object weight in a liquid state at temperatures between 0° and 100° C gravitational field. On Earth, objects of great mass also (32°­ 212° F) at sea level. If a water­rich, Earth­like have great weight, because gravity acts on mass. planet orbits a star within this zone, liquid surface Objects of low mass have low weight. But out in space, water would be possible (if we consider temperature as where there is little or no gravity, the same objects will the primary constraint). Since our unavoidable bias still have their original mass although they may be recognizes that liquid water is essential for life, worlds weightless. Just watch the space shuttle astronauts try that possess it would be the most habitable – especially to move a weightless satellite. It’s really hard because if we compare such moderate worlds with extremely the satellite is very massive compared to the astronaut. cold or hot counterparts such as icy Pluto or baking So, mass can be used as a convenient way to describe Mercury. the ‘bulk’ of an object anywhere. Huang (1959) first described this region in space as the The mass of a star (stellar mass) influences three main habitable zone. The location of the habitable zone circumstances that we are concerned about: 1) stellar depends on the magnitude of a star’s luminosity. For luminosity and the position of circumstellar habitable stars with low luminosities, habitable zones will be zones; 2) the time that the star will stay in its main closer to the star. Stars with great luminosities will have sequence stage; 3) the orbital period for a planet at any more distant habitable zones. given orbital radius. Anything that influences stellar luminosity also 5.3 Stellar Luminosity and Circumstellar Habitable Zones influences the habitable zone. Mass matters.