24 NGC 3532
If you love common names for deep-sky objects, NGC 3532 in the constellation Carina the Keel
will give you all you want. Various amateur astronomers through the years have called it the
Firefly Party Cluster, the Football Cluster, the Wishing Well Cluster, the Pincushion Cluster, and
the Black Arrow Cluster. More formally, it’s also listed as Caldwell 91 and Melotte 103.
No matter how you refer to it, this fabulous open cluster sits in a gorgeous star field 4.7°
south-southwest of magnitude 3.9 Pi (π) Centauri. You’ll see it immediately without optical aid
because it glows at 3rd magnitude and it spans 55'. The 150 or so stars NGC 3532 contains,
however, are too faint to see individually (averaging approximately magnitude 7.5), so the
impression is of a bright glow within the Milky Way.
French astronomer Nicolas Louis de Lacaille discovered NGC 3532 in 1751 through his ½-inch
refractor. It was one of many discoveries he made from the Cape of Good Hope in South Africa
with this instrument.
A 4-inch telescope with an eyepiece that yields a magnification of 100x will reveal more than
half of NGC 3532’s stars. Back off the power a bit, and you’ll see several dark lanes between
clearly delineated lines of stars. Go for an even wider field of view and numerous colorful stars
will pop into view past the cluster’s tightly packed core. A blue and a deep red one lie at the
cluster’s northeast end. The blue star is SAO 238839; its red counterpart is SAO 238855.
A bit of interesting trivia connects this cluster with the Hubble Space Telescope. On May 20,
1990, NGC 3532 became the first target imaged by Hubble’s Wide Field and Planetary Camera.
Well, not the entire cluster — the shot captured a region of sky near the magnitude 8.4 star
HD 96755 that measured only 11" by 14", which is a minuscule 0.006 percent of the area occu-
pied by the whole cluster. — M.B.
26 Mizar and Alcor
For most people living in the Northern Hemisphere,
the first double star we notice is Mizar (Zeta [ζ] Ursae
Majoris), the middle star in the Big Dipper’s handle.
Located just to the northeast is Mizar’s faint cohort,
Alcor (80 Ursae Majoris). Mizar shines at magnitude
2.3, while Alcor is magnitude 4. The pair are sepa-
rated by 11.8', which is resolvable with the unaided
eye if the sky is dark enough and your eyesight is
good enough. In ancient times, some cultures even
used these stars as a test of visual acuity.
Alcor and Mizar are not true physical companions,
however. Alcor is 82 light-years away, while Mizar
is 83 light-years distant. That’s close, but not close
enough to form a true binary star. They do, however,
share a common direction and speed through our
galaxy. Both Alcor and Mizar, as well as the Dipper
stars Merak (Beta [β] Ursae Majoris), Phecda (Gamma
[γ] Ursae Majoris), and Megrez (Delta [δ] Ursae
Majoris) — and about 100 others — form a loose star
cluster called the Ursa Major Moving Group.
Although Alcor is not Mizar’s real partner, one look
at Mizar through a telescope will reveal it as a binary
star, a fact discovered by Italian astronomer Giovanni
Riccioli in 1650. Studies have since shown that Mizar
A and Mizar B are both binaries themselves, bringing25 NGC 2516
NGC 2516 is an open star cluster in the southern
constellation Carina the Keel. Amateur astronomers
refer to it as the Southern Beehive because of its
resemblance to M44 (see #94), the open cluster in
Cancer the Crab. It lies some 1,300 light-years away
and has an apparent diameter of 30', similar to that
of the Full Moon. Various teams of astronomers
have estimated its age at between 110 million and
135 million years.
French astronomer Nicolas Louis de Lacaille
discovered it in 1751, using a ½-inch refractor.
That may not seem like a large scope, but such an
instrument gathers 3.3 times as much light as a
fully dark-adapted human eye. And that increase
in light-gathering power was just enough to allow
Lacaille to find objects either too faint for the eye or
just dense enough that the eye can’t quite resolve
them into stars.
To find this spectacular cluster, look 3.3° west-
southwest of magnitude 1.9 Avior (Epsilon [ε]
Carinae), the southwestern star in the asterism
known as the False Cross. Because NGC 2516 glows
at magnitude 3.8, you’ll have no trouble spotting
it with your naked eyes — it’s one of the sky’s 20
brightest open clusters.
If you point a 6-inch telescope at this cluster, you
might be able to count 75 stars, but it won’t be easy.
That’s because the stars fall into two brightness
ranges. The upper range starts with SAO 250055
— a magnitude 5.8 red giant, making it the cluster’s
brightest star — and includes those members
brighter than magnitude 8. To see the lower range
of stars, crank the power beyond 250x to spread the
bright stars out. If you don’t, their light will hide the
many faint stars within the cluster. When using high
power on NGC 2516, you’ll come across several dou-
ble stars (and even one triple star) whose brightness
straddles the line between the two classes. — M.B.FERNANDO OLIVEIRA DE MENEZESBERNARD MILLER