Red Dwarf companion at Sunset?

[Algr]

If our star, (or one just like it) had a red dwarf companion, what would it look like?

I've got this about 1 million miles away from the sun. I understand that red dwarfs are not really red like a stoplight, but are more like the color of a dimmed incandescent lightbulb. But our sun in the photo above is already reddened by the atmosphere, so how red should I go? Would the atmosphere stop having any further effect? Is the brightness-per-degree similar to a bigger star? Or would a red dwarf star seem dimmer even if we were close enough for it to have the same apparent size?

There is lots of data about a red dwarf's mass, but how big are they in size? In these pic, I am assuming a density similar to the sun, but I don't know if that is right.

Originally I was going to ask if the red dwarf would stretch the sun into an oval, but looking at this, it seems unlikely. A sun could do this by itself, just by spinning quickly. Or would that render the planet uninhabitable?

The angle of the big sun to the dwarf is meant to suggest we are at around 50°-60° latitude on the planet. Is that right? Of course that would be thrown off by the angle of tilt by the planet, so throughout the year, viewers would see the angle change. If you were on the equator during the equinox, the dwarf would be directly above or below the big star. (Or invisible in front or behind it.).

 

[DaveC426913]

There is lots of data about a red dwarf's mass, but how big are they in size? In these pic, I am assuming a density similar to the sun, but I don't know if that is right.

Some quick Googling on density suggests they can be 30-100 times denser than our sun, which would make its disk a lot smaller than your depictions - more like planet-sized in that shot.

(At that size, I suspect its colour would be virtually impossible to see - certainly too small to try to establish and objective colour against an already very red sky). I suspect, to the eye it would appear to be white. (Not true white just white-in-a-red-light white)

Would the atmosphere stop having any further effect?

It would stop having any further effect.

The light from our sun is not being red-shifted; it is being selectively filtered, removing (scattering) bluer light and leaving red. Thus, light from a red star would come through without (much) further filtering.

Originally I was going to ask if the red dwarf would stretch the sun into an oval,

No.

The angle of the big sun to the dwarf is meant to suggest we are at around 50°-60° latitude on the planet. Is that right?

Sure, yeah.

Assuming the red dwarf orbits along with the planets in the Sun's/Solar System's plane.
One thing I am not sure about is whether the dwarf would have a halo of red around it, as in your depictions.

The only reason I can think of for it to have a halo is cloud reflection, but I suspect that it is too dim to noticeably reflect light off nearby clouds, especially enough to overwhelm the clouds already being lit by sunlight, so I think this is might be more accurate:

(Which might be kind of a let-down spectacularity-wise.)

 

[Bandersnatch]

The light from our sun is not being red-shifted; it is being selectively filtered, removing (scattering) bluer light and leaving red. Thus, light from a red star would come through without (much) further filtering

It is being >preferentially< scattered in the blue part of the spectrum. The scattering removes all wavelengths, including red. An already red source gets dimmed as a result. With enough atmosphere in the way all of the light could get scattered.

There is lots of data about a red dwarf's mass, but how big are they in size? In these pic, I am assuming a density similar to the sun, but I don't know if that is right

For main sequence stars there exists an empirically-established relationship between mass and radius, where $R\propto M^a$, in solar units. The exponent $a$ is around 0.8-0.9.
If you simply went with the disc being as many times smaller as the star is less massive, you wouldn't be far off the mark.
Remember, at the lowest mass end you're looking at ~Jupiter-sized discs.

 

[hutchphd]

If the red dwarf were quite red would there be a noticeable reflection of sunlight from some level in the atmosphere? For example could you detect a quarterphase dwarf as such?

 

[snorkack]

If our star, (or one just like it) had a red dwarf companion, what would it look like?

Is the brightness-per-degree similar to a bigger star? Or would a red dwarf star seem dimmer even if we were close enough for it to have the same apparent size?

Dimmer.
For a small red dwarf, see the derivation here:
https://www.physicsforums.com/threads/disc-of-quiet-and-active-proxima.1014634/

There is lots of data about a red dwarf's mass, but how big are they in size? In these pic, I am assuming a density similar to the sun, but I don't know if that is right.

Red dwarfs are dense but they are not that small.
In the 20 ly closest to Sun, the brightest red dwarf is AX Microscopii at +8,69 - this is the matter of defining "red dwarf", that is the line between K and M. Proxima is +15,53. Red dwarfs go to maybe +19.
AC Microscopii is quoted as 0,51 solar radii and 0,029 times visual brightness. Which means it has about 11 % surface brightness. Proxima, as in the quoted post, has 0,2% surface brightness of Sun.

If you were on the equator during the equinox, the dwarf would be directly above or below the big star. (Or invisible in front or behind it.).

When you are speaking of a red dwarf 50% the radius of the primary... High in the sky partial solar eclipses often go unnoticed because Sun in clear sky is too bright to stare at. But low on the horizon the thick air offers some protection (though people are warned not to rely on it too long). Which means that a red dwarf is easily big enough to be a visible dark patch in front of sun.

 

[DaveC426913]

It is being >preferentially< scattered in the blue part of the spectrum. The scattering removes all wavelengths, including red. An already red source gets dimmed as a result. With enough atmosphere in the way all of the light could get scattered.

Yeah.

For main sequence stars there exists an empirically-established relationship between mass and radius, where $R\propto M^a$, in solar units. The exponent $a$ is around 0.8-0.9.
If you simply went with the disc being as many times smaller as the star is less massive, you wouldn't be far off the mark.
Remember, at the lowest mass end you're looking at ~Jupiter-sized discs.

Huh. That seems to fly in the face of the idea that dwarfs are 30-100x more dense.

 

[Bandersnatch]

Huh. That seems to fly in the face of the idea that dwarfs are 30-100x more dense.

1/10 the mass in 1/10 the radius means... how much higher density?

 

[Vanadium 50]

(1) Yes, the radius is not that small - 0.1 to 0.15 solar radii. I don't know why people didn't just look that up rather than messing with density.

(2) A million miles is way, way close. The sun's diameter isn't much smaller. You will have enormous tides and likely enormous mass transfer. I don't think your stars will be round, and their evolution will be very weird.

 

[Algr]

(Which might be kind of a let-down spectacularity-wise.)

Since this is Science Fiction and Fantasy Media, it makes sense to start with a cool looking image and then try to justify it. So I'll try a bigger star?

...halo of red around it, as in your depictions. The only reason I can think of for it to have a halo is cloud reflection...

Yes, that is what I had in mind. It is a smaller version of what the big star is doing. Lightening the clouds in that spot produced red, so I guess I need to actively put more orange in.

For example could you detect a quarterphase dwarf as such?

Do you mean phases like venus? I don't think a radiant object would have that. Or could reflected light be of comparable brightness to the radiant light? If this happens at all, it might be too subtle to bother with.

they can be 30-100 times denser than our sun, which would make its disk a lot smaller than your depictions - more like planet-sized in that shot.

(1) Yes, the radius is not that small - 0.1 to 0.15 solar radii. I don't know why people didn't just look that up rather than messing with density.

My impression was that density would drop dramatically as soon as fusion ignites. When the experts disagree, go for rule of cool!

(2) A million miles is way, way close. The sun's diameter isn't much smaller. You will have enormous tides and likely enormous mass transfer. I don't think your stars will be round, and their evolution will be very weird.

Uh oh! I went for a little over 2x solar diameters distance. (432,000 miles) If a solar system is chosen for terraforming, then it makes sense that it would not be a typical system, but one with extraordinary properties that improve its potential for human habitation.

Thank you!

 

[Vanadium 50]

I'm not sure this makes things more habitable.

Red dwarfs are often - perhaps always - flare stars.
As I said, with that much mass transfer, the evolution of both stars will be weird. I suspect that the ner effect will be to cause the sun-like star to evolve off the main sequence quickly.
The red dwarf's orbintal period will be around 9 hours. It's not clear the planet's orbit will be stable, the rotation will almost surely be tidally locked, and I'd worry a lot about tidal heating.

 

[Algr]

Well the planet is roughly 93 million miles away from the other two, because the big one is the same as Earth's sun. So flares would be too far away, and no reason for tidal locking.

That nine hour figure is important, because we then can't keep using the same shot. It would have to be planned out from the script to be in different places all the time. Of course in mid day it might be lost in the glare of the big star.

I suspect that the ner effect will be to cause the sun-like star to evolve off the main sequence quickly.

This still means billions of years though, right?

 

[snorkack]

As I said, with that much mass transfer, the evolution of both stars will be weird. I suspect that the ner effect will be to cause the sun-like star to evolve off the main sequence quickly.

I actually suspect the opposite.
For sizes of Roche lobes see the approximations at https://en.wikipedia.org/wiki/Roche_lobe
q is in the range between 2 and 10?
For q=8, I get r₁/A around 1,96/(2,4+ln3) which is close to 0,6. So at "million miles", Sun should not fill its Roche lobe. And on your images, the distance looks rather bigger than a million miles.
Now suppose Sun does have a dense companion and does fill its Roche lobe. How would Sun evolve?
In the absence of Roche lobe, what Sun is doing on main sequence is slowly accumulating He, with the result that it slowly brightens and expands already on main sequence. (When subgiant and giant, brightening and expansion will be fast).
In the presence of Roche lobe, if Sun at 1,00 solar masses slowly brightens and spills over mass to its red dwarf companion, Sun becomes less massive and therefore dimmer. Gradual loss of mass will counteract the gradual buildup of He and thus counteract increase of luminosity, in long term!As for density: no, ignition of fusion at the start of main sequence does not cause a dramatic decrease of density. It stops a gradual increase of density and stabilizes the density at its main sequence levels.

 

[Vanadium 50]

So flares would be too far away, and no reason for tidal locking.

If it were orbiting one star, I agree. But its orbiting two: the torque is (ballpark guestimate) 100,000x larger on the planet. The best you can hope for is for the planet to lock to the 9 hour orbit of the red dwarf. The worst? Maybe that tidal heating melts your planet's crust.

 

[Vanadium 50]

This still means billions of years though, right?

That is a very good question.

The red dwarf is fully convective, so it has helium in the outer layers. Transfer of matter from the red to the yellow star is helium-rich, and in the other direction, helium-poor. S o even at equilibrium, the net effect is to move helium from the red to the yellow, aging it.

On the one hand, the red dwarf isn't making all that much helium. On the other, the effect on the yellow star is highly non-linear. Worse, the yellow star will surely becomes a fast-rotator, and these are usually heavy and short-lived. I don't know if people even model fast-rotating G-class main sequence.

What is knwon is that there are a class of stars in globular clusters called "blue stragglers", which have had a lot of mass transfer. They have gone from long-lived red stars to shorter-lived bluer stars,

 

[snorkack]

If it were orbiting one star, I agree. But its orbiting two: the torque is (ballpark guestimate) 100,000x larger on the planet.

Er, why?
Earth is perceptibly tidally influenced by two bodies: moon and sun. Tides due to sun are there but they don´t add 100 000x bigger torque - they add 20% to the torque.
Second star should have a modest tidal influence.

The red dwarf is fully convective, so it has helium in the outer layers.

True but irrelevant. Over the lifetime of a yellow star, a convective red dwarf - which a large part of red dwarfs are not - does not build up any notable amounts of He.

Transfer of matter from the red to the yellow star is helium-rich, and in the other direction, helium-poor. S o even at equilibrium, the net effect is to move helium from the red to the yellow, aging it.

The red dwarf is denser. Transfers will be one way - and loss of mass will counteract aging.

 

[Algr]

The red dwarf is fully convective, so it has helium in the outer layers. Transfer of matter from the red to the yellow star is helium-rich, and in the other direction, helium-poor. S o even at equilibrium, the net effect is to move helium from the red to the yellow, aging it.

But the red dwarf just isn’t that big. It’s entire mass is just a few percent of the yellow, so how could it have such a big effect.

Also, the system would likely be younger than Earth/Sol, so how much time will all this transfer take?

The oval sun version seems to have increased the distance between the two bodies somewhat. But I am still suspicious that the effect would be that large.

 

[snorkack]

But the red dwarf just isn’t that big. It’s entire mass is just a few percent of the yellow, so how could it have such a big effect.

If it is a red dwarf, it is more than a few %.
You could also have a brown dwarf. How strong is the brightness contrast between dark and light side of a hot Jupiter?

 

[Vanadium 50]

so how could it have such a big effect.

There are some really high powers in stellar physics. I recall an answer to a problem set asking me to calculate some functional form (but forget the exact problem) and it went as the 18th power. So I am very hesitant to say something doesn't matter because it's only 10%.

And everything is coupled - the helium goes up, so he opacity goes up, so the radiative transfer goes down, so the temperature goes up, so the fusion rate goes up, so the helium goes up...

It may well be that the effect is small. It also may well be that the effect is large. For blue stragglers, which probably but not certainly have had a bigger impact, their lifetimes are in the one-billion-year ballpark.

How fast will the mass transfer take? Hard to tell, as a) I don't do such modeling, and b) I am not sure that two stars would form this close. However, you can look up W Ursae Majoris variables, which have the sorts of distances and periods you want. Most of the secondaries are a little heavier than what you propose (as I said, I think it's unlikely to form this way) and because the mass transfer is so large, they have a single envelope and thus a single spectral class.

By the way, the tidal forces on your stars are about 2g from one side to the other, for both stars. I would expect the large star to be fairly aspherical.

 

[Algr]

So I take it that this doesn't work either?

Wikipedia says that AB Andromedae is 5.5 billion years old, and those stars are close in size to what I was working on. I'm wondering how literal the term "Contact Binary" is meant to be, given the distances involved.

 

[snorkack]

The list of W UMa variables is here:
https://en.wikipedia.org/wiki/W_Ursae_Majoris_variable

 

[Vanadium 50]

Wikipedia says that AB Andromedae is 5.5 billion years old,

True, but it also says the uncertainty is 2 Gy. I have no reason to distrust the central value, but the large error is probably real. AB Andromedae has the right period (check) and the closeness is mass makes formation less problematic (check), but the two stars share a common envelope, and that's not what your picture shows.

Also, the smaller star is impacted by the larger star. On its own, it should be a K, maybe a K5. But because it shares an envelope it is a G2.

Of course, the heavier you make the 2nd star, the more dramatic the tidal forces will be.

 

[Vanadium 50]

The list

A list.

 

[sophiecentaur]

Is it not true that in a scenario such as this thread is discussing, the whole Solar system would be much less stable and the planetary orbits would be 'all over the place'. That would involve massive temperature swings over millennia. How would that affect the development of life on our planet?

I recently read The Three Body Problem. It's a SciFi book by a Chinese author who describes this sort of situation in an immersive computer game. Aliens were also involved. To make it all work, they needed to have crazy life styles / cycles which involved extreme hibernations and aestivation (the equivalent in extreme summers). Total nonsense but it made me think. I won't be reading the sequels.

 

[snorkack]

Is it not true that in a scenario such as this thread is discussing, the whole Solar system would be much less stable and the planetary orbits would be 'all over the place'.

I doubt it. Consider that a lot of three body systems are quite stable in long term where the periods are well separated, such as Sun-Earth-Moon.

 

[sophiecentaur]

I doubt it. Consider that a lot of three body systems are quite stable in long term where the periods are well separated, such as Sun-Earth-Moon.

When you consider that our disastrous climate change is due to just a few degrees of temperature, is there evidence from any binary system elsewhere that the planets have stable temperatures? Can we be that accurate in a few decades of observations

 

[snorkack]

When you consider that our disastrous climate change is due to just a few degrees of temperature, is there evidence from any binary system elsewhere that the planets have stable temperatures? Can we be that accurate in a few decades of observations

We don´t have actual long observations of circumbinary planets, true, but we do have celestial mechanics.
Who can make more informed comments on the stability of systems with widely separated periods?

 

[Algr]

When you consider that our disastrous climate change is due to just a few degrees of temperature, is there evidence from any binary system elsewhere that the planets have stable temperatures? Can we be that accurate in a few decades of observations

The problem with human-caused climate change is the speed at which hit happens, not the specific temperature changes. Earth has massive temperature and condition from equator to poles, but both can have complex life. Presumably the red dwarf companion would have always been there - we aren't talking about introducing one suddenly.

 

[DaveC426913]

The problem with human-caused climate change is the speed at which hit happens,

The other problem with it is the assumption that climate change should not happen. Extinctions and ecological upheaval happen all the time in deep time. We humans just don't want it to happen on our watch.
(But this is a verboten subject, so let's tie this off.)

 

[Algr]

 

[Vanadium 50]

I think the lobes are thought to be filled, a la Wikipedia.

As far as climate change, the question is "from what to what?" It may be what you need to make the planet habitable.

I still have doubts that a planet will not tide-lock.

 

[Vanadium 50]

Just for fun, I calculated what Proxima Centauri would look like from a planet orbiting Alpha Centauri A. It would be somewhere between 3rd and 4th magnitude. Maybe noticed by the natives (if any), maybe not.

 

[Algr]

I still have doubts that a planet will not tide-lock.

My understanding of gravity is that once you are a certain distance away, it won't matter if the mass you are orbiting is all in the center or in that of two stars. Perhaps if it was a 4:1 or 10:1 ratio, but I was describing a 93:1 ratio.

Just for fun, I calculated what Proxima Centauri would look like from a planet orbiting Alpha Centauri A. It would be somewhere between 3rd and 4th magnitude. Maybe noticed by the natives (if any), maybe not.

Interesting. Not many planets anywhere will have Red Dwarfs naked-eye-visible at all.
So I'm looking this up on Wikipedia and the skies from any planet in that system could never look like Tatooine. My drawing up top might be possible at some times of the year, if a planet was in the habitable zone of A or B. But then the distant star would appear far from the large one most of the year, and could be on the opposite side of the sky.

 

[Vanadium 50]

The degree of torque on the planet depends on how aspherical the body they are orbiting. The sun is spherical (mostly). Two suns are not.

Yes, Proxima will appear as a star in the sky, and as such the primary sun will move through its equivalent of the zodiac.

 

[snorkack]

The degree of torque on the planet depends on how aspherical the body they are orbiting. The sun is spherical (mostly). Two suns are not.

No. The torque depends on how aspherical the planet is.

 

[sophiecentaur]

I still have doubts that a planet will not tide-lock.

Stars are pretty fluid objects so I'd imagine that they would form a fast rotating spheroid. Would the presence of a nearby mass disturb this, bearing in mind that the orbit period would have to be many times greater than the rotation period (years vs hours)? That implies no locking(?).