Faster than a cannonball

Kepler supernova: Watch a 400-year-old cosmic explosion captured by NASA

The debris is traveling at speeds faster than 20 million miles per hour.

In 1604, astronomers glimpsed the fiery death of a star in the distant universe by its supernova. Today, more than 400 years later, the debris of that explosion can still be seen. This aftermath is known as Kepler's supernova.

Recently, NASA's Chandra X-ray Observatory captured images of material blasting from this supernova explosion at speeds faster than 20 million miles per hour, which is around 25,000 times faster than the speed of sound on Earth.

On Wednesday, NASA released a cosmic video sequence consisting of four Chandra images, detailing moving knots linked to the supernova. The related findings were published in May in the journal Astrophysics.

The star in question is a white dwarf star located about 20,000 light-years away in the Milky Way galaxy. Its fiery death was dubbed as the Kepler supernova after German astronomer Johannes Kepler, who was among the group of astronomers to first observe the explosion.

The debris left over from the site of the supernova was captured in stunning detail by Chandra.

NASA/CXC/Univ of Texas at Arlington/M. Millard et al.

The explosion is known as a Type la supernova. This is when a dense white dwarf star exceeds a critical mass limit when it interacts with a companion star. As a result, the white dwarf star is shattered and its remains are launched outwards.

Tracking the speed of debris — Using the latest observations by Chandra, the study team tracked the speed of 15 small knots of debris erupting from the supernova.

The fastest knot from the Kepler supernova had a startling speed of 23 million miles per hour, the highest speed ever detected of supernova remnant debris in X-rays, according to the study.

Meanwhile, the average speed of the knots is about 10 million miles per hour, and the blast wave is expanding outwards at about 15 million miles per hour.

These high speeds have only been measured days or weeks after the initial explosion of a supernova. However, as the Kepler supernova has been going strong for more than 400 years now, the images indicate that the debris has not been slowed down by any collisions with surrounding material since the initial catalyst.

The researchers also used Chandra images obtained in the years 2000, 2004, 2006, and 2014 to detect changes in the positions of the knots in order to be able to measure their speed.

However, they are still not quite sure how to explain the high-speed material. The assumption, for now, is that the supernova was particularly bright, or that the surrounding material is "clumpy," allowing the debris to escape areas that have low density and not be slowed down.

Abstract: We report our measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Kepler's Supernova Remnant (SNR). We measure the Doppler shift of the He-like Si Kα line center energy in the spectra of these knots based on our Chandra High-Energy Transmission Grating Spectrometer (HETGS) observation to estimate their radial velocities. We estimate high radial velocities of up to ∼ 8,000 km s−1 for some of these ejecta knots. We also measure proper motions for our sample based on the archival Chandra Advanced CCD Imaging Spectrometer (ACIS) data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are almost freely-expanding after ∼ 400 years since the explosion. The fastest moving knots show proper motions up to ∼ 0.2 arcseconds per year. Assuming that these high velocity ejecta knots are traveling ahead of the forward shock of the SNR, we estimate the distance to Kepler's SNR d∼ 4.4 to 7.5 kpc. We find that the ejecta knots in our sample have an average space velocity of vs∼ 4,600 km s−1 (at a distance of 6 kpc). We note that 8 out of the 15 ejecta knots from our sample show a statistically significant (at the 90% confidence level) redshifted spectrum, compared to only two with a blueshifted spectrum. This may suggest an asymmetry in the ejecta distribution in Kepler's SNR along the line of sight, however a larger sample size is required to confirm this result.
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