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Binocular Highlight by Mathew Wedel
W
e expect kings to be adorned with jewels,
and the constellation Cepheus, named for
the mythical king of Ethiopia, doesn’t disappoint.
One glittering gem at the heart of the constellation
is the open cluster NGC 7160. I fi rst observed this
cluster more than a decade ago, writing in my note-
book, “small, comp act, surprisingly bright, with stars
packed so close together that they are diffi cult to
resolve despite their brightness.”
It’s fun for me to revisit those notes, because I
unknowingly had stumbled upon an astrophysical
truth: NGC 7160 is really small as open clusters go.
It’s only 7 arcminutes in diameter as seen from Earth,
corresponding to a physical diameter of just 5 light-
years. For comparison, the average diameter of the
open clusters that I’ve discussed in previous columns
is about 20 light-years, and NGC 1893 in Auriga, the
Charioteer, is a whopping 80 light-years across.
NGC 7160 isn’t just small, it’s also quite young,
between 10 and 20 million years old. That’s a lot
of time in human terms — the lineages that led to
humans and the other great apes hadn’t even sepa-
rated from their common ancestor when NGC 7160
formed — but it’s just a tick of the clock in cos-
mic terms, around^1 / 1000 of the age of the universe.
NGC 7160 is part of the Cepheus OB 2 Association,
a vast cloud of young stars that stretches between
Alpha (α) and Delta (δ) Cephei, and most of the
cluster’s brighter members are hot young B-type
stars. There are no more O-type stars in NGC 7160
— those blow themselves up long before their
15-millionth birthdays — and the B-types won’t last
forever. So go have a look at NGC 7160 to see one of
the galaxy’s newer additions.
¢When he’s not stargazing,MATT WEDEL works
on dinosaur bones ten times the age of NGC 7160.
New Kid on the Block
skyandtelescope.com • SEPTEMBER 2019 43