SETI bioastro: Stars and their Habitable Zones 2

From: Larry Klaes (
Date: Fri Apr 28 2000 - 10:56:26 PDT

From: "Ellery, Alex" <>
To: "''" <>
Subject: RE: Star questions
Date: Fri, 28 Apr 2000 10:56:08 +0100
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Lower limit to F star (which hotter than G star such as the sun) around 1.2
solar masses. The sequence goes OBAFGKM in order of decreasing size,
increasing main sequence lifetime and decreasing surface temp. OBA are high
mass stars with radiative envelopes and are generally considered to be too
short-lived for complex life. F marks the start of the low mass type (I
think their upper limit is something like 1.8 solar masses if memory serves
- check these numbers as they're off the top of my head). M stars are
considered to have no habitable zone as habilable zone shrinks in width with
decreasing stellar surface temperature. That leaves F, G and K stars. Mid-F,
ie. F5 and up is considered potentially habitable (F0-F4 have something
wrong with them but can't remember what - I think they also have radiative
envelopes). Late F tend to have high X-ray and uv flux outputs though which
is potentially damaging to life. That leaves G and K. I think K drops off at
the bottom at 0.7 solar masses (check this though). The Salpter initial mass
function defines a power law for stellar size distributions, with increasing
populations at the low mass star end. A giant molecular cloud will form
several low mass stars until a high mass star forms which subsequently
disrupts giant molecular clouds preventing further star formation.

Metallicity is to do with Population (which defines age) rather than
spectral type.

Planets around other stars are generally not similar to our own but that may
be a selection effect. Not much known about that until further observational
data on non-solar terrestrial-type planets is obtained. But no reason to
think they should be different to our own - the laws of chemistry which
govern the condensation sequence from the accretion disk are universal. Hope
that helps.


> ----------
> From: Jayme Blaschke[SMTP:jblaschke@SWMAIL.SW.ORG]
> Reply To:
> Sent: 27 April 2000 20:17
> To:
> Subject: Star questions
> Two off the wall queries here:
> First, can someone give me a star that lies pretty much directly between
> Sol and our galaxy core?
> And second, any rough guess as to the lower limit of the size of F-type
> stars that could viably support life in a solar system? Would planets of
> these parent stars differ significantly from our solar system? Would they
> be metal poor, smaller, etc? (I know they'd basically be colder).
> Inquiring minds want to know.
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