Making a planet habitable for humanoids: The star

Question

The question:

What characteristics are necessary for a planet to be habitable for humans? What should the generic star and planet be like?

The life forms are human, so they

• Need to have access to water (they can melt snow or ice in their mouths, given sufficient surplus metabolic energy).
• Breathe some form of air containing the right amount of oxygen (and not too much carbon dioxide) at the correct pressure (below the death-zone).
• Live in a place with a temperature range similar to that on Earth. They can live in any climate zones, from the tundra to the tropics.
• Live exclusively on the ground, as human ancestors began to do millions of years ago.
• Eat natural foods similar to the ones humans eat - vegetables, fruits, meat, etc.
• Need to live in a natural environment, i.e. not something constructed by another species, such as a space station or protective dome. They should be able to live without protective gear that they can't construct with simple tools (parka, igloo, etc are fine - oxygen concentrators are not)
• Need to have evolved on the planet and not simply left there, as with colonization.

This question is designed to cover the characteristics the star must have to support life on the planet, in order to make the answer shorter, easier to browse through, and less confusing.

Related: Making a planet habitable for humanoids: The planet

---

This is the result of the meta question http://meta.worldbuilding.stackexchange.com/questions/1750/should-there-be-a-canonical-habitability-question In that, I put forward arguments for a canonical question and answer addressing general aspects of the habitability of a planet in order to avoid rehashing the same points over and over in answers to specific questions. This is that canonical question and answer.

The answer will be community wiki, so anyone can edit it and add to it. I'd like to make it organized, though, so please adhere to some basic guidelines to make it neat:

• Use Bold to denote the title of a subsection, and large (#Large) text to denote the title of a major section (e.g. Planet and Star). Formatting examples are given in the answer.
• Use $\LaTeX$ for mathematics.
• Add in links to sources such as Wikipedia and NASA using either in-text links ('[Site name] (Site URL)') or footer links ('[Site name][#]' with '[#]:Site URL' at the bottom). Use '![Description] (Image URL)' for images, though make sure that the image is available for use. Wikipedia images are always usable.
• Resolve any disputes over accuracy in chat and not in an edit war.
• Cite your sources and be accurate! Papers and pre-prints are always nice (see for example arXiv), though Wikipedia should also be okay.
• Don't create separate answers - add to the one answer already provided.

Answer 1

• Age: The time a star spends on the main sequence is roughly inversely proportional to the luminosity, as given by the formula

  $$T \approx \ 10^{10} \text{years} \cdot \left[ \frac{M}{M_{\bigodot}} \right] \cdot \left[ \frac{L_{\bigodot}}{L} \right] =10^{10} \text{ years} \times \left[\frac{M}{M_{\odot}} \right]^{-2.5}$$

  where $M$ and $L$ are the mass and luminosity of the star, respectively, $M_{\bigodot}$ is a solar mass, $L_{\bigodot}$ is the solar luminosity and $T$ is the star's estimated main sequence lifetime See also Age in the section on the planet itself.

• Classification: Stars are generally classified according to their spectral type. 8 spectral classes are generally used: O, B, A, F, G, K, M, L, and T. O stars are the hottest, while T stars are the coolest. Here is an image of sample spectral lines:

  

  Star types are often illustrated in a Hertzsprung-Russell (H-R) diagram, as shown here:

  

  Some types of stars are better suited for supporting life than others. Here's a basic rundown:

  • O-type: These stars are the hottest of the bunch, with surface temperatures of 30,000-50,000 K. Most are very short-lived (with the exception of white dwarfs, which are stellar remnants). This, as discussed in Age, is not conducive to life. O-type stars often end their lives as supernovae, living only about 4 million years. They're very large and massive - up to a couple hundred times the mass of the Sun - and hot, which makes the habitable-zone/liquid-water zone more difficult. O-type stars are not conducive to life.
  • B-type: These stars are cooler than O-type stars (10,000-30,000 K) and much less massive, no more than 20 times the mass of the Sun. They live longer, but not for much more than O-type stars. They, too, die as supernovae. They're not too conducive to life.
  • A-type: These stars are only 7,000-10,000 K and on the order of 1.5-2 solar masses. They live for only a short while, but still much longer than more massive stars - a few hundred million years. Unfortunately, this still isn't long enough for life to form, and they're still too hot for life to have a good chance. Far out from the star, though, planets can be habitable.
  • F-type: These stars are about 6,000-7,000 K and on the order of 1-1.5 solar masses. They live for much longer than more massive stars, and can easily form planets. F-type stars aren't as hostile to life as O-type or B-type stars, and they shouldn't pose any problems for potentially habitable planets.
  • G-type: G-type stars are typically regarded as being the most conducive to life. In fact, our Sun is a G2 star. G-2 stars may live for about 10 billion years - plenty of time for life to develop - and are between 5,000-6,000 K, as well as 0.8-1.2 solar masses. They die gently, as planetary nebulae.
  • K-type: These stars are smaller and cooler than the Sun. They range from 4,000-5,000 K on the surface, and live for 15-30 billion years. This means that life on planets around K-type stars has plenty of time to develop. They are stable and very common.
  • M-type: M-type stars are typically either red dwarfs or red giants. They are the most common type of stars. Red dwarfs are cool - less than 4,000 K on the surface - and long-lived. Some may live for trillions of years. This can be good for life, but red dwarfs are also dim, and may emit powerful flares. Red giants are stars in a certain stage of their life where they are near death (Our Sun will become a red giant one day). They are large, and may envelope planets orbiting them.

  To summarize: Massive, short-lived, luminous and hot stars are not good. Smaller, cooler, and longer-lived stars are better for life - and, fortunately, much more plentiful. With hotter types being too short lived and open questions existing about the habitability of the coolest types, search for habitable planets is currently focused on spectral classes F, G, and K.

• Stellar Variability: Some stars change in luminosity over time, creating periodic increases and decreases in brightness. These stars are called variable stars. The changes can be due to pulsations, eruptions, or even a companion in a binary system.

• Stellar Metallicity: The metallicity of a star refers to the fraction of it composition that is not hydrogen or helium. In most stars in the current era of the universe, this is quite low.

  The metallicity $Z$ is calculated by $$Z=[\text{Fe/H}]= \log_{10} \left(\frac{N_{\text{Fe}}}{N_{\text{H}}} \right)_{\text{star}} - \log_{10} \left(\frac{N_{\text{Fe}}}{N_{\text{H}}} \right)_{\text{Sun}}$$

  Stars can be grouped according to their metallicity into populations. Population I stars have high metallicity and are young, Population II stars have moderate metallicity and are older, and Population III stars have very low metallicity and are largely hypothetical because they lived and died early in the universe's life

  Metallicity is not a cause of habitability, but it does indicate which stars could and could not have planets supporting life.