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Science Forum Index » Astro Forum » Astrophysicists explain the differences in the brightness of
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| Andrew Yee |
 Posted: Fri Feb 09, 2007 12:42 pm |
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Press and Public Relations Department
Max Planck Society for the Advancement of Science
Munich, Germany
Contact:
Prof. W. Hillebrandt
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2200
February 9th, 2007
News SP / 2007 (15)
Supernovae -- Cosmic Lighthouses
Astrophysicists explain the differences in the brightness of supernova
explosions
Supernovae stand out in the sky like cosmic lighthouses. Scientists at the
Max Planck Institute for Astrophysics and at the National Astronomical
Institute of Italy have now found a way to use these cosmic beacons to
measure distances in space more accurately. The researchers have been able
to show that all supernovae of a certain type explode with the same mass
and the same energy -- the brightness depends only on how much nickel the
supernova contains. This knowledge has allowed the researchers to
calibrate the brightness of supernovae with greater precision. This means
that in the future, they will use the brightness of a supernova that they
are observing through their telescopes to determine more accurately how
far away from the Earth the cosmic lighthouse is emitting its rays
(Science, 9 February 2007).
The end of a star's life, when the star has become heavy enough, is marked
by a huge explosion -- a supernova. For a few weeks, a supernova looks
almost as bright as a whole galaxy containing billions of stars.
Physicists designate the brightest of these supernovae as Type Ia. Their
brightness, measured from the Earth, is a measure of their distance from
us -- but there are several uncertainties. "The question still remains:
how suitable are supernovae really for measuring distance? For example,
the knowledge that the Universe is expanding rapidly is largely based on
observations of supernovae," explains Prof. Wolfgang Hillebrandt. All Type
Ia supernovae exhibit similar levels of brightness, but they are not
exactly consistent.
Scientists from the Max Planck Institute for Astrophysics and the National
Astronomical Institute of Italy have now made a breakthrough. They have
come to the conclusion that the explosion energy of the Type Ia supernovae
is almost consistent -- it is equivalent to the fusion energy which a
white dwarf with around one and half times the mass of the Sun can
develop. However, the amount of radioactive nickel and medium-weight
chemical elements such as silicon vary from supernova to supernova and
explain the difference in their brightness. The more nickel a supernova
contains, the brighter it shines.
In the explosion, nuclear fusion of carbon and oxygen creates large
quantities of radioactive atomic nuclei; in some supernovae, this is
mainly the radioactive isotope 56 of the element nickel. The energy from
its radioactive decay is converted to light in the supernova. The fusion
therefore supplies both the energy and the light for the explosion. The
nuclear fusion, however, can end with lighter atomic nuclei like silicon,
for example. This creates the same amount of energy, but the supernova is
not so bright. The researchers identify this situation when they also see
the silicon in the light spectrum of the supernova.
Over the last four years, in a study forming part of a European joint
venture lead by the Max Planck Institute for Astrophysics, scientists have
looked at 20 Type Ia supernova explosions, following each one for several
weeks. Using spectroscopic and photometric data and complicated numerical
simulations, they arrived at results that now make it possible to refine
existing calibration methods. Astronomers calibrate the differences in
brightness of the supernovae with their light curves; that is, the way the
brightness develops over time in newly discovered supernovae. The light
curves of brighter supernovae diminish more slowly than those of less
bright supernovae. Up to now, the weakest link in this calibration method
has been limited knowledge about the supernova explosions themselves: what
causes the differences in brightness and are the corrections made to them
justified? The supernovae that play a part in cosmology in measuring
distances exploded just as our solar system was coming into existence, or
even earlier. Consequently, there is no guarantee that these are the same
explosions as those for which the light curves have been calibrated.
In order to exclude possible systematic differences, scientists need to
have a very good understanding of the explosions, and the scientists from
the Max Planck Institute for Astrophysics and the National Astronomical
Institute of Italy have now made a large contribution to this. "Our
surprising results have for the first time delivered a solid basis on
which we can use supernovae to measure distances in space," says Wolfgang
Hillebrandt. "We now understand the differences in the brightness of
supernovae better and can calibrate this cosmic yardstick accurately in
the future." These findings will also benefit cosmologists who use the
brightness of supernovae to deduce dark energy. Scientists believe that it
is this dark matter that is responsible for the rapid expansion of the
Universe.
Original work:
Paolo A. Mazzali, Friedrich K. Röpke, Stefano Benetti and Wolfgang
Hillebrandt
A Common Explosion Mechanism for Type Ia Supernovae
Science, 9. February 2007
IMAGE CAPTION:
[http://www.mpg.de/bilderBerichteDokumente/multimedial/bilderWissenschaft/2007/02/Hillebrandt0701/Web_Zoom.jpeg
(109KB)]
The arrow points to the supernova 2002bo, the explosion of a white dwarf
in the galaxy NGC 3190 in the Leo constellation -- 60 million light years
away from earth.
Image: Benetti et al., MNRAS 384, 261-278 (2004) |
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