If you get away from city lights and witness the astonishing beauty of the night sky, most of the stars you can see are part of what is known as the “main sequence”. The few that aren’t part of it get attention out of proportion to their numbers, but sometimes we should step back and consider the stars that make up most of the galaxy, our Sun included.
Previous explainers have answered the question of what K-type and B-type stars are, in both cases also looking at why they matter. Both of these are subclasses of the broader category, main sequence stars.
The Lifecycle Of A Star
The galaxy was once nothing but a vast cloud of hydrogen, a little helium and lithium, and almost certainly lots of dark matter. Where the gas was densest, gravity drew in more material, and caused that density to increase in an initially self-sustaining process.
Eventually, the density reached a point where something happened that would take our breath away if we were not so familiar with its outcome: fusion ignited. So much gas became concentrated in a small area that the gravitational force became large enough to overcome the electrostatic repulsion between protons, and there was light.
The same process occurred at millions of locations across the galaxy, sparking the first stars.
For most of its life, the star mainly does one thing – turn hydrogen into helium.
Each helium atom has slightly less mass than the four hydrogen atoms that went into making it. Some of that mass deficit becomes particles like neutrinos, but most of it is turned into energy. By the formula E=mc2, a tiny amount of mass becomes quite a lot of energy when multiplied by the speed of light squared.
We see some of that energy when we look into the night sky, and feel some of it in daylight. The energy also provides an outward force that counteracts gravity, preventing the star from collapsing further.
As long as this is the primary thing a star is doing, it’s considered to be on the main sequence.
As a star runs out of hydrogen, things start to change. First, the core runs out of hydrogen, but fusion continues in a shell around it. Then stars start fusing helium into beryllium and carbon, which then mostly get converted to heavier elements – but around this time processes get more complex, and stars’ paths diverge. At this point, they cease to be considered part of the main sequence.
Identifying The Main Sequence
When astronomers first started trying to make sense of the differences between the stars they could see, they didn’t know any of this.
Instead, they observed that stars had a wide range of temperatures, indicated by their color, which is caused by the wavelength at which they emit the most light.
To make sense of this, two astronomers – Ejnar Hertzsprung and Henry Norris Russell – independently made a chart of stars’ color versus their brightness. Now known as the Hertzsprung-Russell diagram, this shows most stars sticking close to a long, somewhat wobbly, line running from top-left to bottom-right. A minority of stars either veer off towards the top right (giant phase) or sit far away at the bottom left (white dwarfs, considered dead stars).
There are many versions of the Herztprung-Russell diagram, but few are this pretty, as well as accurate and informative.
The line became known as the main sequence.
Competing groups of astronomers came up with different ways to categorize main sequence stars primarily by temperature. The one that became widely adopted, illogical as it is, uses the letters O, B, A, F, G, K, M, starting with the hottest and ending with the coolest.
A pattern became apparent: For most stars, there was a relationship between the temperature and the mass. Larger mass stars are able to fuse their material far more rapidly, releasing vastly more energy per second than their smaller counterparts. A star’s lifetime goes according to a formula of L ∝m-2.5 where L is lifetime and m is mass. The formula is not exact, varying a little with initial chemical composition.
For most of its lifecycle, you can get a pretty good estimate of a star’s mass from its color.
This doesn’t work, however, for stars at the very beginning or end of their lives. Stars do not jump straight to their peak brightness, starting off relatively faint and slowly brightening, so a very young but very massive star can still be quite cool, by the standards of stars, at the beginning.
Things get even more complex at the end, and stars can take many paths to their deaths. The most famous is to puff up to become a red giant. Based on color alone, red giants or supergiants look like M-type stars, also known as red dwarfs – although their temperature is the same, they have millions of times the volume, and the total energy released is therefore much greater. That’s why we can see Betelgeuse with the naked eye, but not Proxima Centauri, despite being more than a hundred times further away.
It would be silly to put such different stars together in the same class.
Consequently, the most important division in classifying stars is between the main sequence and the rest. In the first case, hydrogen fusion in the core is the dominant process, and temperature and mass relate (with some variation by age and chemical composition), while things are very different for the rest.
Where Does The Sun Fit In?
The Sun is very much a main sequence star, in the G-type category (G2V to be precise). It’s been fusing hydrogen for 4.6 billion years, but it still has almost three times as much as it has helium.
Nevertheless, it’s thought the Sun is almost halfway through its “life”, where its future state as a white dwarf does not count. Only about a billion years of that will be in the red giant phase.
The sequence is “main” because it is where stars spend the bulk of their lives.
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