Historical Supernova's 'Echo' Arrives 400 Years Late

On Nov. 11, 1572, astronomer Tycho Brahe observed a bright "new star" — now known as a supernova — in the constellation Cassiopeia.

Brahe observed the star, which outshone even Venus in the night sky until it faded from sight in March 1574.

Most astronomers then believed that the stars were part of a fixed, unchanging dome equally far from Earth at all points. Brahe argued that the "new star" showed the heavens could change, and that each star had an individual distance.

Now, more than 400 years later, astronomers have use the Subaru Telescope in Hawaii to observe "light echoes" from the stellar blast to determine its origin and type and relate that information to what they see in the supernova remnant today.

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A supernova occurs when a star dies violently, sending out an extremely bright outburst of energy.

Some of the light from the original supernova event 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. These reflections are called "light echoes."

In September, scientists used the Faint Object Camera and Spectrograph (FOCAS) instrument at Subaru, to break apart the light echoes of Supernova 1572 into the signatures of atoms (spectra) present when the star exploded, bearing all the information about the nature of the original blast.

"Using light echoes in supernova remnants is time-traveling in a way, in that it allows us to go back hundreds of years to observe the first light from a supernova event," said Tomonori Usuda, lead project astronomer at Subaru. "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."

This same team used similar methods to uncover the origin of supernova remnant Cassiopeia A in 2007.

The results of the Subaru study, detailed in the Dec. 3 issue of the journal Nature, showed clear absorption of once-ionized silicon and absence of the hydrogen H-alpha emission in the light echoes — signatures typical of a Type Ia supernova observed at maximum brightness of its outburst.

Type Ia supernovae are generally thought to originate from white dwarf stars in a close binary system. As the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst.

These supernovae are also the primary source of heavy elements in the universe, and play an important role as cosmological distance indicators

The Subaru study found 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.

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