Someone wrote a while back about Stars and how they affect life on their
planets. Here's my 0.02 800-pound chariot wheels on the subject...
Hotter stars emit more harmful radiation than cooler stars - this means
that life on planets orbiting Type F stars (and G0 and G1 V stars) would
either mutate more quickly (assuming their planet's ozone layer was about
as thick as ours), or there would have to be a thicker ozone layer over
the planet. Presumably, type K and M stars would emit much less UV
radiation (being redder), so ozone layers wouldn't have to be quite as thick.
Potted guide to planetary systems (a la Hard Science)...
The more massive the star, the more short-wavelength radiation it emits (ie.
UV - although there's not that many stars that are hot enough to emit
X-rays!). However, the more massive the star, the shorter it lives - it burns
up its fuel much faster. From what I can gather, a Type O0 V star will have
a total lifespan, from birth to death as a black hole, of less than 1 million
years!. On the other hand, the lowest mass stars (ie. M9 V's and VI's) will
stay as Type V's for well over 60,000 BILLION years. This is far greater than
the age of the Universe (about 15 billion years), which means that any Type M
stars born 15 billion years ago (at the start of star formation in the
Universe) would still be in their infancy.
The age and lifespan of the star makes a big difference to the formation of
planets around it. High mass stars (Type O and B) burn their fuel so rapidly
that they destroy themselves as Supernovae well before any planets even have
a chance to form in the dust clouds that surround them, so no planets will be
present around these stars (apart from the odd captured planet). Certainly,
no life can exist around these stars (if only due to the torrent of UV
radiation emanating from these stars) - no ozone layer could shield a planet
in this situation.
Type A stars are more moderate than their larger cousins. They can
exist for timescales of a few million years, while late Type A stars (A5 V +)
break through the one billion year barrier. These stars probably have planets
in tow, there being enough time for them to coalesce around their stars.
However, these worlds probably won't have any life. Life on Earth has taken
about 3.8 billion years to reach the level it has today - advanced multi-
cellular lifeforms first appeared about 700 million years ago at the
earliest. Assuming this is the case for all potentially life-bearing
worlds, the planet (and oceans) would have to exist for at least 3 billion
years to evolve multi-cellular life, while single-celled organisms and amino
acids could start to form around 800 million - 1 billion years after the
planet formed. So, the earliest Type A stars (ie. A0V to A5V) won't have life
(probably) while the later ones (say, A7V - A9V) will probably have simple
single-celled organisms, assuming oceans are present on the planet. However,
these organisms will not have time to evolve into higher organisms because
their Primary star will turn enter its Subgiant and Giant phases only a
billion years after its formation, killing off all life on the planet.
Type F and G stars are those most likely to possess highly evolved
life, having a long enough lifespan and emitting bearable amounts of radiation.
I think that F8 V stars are the first to have main Sequence Lifespans longer
than 4 billion years, which means that life may have evolved to current (Terran)
levels by the end of the star's useful lifetime. Type F stars emit more UV than
our own Sun, which is a Type G2 V star. The Sun has a total lifespan of about
10 billion years, and we're about halfway through. It has existed for 4.6
billion years or so.
Stars less massive than Sol (the Sun) have a longer lifespan - they
also emit less radiation. As a result of their longer lifespan, some examples
of these stars were born early in the history of the Universe. There are two
types of star - Population I and Population II. The earliest known stars
are Population I, and most of these are Type K and M V stars. These are
'first-generation' stars, and as such are composed entirely of Hydrogen
and Helium, with very little heavy elements. This is because they formed
before any massive stars had exploded as supernovae, releasing heavy elements
into the interstellar medium. (Actually, the more I think about it, the more
I think that Population I stars are actually Subdwarfs (type VI), since these
stars are also metal-poor). The net result is that if the stars are composed
of Hydrogen and Helium, then so too should their planets. As a result, all the
planets of these stars would probably be Gas Giants, with very few solid bodies
in the system, since no rock or metal is around for them to form.
Population II stars are those stars that formed in a metal-rich
intersellar environment, such as the one present today. These include all stars
that have formed in the past 10 - 12 billion years. Since low-mass stars can
form in this time period (not all had to form in the earliest days of the Unive se), then there are plently of Type K and M stars out there with rocky
planets around them.
Type K stars are probably the borderline for habitable planets. Type M
stars are just too dim to generate enough heat to warm planets at distances
beyond 0.2 AU. HOwever, it is interesting to note that during their Subgiant
phase (type IV), Type M stars spend a total of about 4 - 5 billion years. In
this stage, they can warm planets for long enough to allow life to form. THe
only problem is that no Type M star (not even one born 10 billion years ago)
will turn into a Subgiant for another 50 billion years!!!!
OK, that's it. Any comments, opinions etc are welcome. Hope it helped.
TTFN, Constantine