Help with star catalogs requested

This might belong in alt.astronomy, but glancing in there, I hope not.

I'm looking for a set of information about visible stars that would be
useful in a work of fiction, and I'm finding it slow going. I can find
resources (like simbad) that can tell me that alpha Orionius is
Spectral type M2Iab. What I really want to know is what color it would
appear if you were on a hypothetical planet in the star's liquid-water
zone, how long such a planet's year would be, if it's a multiple star
system and how the other system members would appear to the naked eye,
if the star pumps out anything that would make it especially nasty to
live near, the metals content of the star relative to the Sun... that
kind of stuff. Picture the details you'd want to see gotten right in
reasonably hard sci fi, and you get the idea.

I'm happy to derive my facts and I can write software to do any
required calculations, but I haven't got a clue how to get from "type
M2" to "red-orange, and a planet better be at least 10 au out if you
don't want metals to boil on the surface."

Pointers to more appropriate newsgroups welcome.

"ScottM" wrote in message
oups.com...
This might belong in alt.astronomy, but glancing in there, I hope not.

I'm looking for a set of information about visible stars that would be
useful in a work of fiction, and I'm finding it slow going. I can find
resources (like simbad) that can tell me that alpha Orionius is
Spectral type M2Iab. What I really want to know is what color it would
appear if you were on a hypothetical planet in the star's liquid-water
zone, how long such a planet's year would be, if it's a multiple star
system and how the other system members would appear to the naked eye,
if the star pumps out anything that would make it especially nasty to
live near, the metals content of the star relative to the Sun... that
kind of stuff. Picture the details you'd want to see gotten right in
reasonably hard sci fi, and you get the idea.

I'm happy to derive my facts and I can write software to do any
required calculations, but I haven't got a clue how to get from "type
M2" to "red-orange, and a planet better be at least 10 au out if you
don't want metals to boil on the surface."

Pointers to more appropriate newsgroups welcome.


I can't help you that much, but here goes:

* Assuming the atmosphere is transparent, the colour you see on the planet
is the same colour as we see from Earth.

* You can work out how far the planet will be from its Sun if you know the
star's absolute magnitude. Each change of 1 in absolute magnitude is an
increase of a factor of 2.5 in light energy output. The amount hitting the
planet varies as the square of the distance, so being 1.0 greater absolute
magnitude than our Sun means the planet needs to be about 1.58 (sqrt(1*2.5))
times as far away. If its 2.0 greater, then it needs to be 2.23 times
further out (sqrt(2*2.5)), etc. Unfortunately, the period of the year will
also depend upon the mass of the star, which is mostly pretty speculative
(we can measure how bright a star is, and how far away, but until we find a
planet we cannot estimate mass). However, if you can find tables of the mass
of different stars, I can tell you the fomulas that you would need.

* be careful of multiple star systems. Not all potential systems allow
stable orbits for planets. You are probably OK if the planet closely orbits
one star, or is a Lagrangian system with two others. Generally speaking, you
can't have a planet in a stable orbit around two or more close binaries -
they get perturbed into chaotic orbits and eventually flung out of the
system.

* Assuming the atmosphere is transparent, the colour you see on the planet
is the same colour as we see from Earth.

This helps. Is there any simple mapping from spectral type to observed
color? I remember in school (this was, um, a while ago) that G's were
yellow and M's were red but I suspect some broad simpifications were
going on.

* You can work out how far the planet will be from its Sun if you know the
star's absolute magnitude. Each change of 1 in absolute magnitude is an
increase of a factor of 2.5 in light energy output. The amount hitting the
planet varies as the square of the distance, so being 1.0 greater absolute
magnitude than our Sun means the planet needs to be about 1.58 (sqrt(1*2.5))
times as far away. If its 2.0 greater, then it needs to be 2.23 times
further out (sqrt(2*2.5)), etc. Unfortunately, the period of the year will
also depend upon the mass of the star, which is mostly pretty speculative
(we can measure how bright a star is, and how far away, but until we find a
planet we cannot estimate mass). However, if you can find tables of the mass
of different stars, I can tell you the fomulas that you would need.

One of the pleasant aspects of fiction is that if no one knows
something for certain, the author can't be presumed wrong. I'll happily
take the formulas.

* be careful of multiple star systems. Not all potential systems allow
stable orbits for planets. You are probably OK if the planet closely orbits
one star, or is a Lagrangian system with two others. Generally speaking, you
can't have a planet in a stable orbit around two or more close binaries -
they get perturbed into chaotic orbits and eventually flung out of the
system.

Interesting - given that doubles seem to be common, that would argue
for planets either being closely bound to one (and presumably baking),
or stuck at L4 or L5. Given two large solar masses, can a smallish
planet at L3 have a stable orbit, or is it hopeless?

If a planetary mass is in an extremely large orbit (much further out
than L3) around two close binaries, is it still unstable? I think of
Pluto, which trundles around happily despite the Sun's and Jupiter's
mass - I wonder if that kind of thing can "scale up".

Pointers to moderately math-rich sites welcome. The story's not
intended as hard science fiction, but I'd very much like to avoid
"making the Kessel run in 12 parsecs" syndrome.

"ScottM" wrote in news:1133063494.173458.267760
@z14g2000cwz.googlegroups.com:

This might belong in alt.astronomy, but glancing in there, I hope not.

I'm looking for a set of information about visible stars that would be
useful in a work of fiction, and I'm finding it slow going. I can find
resources (like simbad) that can tell me that alpha Orionius is
Spectral type M2Iab. What I really want to know is what color it would
appear if you were on a hypothetical planet in the star's liquid-water
zone,

Not much different to the way the Sun appears on Earth - still far too
bright to look at with the naked eye. The general cast of colours would be
a bit between normal daylight and what you see inside under a tungsten lamp
where the filament temperature is about 2,800 K, whereas the surface
temperature of alpha ori is around 3,100 K. So the view will have a
yellowish cast but your eyes tend to automatically compensate.

how long such a planet's year would be, if it's a multiple star
system and how the other system members would appear to the naked eye,

Forget habitable planets around massive stars like alpha ori (est 16 - 20
Msun). During the main sequence phase of such stars their radiation in the
UV is proportionately much higher than that of our Sun. This particular red
giant probably only started out on the main sequence as a blue giant about
six million years ago, not long enough for any terrestrial planets to have
even settled down. The red giant phase is also highly variable meaning that
your "habitable zone would not be stable for long.

Klazmon.



SNIP

In message , Llanzlan
Klazmon writes
"ScottM" wrote in news:1133063494.173458.267760
:

This might belong in alt.astronomy, but glancing in there, I hope not.

I'm looking for a set of information about visible stars that would be
useful in a work of fiction, and I'm finding it slow going. I can find
resources (like simbad) that can tell me that alpha Orionius is
Spectral type M2Iab. What I really want to know is what color it would
appear if you were on a hypothetical planet in the star's liquid-water
zone,

Not much different to the way the Sun appears on Earth - still far too
bright to look at with the naked eye.

Are you sure of that? I really don't want to sit down and work out how
far away a habitable planet would be, but I wonder if the radiation per
surface area is low enough for it to be tolerable. It's a fascinating
system, with several companion stars which may even be entering the
outer parts of the red giant.

The general cast of colours would be
a bit between normal daylight and what you see inside under a tungsten lamp
where the filament temperature is about 2,800 K, whereas the surface
temperature of alpha ori is around 3,100 K. So the view will have a
yellowish cast but your eyes tend to automatically compensate.

how long such a planet's year would be, if it's a multiple star
system and how the other system members would appear to the naked eye,

Forget habitable planets around massive stars like alpha ori (est 16 - 20
Msun). During the main sequence phase of such stars their radiation in the
UV is proportionately much higher than that of our Sun. This particular red
giant probably only started out on the main sequence as a blue giant about
six million years ago, not long enough for any terrestrial planets to have
even settled down. The red giant phase is also highly variable meaning that
your "habitable zone would not be stable for long.


Anyway, it's been done :-) Poul Anderson wrote a story ("Honorable
Enemies") set on a planet of Betelgeuse, and added a bit explaining how
it has habitable planets for the collection "Agent of the Terran
Empire".

I'd better reel this line of inquiry in a bit. For the story purposes,
"habitable planet" isn't too germaine - people tend to live underground
almost everywhere, and they expect the surface to be either too hot or
cold to visit at all, or at least so hot or cold that very special
precautions are taken. I do assume a scattering of temperate planets
around some stars, but not too many. Part of this project is to figure
out how many of those I can get away with.

Put simply, people haven't travelled to new worlds because it was fun;
they've done it because a technological windfall discovery made it
almost trivial, and there were economic advantages to be had, such as
mining. So I can get away with a lot of planets with absolutely
miserable surface conditions or relatively unstable (in the long term)
conditions. I'd just "accurately" like to know what kind of conditions
I can describe. :-)

Jonathan Silverlight wrote
in :

In message , Llanzlan
Klazmon writes
"ScottM" wrote in news:1133063494.173458.267760
:

This might belong in alt.astronomy, but glancing in there, I hope not.

I'm looking for a set of information about visible stars that would be
useful in a work of fiction, and I'm finding it slow going. I can find
resources (like simbad) that can tell me that alpha Orionius is
Spectral type M2Iab. What I really want to know is what color it would
appear if you were on a hypothetical planet in the star's liquid-water
zone,

Not much different to the way the Sun appears on Earth - still far too
bright to look at with the naked eye.

Are you sure of that? I really don't want to sit down and work out how
far away a habitable planet would be, but I wonder if the radiation per
surface area is low enough for it to be tolerable. It's a fascinating
system, with several companion stars which may even be entering the
outer parts of the red giant.

Interesting question. I would guess that the total radiation flux would
have to be roughly the same as we have on earth. From the SB law the total
radiation ~ 4th power of temperature. You can conclude that the surface
brightness is of course less than the Sun by about a factor of 16? That
means that the star must show as a disk of about two degrees angular
diameter to compensate. I would say that the image of the star on the
retina would be a bit easier to cope with, as although the total energy on
the retina should be the same as for the image of the Sun here on earth,
the larger area would mean more blood vessels to dissipate the heat and
also the longer wavelength radiation may be less damanging to the retina.
Any physiologists out there? I suppose you would get the same effect from
staring at a tungsten disk heated to 3,100K at the appropriate distance to
get the right angular size. It would be at least dazzling anyway.


The general cast of colours would be
a bit between normal daylight and what you see inside under a tungsten
lamp where the filament temperature is about 2,800 K, whereas the
surface temperature of alpha ori is around 3,100 K. So the view will
have a yellowish cast but your eyes tend to automatically compensate.

how long such a planet's year would be, if it's a multiple star
system and how the other system members would appear to the naked eye,

Forget habitable planets around massive stars like alpha ori (est 16 -
20 Msun). During the main sequence phase of such stars their radiation
in the UV is proportionately much higher than that of our Sun. This
particular red giant probably only started out on the main sequence as a
blue giant about six million years ago, not long enough for any
terrestrial planets to have even settled down. The red giant phase is
also highly variable meaning that your "habitable zone would not be
stable for long.


Anyway, it's been done :-) Poul Anderson wrote a story ("Honorable
Enemies") set on a planet of Betelgeuse, and added a bit explaining how
it has habitable planets for the collection "Agent of the Terran
Empire".

Yes. Other SF writers have even described aliens whose home world orbited
stars like Rigel ;-).

Klazmon.

In article .com,
"ScottM" writes:
Is there any simple mapping from spectral type to observed
color?

There's a diagram translating spectral type to temperature in Martin
Zombeck's _Handbook of Space Astronomy and Astrophysics_, available on
the web at:
http://ads.harvard.edu/cgi-bin/bbrow...ap=2&page=0066

There may be tables elsewhere in the book, and there are quite likely
other sources on the web as well.

Do remember, though, that all but the most extreme cool stars are
hotter than your incandescent light bulb, and colors observed with
the human eye will look normal regardless of the temperature of the
star. Photographers would notice a difference ("white balance" on
your digital camera) and so perhaps would artists and fashion
designers, but to the average person colors would be pretty much the
same regardless of the temperature of the star a planet is orbiting.
You don't see colors change going from indoors to outside, do you?

One difference you would notice is sharpness of shadows. Shadow
sharpness depends on the angular diameter of the sun. On earth you
see very sharp shadows at the extreme partial phase of an eclipse.

Given two large solar masses, can a smallish
planet at L3 have a stable orbit, or is it hopeless?

I'd guess L3 is hopeless, but you could have two stars in a close
orbit and a planet orbiting far away.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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