From: tommi@rieska.oulu.fi (Tommi Kaikkonen)
Subject: Usenet University vastaa: Using star catalogues on computer for a simulation
Organization: Taidonkaari 1 E 36, 90570 Oulu, FINLAND
Distribution: sfnet
Date: Wed, 6 Jan 1993 14:27:37 GMT

Heitin Usenet University konsulteille (astro(fysiikka)) saman kysymyksen
kun t{nnekin ja t{ss{p{ on vastaus jonka sain Glenn Spiczakilta Indianan
Yliopistosta.
--tommi

Date:    Tue, 5 Jan 93 16:18:40 PST
From: spiczak%pulsar%ind.hepnet@lbl.bitnet (Glenn Spiczak--Indiana University Astrophysics)
Subject: star catalogues and such
To: tommi@rieska.oulu.fi
X-St-Vmsmail-To: IND::LBL::"tommi@rieska.oulu.fi"
X-St-Vmsmail-Cc: 

Greetings,

   I am replying to the following mail message.  Sorry it has taken a while
for my reply, it is a busy time of year.


*************** text of your message *************************************

From:   IND::LBL::"tommi@rieska.oulu.fi" 23-DEC-1992 12:54:06.77
To:     IND::PULSAR::SPICZAK
CC:
Subj:   UU: Using star catalogues on computer for a simulation

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Date: Wed, 23 Dec 92 19:49:39 +0200
From: tommi@rieska.oulu.fi (Tommi Kaikkonen)
Message-Id: <9212231749.AA18940@rieska.oulu.fi>
To: joe@astro.as.utexas.edu
Cc: bbbehr@sunspot.sunspot.noao.edu, gawne@stsci.edu,
        jsmill01@ulkyvx.louisvill.edu, metzler@pablo.physics.lsa.umich.edu,
        thomas@astro.phys.uwec.edu, spiczak%pulsar%ind.hepnet@lbl.gov,
        burtt@lynch.nscl.msu.edu, kmarvel@nmsu.edu, bazan@astro.as.utexas.edu,
        tommi@phoenix.oulu.fi
Subject: UU: Using star catalogues on computer for a simulation

Hello, I am building a space flying simulator which is partly a game
for many people and partly an experiment of demonstrating the
star sky from different perspectives in the space.

My question are:
- where can I find the best and accurate on-line star databases?
- how can I calculate attributes of the stars not given directly in the db?

I am currently using nearstar.lis which has some 748 entries in the
form x-, y-, z- coordinates, number of stars in the system,
spectral type, luminosity class, name of the star.

My problem is that I don't know how to determine the diameter of a star
by its distance and luminosity class (should I need spectral type also?).

I have been getting only some weak pointers to litterature, but because
I am mere computer specialist I don't understand them so much.

If anybody of you could be so kind and mail me some sort of formula
or table how to calculate the diameter of star I would be the most grateful!

Tommi Kaikkonen, dept. of Electrical Engineering, 4th year student of
computer and information technology.

*************** my response *******************************************

   As expected, the answer to your question is not as straightforward
as it may seem at first glance.  In order to compute the stellar radius
you need to know not just it's absolute magnitude (deduced from knowledge
of the apparent magnitude and distance of the star), but you need to
know the spectral type of the star (Main Sequence, Giant, Supergiant,
White Dwarf, etc.).  These spectral type groupings of stars are the most
important because the underlying physics which is taking place within
the star are similar for all stars within the same type (for example,
all Main Sequence stars are burning Hydrogen within their cores to form
Helium, while Giants are burning Helium and heavier elements and White
Dwarfs have exhausted all of their fuel and are unable to ignite their
remaining elements).

   There is a nice table in the text titled "Astrophysics I:  Stars" by
Bowers and Deeming, Jones & Bartlett Publishing, Inc.  Boston
ISBN 0-86720-018-9 which lists characteristic parameters such as
mass, radius, luminosity, absolute visual magnitude, etc. for stars of
various spectral types.  This is table 3.1 in my edition (1986 I believe).

   This may be good enough for your purposes, and will at least allow you to
be in the ballpark with most of these parameters.

   An accurate calculation of a stellar radius requires a stellar model in
which the luminosity (energy generation rate) of the star is a constraint.
The spectral type tells which physical process are going on at the
stellar core and the spectral class tells the temperature at the stellar
surface.  Thus, the mass and radius of the star are deduced knowing that
they must be such that the correct luminosity and color are produced as
a result of the inward pull of gravity collapsing the star upon itself is
balanced by the outward push of energy generated from within the star.

   In general after completing a particular set of stellar models one can
come up with fairly straightforward relations between the mass and radius
of the stars of that type, or between the mass and luminosity of the stars
of that type.  Typically, the radius is proportional to some power of the
mass (where the mass and radius are in units relative to the mass and
radius of the sun) and the luminosity is also proportional to some power
of the mass (relative to solar values).  These powers are what changes
for stars of different types.  In addition, the composition of the star
is somewhat important and this appears in these relationships as the
mean molecular weight of the material raised to some power which again
depends on the type of star.  This relates to what is called the
metallicity of the star (stars born later have higher metallicities
because previous generations of stars have enriched the material out
of which newer stars form).

   For example, for a star beginning to consume Hydrogen on the Main Sequence
Bowers and Deeming show that for a particular set of assumptions you get
        R_star/R_sun = .451 u**.395 (M_star/M_sun)**.697,
and
        L_star/L_sun = 43.5 u**7.3  (M_star/M_sun)**5.18,
where u is the mean molecular weight, R is the stellar radius, M is the
mass , and L is the luminosity.  For completeness, the mean molecular
weight is defined by
        u = 1/(2 X + 3/4 Y + .5 (1-X-Y),
where X and Y are the hydrogen and helium mass fractions, respectfully
(which are approximately .75 and .25 in most instances).

In the text titled "An Introduction to the Study of Stellar Structure" by
S. Chandresekhar, Dover Publishing, Inc. New York, NY ISBN 486-60413-6
he shows that
        L is proportional to (M**5.5 / R**.5),
which gives a slightly different relation between M, R, and L than above,
so these powers are clearly not well known for a particular star without
a more detailed analysis based on the parameters of that individual star.

   It is my feeling that all of the information you need can be found
in one of the many Astronomical databases which are now available, so below
I have included the information contained in a couple mail messages
which I received regarding a similar question.  The Hubble Guide Star
Catalog may be useful and I imagine that the COMBINED catalog among the
Selected Astronomical Catalogs will be most beneficial.

   Please let me know if you need further information and I will be willing
to see if I can help.

   In case you need it, the relation between the distance of a star and
its apparent and absolute magnitudes is given by
        m - M = 5 log d - 5,
where m is the apparent magnitude, M is the absolute magnitude (defined as
the apparent magnitude which would be seen if the star were at a distance
of 10 parsecs) and d is the distance measured in parsecs (1 parsec =
3.26 light years).  The logarithm is base 10, because a difference of 5
magnitudes corresponds to an intensity ratio of 100, and the apparent
brightness falls off as the square of the distance from the source.
However, the magnitude is in practice only defined for a limited range of
frequencies, so a Bolometric Correction is often listed which must be
added to the visual magnitude in order to determine the bolometric
magnitude (the Bolometric Correction is always negative since there is
more energy in the whole spectrum than in a restricted part of it, recall
that brightness increases as the magnitude decreases).

                                                Glenn Spiczak

************* text of message regarding online database ******************

In article <z9dlxbk@lynx.unm.edu>, opus%pioneer.unm.edu@lynx.unm.edu
(Colby Kraybill) writes:
Newsgroups: sci.astro.hubble