The Caldwell Objects

halfway between M37 in Auriga
(200 million years old) and M44 in
Cancer (400 million years). NGC
3532 weighs in with at least 2,000
solar masses, and its brightest stars
huddle in its central region. In the
1930s Harlow Shapley found the
cluster unusually rich in bright
members of the A-type spectral
class. Surprisingly, the cluster
received little professional
attention since then, until interest
picked up again in the 1980s.
Today we recognize 677 stars as
cluster members. The vast majority
of these are Α-type stars, though
there are also eight K-type stars.
As Herschel suspected,
the cluster is also binaryrich
(he noted "several
elegant double stars,
and many orangecoloured
ones"). NGC
3532's metallicity is
essentially the same as
the Sun's. In a 1993
Astronomy and Astrophysics
article, Detlev
Koester (University of
Kiel, Germany) and
Dieter Reimers discovered
seven white-dwarf
candidates and confirmed
that three of
them were in fact white
dwarfs, with temperatures of about 28,000°
Kelvin. Two of these white dwarfs have 0.6 solar
mass, while the third is a 0.9-solar-mass stellar
cinder. White dwarfs are the evolutionary
endpoints of relatively low-mass stars like our
Sun; more massive stars explode as super-
91
novae. Where exactly is the dividing line between
white-dwarf precursors and super-novae-to-be?
According to Koester and Reimers, it lies
somewhere between 6 and 8 solar masses.
Interestingly, Koester and Reimers found no
white dwarfs in the cluster's center, where
The Caldwell Objects 359