There are five basic aspects of determining if a planet is a suitable candidate for life:
* Location Within the galaxy (for many reasons, half-way between the core and the edge seems ideal)
*Type of star (largely dictated by its mass)
*Planetary Orbit (obviously)
*Planetary Mass (can’t be too small or too large, for many reasons)
*Time (the planetary system can’t be too young or too old).
The last three listed characteristics will be discussed in Part 2. For now, we’ll look at why which kind of star matters in the search for life outside this solar system.
STELLAR CHARACTERISTICS . To fully grasp why certain stars are more suitable candidates for life than others, we first have to look at the nature of stars. Not all stars are the same. In fact, they vary enormously in many characteristics – some of which either increase or decrease the odds of life arising on a planet even if that planet is similar to Earth in all other respects. For now, let’s look at the relevant characteristics that influence a star’s suitability for hosting an ecosystem of similar complexity as Earth’s.
How Stars Shine. In a sentence, stars shine by compressing their own gases. At gas pressures experienced in stars, several things come into play. Firstly, the more compressed a gas gets, the hotter it gets. Secondly, the hotter an object, the faster its atoms or molecules travel. Thirdly, at a certain very high temperature, the heat will tear the atom’s electrons from its orbits, leaving only the naked nucleus of the atom*. Fourthly, without the electron cloud surrounding the nucleus, the atoms of the gas can collide with each other given high enough temperatures and pressures.** Fifthly, all the above means that the nuclei in the hot gases travel so fast that they overpower the proton’s tendency to repel each other, and in the meantime fuse to become a new element (e.g. hydrogen nuclei fusing to form helium nuclei)**. These collisions release tremendous amounts of heat and energy, which enables the sun to give off heat, light, and other forms of radiation; and therefore enable it to supply a planet with sufficient heat to support the ecosystem.
Given the above, it’s easy to see how larger stars, which have higher pressures in their core, fuse their hydrogen fuel much faster than smaller stars and therefore much hotter. However, as should be obvious by now, the old saying “All good things must come to an end”, applies to stars along with everything else.
A perfect rule-of-thumb for telling how massive, hot, and long-lived a star is/will be is simply to look at its color. Just as with iron, red stars are the less massive, cooler, and longer live ones; the blue stars are the most massive, hotter, and live shorter lives. The color order is as follows: Red, Orange, Yellow, White, Blue.
However, this color-to-characteristic sequence holds only for stars on the so-called Main Sequence, which 85% of all known stars are on. A Main Sequence star is one that fuses hydrogen nuclei into helium nuclei. This is important because the smaller the nucleus, the more heat and pressure required to fuse that nucleus into a larger nucleus (i.e. it takes less heat and pressure to fuse hydrogen into helium than it does helium into carbon; and still more energy to fuse carbon and other nuclei into neon or calcium or other heavier nuclei). Furthermore, the heavier the nuclei involved in the fusion, the less energy they release from the collision in proportion to the energy required to produce that collision. Therefore, because, hydrogen->helium fusion is the most common reaction taking place in the cores of stars. Incidentally, it’s also the most efficient form of fusion in terms of energy output to energy input of the fusion..
In addition to a star’s expected lifetime on the Main Sequence, it’s temperature (reflected in it’s color), the star’s mass also reflects its radiation output. Not surprisingly, the larger the star, the greater its radiation output.
So far, we have the following traits associated with a star’s mass:
Low mass: red color, low temperature, long lifetime, low radiation output (with exceptions)
High mass: blue color, high temperature, short lifetime, high radiation output.
Now, let’s look
In the next post, we’ll we get to the pros and cons of each kind of star, plus delve into a few other advantages and disadvantages of a small cool star versus a large hot one:
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NOTES
*Electrons repel each other when they come close together, similar to the way some magnets sometimes repel each other if you place them closely together . That’s why your hand doesn’t pass straight through a table. It’s because the electrons in the table and the electrons in your body are pushing each other away.).
**Protons repel each other to, if they come too close together, unless the gas is so dense that the combination of speed and pressure forces the nuclei of atoms together.
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