* Astronomy

Scientists have found that the type II supernova SN 1979C that exploded in 1979 in the galaxy M100 is as bright today in X-ray light as it was when it was discovered years ago, a surprise finding because such objects usually fade significantly after only a few months.

Using ESA’s XMM-Newton space observatory, a team of astronomers has discovered that this supernova, called SN 1979C, shows no sign of fading. The scientists can document a unique history of the star, both before and after the explosion, by studying rings of light left over from the blast, similar to counting rings in a tree trunk.

M100 is one of the brightest member galaxies of the Virgo Cluster of galaxies, and lies 60000 kly away.
The spiral galaxy, like our Milky Way, is tilted nearly face-on as seen from earth.
Four supernovae have been observed in M100:
• 1901B, a type I, mag 15.6 in March 1901;
• 1914A of undetermined type, mag 15.7 in Feb/Mar 1914;
• 1959E of type I, mag 17.5 in Aug/Sep 1959, 58"E and 21"S of the nucleus, discovered February 21, 1960 and observed through June 17, 1960; and
• 1979C of type II, mag 11.6 on April 15, 1979, which however faded quickly.



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This is an image of X-ray light from the galaxy M100. No longer do we see the prominent spiral arms of the galaxy, apparent in optical images. Instead, the X-ray image reveals high-energy activity throughout the galaxy. The most active region is the galaxy centre.
The red and orange regions are sources of very hot, diffuse gas between stars. SN 1979C is the orange hotspot about 7 o'clock from the white galactic centre.
This XMM-Newton image is a composite of three X-ray energy bands: soft, 0.3-1.5 keV (in red); medium, 1.5-4 keV (in green); and hard, 4-10 keV (in blue, which is blended into the white core).

“This 25-year-old candle in the night has allowed us to study aspects of a star explosion never before seen in such detail. All the important information that usually fades away in a couple of months is still there” - Dr Stefan Immler, leader of the team, from NASA’s Goddard Space Flight Center, USA.

Among the many unique finds is the history of the star’s stellar wind dating back 16 000 years before the explosion. Such a history is not even known about our Sun. Also, the scientists could measure the density of the material around the star, another first. The lingering mystery, though, is how this star could fade away in visible light yet remain so radiant in X-rays.
Stars explode when they run out of fuel to burn. Stars more than 10 times the mass of our Sun will explode in an event called a ‘core-collapse supernova’.
Without fuel and thus energy to support their gravity, such stars first implode. The core reaches a critical density, and much of the collapsing matter gets bounced back out violently into space by powerful shockwaves.
Supernovae can outshine an entire galaxy and are often easily seen in neighbouring galaxies with simple amateur telescopes. Supernovae are typically half as bright after about ten days and fade steadily after that, regardless of the wavelength.


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This optical-ultraviolet image shows a face-on view of the 'grand-design' spiral galaxy M100, about 56 million light years away in the Virgo galaxy cluster. The galaxy is about 100 000 light years across, much like our own spiral galaxy. The position of supernova SN 1979C is marked with a white circle.

Position(2000): RA 12 : 22.9 Dec +15 : 49

SN 1979C has in fact faded in optical light by a factor of 250 becoming barely visible with a good amateur telescope. In X-rays, however, this supernova is still the brightest object in its host galaxy, M100, in the constellation ‘Coma Berenices’.
In identifying the history of the star that created SN 1979C, the team found that this star, about 18 times more massive than our Sun, produced fierce stellar winds. That material was flung into space for millions of years, creating concentric rings around the star.
The X-rays - produced after the explosion when the supernova shock caught up with the stellar wind and heated it to a temperature of several million degrees - illuminated 16 000 years’ worth of stellar activity.


“We can use the X-ray light from SN 1979C as a ‘time machine’ to study the life of a dead star long before it exploded” - Dr Stefan Immler.

The detailed analysis was only possible because SN 1979C has not yet faded away. Scientists have 25 years’ worth of data in a variety of wavelengths, from radio waves through to optical/ultraviolet and X-rays.
They speculate that the abundance of stellar wind has provided ample material to keep SN 1979C glowing so brightly.


The team also captured a rare glimpse of the ultraviolet radiation from the supernova using XMM-Newton. The ultraviolet image independently confirms what the X-ray analysis found: that the circumstellar material – covering a region 25 times larger than our Solar System - has a relatively high density of 10 000 atoms per cubic centimetre, or about 1000 times denser than the wind from our Sun. The ultraviolet image also shows galaxy M100 in detail never seen before.

“XMM-Newton is known among scientists as a superior X-ray observatory, but the study of SN 1979 demonstrates the importance of the satellite's simultaneously observing ultraviolet and optical telescope” - Dr Norbert Schartel, XMM-Newton Project Scientist at ESA's European Space Astronomy Centre (ESAC) in Spain.

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