This false-colour image of Tycho's 1572 supernova remnant is comprised of data gathered at different wavelengths. It shows the gas cloud given off following a natural nuclear explosion on a white dwarf star.
Astronomers use a supernova to time travel
5 Dec 2008
When we look up into the night-time sky, we see stars, planets, and galaxies across a sea of darkness. The movements of planets and seasonal variations to the constellations have been much the same for thousands of years.
But what if the sky changed overnight and a new star, brighter than any other, appeared? Would it be noticed if it happened in the 16th century?
A few months ago astronomers using Japan's Subaru Telescope went back in time and observed light from a 'new star' that originally was seen on November 11, 1572 by astronomer Tycho Brahe and others.
What Brahe saw—a bright star in the constellation Cassiopeia, outshining even Venus—was actually a rare supernova event where the violent death of a star sends out an extremely bright outburst of energy.
Brahe studied the brightness and colour of the 'new star' until March 1572 when it faded from view. The remains of this milestone event are seen today as Tycho's supernova remnant (see the image at the top of this page).
An international team of astronomers recently completed a study using Subaru that focused on the 'light echoes' from Tycho's supernova to determine its origin and exact type, and relate that information to what we see in its remnant today.
A 'light echo' is light from the original supernova event that bounces off dust particles in surrounding interstellar clouds and reaches Earth many years after the direct light passes by; in this case, 436 years ago.
How light echoes work. Light from the explosion travels directly toward the observer, but it also shoots off in other directions. Some of that extra light gets reflected from interstellar gas clouds and eventually makes its way to the observer many years later as a 'light echo'. In this case, the original light arrived at Earth in 1572 (blue arrow), while the light that was scattered by dust clouds around the supernova arrived in 2008 (yellow arrows).
This same team used similar methods to uncover the origin of another supernova remnant, called Cassiopeia A, in 2007.
Lead project astronomer at Subaru, Dr. Tomonori Usuda says, 'using light echoes in supernova remnants is time-travelling in a way, in that it allows us to go back hundreds of years to observe the first light from a supernova event. We got to relive a significant historical moment and see it as famed astronomer Tycho Brahe did hundreds of years ago. More importantly, we get to see how a supernova in our own galaxy behaves from its origin.'
On September 24, 2008, using the Faint Object Camera and Spectrograph (FOCAS) instrument at Subaru, the light echoes were broken apart into the signatures of the atoms (spectra) present when Tycho's supernova exploded, bearing all the information about the nature of the original blast. The results showed clear absorption of once-ionised silicon and absence of the hydrogen H-alpha emission. The findings are very typical of a Type Ia supernova observed at maximum brightness of its outburst.
For Type Ia supernovae, a white dwarf star in a close binary star system is the typical culprit. As gas from the companion star is pulled onto the white dwarf, the white dwarf gets progressively heavier, becomes compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst. (Other supernova—Types II, Ib, Ic etc—are different kinds of explosions resulting from the death of a massive star.)
However, as Type Ia supernovae with luminosities both brighter and fainter than standard ones have been reported recently, the understanding of the supernova outburst mechanism has come under debate. In order to explain the diversity of the Type Ia supernovae, the Subaru team studied the Tycho outburst mechanisms in detail.
What they found is that Tycho's supernova shows indications that it blew up in a non-symmetrical way, which is information that helps astronomers to improve their explosion models. In addition, follow-up comparisons with Type Ia supernovae found outside our Galaxy shows that Tycho's supernova belongs to the majority class of Normal Type Ia, and, as such, is now the first confirmed and precisely classified supernova in our galaxy.
This is important, because Type Ia supernovae are not only the primary source of heavy elements in the Universe, but they also play an important role as cosmological distance indicators—astronomers use them as 'standard candles' for measuring huge distances in space, because their intrinsic brightnesses are always the same, and thus their apparent brightnesses can be used to estimate distance.
This study has demonstrated how light echoes can be used to study supernovae outbursts that occurred hundreds of years ago. When seen from different angles, the echoes enabled the team to look at the supernova in a three dimensional view. This 3D aspect will accelerate the study of the mechanisms of supernova based on their explosion's shape, which, to date, has been impossible with supernovae in galaxies outside our Milky Way.
Adapted from information issued by the Subaru Observatory / Spitzer Space Telescope / Chandra X-Ray Observatory.
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