Birth of a neutron star possibly spotted
Astronomers have followed supernova 2012au for several years
B Y LISA GROSSMAN
For the first time, astronomers may have
watched a massive stellar explosion give
rise in real time to a superdense corpse
called a neutron star.
Observations of supernova 2012au,
first spotted in 2012 in a galaxy 77 million light-years from Earth, show charged
oxygen and sulfur atoms fleeing the scene
of the explosion at 2,300 kilometers per
second. That visibility suggests that
the shells of gas surrounding the dense
remains of the original star are being lit
up from within by a pulsar, a type of fast-spinning, radiation-spewing neutron star,
researchers report online September 12
in Astrophysical Journal Letters.
“It’s proof positive, a smoking gun,”
says coauthor Dan Milisavljevic, an
astrophysicist at Purdue University in
West Lafayette, Ind. “We’ve seen this
supernova from the explosion up until
this transformation into the neutron
star.” Now astronomers have a chance
to test theories about how supernovas
and their aftermaths evolve in real time.
Milisavljevic and colleagues first monitored SN 2012au for a year following the
explosion, and found that it faded more
slowly than most supernovas of its kind.
That could have meant that a pulsar was
contributing energy to the explosion,
Supernova 2012au, which exploded in the galaxy NGC 4790 (shown), was absent in a 2001
keeping the lights on longer.
Sometimes supernovas appear to
brighten again when the dead star’s outer
layers of gas slam into hydrogen atoms
that float between stars. So Milisavljevic
and colleagues followed up in June. Six
years after the explosion, SN 2012au was
still relatively bright. The team saw no
signs of hydrogen in the wavelengths of
light around it. Instead, the team found
the lit-up ionized oxygen and sulfur
atoms making a quick getaway. Those
heavier atoms would trail hydrogen as
material leaves a supernova explosion,
forming an inner shell of ejected gas.
Astronomers have seen other pulsars
lighting up their surroundings in our
galaxy. But this would be the first time
the phenomenon has been seen outside
the Milky Way and so soon after the
explosion, says astrophysicist Samar
Safi-Harb of the University of Manitoba
in Canada, who was not involved in the
new observations. “We don’t know what
happens between the explosion itself
and those remnant stages,” she says.
If the findings prove true, “we have an
example of how [a supernova remnant]
manifests itself in those early stages.”
It’s still possible something more exotic
is happening in SN 2012au, Safi-Harb
says. “Only time will tell.” s
didn’t make fat or other tissues. That
result shows that these stem cells are
specific to the skeleton.
Longaker’s team tracked down adult
skeleton stem cells by cataloging the
types of RNA produced in the fetal skeletal stem cells. Different types of RNA
reveal the genetic instructions encoded
in DNA, which determine a cell’s identity
and function. Skeletal stem cells in adult
bones and fat that make the same RNAs
also made bone, cartilage and spongy
bone when transplanted into mice.
The team also manipulated embryonic-like stem cells, or induced pluripotent
stem cells, into becoming the skeletal
stem cells. The ability to grow skeletal
stem cells in a dish “is particularly promising, as you can grow as many of these as
you wish,” Tabin says. s
“under extremely clean conditions
to avoid contaminants,” says study
coauthor Jochen Brocks, also of Australia
As a control, the team extracted
organic material from the rocks surrounding the Dickinsonia fossils. Steroids isolated from these rocks turned
out to be about 70 percent stigmasteroid,
an organic molecule found in algae.
That’s consistent with the likely marine
setting for the Ediacarans: The creatures
were thought to dwell in shallow waters,
atop microbial mats.
But the fossils themselves were strikingly high in cholesterol relative to other
steroids: Cholesterol made up between
about 85 and 93 percent of all steroids
in different fossils of the genus.
“The nearly 100 percent proportion
of cholesterol in the Dickinsonia fossils
tells us it must have been an animal,”
Brocks says. Today, cholesterol is produced by bacteria in the guts of animals.
Next, the team plans to look for biomarkers from other Ediacarans in the
White Sea rocks — and there are plenty
of intriguing options, Brocks says. “The
coolest target is the rangeomorphs,”
he says. “These weird creatures are
constructed like a fractal. No modern
animal is built like that.” s