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How visible is Sirius B from Sirius A?



 
 
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  #1  
Old March 3rd 14, 03:05 AM posted to sci.astro
Kate Orman
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Posts: 1
Default How visible is Sirius B from Sirius A?

Hi. I'm an SF writer, and many years ago I asked a pretty basic question here in sci.astro and got some extremely helpful answers, so I thought I'd have another go!

My story's set on a planet fairly closely orbiting Sirius A. My question is: how well (on average) could the inhabitants see Sirius B? Would it be too far away to seen without a telescope, or would it shine brightly enough to be seen by the naked eye?

Many thanks for any help anyone can give me!

Cheers,
- Kate
  #2  
Old March 3rd 14, 04:34 AM posted to sci.astro
Yousuf Khan[_2_]
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Posts: 1,692
Default How visible is Sirius B from Sirius A?

On 02/03/2014 9:05 PM, Kate Orman wrote:
Hi. I'm an SF writer, and many years ago I asked a pretty basic
question here in sci.astro and got some extremely helpful answers, so
I thought I'd have another go!

My story's set on a planet fairly closely orbiting Sirius A. My
question is: how well (on average) could the inhabitants see Sirius
B? Would it be too far away to seen without a telescope, or would it
shine brightly enough to be seen by the naked eye?

Many thanks for any help anyone can give me!


Sirius A and B orbit each other between 8.2 to 31.5 AU, so basically the
orbit ranges from around the orbit of Saturn to around the orbit of Pluto.

I'd say at its closest approach, Sirius B should be more luminous than
Saturn. And even at its furthest approach it should be luminous enough
to be seen at least with binoculars. Usually the planets further than
Saturn aren't visible to the naked eye, but this is a star after all so
it generates its own light.

Yousuf Khan
  #3  
Old March 3rd 14, 09:55 AM posted to sci.astro
Mike Dworetsky
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Posts: 715
Default How visible is Sirius B from Sirius A?

Yousuf Khan wrote:
On 02/03/2014 9:05 PM, Kate Orman wrote:
Hi. I'm an SF writer, and many years ago I asked a pretty basic
question here in sci.astro and got some extremely helpful answers, so
I thought I'd have another go!

My story's set on a planet fairly closely orbiting Sirius A. My
question is: how well (on average) could the inhabitants see Sirius
B? Would it be too far away to seen without a telescope, or would it
shine brightly enough to be seen by the naked eye?

Many thanks for any help anyone can give me!


Sirius A and B orbit each other between 8.2 to 31.5 AU, so basically
the orbit ranges from around the orbit of Saturn to around the orbit
of Pluto.
I'd say at its closest approach, Sirius B should be more luminous than
Saturn. And even at its furthest approach it should be luminous enough
to be seen at least with binoculars. Usually the planets further than
Saturn aren't visible to the naked eye, but this is a star after all
so it generates its own light.

Yousuf Khan


I'm not sure how you are doing this calculation. The absolute magnitude of
Sirius B is (from references) about +11.5, so at 31.5 AU, log(d) = -3.82 (d
in parsec). The usual distance modulus formula m - M = 5logd -5 then gives
apparent magnitude m = -12.6, roughly as bright as the average full moon
from Earth. At 8 AU it should be about 15 times brighter than that.

Incidentally, Sirius is around 25 times more luminous than the Sun, so if
the supposed planet of the OP is meant to have Earthlike conditions, it
needs to be about 5 AU from the primary, so the distance to Sirius B would
range from around 3 AU to 26.5 AU at opposition (if the orbit is roughly
circular).

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

  #4  
Old March 3rd 14, 10:28 PM posted to sci.astro
Steve Willner
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Posts: 1,172
Default How visible is Sirius B from Sirius A?

In article ,
"Mike Dworetsky" writes:
I'm not sure how you are doing this calculation. The absolute magnitude of
Sirius B is (from references) about +11.5, so at 31.5 AU, log(d) = -3.82 (d
in parsec). The usual distance modulus formula m - M = 5logd -5 then gives
apparent magnitude m = -12.6, roughly as bright as the average full moon
from Earth. At 8 AU it should be about 15 times brighter than that.


Looks right to me. Worth noticing is that Sirius B would be a point
of light, not extended. Looking at it with human eyes would probably
burn a blind spot in one's retina. A single spot would be tiny and
therefore unnoticed in normal life, but an accumulation of blind
spots would significantly decrease vision. Presumably the
inhabitants of such a planet would have evolved some protection
against this danger, though I'm not sure quite what. Maybe just a
reflex aversion to looking at bright lights would suffice.

Incidentally, Sirius is around 25 times more luminous than the Sun, so if
the supposed planet of the OP is meant to have Earthlike conditions, it
needs to be about 5 AU from the primary, so the distance to Sirius B would
range from around 3 AU to 26.5 AU at opposition (if the orbit is roughly
circular).


Hard to imagine a planet 5 AU from Sirius A having a stable orbit.
There must be some calculation of how small the orbit would have to
be, but I don't know where to find it. Even 1 AU, where mean
temperature would be about 330 deg_C, seems dubious to me.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #5  
Old March 4th 14, 10:13 AM posted to sci.astro
Mike Dworetsky
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Posts: 715
Default How visible is Sirius B from Sirius A?

Steve Willner wrote:
In article ,
"Mike Dworetsky" writes:
I'm not sure how you are doing this calculation. The absolute
magnitude of Sirius B is (from references) about +11.5, so at 31.5
AU, log(d) = -3.82 (d in parsec). The usual distance modulus
formula m - M = 5logd -5 then gives apparent magnitude m = -12.6,
roughly as bright as the average full moon from Earth. At 8 AU it
should be about 15 times brighter than that.


Looks right to me. Worth noticing is that Sirius B would be a point
of light, not extended. Looking at it with human eyes would probably
burn a blind spot in one's retina. A single spot would be tiny and
therefore unnoticed in normal life, but an accumulation of blind
spots would significantly decrease vision. Presumably the
inhabitants of such a planet would have evolved some protection
against this danger, though I'm not sure quite what. Maybe just a
reflex aversion to looking at bright lights would suffice.


Yes, I was aware of this issue, but it pales into insignificance compared to
the next one.


Incidentally, Sirius is around 25 times more luminous than the Sun,
so if the supposed planet of the OP is meant to have Earthlike
conditions, it needs to be about 5 AU from the primary, so the
distance to Sirius B would range from around 3 AU to 26.5 AU at
opposition (if the orbit is roughly circular).


Hard to imagine a planet 5 AU from Sirius A having a stable orbit.
There must be some calculation of how small the orbit would have to
be, but I don't know where to find it. Even 1 AU, where mean
temperature would be about 330 deg_C, seems dubious to me.


I agree, but I did want to go into all those details at once. The period of
the 5AU hypothetical would be fairly short (rough Kepler's third law calc
yields around 6-7 years) compared to the longer Sirius B orbit. But an
occasional regular peri-sirion by a massive companion star (around 1 solar
mass for B) ought to make stability difficult and a circular orbit
impossible.

So a close orbit is required for stability but makes the surface conditions
approach those of Venus.

The evolutionary state of the Sirius system would make habitability
questionable anyways--Sirius B evolved first, and would have been a red
giant, cooking any planet in the system some time in the past few hundred
million years.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

  #6  
Old March 4th 14, 04:49 PM posted to sci.astro
Yousuf Khan[_2_]
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Posts: 1,692
Default How visible is Sirius B from Sirius A?

On 04/03/2014 4:13 AM, Mike Dworetsky wrote:
Steve Willner wrote:
Hard to imagine a planet 5 AU from Sirius A having a stable orbit.
There must be some calculation of how small the orbit would have to
be, but I don't know where to find it. Even 1 AU, where mean
temperature would be about 330 deg_C, seems dubious to me.


I agree, but I did want to go into all those details at once. The
period of the 5AU hypothetical would be fairly short (rough Kepler's
third law calc yields around 6-7 years) compared to the longer Sirius B
orbit. But an occasional regular peri-sirion by a massive companion
star (around 1 solar mass for B) ought to make stability difficult and a
circular orbit impossible.

So a close orbit is required for stability but makes the surface
conditions approach those of Venus.

The evolutionary state of the Sirius system would make habitability
questionable anyways--Sirius B evolved first, and would have been a red
giant, cooking any planet in the system some time in the past few
hundred million years.


Actually, the entire evolutionary timeline of the Sirius system would
make it difficult for a native race of sentients to evolve on it. It's
only a quarter of a billion years old, it's main sun has already
undergone a red giant transition a few million years ago, and it's
current main sun only has another 1 billion years left before it too
goes red giant. However, it's possible that an emigrant race of
sentients could've settled on a planet in that system and terraformed it.

Yousuf Khan
  #7  
Old March 4th 14, 07:38 PM posted to sci.astro
Brad Guth[_3_]
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Posts: 15,175
Default How visible is Sirius B from Sirius A?

On Sunday, March 2, 2014 6:05:16 PM UTC-8, Kate Orman wrote:
Hi. I'm an SF writer, and many years ago I asked a pretty basic question here in sci.astro and got some extremely helpful answers, so I thought I'd have another go!



My story's set on a planet fairly closely orbiting Sirius A. My question is: how well (on average) could the inhabitants see Sirius B? Would it be too far away to seen without a telescope, or would it shine brightly enough to be seen by the naked eye?



Many thanks for any help anyone can give me!



Cheers,

- Kate


Sirius(b) is mostly emitting photons in the UV spectrum, so don't expect any human visual capability of even telescope aided views of that one.
  #8  
Old March 5th 14, 01:11 AM posted to sci.astro
Bernard Isker
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Posts: 23
Default How visible is Sirius B from Sirius A?

How about the Dogon who indicate that there is a Sirius C? If it exists it
must be very dim and orbiting far from Sirius A+B. I would think it would
have been found by now if it exists. But you never know.

I doubt if Astonomers have spent a lot of large telescope observatory time
looking.

As far as I know the Dogon don't give much detail on this star since their
religion shrouds it in mystery.


"Brad Guth" wrote in message
...
On Sunday, March 2, 2014 6:05:16 PM UTC-8, Kate Orman wrote:
Hi. I'm an SF writer, and many years ago I asked a pretty basic question
here in sci.astro and got some extremely helpful answers, so I thought
I'd have another go!



My story's set on a planet fairly closely orbiting Sirius A. My question
is: how well (on average) could the inhabitants see Sirius B? Would it be
too far away to seen without a telescope, or would it shine brightly
enough to be seen by the naked eye?



Many thanks for any help anyone can give me!



Cheers,

- Kate


Sirius(b) is mostly emitting photons in the UV spectrum, so don't expect
any human visual capability of even telescope aided views of that one.



  #9  
Old March 5th 14, 06:31 AM posted to sci.astro
Brad Guth[_3_]
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Posts: 15,175
Default How visible is Sirius B from Sirius A?

On Tuesday, March 4, 2014 4:11:21 PM UTC-8, Bernard Isker wrote:
How about the Dogon who indicate that there is a Sirius C? If it exists it

must be very dim and orbiting far from Sirius A+B. I would think it would

have been found by now if it exists. But you never know.



I doubt if Astonomers have spent a lot of large telescope observatory time

looking.



As far as I know the Dogon don't give much detail on this star since their

religion shrouds it in mystery.





"Brad Guth" wrote in message

...

On Sunday, March 2, 2014 6:05:16 PM UTC-8, Kate Orman wrote:


Hi. I'm an SF writer, and many years ago I asked a pretty basic question


here in sci.astro and got some extremely helpful answers, so I thought


I'd have another go!








My story's set on a planet fairly closely orbiting Sirius A. My question


is: how well (on average) could the inhabitants see Sirius B? Would it be


too far away to seen without a telescope, or would it shine brightly


enough to be seen by the naked eye?








Many thanks for any help anyone can give me!








Cheers,




- Kate




Sirius(b) is mostly emitting photons in the UV spectrum, so don't expect
any human visual capability of even telescope aided views of that one.



Supposedly some efforts have been made to discover Sirius(c), or anything else associated with Sirius(a) and (b). At least so far, no luck.

What's needed is a good StarShade.
http://en.wikipedia.org/wiki/New_Worlds_Mission
http://www.newscientist.com/article/...l#.Uxa0_vldV0w

However, we should be capable of getting a very good look-see at the photosphere of Sirius(a), because more than sufficient optics with ample magnification and optical filters have existed for more than a decade, and yet not one example of that enormous photosphere can be found (seems a bit odd).

  #10  
Old March 5th 14, 09:15 AM posted to sci.astro
Mike Dworetsky
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Posts: 715
Default How visible is Sirius B from Sirius A?

Brad Guth wrote:

Sirius(b) is mostly emitting photons in the UV spectrum, so don't
expect any human visual capability of even telescope aided views of
that one.


Which of course does not explain why Sirius B was discovered visually in the
1860s during testing of a new Alvan Clark refractor, and I myself saw it
through the Mt Wilson 60-in telescope in the 1970s. It's faint, but the
main viewing difficulty is its proximity to bright Sirius, not its low
luminosity.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

 




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