Life in the Universe part 1: How many suitable stars are born each year?

Our star, the Sun, is the most important thing with regards to supporting life on planet Earth. Without it we would have no light, no heat, no energy; life as we know it would never have been able to get started. So what makes a life-giving star, and how often do such stars crop up?

What makes a suitable star?
Not just any old star can make it as a life-giver, but to answer this question we need to consider what we mean by 'life'. To simplify things, we'll look at life that is likely to be similar to that which has arisen on our planet: scientists assume that most life would be based around similar elements and conditions as us. Many people have considered life in other forms living in very different conditions, but that's a post for another day! We'll also consider the development of life that is capable of reaching at least our level of development- because that's where things would get really interesting!

A star capable of nurturing life needs to fulfil the following conditions:
  • It must be a stable source of heat for long enough to allow life to develop.
If a star's heat output fluctuates too far and too fast, conditions will change too much and too quickly for life to take hold. If it doesn't have a stable heat output for long enough, any life that has arisen will be snuffed out before it has a chance to develop into intelligent civilisation. From the only example that we can study, life takes several billion years to develop to a level at which it is capable of extraterrestrial communication*. Really big stars use up their fuel much faster than smaller stars and so are less likely to provide the right conditions for life for long enough.
  • It must produce enough heat for life to develop.
Smaller stars produce less heat and so a planet would have to be closer to it before life could start to develop. If a planet gets too close to its parent star it is in greater danger of being bathed in levels of radiation that would work against the formation of life. Also, the closer a planet is to its parent star, the greater the risk of it becoming tidally locked (the same face facing the star all the time- like the Moon as it orbits the Earth). This wouldn't be good for the formation of life.

How often do suitable stars form?
Stars form in the Milky Way at a rate of about four per year. Stars of types known as F, G and K (the Sun is a type G star) are thought to be the best bet for parenting life-friendly solar systems, and comprise about one tenth of all stars. Such stars are therefore formed at a rate of about 0.4 per year.

Life in the Universe
This post is the first in a series about life in the universe, leading up to a discussion about the Drake Equation. Next up will be a post discussing the question "how many suitable stars have planets orbiting them?"

* We're only just reaching this point ourselves.


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