Stars vary in size, mass,
temperature and brightness (luminosity). Some stars could be
good suns for life-bearing worlds, some could not. Here is some
information to help you to choose your star.
Stars have life cycles. They
begin in clouds of stellar dust and gas, accumulating mass by
gravitational attraction. As the dust and gas contract, energy
is released, and the mass heats up and begins to glow. Very massive
protostars contract rapidly, perhaps taking only ten thousand
years to begin the next phase. Smaller stars, like our Sol, may
take tens of millions of years to reach it.
In the center of the protostar,
the heat increases until nuclear fusion begins. The star is now
generating its own heat energy, fusing the nuclei of atoms together
to make helium from hydrogen. Carbon, and even iron, can be formed
inside very hot stars. As these changes take place, energy is
released as heat and light.
Most stars fit a predictable
pattern. Once they have started to burn their nuclear fuel, they
can be assigned a place on The Main Sequence Diagram (The Hertzsprung-Russell
Diagram). These stars have predictable lifetimes and are consistent
in their energy generation.
As you can see, there is a sort
of diagonal line of stars going from right to left. These are
main sequence stars. About nine out of every ten stars are on
the main sequence. On the lower right you see the red dwarfs
-- small, dim stars with long lifetimes. Most stars have less
mass than our sun. Close to the center we see our sun, and other
stars like it: their lifetimes are about ten billion years. In
the upper left we see big, bright, hot stars: these burn up quickly,
and there is not enough time for life to develop on their planets.
Finally, the star burns up most
of the hydrogen in its core and begins to die. Our own sun will
grow hotter (due to gravitational forces) and expand almost to
the orbit of the earth, destroying any life that may be on the
planet then. The sun will lose perhaps half its mass, and grow
smaller and fainter as it cools.
Life forms adapt to their environments,
and so must have a dependable environment to adapt to. Main sequence
stars are stable as they live out the mature period of their
lives. Stars are grouped by temperatures. Going from hottest
to coolest, the groups are called O, B, A, F, G, K, and M.
Each group is divided into sub-categories with 0 the highest
and 9 the lowest. The numbers below are really a
the stars in each category are not all
exactly the same size.
||Temperature in Kelvins
Life Span in Years
|above 30,000 K
||Very, very hot stars that burn up fast
- 30,000 K
||Not enough time for life
- 10,000 K
||Not enough time for life.
- 7,500 K
||Life could get started here.
- 6,000 K
||Our sun is a G2 star.
- 5,000 K
||Lots of time, not much heat.
|Below 3.500 K
||Lots of time, not much heat
More than 100 Billion years
As you can see, our life bearing
planets probably orbit G or K type stars. These stars have long
enough lives to permit the development of life on their planets,
provided that the planets are in the life
zone. The life zone is that region where a planet orbiting
the star will experience temperatures that permit liquid water
to exist. It is possible that life might be able to evolve with
some other compound than water, but we must leave that possibility
to those who have a background in organic chemistry.
1998, 2003. Elizabeth
Anne Viau. All rights reserved. This material may be used by
individuals for instructional purposes but not sold. Please inform
the author if you use it at