Kepler's Supernova Remnant

Kepler's SNR, as seen from Hubble, Chandra, and Spitzer

Above is an image of Kepler's Supernova Remnant, one of the most famous objects of its kind. It is the remnant of star that exploded in October of 1604, as seen from Earth. It is named after Johannes Kepler, whose picture is seen here. He did not "discover" the supernova, as the telescope had not yet been invented, and the "stella nova" (new star) was visible to anyone who simply looked up in the sky. At peak brightness, it was brighter than anything else in the night sky except Venus, peaking at a magnitude of around -2.5! Kepler actually didn't even see it until more than a week after it was first observed in various parts of the world. The autumn skies in Prague are not always kind to astronomers, and it wasn't until October 17th that the clouds gave way and Kepler documented the new "star" in the foot of Ophiuchus. We now know it as Kepler's supernova because Kepler kept the most detailed records of the star until it faded from view over the next year or so. He later published a book on his observations, De Stella Nova. The image on the right is a drawing from De Stella Nova, showing the position of the new star. If you look closely, you can see an "N" in the foot of Ophiuchus, which marks the position of the supernova. Of course, at the time, no one knew anything about supernovae, which is why Kepler simply called it a "new star" (stella nova).

The remnant of Kepler's supernova was discovered in the mid-20th century. Baade (1943) reported the discovery of "a small patch of emission nebulosity, which is undoubtedly a part of the masses ejected during the outburst." Constructions of the lightcurve based on Kepler's observations and Chinese observations led Baade to classify the supernova as a type I. This classification was later called into doubt by multiple people, and until recently, Kepler's SNR was the only "historical" supernova remnant whose origin remained a mystery.

Kepler Across the Spectrum

Kepler's SNR has been observed for over 50 years now across the electromagnetic spectrum. Baade's detection in 1943 was in optical wavelengths, but subsequent observations have been made in radio waves, X-rays, microwaves, and most recently, infrared.

A radio image of Kepler from the VLA (Very Large Array) in 1997 at 6 cm. The colors correspond to different intensities, with the redder regions being brighter. Thus, the northern shell of the remnant is easily the brightest part of the remnant. This holds true for all wavelengths. Emission from supernova remnants like this one in radio waves comes from synchrotron emission resulting from electrons spiraling at relativistic speeds in the magnetic field created by the shock wave.

The Hubble Space Telescope image of Kepler. Notice that there really isn't a lot to see here, just some bright knots here and there. You can see most of the bright northern shell and parts of the interior "belt," but that's about it. Supernova remnants are not generally bright in visible light. Optical emission only comes from the densest parts of the remnant.

In 2006, the Chandra X-ray Observatory observed Kepler for an incredible 750,000 seconds (nearly 9 days) of observing time. The result was this image, which contains ~30 million X-ray photons. As I said above, Kepler had been the only historical supernova remnant whose origin remained a mystery. Analysis of the X-ray image now leads us to believe that Kepler is the result of a type Ia supernova that is interacting with an unusually dense circumstellar medium. Emission in this image comes from multiple sources, including emission lines from high-ionization states of ejecta elements like iron, thermal bremsstralung emission from the hot plasma, and non-thermal synchrotron emission from the outer reaches of the shock wave. You can read more about this in the Chandra press release here.

In October of 2004, Kepler was observed with the newly launched Spitzer Space Telescope. This also marked the 400th anniversary of the supernova itself. Kepler was detected in nearly all of Spitzer's instruments, and the image of the left combines 24 µm emission in red, 8 µm emission in green (which looks sort of orange in the image), and 3.6 µm emission from starlight in blue. It is by far the most detailed image of Kepler in the infrared. Most of the emission in this image comes from warm dust that has been heated by the blast wave from the supernova. I had the pleasure of being part of the collaboration that was responsible for obtaining and analyzing the infrared images from Spitzer. Our paper (Blair et al. 2007) can be found here.

For more information on Kepler's Supernova Remnant, visit Bill Blair's Kepler page.

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