bert wrote:There are a very large number of stars in this photograph that form definite strings. It that due to the time delayed photographic process, or do stars actually form in strings accross space?
More often than not, long strings of stars are probably chance alignments.
Take a look at this David Malin/Anglo-Australian Observatory/Royal Observatory of Edinburgh image of the Pleiades:
A good thing about it is that it is labelled.
One of the more obvious "strings" of stars in the Pleiades seems to start some distance below Alcyone and extend downward and to the left, passing Merope. However, this string is partly a chance alignment.
The topmost star in this string, HD 23631, is probably a true Pleiad. Its parallax would seem to be wrong, but it hasn't been well measured, and its proper motion agrees well with the average proper motion of the Pleiades. Its color and brightness is also right for a member of the Pleaides of spectral class A.
The second star from the top, HD 23632, is definitely a Pleiad. Its parallax has been well measured and agrees very well the average parallax of the Pleiades. Its proper motion is just right, as is its color, brightness and measured spectral class, A1V.
The third star from the top, HD 23609, is not
a Pleiad. Its parallax has been well measured, and it is only about half as far away as the true members of the Pleiades. Unsurprisingly, its proper motion also disagrees with the Pleiades, and its color and brightness is wrong for a star at the distance and general reddening of the Pleiades.
The fourth star from the top, HD 23643, is a Pleiad. Its parallax does not make sense, but it has not been well measured. Its proper motion is just right, as is its color and brightness for a member of the Pleaides of spectral class A.
The fifth star from the top, HD 23654, is not a Pleiad. Its measured parallax is unrealiable, but its proper motion is wrong, as is its color and brightness.
The sixth star, HD 23665, is also not a Pleiad. Again its parallax is unreliable, but again its proper motion is wrong, as is its color and brightness.
How do we know what the color and brightness of a star belonging to the Pleiades cluster "ought" to be? We can figure that out by constructing a color-magnitude graph, where we plot the color and the brightness of all the members of the Pleiades. In this graph, the vertical axis measures the brightness (or V magnitude) of the members of the Pleiades, and the horizontal axis measures the B-V color index of the stars.
We can see from this graph that the brightest stars of the Pleiades are the bluest, and the fainter a member of the Pleiades is, the redder it is. Not only that, but we can tell from this graph exactly how bright a member of the Pleiades ought to be if it is a certain color, and in the same way, we can say what color it ought to be if it is of a certain brightness.
If we find stars which appear to be located in the Pleiades cluster, but whose position in the color-magnitude graph is wrong, and if we can determine that their proper motion is also wrong, then we can be sure that these stars are not members of the Pleiades. Instead, they are either background or a foreground stars.
Therefore, many long strings of stars are really chance alignments.
Last edited by Ann on Thu Sep 22, 2011 8:59 am, edited 2 times in total.