231
from each other by at least 1 arcmin. Most people are unable to see Pleione because
of two other factors.
First, the 1 arcmin resolution is only obtained in bright light, when the resolution,
or point spread function (psf), of the human eye is dominated by the cones of the
retina. In faint light, human vision relies more on the rods that are sparsely distributed around the retina, and have a much broader psf.
A second factor, called “glare function” by physiologists, is what prevents you
from seeing details next to car headlights pointing at you. The glare function
depends on the dynamic range and psf of the human eye. Imperfections in the
human eye give it a psf which has a broad base a few arcmin wide (Ginis, Perez,
Bueno, & Artal, 2012), which in turn limits the dynamic range.
As a result, faint stars cannot be seen within a few arcmin of bright stars. The
precise value of the measured half-width half-maximum (HWHM, at which the psf
falls to half of its peak value) of the human psf depends on age, ethnicity, eye colour,
and pupil dilation. For example, Australian Aboriginal people have statistically better acuity than Europeans (Taylor, 1981), although it is not known whether this
affects the glare function. Here we assume the results from Fig. 5 of Ginis et al.
(2012), from which HWHM appears to be in the range 3–4 arcmin for most people.
Pleione is 5 arcmin from the star Atlas, which is about four times brighter than
Pleione, and the resulting glare from Atlas prevents most people from seeing
Pleione.
6 Discussion
6.1 The Lost Pleiad
Although the Pleiades do not appear as seven stars to most humans, could they have
appeared as seven stars in the past? There are two potential reasons why they may
have done. First, we have already noted that many of the Pleiades are B stars, which
are often variable. While we have no evidence of any long-term major changes in
brightness, and the long-term variability of B stars is poorly understood, we cannot
discount the possibility that one of the faint stars was much brighter in the past.
Here we suggest an additional reason. Because of Pleione’s measured proper
motion, Pleione was further from Atlas in the past, as shown in Fig. 3. In 100,000 BC
it was 8.4 arcmin away, significantly decreasing the glare from Atlas. Figure 4
shows a simulated image of the two stars for an individual with HWHM of
3 arcmin. Even ignoring variability, Pleione was visible as a separate star from
Atlas in 100,000 BC, so that the Pleiades would appear as seven stars to normal
human eyes.
Why Are There Seven Sisters?
from each other by at least 1 arcmin. Most people are unable to see Pleione because
of two other factors.
First, the 1 arcmin resolution is only obtained in bright light, when the resolution,
or point spread function (psf), of the human eye is dominated by the cones of the
retina. In faint light, human vision relies more on the rods that are sparsely distributed around the retina, and have a much broader psf.
A second factor, called “glare function” by physiologists, is what prevents you
from seeing details next to car headlights pointing at you. The glare function
depends on the dynamic range and psf of the human eye. Imperfections in the
human eye give it a psf which has a broad base a few arcmin wide (Ginis, Perez,
Bueno, & Artal, 2012), which in turn limits the dynamic range.
As a result, faint stars cannot be seen within a few arcmin of bright stars. The
precise value of the measured half-width half-maximum (HWHM, at which the psf
falls to half of its peak value) of the human psf depends on age, ethnicity, eye colour,
and pupil dilation. For example, Australian Aboriginal people have statistically better acuity than Europeans (Taylor, 1981), although it is not known whether this
affects the glare function. Here we assume the results from Fig. 5 of Ginis et al.
(2012), from which HWHM appears to be in the range 3–4 arcmin for most people.
Pleione is 5 arcmin from the star Atlas, which is about four times brighter than
Pleione, and the resulting glare from Atlas prevents most people from seeing
Pleione.
6 Discussion
6.1 The Lost Pleiad
Although the Pleiades do not appear as seven stars to most humans, could they have
appeared as seven stars in the past? There are two potential reasons why they may
have done. First, we have already noted that many of the Pleiades are B stars, which
are often variable. While we have no evidence of any long-term major changes in
brightness, and the long-term variability of B stars is poorly understood, we cannot
discount the possibility that one of the faint stars was much brighter in the past.
Here we suggest an additional reason. Because of Pleione’s measured proper
motion, Pleione was further from Atlas in the past, as shown in Fig. 3. In 100,000 BC
it was 8.4 arcmin away, significantly decreasing the glare from Atlas. Figure 4
shows a simulated image of the two stars for an individual with HWHM of
3 arcmin. Even ignoring variability, Pleione was visible as a separate star from
Atlas in 100,000 BC, so that the Pleiades would appear as seven stars to normal
human eyes.
Why Are There Seven Sisters?