Binary Star : Mean Proper Motions and True Motions

Bug #1271270 reported by Alexander Wolf on 2014-01-21
This bug affects 3 people
Affects Status Importance Assigned to Milestone

Bug Description


One of this continuing issues that I find with Stellarium is that moving into the distant future or sitant past shows the proper motions and true of binary stars, like Alpha Centauri, through space.

[Other examples are; Acrux, Gamma Centauri, Mu Verorum, Delta Velorum, y Centauri [not letter Gamma] or Beta Muscae. Northern examples, Delta Cygni, Alpha Piscium, Epsilion Hydra, The worst example is 61 Cygni, which at cuurrent date has the stars already too far

Here, in case of Alpha Centauri, A and B stars have different proper motions, using the HIP data, and these are displayed as two different diverging stars, flying apart, which simply get wider and wider as separate stars. (All these stars above do too!)

Alpha Centauri is the principle example, as it is the nearest star with very high proper motions. Using Stellarium as a planetarium for the past or future sky shows very poor educational value if the stars are not behaving as they should.

Exampled, for Alpha Cen A (HIP 71683) have individual proper motions pmRA= −3679.25 and pmDec.=+473.67 mas per year and Alpha Centauri B (HIP 71681) as −3614.39 and +802.98 mas per year.

The mean of these motions should adopted as a first approximation, I.e. When combined as −3646.82 and +638.25 mas per year. Hence, the proper motions for both stars would be better expressed as this value.

Usually, there are inherent problems with the companions proper motions, as their individual deviations in the directions they are travelling are not constant due to the binary system's orbital motion. Nominally, the true measure of the proper motion should be when both stars are travelling together (a line perpendicular to position angle of the direction the system is moving. The quoted proper motions here are not under this condition. [There are serious doubts on the given proper motion in declination for the B-star, and it could be easily argued that the A star motion should apply to both. Reasoning to explain this is not easy to say in a few words - at least in this expression of a problem.]

There are more serious issues than proper motions here, as the differing radial velocities of both stars. This changes over the 79.9 year orbital period, whose velocity varies by the orbital eccentricity / true distance the stars are apart. It is the mean or average radial velocity that is important here, being 25.1+/-0.3 km per second.

The other issue is that Stellarium data does not have radial velocity information, which is required to show not only the proper motion changes, but the real changes in distances by their true individual motions in space. Hence, the parallax and proper motions are actually variables not constants over time, where knowing the radial velocity means you can simply calculate these true motions.

If Stellarium had this too, when you advance or return in time, for example, the star magnitudes would change as they move towards or move away from the Sun. Like Aplha Centauri, the system is moving towards us, decreasing in distance but increasing in brightness. From current data, the maximum will be reached in 29,240+/-1,370 AD when the parallax will be +1098 mas. and Alpha Centauri shines at −1.05 than −0.27 magnitude today.

At present, the motions of stars into the future and past just stay the same in magnitude only taking into account a constant proper motion across the sky. [The only other addition is precession, which is nothing to do with true stellar motions.]

Suggested Reading to solve this is something like;

Matthews, R.A.J., "The Close Approach of Stars in the Solar Neighbourhood", QJRAS. (Quarterly Journal of the Royal Astronomical Society), 35, 1 (1994) @….1M

The maths to do this is simple (pg.2) and velocities (pg.7)

In some future update, if the radial velocities were just added to the star data file, and the individual proper motions combined for binary stars, Stellarium would be a much more realistic software for the past and future sky. Education wise it is a must-do!!!

In the meantime, Alpha Centauri and the stars given above should be expressed with mean proper motions and mean parallaxes.

Note: If there were a bigger wish list, these binary stars should show their orbital motions over time.

summary: - Binary Star : Mean Proper Motions and True Motions Binary Star : Mean
- Proper Motions and True Motions
+ Binary Star : Mean Proper Motions and True Motions
Changed in stellarium:
importance: Undecided → Wishlist
gzotti (georg-zotti) wrote :

Good points, I would like to have better historical accuracy (from true spacial motion) as well! This should go at least into a blueprint!

Changed in stellarium:
status: New → Triaged
Ariane Stevens (sjstar) wrote :

I would not call this a bug, it is just a limitation.

Radial velocities added to the database would have to be added before it could apply to all stars.

As the timeframe changes, the positions, magnitudes, distances, parallaxes and proper motions would all change based on the date. The motion also would also accelerate or slow down as they respectively approach us or recede from us.

However, this would not change binary stars. To fix this you would need the mean proper motions, and the mean radial velocity (the components oscillate above and below the mean velocity, termed /gamma/ as seen is spectroscopic binaries. It could be estimated based on the eccentricity, and even the inclination as well. This would not be simple. Some average value would suffice as general use of the software.

Other programs, like Voyage 4 apply the radial velocity so you can see all these changes, but the binaries still fly apart moving off in their programmed proper motions.
Radial velocities for all the Hipparcos stars would use either "Radial velocities for XHIP catalogue (de Bruijne+, 2012)" or "Extended Hipparcos Compilation (XHIP)" (Anderson+, 2012) "Another useful one is General Catalog of mean radial velocities." (Barbier-Brossat+, 2000) Stand for most is the "Pulkovo radial velocities for 35493 HIP stars" (Gontcharov, 2006). This might suffice!)

For the more distant stars, these effects are trivial, so applying this to down to 9th and 10th magnitude would be sufficient. Applying it to all star with 200 pc, would be desirable, as the +/-99999 years, makes little difference to stellar positions.

All data would be available using the CDS Vizier.

Thankx, Ari

tags: added: binary-stars
tags: added: double-stars
removed: binary-stars
tags: added: catalog
Alexander Wolf (alexwolf) wrote :

The bug report was moved to github:

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