Home / Binocular Universe: Northern Exposure
by Phil Harrington 02/28/13 | Email Author
Download this Document (493 kilobyte)
Voice your opinion about this topic in the forums
northern circumpolar sky holds many binocular targets that we can enjoy
throughout the year. This month,
let's take aim at the constellation Ursa Minor, the Little Bear.
You may know it better as the Little Dipper, an asterism made up
seven brightest stars in the Little Bear.
Above: Winter star map from Star
by Phil Harrington.
Finder chart for this month's Binocular Universe. Chart
adapted from Touring
Binoculars Atlas (TUBA)
what you will, this star group is
most famous as the home of the North Star, Polaris
Minoris). Earth's rotational axis is aimed
three-quarters of a degree away from Polaris, causing it to trace out a
tiny circle around that invisible point every 24 hours.
The North Celestial Pole is slowly moving
closer to Polaris. It will continue to
close to within 14 minutes of arc around the year 2105, when it will
start to pull away.
Polaris is currently the pole star, the
26,000-year wobble of Earth's axis, called precession, causes the
Pole's aim to trace a 47° circle in the sky. For instance,
building of the pyramids nearly 4,600 years ago, the North Pole was
toward the star Thuban in Draco.
Fast forward 5,200 years from now and the pole
will be point near Alderamin
us at one time or another have
heard someone misspeak by referring to Polaris as the brightest star in
night sky. One look its way will quickly
dispel that myth, however. In fact,
2nd-magnitude Polaris is the 48th brightest star in the night sky.
find that disappointing, take heart
that despite its dimness, there is more to Polaris than meets the eye. Polaris is a Cepheid variable star, an
important class of star that has proven invaluable to our understanding
in the universe.
Cepheid variables, including Polaris,
are yellow giant stars that pulsate with precise regularity. Cepheids have a very well established
relationship between their periods and their luminosity, or inherent
brightness. The longer a Cepheid's
period of variability, the greater its luminosity.
By measuring a Cepheid's periodicity, we can also
determine its luminosity, or it's "wattage" if you prefer.
Once we know that as well as the star's
apparent brightness, or magnitude, its distance can be calculated very
precisely. From studies, Polaris, which
takes 3.97 days to complete one cycle, is known to be 430 light years
Unfortunately, it only changes in brightness by about 0.03 magnitudes. That's far too subtle to notice through
night you're out with your
binoculars, check out Polaris nonetheless.
If you look carefully, you should see that it belongs to a
faint stars. I first read about this
asterism in Burnham's Celestial Handbook, where it is described
heavenly Engagement Ring. It's
plotted on the finder chart here as Hrr
1, a reference to it being the first of a dozen asterisms described in
my book Touring
the Universe through Binoculars. Judging
by how bright Polaris is compared to the other stars, that's a lot of
bling! Of course, in reality, these stars
all lie at
very different distances away from Earth, and so do not form a true
Yellowish Polaris highlights the Engagement Ring asterism in this
sketch made by the author through 10x50 binoculars.
second brightest star in Ursa Minor, Kochab (Beta
Ursae Minoris), is just
a tenth of a magnitude fainter than Polaris.
Both look yellowish through binoculars, but you may have to
images slightly to enhance the delicate color.
spot another yellowish star,
4th-magnitude 5 Ursae Minoris, lying
just to the right (west, as the chart is oriented) of Kochab? Together, they remind North Carolinian
observer Tomm Lorenzin as "double caution lights."
Ursae Minoris to the southwest of
5 Ursae Minoris and 4 Ursae Minoris to the northwest join in to form
distinctive right triangle of stars.
Using your averted vision, look just to the west of the line
3 to 4. Can you see a hook-shaped group
of 7th- and 8th-magnitude stars? In his
book Observing the Night Sky with Binoculars, author Stephen
calls this hook-shaped asterism the Fishhook.
have V Ursae Minoris, a semi-regular variable star that is
fun to monitor through binoculars. The
American Association of Variable Star Observers (AAVSO) includes it as
one of 153
variable stars in their Binocular
Program. As they explain on their
web site, most of the stars in this program are either long-period
(like Mira in Cetus) or semi-regular stars, like V.
Most of the stars in the program fluctuate
between magnitudes 3 and 9.5, so can be monitored continuously through
binoculars. As our example here, V UMi,
as it is written in shorthand, usually flutters between magnitudes 7.1
with an average period of 73 days. Use
the finder chart below from the American Association of Variable Star
Observers, or if you'd prefer, generate your own custom version using
generator at http://www.aavso.org/vsp
Finder chart for V Ursae Minoris, created with the
AAVSO's chart-generator web site.
finally, we jump the border into
northern Cepheus for the open cluster NGC
188. Fair warning up front: this is
a tough catch through binoculars, even with 70- and 80-mm giants. That's why I include it as "Big
Binocular/Small Scope" challenge #64 in my book Cosmic Challenge. To try your own luck, head back to
Polaris. NGC 188 lies only 4 degrees
away. Under exceptional skies, some have
reported seeing the cluster's gentle glow through 50-mm, and even
binoculars. The best I can claim,
however, is spotting a very subtle hint through 16x70 binoculars under
from the Stellafane convention many years ago.
is unique in more ways than its
unusual location. Looking at its
individual stars reveals that its hottest main sequence star is
F2, with many others rated as spectral classes G and K.
That's in sharp contrast to what we find in
most star clusters, which are made up of spectral type O, B, and A
stars. All of these are far hotter and
massive. They live life in the fast
lane, consuming their hydrogen fuel at a furious rate, only to evolve
main sequence in several million years.
Lower mass stars like our Sun and those in NGC 188 survive for
of years, as they use up their hydrogen far more judiciously. Therefore, from these studies we know that
NGC 188 is unusually old for an open cluster.
Current estimates put it at 5 billion years old.
That makes it the second oldest open star
cluster north of the celestial equator.
Only NGC 6791
Lyra is older.
you can see, there are
plenty of binocular targets awaiting us in the north circumpolar sky. I have discussed only a few.
The list below includes many more.
of all, each can be found every night of the year, at least for most of
us. So, wait for that special
night! That's when you will find that,
for northern exposure, two eyes are better than one.
About the Author:
Harrington is the author of nine books on astronomy, including Touring
the Universe through Binoculars and Cosmic Challenge. Visit his web site at www.philharrington.net .
copyright 2013 by Philip S. Harrington. All
rights reserved. No
reproduction, in whole or in part, beyond single copies for use by an
individual, is permitted without written permission of the copyright