Main Sequence Star

**Main Sequence Stars** are defined as stellar objects that have ongoing hydrogen fusion at the star’s core, and have a stable balance of outward pressure due to the fusion process and inward gravitational pressure. They are very populous (around 70% of the Milky Way’s stars are in their main sequence).

Main Sequence stars are classified into 7 spectral classes: O, B, A, F, G, K and M.  Each class is further subdivided into 0 – 9, where 0 is the hottest stars of that class and 9 are the coolest. The Sun, for example, is classified as G2. Fractions, such as A9.5 or K3.7, are also used. Stars with low numbers in their class are known as early, and stars with high numbers in their class are known as late.

## Physical Properties

Each stellar class has physical parameters, which are summarized in the following table:

Given the mass of the star, we can work out many other properties.

The radius of an O, B, A or F star can be calculated using the following formula:

The radius of a G, K or M star can be calculated using the following formula:

From the above, the circumference (C), surface area (A), volume (V) and density (ρ) can be calculated, using the following formulae:

The luminosity (the amount of energy the star releases) is also related to the mass of the star, via the following relationship:

The amount of time the star spends on the main sequence (its “main sequence lifespan”) is determined by the following formula:

The effective temperature (surface temperature) of a star is given by:

## Habitability

    There are several factors to take into consideration when considering habitability of any particular main sequence star. Here are some of those considerations:

* **Stellar Lifetime**: Any planet needs enough time for life to evolve on it. This means that O, B and A stars, which spend less than 1 billion years on the main sequence, are poor candidates for life-bearing worlds.
* **Radiation**: The star needs to emit enough UV radiation to trigger atmospheric dynamics such as an ozone layer, but not enough that the radiation kills off any potential life.
* **Tidal Locking**: The habitable zone of the star must be far enough away from the star that planets are not tidally locked to the star, which would inhibit life.
* **Metallicity**: Research suggest only mid-high metallicity stars can host planets. Therefore, the star must be reasonably metal-rich.

Taking these into consideration, the best candidates for life as we know it are K, G and F stars, with K stars being the most likely to house life-bearing worlds.

## Worldbuilding in Practice

> Agamemnon is a bright, white, metal-rich F class star in the Tyson cluster and is one of the most recently colonized systems in the Armstrong Sector. It is about 1.38 M☉ and is roughly 17% larger than the Sun. Despite this, it is just over three times as bright and is a sweltering 7066 K on its surface. Despite the age of its colony, it is one of the most thriving systems in human space, and boasts not only a “shirt-sleeves” habitable planet, but also a number of easily accessible giant planets and mineral-rich asteroids.
>
>

> Not all systems are as fortunate as Agamemnon, however. The red dwarf Eitri is one such example. It is a tiny M class star of about 0.247 M☉ and is about a third of the size of the Sun, and only a mere 0.07% as luminous! With only a handful of terrestrial planets and a complete lack of giant planets, it barely features on star maps, and is only a “stepping stone system” to the blue giant Hrimnir.
>
>

> OKE4-19a exists at the other end of the spectrum from Eitri. A massive, bright blue O class star, it weighs a whopping 43.8 M☉ and has an enormous radius of 6.6 R☉. It is 556,108 times as luminous as the Sun and is over 10 times as hot. Evidence suggests it is surrounded by many planets, but governments have deemed it “unprofitable to colonize”, as the combined cons of a high radiation and potentially very long travel times (not to mention a possible supernova risk from its white dwarf companion) have outweighed the possible pros of such an extensive planetary system. Nevertheless, it is a source of avid study by astronomers in the nearby Tyson cluster.
>
>

## References

1. ↑ Wikipedia: Main Sequence
2. ↑ Crash Course Astronomy: Stars
3. ↑ Crash Course Astronomy: The Sun
4. ↑ Crash Course Astronomy: Low Mass Stars
5. ↑ Crash Course Astronomy: High Mass Stars
6. ↑ Wikipedia: Nuclear Fusion
7. ↑ Artifexian: How to Build 1000 Stars