The Caldwell Objects

condensed, almost to the very margin. This is the in "metals" they contain RR Lyrae variables,
brightest small nebula that I have seen. I tried several which astronomers use to measure distances. In a
magnifying powers on this beautiful globe; a 1998 issue of the Astronomical Journal, Alistair R.
considerable portion round the margin is resolvable,
Walker (Cerro Tololo Inter-American
but the compression to the centre is so great, that I can-
Observatory) revealed that 29 of the cluster's RR
not reasonably expect to separate the stars. I compared
this with the 68 Conn, des Terns, and this nebula greatly
exceeds the 68 in condensation and brightness.
Lyrae variables yield a distance of 39,800 lightyears,
in very close agreement to William Harris's
distance for the cluster, which appears in the
Dunlop's mention of the "68 Conn, des T refers e r table n on page s 288. " NGC 1851's true physical
to the 68th object — a 7th-magnitude globular diameter, then, is about 140 light-years. Mixed in
cluster in Hydra — cataloged by Charles Messier with NGC 1851's old stars is an apparently
and listed in the French almanac Connaissance des younger population, which includes a clan of 13
Temps in 1783. Like M68, Dunlop's new object is a helium-burning rebels called blue stragglers.
7th-magnitude globular cluster, and it is the (Blue stragglers form either when double-star
brightest and most intriguing deep-sky object in systems slowly coalesce or when two unrelated
Columba. Some 54,500 light-years from the stars collide.) NGC 1851 is also the site of an
galactic center, NGC 1851 belongs to a breed of intense X-ray source that could be signaling the
old globular clusters in the galaxy's halo. By presence of a black hole. The source was one of
studying the chemical composition, motions, and the first discovered in a globular cluster, and is as
other physical characteristics of globular clusters, old as satellite X-ray astronomy. The X-ray
astronomers have learned much about their source remained unidentified at optical (visual
evolution. For instance, it appears that the oldest and near-ultraviolet) wavelengths for a quarter
globular clusters appear to populate the halo century, but a 1996 report in the Astrophysical
while the younger ones lie closer to the plane of Journal revealed that the Hubble Space Telescope
the galaxy. Globular-cluster ages range from had identified a 21st-magnitude candidate
about 12 to 16 billion years. NGC 1851 is 14 counterpart only 2" from it.
billion years old. Its age (comparable to that of NGC 1851 resides in a star-poor region of
the universe) is mirrored in its low metallicity. sky, so it can be a challenge to locate. I suggest
Each of the cluster's member stars contains, on starting with Phact. Confirm with binoculars that
average, about 1/17 as much iron (per unit of Phact is the northern apex of a triangle with Beta
hydrogen) as does our Sun, attesting to their and Epsilon Columbae. Now move your
formation from primordial gases that had binoculars about 3° south-southwest of Epsilon
undergone little nuclear processing within Columbae and you should see a right triangle of
previous generations of stars. The cluster's three 5th- to 6th-magnitude stars. Center your
integrated spectral type is F7, and its radial finderscope on the southernmost star in that
velocity has been measured at 320 km per triangle (it is an irregular variable) and look in
second.
your telescope's low-power eyepiece. You will
Globular clusters are dominated by aged red- see that that magnitude-5.7 star is the brightest
giant stars, and when they are not too poor
member of a 30'-wide triangle with two roughly
magnitude-7.5 stars. NGC 1851 is 1¼° southwest
of the westernmost star
The Caldwell Objects
73
289