Expert: Tom Whiting - 1/18/2007

Question
Hi.
I find star distances to be stated inconsistently in various sources. For example, I just encountered a Web site (put up by a professional astronomer) which gives the distances of all three of Orion’s belt stars as  1500 LY, whereas in other sources I see the three stars at different distances: e.g., 800,900 and 1300 LY. Is there some authoritative source, hopefully on the Web, that I could turn to in order to be sure of the right figure?
Thanks
David  

Answer
Hi David,
Yes, stellar distances (especially those beyond 100 lightyears) are one of the toughest quanitities to measure with any kind of absolute accuracy.  In fact, we can effectively say that until we get a spaceship there, we will never know their exact absolute distance.  Right now, the most "authoritative" source is from the Hipparcos Satellite of the 1990's that measured the stellar parallax to an accuracy of 0.002 arcseconds in most cases.  This extends our "fairly accurate" picture from say 100 lightyears out to about 500 lightyears, but the stars in question, Orion's Belt stars, are much farther, so we just, at this time, cannot be super-accurate like we can for the "nearby" stars.

The reason for this is historical.  It's all built on a "ladder"
of distance measurement.  When Friedrich Bessell first measured 61 Cygni using stellar parallax in 1838, he found a method that is quite accurate, but only good out to about 100 lightyears.  This is still the preferred method for high accuracy...the only problem is, stars beyond 100 ly have such
a small parallax angle, one just cannot measure this small
angle.  So the next ladder system was use of the H-R diagram,
whereby astronomers determine both the absolute and the
apparent magnitudes, subtract the two, and determine the
distance indirectly.  (average accuracy, but it introduces a
known error of about 10-20%...but it's the only way we can
do it.)  Next is the Cepheid Variable method, but this introduces another 10% error, then next is the red shift and
still another 10% error....and so on.  Each rung of the distance
ladder and you sacrifice more accuracy...that's just the way
it works, and nothing we can do about it.

Bottom line is, today, beyond 500 lightyears, there is just no
good method known to determine a strict, absolute accuracy
to a stellar distance.  AND, this is WHY you see various
sources list different results.
Prior to Hipparcos Satellite measurements, we could only
determine the "Orion" grouping with a rough estimation.....
and since the Orion Nebula (M-42) measured out at around
1500 ly, most astronomers placed most of the Orion bright
blue white giants, including Rigel,  at that same distance of 1500 ly.  But today, I think the Hipparcos measurements refined that distance to 900, 1300, and 820 ly, west to east (or right to left as seen from the N. Hemisphere) respectively for  Delta, Epsilon, and Zeta Orionis.....or common names Mintaka, Alnilam, and Alnitak respectively.   Realize, even with the Hipparcos results, these stars are beyond the accurate 500 ly limit, so I would automatically impose a + or -
10-20% unknown error to those distances.   I've even seen
one source where the distance to Delta Orionis quoted as
916 lightyears....3 significant figures!! which is ridiculous,
because the distance cannot be found to 3 Sig. figure
accuracy!!  (He probably just averaged a bunch of figures
to obtain that result, but he should not have used the 3
significant figure number......900 to 920 is more appropriate
for that star.
So bottom line is....the very best we can do right now is
900, 1300, and 820....right to left...Delta, Epsilon, and Zeta,
repectively.  (I, and most astronomers, prefer the Greek letter
designation for most of the stars as opposed to their
common names...then everyone worldwide...knows which
star you are talking about because that is the standard,
recognized system of naming stars).

PS...I think most astronomers don't get too "up tight" about
super-accurate stellar distances beyond 500 lightyears, because, well,  we aren't going to be visiting these bodies anytime soon anyway!  If we know their distances to an accuracy  of + or - 10%, that's close enough for now.
But in reality, there is just no good way of getting the
accuracy any better with today's technology.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA