Sky & Telescope - May 2021 - 45
Evenings with the Stars by Fred Schaaf
Anatomy
of the
Amazing
Big Dipper
Alkaid
Mizar
Alioth
Megrez
There's more to this
distinctive collection of
stars than meets the eye.
Dubhe
Phecda
A K IR A FUJII
T
he Big Dipper is the most famous
asterism. It's so eye-catching that
few of us focus on the individual stars
that make it up. However, there's much
to see and to learn about them.
The Big Dipper is part of the larger,
official constellation Ursa Major, the
Great Bear. Most constellations have
stars with Greek letter designations that
are sequenced in order of diminishing
brightness, with Alpha being brightest,
Beta second brightest, and so on. However, the Greek letters in the Big Dipper
are arranged by position from the lip of
the dipper bowl, around the bottom of
the bowl, and out to the end of the handle. Thus, starting from the lip of the
bowl, the stars are Alpha Ursae Majoris (Dubhe), Beta (Merak), Gamma
(Phecda or Phad), Delta (Megrez),
Epsilon (Alioth), Zeta (Mizar), and Eta
(Alkaid or Benetnasch).
Given that the Greek letters are out
of order, what are the relative brightnesses of the seven Big Dipper stars?
Most people who've casually observed
the pattern would probably say they're
all roughly the same brightness. That's
almost true. Six of the stars differ by
little more than half a magnitude.
Interestingly, these six fall into two
brightness batches. The brighter batch
includes Dubhe and Alioth at magnitude 1.8, Alkaid at 1.9, and Mizar at 2.2.
That's the lip of the bowl and the three
stars of the handle. The fainter batch
(if you can call two stars a " batch " ) is
Merak
S TEMPORARY DIPPER The Big Dipper is the most recognizable part of the constellation Ursa
Major, the Great Bear. Five of the Dipper's stars are moving through space together, but two
(Alkaid and Dubhe) are not. As a result, over the coming millennia the iconic shape of the Dipper
will eventually become unrecognizable.
Merak at magnitude 2.3 and Phecda at
2.4, which form the bottom of the bowl.
Of course, at nightfall in April and May
the Big Dipper rides high above Polaris
and is oriented upside-down, so the
bottom of the bowl is on top.
So what about the seventh, dimmest
star of the Big Dipper? That's Megrez. It
shines at only magnitude 3.3 - almost
a full magnitude dimmer than the
faintest of the other six. In fact, there
are five stars in other parts of Ursa
Major that are equal to or brighter than
Megrez. Its dimness is easily overlooked
because it's positioned right in the
middle of the Big Dipper, quietly forming the link between the three handle
stars and the rest of the bowl.
Most asterisms are not true clusters.
Instead, they're random line-of-sight
patterns in the sky comprised of stars
that are often at different distances
from Earth. However, in the case of
the Big Dipper, the middle five stars
really do seem to be the remaining core
of a once rich and tight star cluster,
now known as the Ursa Major Moving
Group. These Dipper stars (along with a
handful of additional, fainter ones) are
still travelling through space together
at similar distances, approximately 80
light-years away. The two stars that
aren't part of this collection mark opposite ends of the Big Dipper. Dubhe and
Alkaid are 120 and 104 light-years away,
respectively, and heading in different
directions from the main cluster quintet. As a result of this arrangement, over
the course of a few tens of thousands of
years, the Dipper will have a markedly
different shape than it does today.
A final wonderful difference
between the disparate sets of Dipper
stars is their spectral types. The cluster
five are all between A0 and A3 (tinted
bluish white), while Alkaid is a B3 (hotter, bluer) star and Dubhe is a cooler,
noticeably orange K0 star.
FRED SCHAAF observes the night
sky from the oak-pine fringe of the New
Jersey Pinelands National Reserve.
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1
45
http://www.skyandtelescope.org
Sky & Telescope - May 2021
Table of Contents for the Digital Edition of Sky & Telescope - May 2021
Contents
Sky & Telescope - May 2021 - Cover1
Sky & Telescope - May 2021 - Cover2
Sky & Telescope - May 2021 - 1
Sky & Telescope - May 2021 - Contents
Sky & Telescope - May 2021 - 3
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