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hkenoch
2010-Mar-10, 05:46 PM
Hello Bad Astronomy. :whistle:

I am in the process of editing a book I have written, and I need some answers, or speculation, in regards to a few technical questions. Hopefully you all can help, or point me to old threads here or other web sites that will be helpful.

In short, the story takes place on an extrasolar planet. It is roughly the same size, atmosphere, and temperature as Earth.

The planet has, though, a very large orbit and thus a long year--about 18 years.

It also has a substantial ring system.

So,my questions:

1. At this orbit, what color/temperature sun would this planet have to give it the same relative temperatures as Earth?

1a. How large would this sun appear in the sky?

1b. Assuming the atmosphere is largely the same as Earth's, what color would the sun appear to the people on the ground? What color would the sunsets be?


On to the rings. I tried to research Saturn as a model and read some of the "If Earth had rings" threads here, but did not find any solid answers.

2. From the perspective of the ground, would the rings change color depending on the season or the time of day? I had something in mind that there was a good deal of frozen ice or other substance in the rings, which might melt depending on the season, but I do not know if this is technically appropriate.

3. Do the rings have any effect on ocean tides?

4. How pronounced would the ring-shadow be on the surface of the planet? Would it be like night, or more like a cloud-shadow?

4b. Depending on the season, would this shadow "migrate" or would it be geosynchronous?

Thanks in advance. Feel free to point out any other technical issues that would come to mind.

Ilya
2010-Mar-10, 07:35 PM
If the star has the same mass as Sun, then the radius of an 18-year orbit would be 6.9 AU -- greater than orbit of Jupiter. But the star cannot have same mass as Sun because it must be hotter, thus more massive, thus the 18-year orbit will be bigger. So...

Luminosity of a main-squence star is approximately proportional to its mass to the power 3.5
Insolation of a planet is proportional to luminosity (i.e. to star's mass to the power 3.5) divided by square of distance
Distance, i.e. orbital radius, is proportional to cube root of [star's mass times square of orbital period]

We measure distance D in astronomical units (Earth-Sun distance), insolation I in solar constants (Earth insolation ), orbital period P in years and mass M in solas masses.

Insolation must be 1, so M3.5/D2=1
Orbital period must be 18, so D3=M*P2=M*182=324*M

We are down to two equations with two unknowns:

M3.5/D2=1
D3=324*M

I will solve it when I have time

[Edited]: I highlighter a blooper above

grant hutchison
2010-Mar-10, 07:53 PM
2. From the perspective of the ground, would the rings change color depending on the season or the time of day?They'll change brightness with both season and time of day.
People will see the illuminated side of the rings in local summer, and the unilluminated side in winter. The rings will be edge-on to the sun at the equinoxes, and therefore particularly dark.
Overhead at local noon you'll be seeing primarily the dark sides of the ring particles; overhead at local midnight you'll see the illuminated sides, except where the shadow of the planet falls on the rings.


3. Do the rings have any effect on ocean tides?No. They're uniformly distributed and equatorial (barring something very odd about their formation), so there will be no variation in gravitational potential as the planet rotates beneath them.


4. How pronounced would the ring-shadow be on the surface of the planet? Would it be like night, or more like a cloud-shadow? You get to choose: it depends on the density of the ring.


4b. Depending on the season, would this shadow "migrate" or would it be geosynchronous?The ring shadow moves with the seasons. It will be confined to the equator around the equinoxes, and will then pass into the winter hemisphere. Exactly where the shadow falls will depend on the axial tilt, and the distance to the inner and outer edges of the ring.

Grant Hutchison

IsaacKuo
2010-Mar-10, 08:14 PM
2. From the perspective of the ground, would the rings change color depending on the season or the time of day? I had something in mind that there was a good deal of frozen ice or other substance in the rings, which might melt depending on the season, but I do not know if this is technically appropriate.
Since the planet has Earth-like temperature conditions, the rings will not be made of ice. Ice would have melted quickly. The rings would instead be made of something similar to asteroid material. This could be silicate heavy rock, or nickel-iron ore, or other volatile-poor substances.

Ilya
2010-Mar-10, 08:17 PM
We are down to two equations with two unknowns:

M3.5/D2=1
D3=324*M

I will solve it when I have time
After transformations I get M=3.9, L=117, and D=10.8. A star with 3.9 Sun's mass, 117 times luminosity, and a planet circles it every 18 years on a 10.8 AU orbit. That makes it a Class B white star with diameter about 2 times that of Sun. At 10.8 AU distance it would look about 1/5 Sun's visible diameter, and I think would look white, not yellow. Sky would be incredibly deep blue due to much greater amount of blue in the star's spectrum. Sunsets and sunrises would be "yellow" or possibly even "green" rather than "red" for the same reason.

hkenoch
2010-Mar-10, 08:40 PM
After transformations I get M=3.9, L=117, and D=10.8. A star with 3.9 Sun's mass, 117 times luminosity, and a planet circles it every 18 years on a 10.8 AU orbit. That makes it a Class B white star with diameter about 2 times that of Sun. At 10.8 AU distance it would look about 1/5 Sun's visible diameter, and I think would look white, not yellow. Sky would be incredibly deep blue due to much greater amount of blue in the star's spectrum. Sunsets and sunrises would be "yellow" or possibly even "green" rather than "red" for the same reason.

Thanks! That's incredibly helpful. Green sunsets sound incredibly beautiful.

eburacum45
2010-Mar-10, 08:43 PM
For questions such as this I use an on-line calculator I found on Geocities several years ago; it has disappeared now, but the WebArchive retains a copy of it.

Here it is;
http://web.archive.org/web/20030218125318/www.geocities.com/Area51/Corridor/8611/mseqstar.htm

Now to get the answer to the question you have posed, I set the Star Luminosity at Planet at 1 (to get an Earth-like level of luminosity).

To get the orbital period right I entered a range of luminosities until I found one which produced a Planet Annual Period of 18 years. The required luminosity turns out to be 109.5 x solar, with a mass 3.525 times as massive as the Sun, and a spectral class of B9. (I don't know if this is bolometric luminosity or not, by the way, but somehow I doubt it)

(edit) I see Ilya has worked it out himself; his (no doubt more reliable) figures are in the same rough ball park as those generated by the calculator.

Such a star would be very hot, and put out most of its luminosity in the ultraviolet range.
It would also look very small in the sky, no more than 0.253 x the diameter of the Sun in the sky. Because such a star gives out so much of its energy in UV, it would look relatively dim to human eyes; but it would give you terrific sunburn unless the atmosphere filtered out the rays.

Worst of all, a B9 class star would only last a few tens of millions of years, if I recall correctly; a bit of a short lifetime for a planet to develop life, let alone complex life.

On the other hand...
There are a few, very luminous, massive stars which have relatively Sun-like spectra, which would mean that they would not put out so much dangerous ultraviolet. These are the yellow supergiants, with masses as much as 15 or 20 times solar
http://en.wikipedia.org/wiki/Yellow_supergiant
and you would have to recalculate the orbit for such a star (the higher the mass, the faster the planet goes round the star for any given luminosity). These stars only remain in this state for a very short time cosmologically speaking, as they swing into blue and red phases quite quickly. This instability would not give your planet long enough to evolve a biosphere.

Basically, the 18 year orbit is a real problem, and an Earth-like planet would be unlikely to exist in such a location. I suppose you could have a planet which has been artificially terraformed in the relatively recent past orbiting such a massive star- but it would not be a very good candidate for terraforming, as these massive stars tend to explode after a few million years, ruining all the hard work of terraformation.

hkenoch
2010-Mar-10, 08:45 PM
They'll change brightness with both season and time of day.
People will see the illuminated side of the rings in local summer, and the unilluminated side in winter. The rings will be edge-on to the sun at the equinoxes, and therefore particularly dark.
Overhead at local noon you'll be seeing primarily the dark sides of the ring particles; overhead at local midnight you'll see the illuminated sides, except where the shadow of the planet falls on the rings.

Great information but I am confused by what you mean by "dark side" and "light side." I don't quite understand whether dark and light correspond with the top and bottom of the rings, or with the 'night' and 'day' side.

Am I to understand 'edge-on' as the rings appearing as a very small line in the sky?

hkenoch
2010-Mar-10, 08:48 PM
Thanks eburacum45, good food for thought! I may be able to work some of those "problems" into the story line, though.

IsaacKuo
2010-Mar-10, 08:56 PM
Maybe one possible solution would be to use an orange or red dwarf star, but the planet is in a horseshoe orbit relative to a brown dwarf. Even though the true year is about one Earth year, the period of the horseshoe orbit is 18 years. As far as the inhabitants are concerned, what matters is the 18 year seasonal cycle (summer for 9 years when the planet is closer to the star; winter for 9 years when the planet is further from the star).

hkenoch
2010-Mar-10, 09:02 PM
Maybe one possible solution would be to use an orange or red dwarf star, but the planet is in a horseshoe orbit relative to a brown dwarf. Even though the true year is about one Earth year, the period of the horseshoe orbit is 18 years. As far as the inhabitants are concerned, what matters is the 18 year seasonal cycle (summer for 9 years when the planet is closer to the star; winter for 9 years when the planet is further from the star).

Very interesting... I kind of like that idea. Would the information about Do you know how that would affect the appearance of the sun and sunsets, though?

grant hutchison
2010-Mar-10, 09:10 PM
Great information but I am confused by what you mean by "dark side" and "light side." I don't quite understand whether dark and light correspond with the top and bottom of the rings, or with the 'night' and 'day' side.Each ring particle has its own night (dark) and day (light) side. When your view of the rings is mainly of the dark side of the particles, the rings will appear dark; when you look towards the light side of the particles, the rings will appear bright.


Am I to understand 'edge-on' as the rings appearing as a very small line in the sky?That only happens if you're on the equator, when the rings will form a line in the sky. If you're north or south of the equator, you will have an unchanging view of the rings, but their angle of illumination will change with the time of day and the season. At the equinoxes, the rings will be edge-on to the sun, and therefore the ring surface will be poorly illuminated and will appear dark.
(Perhaps it's worth noting at this point that there will be high latitudes from which the rings are invisible, because they are below the horizon. The cut-off at which that occurs depends on how large you make the outer radius of the rings, compared to the radius of the planet.)

Grant Hutchison

IsaacKuo
2010-Mar-10, 09:15 PM
I'm not sure, but I think the problem in this case is that 18 years is a short time for a horshoe orbit's cycle. This means it requires a short true year, which means a small orbit radius, which means the planet is likely tidally locked to the star. That would mean no sunsets at all--no day/night cycle. I assume this would be unacceptable due to needing to rewrite everything.

Hmm...instead of a brown dwarf companion, the other planet could be Earth-like in mass. That would halve the horshoe orbit period.

Either way the other planet would appear to be very small most of the time, and move very slowly in the sky. It would be culturally important because it starts getting bigger and closer as the switch between the seasons approaches. It appears largest at closest approach, during the season switch, and then recedes away.

hkenoch
2010-Mar-10, 09:22 PM
Ok, Grant, that makes a bit more sense to me.

hkenoch
2010-Mar-10, 09:32 PM
Still a neat idea but, after some speaking, I like the idea of the bluer skies and green/yellow sunsets... if fits in better, culturally.

chornedsnorkack
2010-Mar-10, 09:39 PM
A few tens of millions of years is underestimate.

Consider Ain. Ain b is well proven to exist, although its mass is unknown, as is common for planets, and it might be a brown dwarf.

Ain, like the rest of Hyades, is roughly 600 million year old.

The mass is supposed to be 2,7 times solar, and luminosity is 97 +- 8 times solar.

A planet Ain c with terrestrial temperature would orbit at 9,85 +- 0,4 AU and have orbital period of slightly under 19 years, with error bars of a year and a half or so either way. Right in the bullīs eye. Ain is THE eye of the Bull.

Oh, and Ain is K0. Yellower and bigger than Sun.

eburacum45
2010-Mar-10, 09:47 PM
Nice one. Ain is an 'orange giant', and has left the main sequence so won't stay very long in that condition. But it would be a nice location for a terraformed world on a temporary basis.
http://en.wikipedia.org/wiki/Epsilon_Tauri

antoniseb
2010-Mar-10, 09:53 PM
Still a neat idea but, after some speaking, I like the idea of the bluer skies and green/yellow sunsets... if fits in better, culturally.

One nice thing about IsaacKuo's solution is that if your Science Fiction story involves complex life having spontaneously developing on this planet... That won't happen during the short life of a B-type star.

Another possibility might be that your planet is an Earth-sized moon around a gas-giant in a Sun-like solar system, and that the atmosphere is heated from tidal effects much like Io and Europa are heated around Jupiter. That way, the total radiance of the star is not so important.

Of course Earthquakes would be a daily part of the culture.

hkenoch
2010-Mar-10, 09:55 PM
Nice one. Ain is an 'orange giant', and has left the main sequence so won't stay very long in that condition. But it would be a nice location for a terraformed world on a temporary basis.
http://en.wikipedia.org/wiki/Epsilon_Tauri


How long would it be in that condition?

hkenoch
2010-Mar-10, 09:57 PM
Another possibility might be that your planet is an Earth-sized moon around a gas-giant in a Sun-like solar system, and that the atmosphere is heated from tidal effects much like Io and Europa are heated around Jupiter. That way, the total radiance of the star is not so important.


Can you elaborate on this a little bit more for me?

eburacum45
2010-Mar-10, 10:14 PM
On an incidental note you might like to read this thread about rings around the Earth;
http://www.bautforum.com/astronomy/99795-if-earth-had-saturn-like-rings.html

antoniseb
2010-Mar-10, 10:28 PM
Can you elaborate on this a little bit more for me?

I'm assuming that you need an Earth-like eco-system for your story, I don't know if it has to have developed on its own or whether it is possibly something that an advanced civilization brought to your B star and built by terraforming... but assuming the locally developed Earth-like eco-system is what you're after, all you need is for the surface level atmosphere to be about 300K in some or all places on the surface, and that the atmosphere is thick enough to be an insulating layer to trap the right amount of heat and block radiation.

In an Io or Europa situation, their orbits are made elliptical by gravitational perturbations from the other moons, and this causes flexing of top layers of the moon from Jupiter's differential gravitation. This flexing generates heat. The heat escapes to the surface slowly, and heats the lower atmosphere.

The Moon would have an orbital period around the Gas giant of 20 to 100 hours roughly. The people on the back side would never see the gas giant, but would potentially see constant auroras at night.

If this was around a Sun-like star, plants would photo-synthesize much more slowly, because of the 300 times dimmer sunlight, but other forms of energy collection by life should be possible.

grant hutchison
2010-Mar-10, 10:49 PM
Towards the end of the last century, I generated a couple of small, simple animations of Saturn which may help with visualizing the illumination of rings and planet.

This one (http://www.ghutchison.pwp.blueyonder.co.uk/SATURN.MPG) (650KB mpg) hovers over Saturn's "Tropic of Cancer", viewing the planet from the direction of the northern summer solstice, and observes the seasonal cycle of one Saturnian year. You can see how the rings' shadow on the planet and the planet's shadow on the rings evolve over one orbital period.

This one (http://www.ghutchison.pwp.blueyonder.co.uk/SATURN2.MPG) (710KB mpg) flies around the planet in the ecliptic plane during a moment of southern hemisphere summer.

In both animations, I modelled the rings as a population of large backscattering particles (tiny moons), and a population of small particles which forward-scatter in short wavelengths (like smoke). The interplay of these scattering modes produces the shifts in brightness and colour as the sun angle changes.

(In both cases please forgive the over-exuberant use of "lens flare", which persists long after the source of illumination is out of shot.)

Grant Hutchison

eburacum45
2010-Mar-11, 12:02 AM
How long would it be in that condition?

I don't know; I'm not an astronomer, just an interested person ,like yourself. Perhaps a real 'stronomer will be along in a bit to give the answer.

But if pressed I would guess that it might be about 30 million years. Our own Sun will be in an orange subgiant phase for about 600 million years, before it becomes a red giant.
From here
http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html

Ain seems to be evolving about twenty times as fast as the Sun, so maybe its orange subgiant phase might last 600/20 =30 million years. But that is just a wild guess.

hkenoch
2010-Mar-11, 01:15 AM
I don't know; I'm not an astronomer, just an interested person ,like yourself. Perhaps a real 'stronomer will be along in a bit to give the answer.

But if pressed I would guess that it might be about 30 million years. Our own Sun will be in an orange subgiant phase for about 600 million years, before it becomes a red giant.
From here
http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html

Ain seems to be evolving about twenty times as fast as the Sun, so maybe its orange subgiant phase might last 600/20 =30 million years. But that is just a wild guess.

That's more than enough time, for what the story needs. :lol:

hkenoch
2010-Mar-11, 01:18 AM
On an incidental note you might like to read this thread about rings around the Earth;
http://www.bautforum.com/astronomy/99795-if-earth-had-saturn-like-rings.html

I've seen the thread and found it helpful. I'd been toying with the idea of rings, and decided I had to do it after seeing that video!

hkenoch
2010-Mar-11, 01:41 AM
Towards the end of the last century, I generated a couple of small, simple animations of Saturn which may help with visualizing the illumination of rings and planet.

This one (http://www.ghutchison.pwp.blueyonder.co.uk/SATURN.MPG) (650KB mpg) hovers over Saturn's "Tropic of Cancer", viewing the planet from the direction of the northern summer solstice, and observes the seasonal cycle of one Saturnian year. You can see how the rings' shadow on the planet and the planet's shadow on the rings evolve over one orbital period.

This one (http://www.ghutchison.pwp.blueyonder.co.uk/SATURN2.MPG) (710KB mpg) flies around the planet in the ecliptic plane during a moment of southern hemisphere summer.

In both animations, I modelled the rings as a population of large backscattering particles (tiny moons), and a population of small particles which forward-scatter in short wavelengths (like smoke). The interplay of these scattering modes produces the shifts in brightness and colour as the sun angle changes.

(In both cases please forgive the over-exuberant use of "lens flare", which persists long after the source of illumination is out of shot.)

Grant Hutchison

Those are very nice, very professional looking. It helps to visualize it like that.

m74z00219
2010-Mar-12, 07:30 AM
Hi hkenoch. Though much information can be found online, I still find it useful to have a handy book with all the needed information in one location: my hands. I'd like to recommend to you "World-Building" by Gillett.

I've used it a few times in my writing (I'm also an amateur SF writer) and it's great to read just for fun.

http://www.amazon.com/World-Building-Science-Fiction-Writing/dp/158297134X/ref=sr_1_1?ie=UTF8&s=books&qid=1268378961&sr=1-1

I also recommend the starflight handbook for useful info on the mechanics of spaceflight.

http://www.amazon.com/Starflight-Handbook-Pioneers-Interstellar-Editions/dp/0471619124/ref=sr_1_1?ie=UTF8&s=books&qid=1268379005&sr=1-1


You might also find it helpful to use the Celestia software as a visual aid. It does an excellent job rendering Saturn at its rings (shadow and all).

m74

George
2010-Mar-12, 06:40 PM
Nice thread.


Because such a star gives out so much of its energy in UV, it would look relatively dim to human eyes; but it would give you terrific sunburn unless the atmosphere filtered out the rays. It should look even brighter than the Sun we see at 1 AU because extended objects do not diminish in surface brightness (watts/m2). At 400nm, a 10,000K star 2x the diameter of the Sun will be 3.3x greater in surface brightness. At 700nm it is 1.2x as great. [Corrrection: ~ 13x and 5x, respectively] As you stated, it will be smaller in apparent size but it won't be fun to look at. ;) [I think it would look bluish-white if properly attenuated and without much or any atmospheric effects.]

[Did someone says something about a yellow Sun??? :razz: ]

hkenoch
2010-Mar-12, 09:09 PM
Hi hkenoch. Though much information can be found online, I still find it useful to have a handy book with all the needed information in one location: my hands. I'd like to recommend to you "World-Building" by Gillett.

I've used it a few times in my writing (I'm also an amateur SF writer) and it's great to read just for fun.

http://www.amazon.com/World-Building-Science-Fiction-Writing/dp/158297134X/ref=sr_1_1?ie=UTF8&s=books&qid=1268378961&sr=1-1

I also recommend the starflight handbook for useful info on the mechanics of spaceflight.

http://www.amazon.com/Starflight-Handbook-Pioneers-Interstellar-Editions/dp/0471619124/ref=sr_1_1?ie=UTF8&s=books&qid=1268379005&sr=1-1


You might also find it helpful to use the Celestia software as a visual aid. It does an excellent job rendering Saturn at its rings (shadow and all).

m74

Those look really helpful. I wish there was a version of Celestia where you could make your own planets.

Feel free to PM me if you want to talk writing.

hkenoch
2010-Mar-12, 09:10 PM
Nice thread.

It should like even brighter than the Sun we see at 1 AU because extended objects do not diminish in surface brightness (watts/m2). At 400nm, a 10,000K star 2x the diameter of the Sun will be 3.3x greater in surface brightness. At 700nm it is 1.2x as great. As you stated, it will be smaller in apparent size but it won't be fun to look at. ;) [I think it would look bluish-white if properly attenuated and without much or any atmospheric effects.]

[Did someone says something about a yellow Sun??? :razz: ]

I like blueish--white; do you think it would have a halo though?

George
2010-Mar-13, 01:09 AM
I like blueish--white; Due to its extreme brightness, like our sun, it will look white even if it could be a deep blue star, which can not happen, because our color cones become maxed-out and this produces a very bright white result. A bluish-white disk would only be seen if the intensity were reduced to normal photopic vision levels.


... do you think it would have a halo though? This is typically an atmospheric effect. I suppose you could throw in a somewhat dense asteroid belt for a halo effect. :)

BigDon
2010-Mar-13, 01:24 AM
This is typically an atmospheric effect. I suppose you could throw in a somewhat dense asteroid belt for a halo effect. :)

Or said asteroid belt may have had a recent (geologically speaking) bout of "bumper cars" and be very dusty.

10 AU is far enough out there could be a really energetic inner system astreroid belt that wouldn't endanger the planet in question. That's what? Halfway between Jupiter and Uranus?

George
2010-Mar-13, 01:31 AM
Or said asteroid belt may have had a recent (geologically speaking) bout of "bumper cars" and be very dusty.

10 AU is far enough out there could be a really energetic inner system astreroid belt that wouldn't endanger the planet in question. That's what? Halfway between Jupiter and Uranus? I was thinking of a belt that was only a few AU out.

The bigger problem is that I doubt it would last very long if it had much of any inclination to the eclilptic plane, so it would only look like Saturn's rings when they are inclined towards us.

hkenoch
2010-Mar-13, 02:34 AM
Due to its extreme brightness, like our sun, it will look white even if it could be a deep blue star, which can not happen, because our color cones become maxed-out and this produces a very bright white result. A bluish-white disk would only be seen if the intensity were reduced to normal photopic vision levels.



Would it have subtle blue around the edges, though, like our sun appears yellowy around its edges? Or are you saying that the blue would be too deep for humans to detect, period?

hkenoch
2010-Mar-13, 02:38 AM
I was thinking of a belt that was only a few AU out.

The bigger problem is that I doubt it would last very long if it had much of any inclination to the eclilptic plane, so it would only look like Saturn's rings when they are inclined towards us.

I'm a bit lost on what you are discussing here.

When I said "Halo" I meant more like a sun dog. I thought there might be a more immediate contrast between the sun light and the sky, since the sky is darker.

It sounds like you all interpreted my question as 'does the sun have a ring.'

grant hutchison
2010-Mar-13, 12:26 PM
I wish there was a version of Celestia where you could make your own planets.That's kind of the point of Celestia, as far as many people are concerned: you can make your own planets and star systems.
The Celestia wikibook (http://en.wikibooks.org/wiki/Celestia/Catalog_File_Reference) tells you how.

Grant Hutchison

eburacum45
2010-Mar-13, 12:40 PM
I've made lots;
here's an image (not by me, incidentally) which includes some planets I've made as well as planets by John Dollan and others. You can probably spot the Earth in there somewhere.
http://www.orionsarm.com/im_store/OrionsArm2by1Wyrmshadow1.jpg

It's addictive, or at least I find it so.

chornedsnorkack
2010-Mar-13, 02:22 PM
A main-sequence star at roughly right size, 98 times solar brightness, is Algol A, which has 3,6 times solar mass and B8V spectrum. The whole Algol ABC system has 105 times solar brightness and 6 times solar mass. So Algol d at 10,25 AU orbit should have roughly 13 year orbiting period. The period of Algol C (just under 2 years) is slightly uncomfortable, though.

But the Algol ABC system would look stunning from Algol d....

George
2010-Mar-13, 04:23 PM
I'm a bit lost on what you are discussing here.

When I said "Halo" I meant more like a sun dog. I thought there might be a more immediate contrast between the sun light and the sky, since the sky is darker. I don't think the atmospheric displays would be much different though they might be a little brighter and more narrow since this sun is smaller in apparent size and has greater surface brightness. [I corrected my early surface brightness values since I should not have divided by the gain in area of the larger star.]

Rainbows should appear stronger in violent, indigo and blue compared to the red end of the spectrum. This is due to what eburacum45 mentioned earlier about the spectral shift toward the blue end of the spectrum, which comes from having hotter stars.


It sounds like you all interpreted my question as 'does the sun have a ring.' Such things are atmospheric effects and mostly independent of which star you pick.

George
2010-Mar-13, 04:54 PM
Would it have subtle blue around the edges, though, like our sun appears yellowy around its edges? The Sun does seem to have a bit of yellow along or near its limb, but I don't know why that is since it is an all-white star. Some say it may be due to a contrast effect since the sky is so blue.

Since the sky would be even more blue with a hotter star, it's even less likely it would have blue edges. Perhaps near the edges it will be more yellow assuming it is due to contrast issues, not that I know.


Or are you saying that the blue would be too deep for humans to detect, period? No. The eye has three distinct color cones along with many more rods. [The rods are very, very sensitive and help us see in dim lighting, but they contribute very little to what color we see.] The cones become active when light levels reach about 1 candela/m2. The dynamic range of the human eye is enormous, about 1 trillion. The Sun's surface brightness, however, greatly exceeds this upper limit by many thousands of times, which is why it is painful to look at it. A blue star replacement would be even worse assuming evolution did nothing to help the residents.

When all three of our color cones are observing an intensity of light so bright that they are responding at full output, then the brain has no way to determine if one color is more dominant than the other since there is no longer any relative amounts of color strengths. Thus, white is what the brain tells us it is seeing even though the object, when its excessive intensity is reduced, may appear to have significant color otherwise.

If the Sun were a deep yellow or even a blue star it would still look bright white to any space observer unless they had some way to attenuate the excessive intensity. A simple strobe or neutral filter would reveal its true color, but this has never been done. [Alas, I'm drifting toward another topic. :)]

The reason we see many stars having color is because they are so far away that they are no longer too bright for our color cones and brain (retinex) to register their proper color, though star color is "in the eye of the beholder". I see more green than my wife and others see more red in orange Antares than I do. [I don't know the name of their defect. :razz: ;)]

chornedsnorkack
2010-Mar-13, 05:07 PM
If the Sun were a deep yellow or even a blue star it would still look bright white to any space observer unless they had some way to attenuate the excessive intensity. A simple strobe or neutral filter would reveal its true color, but this has never been done. [Alas, I'm drifting toward another topic. :)]


Are water clouds colour-neutral? When they are the right thickness to resolve the disc of the Sun yet stare straight at it?

George
2010-Mar-13, 05:46 PM
Are water clouds colour-neutral? When they are the right thickness to resolve the disc of the Sun yet stare straight at it?Good question. I understand that they scatter light (Mie scattering) in a neutral way, but even if they are neutral they only seem to float in atmospheres, which are not netural in their scattering. :)

Do you benefit from fogs and thin enough cloud structures to be able to comfortably view the Sun's disk? This is somewhat rare here in central Texas, but I have made it a point to take note when it happens so that I can look for sunspots and image them without a fitler.

When I can see the disk in this way and its high in altitude, it looks white to me.

White clouds and their neutral or near neutral scattering is another argument for a white Sun, though there is one argument that exceeds the rest.

Alpharomeo81
2010-Mar-13, 08:47 PM
This is how I see it.


The planet has, though, a very large orbit and thus a long year--about 18 years.

Jupiter's year: 11.8 Earth years
Saturn's year: 29.4 years


1. At this orbit, what color/temperature sun would this planet have to give it the same relative temperatures as Earth?

If it does not have too much sunlight, it may require a greenhouse effect caused by heavy presence of greenhouse gases, perhaps like mars, or it may require an internal source of heat, perhaps caused by molten and the presence of radioactives in a young planet. The problem I see with a hotter planet would be its more violent weather if orbit rotation is too slow, causing a gradient of temperature between night and day sides. Due to extreme temperatures, life as we know it may be possible only at sunset. If the planet is too big, living creatures should be shorter or even flat due to gravity.

Ring shadow could disrupt temperatures, causing temperature gradients and more violent storms in the daylight side.


1b. Assuming the atmosphere is largely the same as Earth's, what color would the sun appear to the people on the ground? What color would the sunsets be?

It depends on the composition of planet's atmosphere. If it contains carbon dioxide it would look like the atmosphere of Mars. If it has heavy oxygen presence like Earth it would look blue. Notice that Earth was supposed to have heavy presence of carbon dioxide in the past, but stromatolites are suspected of terraforming Earth atmosphere. Having Earth's atmosphere should be quite exceptional.

http://en.wikipedia.org/wiki/Stromatolite

Sunsets would start with daylight colors turn to reddish colors if it has a sun like the one we have.

Read this for a detailed explanation
http://askville.amazon.com/sun-sky-sunset-appears-red-Light-refraction/AnswerDetails.do?requestId=61815&responseId=61910


2. From the perspective of the ground, would the rings change color depending on the season or the time of day? I had something in mind that there was a good deal of frozen ice or other substance in the rings, which might melt depending on the season, but I do not know if this is technically appropriate.

What if the rings were caused by space debris which remained in space for several thousand years, until they formed rings. The race that created the rings might not be able to travel to space because debris would create a Kessler syndrome that may prevent space travel. We are not so far from having a kessler syndrome.

http://en.wikipedia.org/wiki/Kessler_syndrome

If you want to see the current amount of space debris in Google Earth, follow these steps:

1.Download Google Earth addon installer from http://adn.agi.com/SatelliteDatabase/SatelliteDatabase.kmz
2.Double click on the file to start the installation process.

You may find more info on this addon at http://freegeographytools.com/2008/real-time-satellite-visualization-in-google-earth
You also may notice that the number of useless objects outnumbers the active objects in space. You also may notice how it looks like a wasp nest and you may wonder how you could navigate across this field of debris.

hkenoch
2010-Mar-22, 03:48 PM
Thanks for all the answers here, guys! It was really helpful; if I get to add a 'thank you' section I will definitely credit you all!

chornedsnorkack
2011-Oct-24, 10:08 AM
A main-sequence star at roughly right size, 98 times solar brightness, is Algol A, which has 3,6 times solar mass and B8V spectrum. The whole Algol ABC system has 105 times solar brightness and 6 times solar mass. So Algol d at 10,25 AU orbit should have roughly 13 year orbiting period. The period of Algol C (just under 2 years) is slightly uncomfortable, though.

But the Algol ABC system would look stunning from Algol d....

And Kepler 16b just proved that a circumbinary can work at just about 5,6 times the binary period!

So, what about Algol d after all?

Githyanki
2011-Nov-01, 05:45 PM
Given the short lifespans of B-class stars, any "Earth-like" world in a system like that wouldn't be "Earth-like" at all apart from resembling early-Earth with a thick atmosphere warm of carbon-dioxide, green-oceans, no continents and virtually a "Water-World" with a temperature of about 200F.

To make it Earth like, it has to be terraformed. You'd have to pump a lot of oxygen into the atmosphere faster than the iron in the oceans would absorb it. If you give it far-less water than Earth has, you'll still get a world covered with water, but not as deep and more volcanic islands and in areas of massive-volcanism, you might get a bulge which could act as a defacto continent. Any land would probably be stained red from all the O2.

If it's "Terraformed", then the "Belt" could possibly be man or alien-made satellites orbiting or the remains of a generation-ship or even the space-colonies of humans who were living in space for the thousands of years it takes to terraform the surface. If the process took millions of years, they could have evolved into a species what doesn't require an Earth-like world to live and they can be totally adapted to living in space-colonies.

chornedsnorkack
2012-Jun-29, 08:23 AM
Given the short lifespans of B-class stars, any "Earth-like" world in a system like that wouldn't be "Earth-like" at all apart from resembling early-Earth with a thick atmosphere warm of carbon-dioxide, green-oceans, no continents and virtually a "Water-World" with a temperature of about 200F.

To make it Earth like, it has to be terraformed. You'd have to pump a lot of oxygen into the atmosphere faster than the iron in the oceans would absorb it. If you give it far-less water than Earth has, you'll still get a world covered with water, but not as deep and more volcanic islands and in areas of massive-volcanism, you might get a bulge which could act as a defacto continent. Any land would probably be stained red from all the O2.

If it's "Terraformed", then the "Belt" could possibly be man or alien-made satellites orbiting or the remains of a generation-ship or even the space-colonies of humans who were living in space for the thousands of years it takes to terraform the surface. If the process took millions of years, they could have evolved into a species what doesn't require an Earth-like world to live and they can be totally adapted to living in space-colonies.

Ain is 98 times solar luminosity now it is a subgiant. It is 2,7 times solar mass.

Back when it was on main sequence, what was its luminosity compared to Sirius or Vega?