Like shiny
flakes sparkling in a snow globe, over 100,000 stars whirl within the globular
cluster M13, one of the brightest star clusters visible from the Northern
Hemisphere. Located 25,000 light-years from Earth with an apparent magnitude of
5.8, this glittering metropolis of stars in the constellation Hercules can be
spotted with a pair of binoculars most easily in July. Credits: NASA
High in
the summer sky, the constellation Hercules acts as a centerpiece for late-night
stargazers. At the center of Hercules is the “Keystone,” a near-perfect square
shape between the bright stars Vega and Arcturus that is easy to recognize and
can serve as a guidepost for some amazing sights. While not the brightest
stars, the shape of the hero’s torso, like a smaller Orion, is nearly directly
overhead after sunset. Along the edge of this square, you can find a most
magnificent jewel - the Great Globular Cluster of Hercules, also known as Messier 13.
Look up after sunset during summer months to find
Hercules! Scan between Vega and Arcturus, near the distinct pattern of Corona
Borealis. Once you find its stars, use binoculars or a telescope to hunt down
the globular clusters M13 (and a smaller globular cluster M92). If you enjoy
your views of these globular clusters, you’re in luck - look for another great
globular, M3, near the constellation Boötes.
Globular clusters are a tight ball of
very old stars, closer together than stars near us. These clusters orbit the
center of our Milky Way like tight swarms of bees. One of the most famous short
stories, Nightfall by Isaac Asimov, imagines a civilization living on a planet within
one of these star clusters. They are surrounded by so many stars so near that
it is always daytime except for once every millennium, when a special alignment
(including a solar eclipse) occurs, plunging their planet into darkness
momentarily. The sudden night reveals so many stars that it drives the
inhabitants mad.
Back here on our home planet Earth, we are lucky enough to experience skies full of stars, a beautiful Moon,
and regular eclipses. On a clear night this summer, take time to look up into the Keystone
of Hercules and follow this sky chart to the Great Globular Cluster of
Hercules. A pair of binoculars will show a faint, fuzzy patch, while a small
telescope will resolve some of the stars in this globular cluster.
A red giant star and white dwarf orbit each other in
this animation of a nova similar to T Coronae Borealis. The red giant is a
large sphere in shades of red, orange, and white, with the side facing the
white dwarf the lightest shades. The white dwarf is hidden in a bright glow of
white and yellows, which represent an accretion disk around the star. A stream
of material, shown as a diffuse cloud of red, flows from the red giant to the
white dwarf. When the red giant moves behind the white dwarf, a nova explosion
on the white dwarf ignites, creating a ball of ejected nova material shown in
pale orange. After the fog of material clears, a small white spot remains,
indicating that the white dwarf has survived the explosion. NASA/Goddard
Space Flight Center
Bonus! Between Hercules and the ice-cream-cone-shaped Boötes constellation,
you’ll find the small constellation Corona Borealis, shaped like the letter
“C.” Astronomers around the world are watching T Coronae Borealis, also known
as the “Blaze Star” in this constellation closely because it is predicted
to go nova sometime this summer.
There are only 5 known nova stars in the whole galaxy. It is a rare observable
event and you can take part in the fun! The Astronomical League has issued a Special
Observing Challenge that anyone can participate in. Just make a sketch of the
constellation now (you won’t be able to see the nova) and then make another
sketch once it goes nova.
Tune into our mid-month article on the Night Sky Network page, as we prepare for the Perseids! Keep looking up!
by Vivan
White of the Astronomical Society of the Pacific
Source: July’s Night Sky Notes: A Hero, a Crown, and Possibly a Nova! - NASA Science
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