101 SKY OBJECTS
WWW.ASTRONOMY.COM 2946 California Nebula
Today’s brightly lit cities aren’t usually the places where new deep-sky objects are discovered, but in
the 19th century, gas streetlights didn’t produce much light pollution. In November 1885, Vanderbilt
University astronomer Edward Emerson Barnard visually discovered the California Nebula from Nashville,
Tennessee, with a 6-inch refractor. One of the pioneers of astrophotography, Barnard is perhaps better
known for his catalog of dark nebulae.
Designated NGC 1499, the California Nebula is an emission nebula stretching 2½° long and ½° wide. Its
apparent magnitude of 6 suggests it can be seen in small optics. But this nebula’s expansive and diffuse
nature gives it a low surface brightness, making it difficult or impossible to see except under the darkest
skies (i.e., where magnitude 6.5 stars are visible with the naked eye).
As you might expect, the California Nebula is relatively close to Earth — estimates put it between
1,000 and 1,500 light-years away. That places the length of this hydrogen cloud somewhere around
100 light-years from end to end. For comparison, the famous Orion Nebula (M42; see #19) is about
1,600 light-years distant but only 24 light-years in diameter.
HII regions such as NGC 1499 are always illuminated by nearby hot, young stars. The California’s
source of ionizing energy is Xi (ξ) Persei, which sits on the nebula’s Nevada side (what would be the
state’s eastern border). Also called Menkib, this 4th-magnitude star is classified as O7.5 and has a surface
temperature of 62,540 degrees Fahrenheit (34,730 degrees Celsius) — one of the hottest naked-eye stars.
Its distance is 1,200 light-years.
To improve your odds of seeing NGC 1499, you will need the right skies and equipment. As mentioned,
perfect skies are a necessity; a wide field of view is important to pick up the entire nebula, though a
smaller field can bring out one edge. A filter that enhances Hydrogen-alpha (Hα) wavelengths and
suppresses airglow will best show off this gas cloud. — A.G.47 NGC 2403
The 18th-largest constellation, Camelopardalis,
depicts a giraffe that stretches from the
northern border of Perseus up to near Polaris.
Its brightest star, designated Beta (β), is 4th
magnitude. Camelopardalis is rich in galaxies,
and the brightest is NGC 2403 at magnitude
8.9 — bright enough to see in large binoculars
under dark skies.
Some 10 million light-years away, this Scd
spiral lies in the outer regions of the M81 Group,
the closest association of galaxies to the Local
Group. It appears similar enough to M33 in
Triangulum (see #61) that Allan Sandage wrote
a comparison of the two galaxies in his 1961
book The Hubble Atlas of Galaxies. NGC 2403
is 50,000 light-years in diameter and inclined
to our line of sight by 60° — that’s 10,000 light-
years smaller and 5° more inclined than M33.
NGC 2403 contains NGC 2404, a massive HII
region roughly 940 light-years across. For com-
parison, the Large Magellanic Cloud’s TarantulaNebula (see #22) is 1,000 light-years across.
These are all young star-making machines.
While NGC 2404 may require a large telescope
to see well, more modest instruments can reveal
NGC 2403’s spiral structure. Discovered by
William Herschel in 1788, this galaxy isn’t difficult
in a 4-inch and shows its generous 17.8'-by-11'
dimensions in an 8-inch. A 13-inch scope can
tease out some of the spiral structure, and larger
apertures show even more detail, including
mottling from stellar associations and HII regions.
There are faint foreground stars superimposed
on this galaxy that might be mistaken for
supernovae.
NGC 2403 is in an area lacking bright guide
stars. If you want to star hop to find it, the best
way is to find Omicron (ο) Ursae Majoris. Center
on that star, then locate 23 Ursae Majoris. You’ll
find NGC 2403 at the same distance and angle
from that star but in the opposite direction, near
51 Camelopardalis. — A.G.MARK JOHNSTON
TONY HALLASBERNARD MILLER