Sky and Telescope - December 2016 - 27

M35

M45

+20°

The
Hyades

ORION

Star magnitudes

1
2
3
4
5

CE TUS
TAURUS
0°

M77

M78
M43

M42

LEPUS
ERIDANUS
-20°

Orion Nebula

Tang Dyn

asty
ESO / J. EMERSON / VISTA / CAMBRIDGE ASTRONOMICAL SURVEY UNIT; STATUE: WIKIMEDIA COMMONS /
CC0 1.0 UNIVERSAL PUBLIC DOMAIN

Our next step is the Helix Nebula, NGC 7293
(Caldwell 63), 700 light-years away in Aquarius. The Helix
is a planetary nebula, a shell of gas blown oﬀ by a dying
star, and the closest planetary nebula to Earth. That proximity makes it a tough catch, as its light is spread over so
much sky. You'll need a clear night and binoculars or a
rich-ﬁeld scope to see this faint ring of light.
We see light that left the Helix in the early 1300s,
when science was in full ﬂower in the Islamic world.
The greatest astronomer of the age was Ulugh Beg,
born in 1394, who built an immense observatory at
Samarkand and charted nearly a thousand stars. He also
determined the length of the sidereal year to within 25
seconds, and measured Earth's axial tilt more accurately
than either Nicolaus Copernicus or Tycho Brahe.
If the last nebula was elusive, the next one should not
be: it's the famous Orion Nebula, M42/M43. The Orion
Nebula is a "total object," meaning that it looks good at
every aperture and magniﬁcation, from the naked eye
to the Hubble Space Telescope. Recent measurements
have reﬁned the nebula's distance to 1,344 light-years.
That takes us back to the mid-600s, when astronomy
was largely dormant in the West. Classical manuscripts
by Ptolemy and others awaited discovery by Islamic and
later European scholars. But astronomy was thriving in
Tang dynasty China. The Dunhuang Star Chart, which
dates from around the year AD 700, includes 1,300 stars
- more than the catalogs of either Ulugh Beg or Tycho
Brahe (albeit not to the same positional precision). A more
complete celestial atlas wouldn't exist until Johannes Hevelius published his catalog of 1,564 ﬁ xed stars in 1687.
Our next step gets us three for the price of one: open
clusters M36, M37, and M38 in Auriga. The members
of this trio are all at similar distances, between 4,100 and
4,500 light-years. The light we see now left these clusters
in the late third millennium BC. In Egypt's Middle
Kingdom, coﬃn lids were inscribed with diagonal star
tables. Across Europe, stone circles were in use at observatories of impressive sophistication, grave goods from
China recorded dozens of constellations, and early Polynesian explorers were navigating by the Sun and stars.
References to celestial objects were also common in the
oral histories that would eventually be written down in
The Iliad, The Odyssey, and the Hebrew Bible.
Another step of similar length carries us out to the
Double Cluster in Perseus, composed of NGC 869 and
NGC 884. At a little more than 7,000 light-years away,
the Double Cluster is signiﬁcantly closer to the edge
of the Milky Way Galaxy than we are; it resides in the
Perseus Arm rather than our own Orion Spur. Now
we're at 5000 BC, a time when early agricultural societies watched the night sky and charted the motions of
the heavens. The oldest known observatory is at Warren Field in Scotland, where astronomers used 12 pits
to track the phases of the Moon and subsequently, the

A TOTAL OBJECT This wide-ﬁeld composite image, taken with the VISTA
telescope at ESO's Paranal Observatory in Chile, shows an unusual view of a
familiar object, the Orion Nebula. Astronomers used VISTA's infrared capabilities to peer through obscuring dust clouds to capture the 1,344-year-old light
emanating from star-forming regions in the heart of the nebula.

Sk yandTelescope.com December 2016

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Sky and Telescope - December 2016

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