Supernova SN 2014C (X-ray) [annotated]
This image from NASA's Chandra X-ray Observatory shows spiral galaxy NGC
7331, center, in a three-color X-ray image. Red, green and blue colors
are used for low, medium and high-energy X-rays, respectively. An
unusual supernova called SN 2014C has been spotted in this galaxy,
indicated by the white box in the image. Credit: NASA/CXC/CIERA/R.Margutti et al. Annotated image
Supernova SN 2014C
This visible-light image from the Sloan Digital Sky Survey shows
spiral galaxy NGC 7331, center, where astronomers observed the unusual
supernova SN 2014C . The inset images are from NASA's Chandra X-ray Observatory,
showing a small region of the galaxy before the supernova explosion
(left) and after it (right). Red, green and blue colors are used for
low, medium and high-energy X-rays, respectively. Credit: X-ray images: NASA/CXC/CIERA/R.Margutti et al; Optical image: SDS. Hi-res image
Annotated image
We're made of star stuff," astronomer Carl Sagan famously said.
Nuclear reactions that happened in ancient stars generated much of the
material that makes up our bodies, our planet and our solar system. When
stars explode in violent deaths called supernovae, those newly formed
elements escape and spread out in the universe.
One supernova in particular is challenging astronomers' models of how
exploding stars distribute their elements. The supernova SN 2014C
dramatically changed in appearance over the course of a year, apparently
because it had thrown off a lot of material late in its life. This
doesn't fit into any recognized category of how a stellar explosion
should happen. To explain it, scientists must reconsider established
ideas about how massive stars live out their lives before exploding.
"This 'chameleon supernova' may represent a new mechanism of how
massive stars deliver elements created in their cores to the rest of the
universe," said Raffaella Margutti, assistant professor of physics and
astronomy at Northwestern University in Evanston, Illinois. Margutti led
a study about supernova SN 2014C published this week in The
Astrophysical Journal.
A supernova mystery
Astronomers classify exploding stars based on whether or not hydrogen
is present in the event. While stars begin their lives with hydrogen
fusing into helium, large stars nearing a supernova death have run out
of hydrogen as fuel. Supernovae in which very little hydrogen is present
are called "Type I." Those that do have an abundance of hydrogen, which
are rarer, are called "Type II."
But SN 2014C, discovered in 2014 in a spiral galaxy about 36 million to
46 million light-years away, is different. By looking at it in optical
wavelengths with various ground-based telescopes, astronomers concluded
that SN 2014C had transformed itself from a Type I to a Type II
supernova after its core collapsed, as reported in a 2015 study led by
Dan Milisavljevic at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts. Initial observations did not detect hydrogen,
but, after about a year, it was clear that shock waves propagating from
the explosion were hitting a shell of hydrogen-dominated material
outside the star.
In the new study, NASA's NuSTAR (Nuclear Spectroscopic Telescope Array)
satellite, with its unique ability to observe radiation in the hard
X-ray energy range -- the highest-energy X-rays -- allowed scientists to
watch how the temperature of electrons accelerated by the supernova
shock changed over time. They used this measurement to estimate how fast
the supernova expanded and how much material is in the external shell.
To create this shell, SN 2014C did something truly mysterious: it threw
off a lot of material -- mostly hydrogen, but also heavier elements --
decades to centuries before exploding. In fact, the star ejected the
equivalent of the mass of the sun. Normally, stars do not throw off
material so late in their life.
"Expelling this material late in life is likely a way that stars give
elements, which they produce during their lifetimes, back to their
environment," said Margutti, a member of Northwestern's Center for
Interdisciplinary Exploration and Research in Astrophysics.
NASA's Chandra and Swift observatories were also used to further paint
the picture of the evolution of the supernova. The collection of
observations showed that, surprisingly, the supernova brightened in
X-rays after the initial explosion, demonstrating that there must be a
shell of material, previously ejected by the star, that the shock waves
had hit.
Challenging existing theories
Why would the star throw off so much hydrogen before exploding? One
theory is that there is something missing in our understanding of the
nuclear reactions that occur in the cores of massive, supernova-prone
stars. Another possibility is that the star did not die alone -- a
companion star in a binary system may have influenced the life and
unusual death of the progenitor of SN 2014C. This second theory fits
with the observation that about seven out of 10 massive stars have
companions.
The study suggests that astronomers should pay attention to the lives
of massive stars in the centuries before they explode. Astronomers will
also continue monitoring the aftermath of this perplexing supernova.
"The notion that a star could expel such a huge amount of matter in a
short interval is completely new," said Fiona Harrison, NuSTAR principal
investigator based at Caltech in Pasadena. "It is challenging our
fundamental ideas about how massive stars evolve, and eventually
explode, distributing the chemical elements necessary for life."
NuSTAR is a Small Explorer mission led by Caltech and managed by JPL
for NASA's Science Mission Directorate in Washington. NuSTAR was
developed in partnership with the Danish Technical University and the
Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences
Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC
Berkeley, and the official data archive is at NASA's High Energy
Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. JPL is managed by Caltech for NASA.
