K-type stars are the overlooked siblings of the stellar world, but they could offer the best chances to find life, so here's a little about them.

How Are Stars Categorized?

Ancient astronomers noticed that not all stars are the same color. It even influenced what they called some – Antares' name is a reference to its red color resembling that of Mars, which the Greeks called Aries. Intriguingly some classical astronomers described possibly the most famous red star as being yellow, but to the naked eye most stars look white, so there wasn't a lot of categorization to do.

With telescopes we could identify more subtle colors, and spectroscopes gave us more information about stellar differences than the eye alone could manage. A star's color indicates the temperature of its outer layers, and astronomers use this to categorize them.

Most of the stars we see are part of what is known as the “Main Sequence” and these were put into seven types, with the hottest having temperatures over 30,000° C (54,000° F), while the coolest are between 2,100 and 3,400° C (3,812-6,152° F). You might expect these types to start at A, and therefore never even get to K, but the lettering is much more confusing, being O, B, A, F, G, K, M, for historical reasons.

Some stars are off the main sequence, such as white dwarfs and red giants, and don’t fit into this classification system, but these are a topic for another day.

Characteristics Of K-type Stars

K-type stars have temperatures between 3,400° and 4,900° (6,152-8852° F). By comparison, the Sun’s temperature is 5,500° C, placing it in the middle of the G-type stars. K-type stars have radii 10-30 percent smaller than the Sun and are about 60-90 percent as massive.

Even though K-type stars are mostly made of hydrogen, the distinctive hydrogen spectral lines, which are dominant in A-type stars are very weak, and can be hard to see compared to the spectral lines of rarer elements. Based on spectral lines alone, you have to look closely to spot the difference between a K and G-type star. However, in a telescope powerful enough to show the brightness reasonably well, the orange color is often reasonably noticeable compared to the familiar yellow. 

Why Are K-type Stars Important For The Search For Life?

We know that G-type stars can support life around them, and even intelligent life, but the prospects don’t look so good for most other categories. The hotter and more massive a star is, the shorter its lifespan. It’s almost certain that O and B-type stars don’t live long enough for life to develop in their vicinity. Any planets would still be bombarded with enough force to turn their surfaces to magma when the star dies, which stars this massive do in spectacular fashion. A and F stars live longer, and life might develop on the planets they warm, but it’s unlikely conditions would stay stable long enough for complex multi-cellular life to form, let alone anything we could have a conversation with.

At the other end of the spectrum, M-type stars (better known as red dwarfs) live very long lives, so that certainly life will not lack for time. However, debate continues whether their planets can be habitable (at least at the surface) because they are so cool any planet warm enough to have liquid water must orbit very close to the star. M-dwarfs produce very strong flares, and there are fears these would strip nearby planets’ atmospheres and make life impossible. 

Whether this is universal, or if some red dwarfs could have warm planets that escape such fates, remains debated, but many astronomers fear that we won’t find advanced life around M-type stars, if we find life at all. 

In that case, the field would be reduced to G and K-type stars, and K-type stars are more common. If so, unless there is some obstacle to advanced life around K-type stars we have not noticed, the majority of stars in the galaxy that could host complex life belong to this category. Then again, K-type stars do emit a lot of X-rays and UV light early in their evolution, which probably means that they take a long time to be suited to having life nearby.

Why Have K-type Stars Been Overlooked?

If this is the case, you might wonder, why do we almost never hear about K-type stars?

For one thing, being small and cool, they are also faint. A and B type stars are so luminous they make up most of those we can see with the naked eye. O-type stars are exceptionally rare, but their influence on everything around them is so great it would be hard not to notice the few the galaxy has. And of course, humans are biased in favor of G-type stars since we have such a great opportunity to study the one we orbit.

Meanwhile, M-type stars are very common, making up 70 percent of the stars in the galaxy. If you leave out non-main sequence stars like Sirius B almost forty of the nearest 50 stars are M-type, including Proxima Centauri, so they get plenty of attention as well.

That’s not to say K-types never draw attention. In June, the star HD 48948 was found to have a planet that may have the best prospects for an Earth-like climate we’ve yet found. You can guess it’s type. 

Probably the most famous K-type star, is Epsilon Eridani. At 10.5 light years away, this was once considered the nearest star that might have Earth-like planets, until we realized it is less than a billion years old, and any life there would be very basic. There’s a closer K-type star, however. Alpha Centauri B, the equal second closest star beyond the Sun is K-type, but locked in an orbit with a brighter G-type star, and needing a telescope to separate them, this is a star that truly knows what it means to be overlooked.