i am researching material for a book.  can anyone confirm whether sirius- aka the dog star - is moving closer to the sun?

i read that ancient astronomers recorded that sirius was red prior to about 50 bce.  would that indicate that it was moving further away from the sun? does its present color indicate that it is moving towards us?  if all true, what would explain the change in direction?

i have learned over the course of many writing projects not to trust everything i read.  anybody's help on these questions is greatly appreciated.  you may email me at architect77777@aol.com if you like.   thank you.

JA

Correction: Sirius and the Sun are approaching one another at approximately 7.6 kilometers per second.  This radial motion has nothing to do with the red color reportedly associated with Sirius in ancient times. 

A possible scientific explanation is offered at the bottom of this page.  Alternative explanations include an interaction with a third as yet undiscovered star, the possibility that it was simply the use of poetic licence, and the fact that scintillation and atmospheric prismatic dispersion can make Sirius appear red, as well as other colors, when it is close to the horizon.

Dave Mitsky

second sight:

can anyone confirm whether sirius- aka the dog star - is moving closer to the sun?

Welcome to the discussion group, second sight.

Yes, Sirius and the Sun are moving closer to each other by 7.6 km every second.

Thank you for your response but I am left with a problem. 

I heard from another member who said sirius was moving away from the sun at 7.6 kilometers per second.  lol

How is such a thing determined?

oops sorry dave I didn't see that you corrected yourself.  thank you!

My next question.  does anyone know how close sirius and the sun will approach each other and when that would occur?

Dave i read the article regarding possible explainations for Sirius being red in the past.  The companion star changing from a red giant to a white dwarf is clever however I agree that the time period would have been too long for the Dogon tribe to remember.

I just finished reading Walter Cruttenden's book LOST STAR as part of my research for my book FIRST EARTH.  He speculates that Sirius and our Sun orbit each other. The change from red to "blue" might be because Sirius was moving away from the Sun and is now coming back.

I find this speculation fascinating because I wonder if this relationship would possibly provide the means to stir up the ort cloud periodically and send comets towards earth.

Has anyone speculated whether Sirius has any orbiting planets?

Relative to other stars, Sirius is very nearby (which is why it appears to be so bright). Even so, I doubt very much whether the Dogon myth is related to actual sightings of the companion.

Nevertheless the flaw I'd like to point out in your reasoning, as it would affect your story, is that we don't see a star's color as affected by its motion along our line of sight. It is the spectral lines that are shifted. 

The color we detect visually in stars is due to their temperatures, not their relative motion. The "color" attributable to Doppler-shift is well below the level of human visual sensitivity.

interesting.  as you can tell i am not an expert in this field.  so red shift is not visible to the naked eye or even through a telescope?

second sight:

oops sorry dave I didn't see that you corrected yourself.  thank you!

My next question.  does anyone know how close sirius and the sun will approach each other and when that would occur?

If my calculations are correct at a closure rate of 7.6 k/sec, and a distance of 8.5 ly, a fly-by won't happen for about 1.24 trillion years.  Of course both stars, and the Earth,  will be cinders by then.

if sirius were at the far end of an eliptical orbit and beginning to accelerate back to the sun,  would that make any difference in those calculations?  or would we still be talking about everything being in cinders by the time they came near each other?

second sight:

if sirius were at the far end of an eliptical orbit and beginning to accelerate back to the sun,  would that make any difference in those calculations?  or would we still be talking about everything being in cinders by the time they came near each other?

Calculating the acceleration as Sirius gets closer is beyond my skills, but I don't think it will speed up enough to get here before the Sun goes to its red giant phase in about 5 billion years.

second sight:

interesting.  as you can tell i am not an expert in this field.  so red shift is not visible to the naked eye or even through a telescope?

That's right. Not directly, anyway. That is, if you look through a telescope at a star field and see stars of different colors (a good example is Albireo, a double star with primary and secondary of obviously differing colors) your eye is picking up the color difference due to temperature.

Unless the velocity toward or away from the observer is a significant fraction of the speed of light, the change in color due to Doppler shift is below the threshold of the sensitivity of our eyes to color.

However, you can use a direct-view spectrograph to display the absorption lines in the spectra of stars. You calibrate such an instrument by viewing a lamp of known emittance (say, a sodium-vapor lamp) and noting where the emission and absorption lines occur relative to a scale in the instrument. That gives you a "standard" against which to measure the spectrum from a star. If you then aim the telescope such that the image of the star falls across the spectrograph's slit, you can use a micrometer to shift the image left or right along the scale until the absorption lines match up (for sodium, for example). The difference in the micrometer readings for the standard lamp versus the observed star will give you a factor you can apply to the wavelength scale and determine the percentage of shift toward red or blue. An equation will then give you the star's radial velocity.

Even amateurs can do this today, with reasonably inexpensive equipment -- especially if they already have a telescope and CCD imager with the right specifications. Spectrographs capable of doing this with amateur-class instruments range from about $1,800 to about $4,000.

thank you for this information. all the contributions to my questions have been extremely helpful. 

from your description of the science involved it seems to me that the story of ancient observers claiming sirius was red in the past and changed to blue is either a myth or has some other explaination that fits with in the context of what primitive people were capable of observing. the author of Lost Star may need to go back and revise some of his thinking.  i think that is what makes this so interesting in that we fish for possible solutions and eventually get a hit.

sirius does not appear to be a star i can use in my book.  i have to find some other candidate that can come close enough to the sun to shake comets loose and send them towards earth to create the climax of the novel.

i am thinking i will go back to the old nemesis theory of a brown dwarf acting as a companion star to the sun. 

do you or anyone else reading this string know of any evidence supporting that theory?

In the April 2006 star special issue there was an info box entitled Close stellar passages.

The local stars all pass close to the Sun. Sirius' closest pass is around 7.2 Ly.

This will be in 66,000 years.

The main danger from nearby stars is Gliese 710. It will pass just a lightyear away.

This is well into the Oort cloud. Comets will reign into the solar system.

Even the Alpha Centauri system will close to 3.7 Ly before receding. 

At this link, near the end of the page, there is a brief discussion of star perturbations and the stellar companion hypothesis (Richard Muller's idea that the Sun has a dark companion, aka: Nemesis). There is no credible evidence that this is the case despite a number of interesting attempts to locate one. However, the possibility remains ...

The star that Oliver mentioned comes near the outer edge of the Oort Cloud, by some estimates, but not "well within it". It need not, however, since it may have an Oort Cloud of its own and interaction between the two cometary clouds would be enough to shower both systems' inner regions with comets.

Don't hold your breath, however. A light year is about 6 trillion miles. Even at cosmic velocities (tens of thousands of kilometers per second) a disturbed comet is going to take a long time to reach the region of the Earth's orbit.

From the standpoint of your story, you're probably more interested in the date (year) something like an interstellar close encounter might happen, and the amount of time that would elapse before we'd be likely to be hit by cosmic debris. You'd have to find a candidate star (like the one Oliver mentioned), get is position and proper motion/velocity, and calculate the date.

IIRC, Barnard's Star will be the closest star to the Sun in 10,000 years or so.

Dave Mitsky 

Sirius and the Sun do not orbit each other.  Sirius is not headed in a straight line toward the Sun.  It has a “proper motion” component in a direction perpendicular to our line of sight.  Its current radial velocity in the line of sight of -7.6 km/sec will moderate over time.  Sirius is currently 8.6 light-years distant.  According to my calculation Sirius will reach a minimum distance of 7.8 light-years around the year AD 60000.   

The Dogon tribe of Africa amazed some anthropologists during the mid-20^th century due to their knowledge that Sirius has a small companion.  It was later realized that missionaries had visited the tribe during the 1920s, and knowing that the tribesman revered Sirius, they likely would have passed along the latest knowledge about Sirius being a binary star.  Also, some Dogon members served in Europe during World War I.

The purely speculative conjecture of a companion star to the Sun has been overdone on this message board recently due to the hype of a Planet X, Nibiru or Nemesis supposedly coming near us in 2012.  There is no evidence to support such a claim.  End of the world advocates will always be around to grab money from credulous believers.

second sight:

Dave i read the article regarding possible explainations for Sirius being red in the past.  The companion star changing from a red giant to a white dwarf is clever however I agree that the time period would have been too long for the Dogon tribe to remember.

I just finished reading Walter Cruttenden's book LOST STAR as part of my research for my book FIRST EARTH.  He speculates that Sirius and our Sun orbit each other. The change from red to "blue" might be because Sirius was moving away from the Sun and is now coming back.

I find this speculation fascinating because I wonder if this relationship would possibly provide the means to stir up the ort cloud periodically and send comets towards earth.

Has anyone speculated whether Sirius has any orbiting planets?

IMO, the Dogon tribe was pulling a fast one.

To the best of my knowledge, the widest known binary stars have separations of about 1000 astronomical units.  There are  63,240 AUs in one light year.  So Sirius and the Sun would be approximately 544 times farther apart than the most widely separated binaries.  Gravity would be very hard pressed indeed to maintain the two as a binary pair.

Dave Mitsky

Perhaps another story angle you could pursue is that the Sun may have been part of a nearby association in the distant past. An encounter with another star Back In The Day could result in a near-term (future) event that would fit your requirements.

Since stars often form in groups from the same collapsing interstellar gast/dust cloud, the Sun may have been part of such a group (perhaps the Ursa Major Moving Group) ...

Run the clock back a few billion years and theorize that as the individual members of this group dispersed, a close enounter between two of them (not including the Sun) could have ejected an object (like a dwarf star) in a direction which would put it on a collision course with us at some time in the then-distant future.

Then run the clock forward to the present or some point in the not-too-distant future to anticipate the encounter with the relic from our common stellar nursery.

??

thank you oliver.  66,000 years doesn't fit into my novel timeline easily unfortunately.  is it true that we can't see any of the dark stars that might be coming our way such as brown dwarfs?  or is there new methods to detect them that are deployed and scanning?

 by the way, i have visited your city.  i took the tour of John Wesley's church and thoroughly enjoyed the presentation by the gentleman portraying wesley.