Our Average Star

The Sun is an average star. It is a main sequence star, classified as G2V.

From Cornell University's Department of Astronomy

We should start by defining what we mean by Main Sequence.  In the above diagram, called the Hertzsprung-Russell diagram or H-R diagram, you can see that most of the stars that have been classified lay on the band starting in the upper left to the lower right.  The stars in the upper left are hotter, bigger, more luminous but generally younger than those in the lower right. Despite these differences, the stars and all the stars in between on this bad are similar. All these stars are fusing hydrogen in their cores; via the proton-proton chain or the C-N-O cycle*.

*The C-N-O cycle is just another way of converting four hydrogen nuclei into one helium nucleus, using carbon (C), nitrogen (N), and oxygen (O) as catalysts. This cycle is generally found in hotter stars. We will not go into detail here about the C-N-O cycle. If you wish to learn more, leave a comment below.

Generally, the hotter and bigger a main-sequence star (also called dwarf stars, not be confused with white dwarfs), the less time it spends on the main sequence. From the above diagram, the hot stars are classified as O and B, medium stars are A, F, and G, and cooler stars are K, M, L, T, and Y.  The letters were originally meant to represent the strength of their hydrogen lines with A having the strongest and O stars the weakest (L, T, and Y are relatively new classes). Now, the spectral types are divided into temperatures classes. Spectral types can be broken done further in each type. For example, the hottest A stars will be classified A0 and the coolest A9.

Why do hotter stars spend less time on the main sequence than the cooler stars? They do have more mass, therefore more hydrogen to fuse into helium. But, because of their massive size, they must fuse hydrogen much faster to overcome the stars propensity to collapse under its own weight*.

*This process of preventing a star from collapsing under its own gravity is called hydrostatic equilibrium. The force from the energy created in the core equals the force of gravity and keeps the star from completely collapsing. However, in some stars, gravity can overcome the force of photons pushing out. But that is for a different post.

G stars, then, are average. They have hydrogen lines that are medium in strength, stronger than K, M and O stars, but weaker than A, B, and F stars. G stars are noted for having strong Ca-II lines (doubly-ionized Calcium, Ca 2+). They fall in a surface temperature range of 5200 to 6000 K (our Sun is about 5800 K) and are yellowish in color.

Stars are also classified by luminosity. Class 0 stars are hypergiants, Class I stars are supergiants, Class II are bright giants, Class III are giants, Class IV are subgiants, Class V are main sequence or dwarfs, Class VI are subdwarfs, and Class D are white dwarfs (not the same as main sequence)*.

*White dwarfs are the final stage in the life of a lower mass main sequence star. Our Sun will end up as a white dwarf once it begins to evolve off the main sequence in about 4.5 billion years.

So our Sun is average. It is a G2V star, similar to 7.5% of all main sequence stars in the solar neighborhood (i.e. stars within 250 light-years of the Sun). But why is it weird?

1. We have planets in our solar system. Well, so do other stars. But up until the mid-1990s, we thought that we were the only planetary system.
2. The lower mass planets are closer to the Sun than the higher mass stars. From everything we’ve seen, it appears that higher mass planets are closer to their parent star compared to our planetary system. This may be a selection bias as it is easier to find larger planets closer to their parent stars than those farther away or lower mass planets close to their parent star. As technology gets better, this bias may go away.
3. In one word: YOU. You exist and as far as we know, Earth is the only celestial body that has life of any kind. The one thing we are doing now is looking for planets that may have liquid water*.

*Water is important because it is the only substance in the universe that is actually less dense as a solid than as a liquid. At 4°C, water is at its densest. If you take a glass of water and put in ice cubes, the ice floats. This is common knowledge. But scientifically, this makes no sense. Solids are denser than their liquid counterparts, and gases are the least dense. Why is this important? We know that life began in water, and if water followed the rules of phase changes, as the surface water froze, it would sink to the bottom, until the entire body of water was frozen solid. Nothing could survive in that, and life could not evolve.

We could find life around other stars with better technology, but for now, we are the only planet with life.

There are things we do know about planets around other stars.

1. Planets around O, B and in a small way, A main sequence stars, probably do not have life or at least intelligent life because those stars do not spend a lot of time on the main sequence (generally less than a billion years) which would not give life a chance to evolve.
2. Planets around M, L, T, and Y main sequence stars probably do not have intelligent life because in order to have life, liquid water is a must. Planets around these stars need to be close to the star to be warm enough, must be large enough to have a magnetic field, and probably much larger sibling planets to deflect comets or asteroids that could potentially prevent any sort of large life forms from evolving.
1. It must have the right surface temperature to allow liquid water to flow on the surface.
2. It must be large enough to have a liquid molten core to create a strong enough magnetic field to prevent radiation from its parent star from reaching the surface.
3. If there was enough material in the stellar nebula, the star probably would have been bigger.

 Next time, we will get into the planets in our solar system itself, starting with Mercury, our smallest planet.