SETI bioastro: The Ages of Stars

From: Larry Klaes (
Date: Wed Apr 19 2000 - 14:52:35 PDT

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Date: Wed, 19 Apr 2000 14:32:21 -0700
From: Ron Ebert <>
Subject: Re: [ASTRO] The Ages of Stars
Reply-To: Ron Ebert <>

At 10:29 AM 04/18/2000 -0400, Amalendra Anandara wrote:

>Until the Hipparcos data (partially/) resolved the conflict, the ages of
>the oldest stars were estimated to be greater than the age of the
>Could someone tell me how the ages of distant stars are estimated? Can
>we only do so for stars that have left the main sequence? How do we know
>the mass of a star or the relative abundances of elements in it? Thank

We can only do reliable estimates of stars in globular clusters, or open
clusters if we're sure which stars are members of it and which are not.
They are plotted on a Hertzsprung-Russell diagram and we look at the stars
which have turned off the main sequence. We presume that all the stars in
the cluster formed at the same time. More massive stars are shorter lived
and move off the main sequence sooner than their neighbors. Generally
speaking, if most of the stars are on the main sequence then the cluster is
young, but if a good fraction have moved off then the cluster is old. We
can look at the particular turn off point - as we go from "late" spectral
types of small cooler stars to "earlier" spectral types of more massive
stars, we'll stop seeing stars on the main sequence and only find them on
the turn-off branches. This gives us the age of all the stars in the
cluster since we now know what spectral type ends the main sequence in that
cluster, and stellar evolution models tells us the age where the end
spectral type turns into a red giant.

Masses are estimated based on spectral type and absolute luminosity. If you
know the star's distance from other means, then the apparent magnitude will
give you the absolute magnitude. The relative abundance of elements can be
found by examining the star's spectrum. Larger abundances will give you
brighter spectral lines than smaller abundances.

Ron Ebert

The brightest flashes in the world of thought are incomplete until they
have been proved to have their counterparts in the world of fact.

- John Tyndall

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