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Astronomers detected supernova PTF 11kly (location marked by arrow) growing
brighter (above, from left, on August 23, 24 and 25 Universal time), in the Pinwheel
Galaxy. Caused by thermonuclear reactions inside white dwarfs, type 1a superno-
vas are used by scientists to measure the expanding universe.
of the sun, a runaway thermonuclear
reaction ignites and the star violently
combusts, producing a bright cosmic
pockmark.
Because the starting mass of type 1a
supernovas is so uniform, these explo
sions reach predictable peak bright
nesses. Astronomers can therefore use
the brightnesses to measure relative
extragalactic distances and the uni
verse’s expansion rate.
For 80 years, scientists have known
that the universe is growing. In 1998, evi
dence from type 1a supernova measure
ments suggested that the rate of the
expansion is accelerating.
Since then, further data from super
novas and other observations have sup
ported that conclusion; astronomers
now attribute the quickening expansion
to “dark energy,” a mysterious, repulsive
force thought to make up more than 70
percent of the massenergy content of
the universe.
“This class of supernova is the kind
of object that showed us that there was
dark energy, and is one of the best tools
we have for measuring the expansion
rate of the universe,” Riess says.
Riess is planning on using Hubble
telescope observations of M101 to learn
more about Cepheid variable stars, also
important cosmic milemarkers, but
only for measuring shorter distances
than those accessible with faraway
supernovas. Hubble will continue to
peer at M101 through October, record
ing data about both the supernova and
the Cepheids. Since the Pinwheel Gal
axy is so close by, astronomers can take
detailed measurements of both Cephe
ids and the supernova and combine
them to nail down “true” distances.
A stellar explosion observed last month, designated PTF 11kly, is what astronomers
call a type 1a supernova. Supernovas of this type occur after a white dwarf star,
typically not much heavier than the sun, adds enough material to exceed about 1. 4
times the sun’s mass. One likely scenario for acquiring the extra mass is siphoning
it from a nearby star (left). Once the mass limit is exceeded, the white dwarf ignites
in a thermonuclear explosion (center), leaving behind nothing but debris (right).
