Quoted from SEDS:
Probably discovered by Hodierna before 1654.
Independently discovered by Charles Messier 1764.
The Triangulum Galaxy Messier 33 (M33, NGC 598) is another prominent
member of the
Local Group of galaxies. This galaxy is small compared to its big
apparent neighbor, the
Andromeda galaxy M31, and to our
Milky Way galaxy,
but by this more of average size for spiral galaxies in the universe.
One of the small Local Group member galaxies, LGS 3, is possibly a
satellite of M33, which itself may be a remote but gravitationally bound
companion of the
Andromeda galaxy M31.
M33 is approaching us (our Solar System) at 182 km/s according to R.
Brent Tully, or at 179 +/-3 km/s according to
NED.
Corrected for our motion around the Milky Way's Galactic Center, it is
approaching our Galaxy at 24 km/sec.
M33 was probably first found by
Hodierna before 1654 (perhaps together with open cluster
NGC 752).
It was independently rediscovered by
Charles
Messier, and
cataloged by him on August 25, 1764. Nevertheless,
William
Herschel, who otherwise carefully avoided to number Messier's
objects in his survey,
assigned it the number H V.17, on the ground of an observation dated
September 11, 1784. Also because of the cataloging of Herschel, the
brightest and largest HII region (diffuse emission nebula containing
ionized hydrogen) has obtained a NGC number of its own:
NGC 604
(William Herschel's H III.150); it is situated in the northeastern part
of the galaxy; apparently the bright knot near the top of our image.
This is one of the largest H II regions known at all: it has a diameter
of nearly 1500 light-years, and a spectrum similar to the Orion nebula
M42. Hui Yang
(University of Illinois) and Jeff J. Hester (Arizona State University)
have taken a
photograph of NGC 604 with the Hubble Space Telescope, resolving
over 200 young hot massive stars (of 15 to 60 solar masses) which have
recently formed here.
M33 was among the first "spiral Nebulae" identified as such by
William
Parsons, the Third Earl of Rosse;
see his
drawing. It was also among the first "nebulae" identified as
galaxies, in which Cepheid variable stars were found;
Edwin Hubble
published a fundamental study in 1926 (Hubble 1926).
Several other knots in the spiral arms of M33 have been assigned
their own NGC catalog numbers: NGCs 588, 592, 595, and NGC 603 (the
latter is listed as nonexistent in the RNGC though, although they
mention it was listed by Zwicky), as well as ICs 131, 132, 133, 134,
135, 136, 137, 139-40, 142, and 143 (NGC 2000.0 lists IC 134 and 139-40
as stellar, while the
Webb Society
Deep-Sky Observer's Handbook, Vol. 4 [Galaxies] shows IC 139-40 on
the chart on p. 215, which is credited to Ronald J. Buta of McDonald
Observatory, University of Texas). Some of them are identified in
our map
also. Kenneth Glyn Jones notes that they should be visible in 12.5-inch
telescopes. The giant
emission
nebula NGC 595 was investigated by William H. Waller with the HST
(e.g. Astronomy, June 1995, p. 16-18); with Hubble he resolved
the hot massive stars that excite that nebula's gas to shine.
The results of the Hipparcos satellite have lead to a revision of the
cosmic distance scale, therefore also of our distance to M33: The
current value is about 3.0 million light-years. Most sources give a
distance of 2.3 to 2.4 million light-years, but the Sky Catalogue 2000.0
has more than 2.9 million light-years (900 kpc), which by chance may be
closer after the new Cepheid distance recalibration, due to 1997
Hipparcos satellite results. Investigations of Cepheids in M33 of 1991 (Freedman
et.al., 1991) have revealed that M33 is at a slightly greater
distance from us than the
Andromeda Galaxy M31.
With our distance values, the distance of M33 from M31 is about 750,000
light-years. Assuming the former value, its angular dimension of 73 arc
minutes in major axis (about 2.5 times the Moon's diameter) corresponds
to about 50,000 light-years, half the diameter of the Milky Way.
However, the faintest outlayers seem to reach more far out, so that the
true diameter may be at least 60,000 light-years. The mass of the
Triangulum Galaxy has been estimated between 10 and 40 billion solar
masses.
The Triangulum galaxy M33 is of type Sc, and even a "late"
representative of that type so that Tully classifies it as Scd (in the
Nearby Galaxies Catalog). The pronounced arms exhibit numerous
reddish HII regions (including NGC 604), as well as bluish clouds of
young stars. Baade has also discovered Population II stars, and globular
clusters have been found. Although no supernovae have yet been detected
in the Triangulum galaxy, several supernova remnants have, and were
cartographed by radio astronomers with high accuracy. At least 112
variables have been discovered in M33, including 4 novae and about 25
Cepheids. A strong X-ray source is also situated in this galaxy.
For the observer, this galaxy can be glanced with the naked eye under
exceptionally good conditions; for most people, it is the most distant
object visible to the naked eye (there are rare reports that some
eagle-eyed stargazers managed to see
M81 under
exceptional conditions, but this is exceptional with all respects). It
is outstanding in good binoculars, but as its considerable total
brightness is distributed quite evenly over an area of nearly four times
that covered by the full Moon, its surface brightness is extremely low.
Therefore, it is difficult to impossible to view this galaxy in
telescopes which do not allow low magnification - lowest is best for
this object ! The best view of M33 the present author had was with a
6-inch refractor at magnification 25. M33 is also a most rewarding
target for the astrophotographer, who can track down its spiral arms and
brighter nebulae with considerably inexpensive equipment.
More ambitious observers with large telescopes (> 40 cm aperture) may
try to track down some of M33's globular clusters; Rich Jakiel has
tracked down
5 globular
clusters in M33 with a 50-cm telescope.
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