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Astronomers Determine Extent of Extreme Stellar Blast by Observing Light Echoes

Visualize a travel to the Moon within just 20 seconds! Material from a 170-year-old stellar eruption sped away at such a speed from the eruptive, unstable, and exceptionally massive star Eta Carinae.

This sequence of images show’s an artist’s conception of the expanding blast wave from Eta Carinae’s 1843 eruption. The first image shows the star as it may have appeared before the eruption, as a hot blue supergiant star surrounded by an older shell of gas that was ejected in a previous outburst about 1000 years ago. Then in 1843, Eta Carinae suffered its explosive giant outburst, which created the well-known two-lobed “Homunculus” nebula, plus a fast shock wave propagating ahead of the Homunculus. New evidence for this fast material is reported here. As time proceeds, both the faster shock wave and the denser Homunculus nebula expand and fill the interior of the old shell. Eventually, we see that the faster blast wave begins to catch-up with and overtake parts of the older shell, producing a bright fireworks display that heats the older shell. (Image credit: Gemini Observatory)

Astronomers have come to the conclusion that this is the fastest jettisoned gas ever measured from a stellar outburst that did not lead to the complete annihilation of the star.

The blast, which was from the brightest star known in our galaxy, emitted nearly as much energy as a classic supernova explosion that would have left a stellar corpse behind. Yet, in this case, a double-star system endured and had a vital role in the conditions that resulted in the colossal blast.

In the last seven years, a group of astronomers headed by Nathan Smith, from the University of Arizona, and Armin Rest, from the Space Telescope Science Institute, established the magnitude of this extreme stellar blast through the observation of light echoes from Eta Carinae and its surroundings.

Light echoes are generated when the light from short-lived, bright events are reflected off from clouds of dust, acting like distant mirrors that redirect light in the direction of observation. Similar to an audio echo, the reflected light’s arriving signal has a time delay following the original event because of the finite speed of light. In regards to Eta Carinae, the bright event was a major eruption, also known as the “Great Eruption,” of the star back in the mid-1800s, during which a huge amount of mass was expelled. Astronomers were able to use the delayed signal of these light echoes to decode the light from the eruption using advanced astronomical telescopes and instruments, although the original eruption was observed from Earth in the mid-19th century. That was a period before the advent of sophisticated tools such as the astronomical spectrograph.

A light echo is the next best thing to time travel,” stated Smith. “That’s why light echoes are so beautiful. They give us a chance to unravel the mysteries of a rare stellar eruption that was witnessed 170 years ago, but using our modern telescopes and cameras. We can also compare that information about the event itself with the 170-year old remnant nebula that was ejected. This was a behemoth stellar explosion from a very rare monster star, the likes of which has not happened since in our Milky Way Galaxy.”

Eta Carinae was temporarily promoted by the Great Eruption to be the second brightest star that can be observed in our nighttime sky, hugely outshining the output of energy from every other star in the Milky Way, following which the star faded from naked eye visibility. Material with a mass of almost 10 times greater than the mass of the Sun was expelled by the outburst and also formed the bright glowing gas cloud called the Homunculus. This dumbbell-shaped remnant can be observed surrounding the star from the inner side of an immense star-forming region. It is also possible to observe the eruptive remnant from the equatorial regions and Southern Hemisphere of the Earth using small amateur telescopes; however, it is best seen in images captured with the help of the Hubble Space Telescope.

The astronomers decoded the light from the light echoes and gained insights into the expansion speeds in the historical explosion by using instruments on the 8-m Gemini South telescope, Cerro Tololo Inter-American Observatory 4-meter Blanco telescope, and the Magellan Telescope at Las Campanas Observatory. “Gemini spectroscopy helped pin down the unprecedented velocities we observed in this gas, which clocked in at between about 10,000 to 20,000 kilometers per second,” stated Rest. The U.S. National Science Foundation (NSF) has supported the research group, Gemini Observatory, and Blanco telescope.

We see these really high velocities all the time in supernova explosions where the star is obliterated.” noted Smith. Yet, in this case, the star remained and enabled the researchers to reach a new territory. “Something must have dumped a lot of energy into the star in a short amount of time,” stated Smith. Since the material expelled from Eta Carinae travels nearly 20 times faster than anticipated for typical winds from a gigantic star, according to Smith and his colleagues, enlisting the assistance of two partner stars could describe the extreme outflow.

The astronomers proposed that the best and easy way for explaining a broad array of observed facts related to the eruption and the remnant star system observed currently at the same time is using an interaction of three stars, which includes a dramatic event where two of the three stars merged into one monster star. If this is the case, then the current binary system must have originated from a triple system, where one of those two stars swallowed its sibling.

Understanding the dynamics and environment around the largest stars in our galaxy is one of the most difficult areas of astronomy,” stated Richard Green, Director of the Division of Astronomical Sciences at NSF, the major funding agency for Gemini. “Very massive stars live short lives compared to stars like our Sun, but nevertheless catching one in the act of a major evolutionary step is statistically unlikely. That’s why a case like Eta Carinae is so critical, and why NSF supports this kind of research.”

Chris Smith, Head of Mission at the AURA Observatory in Chile and also part of the research group adds a historical perspective. “I’m thrilled that we can see light echoes coming from an event that John Herschel observed in the middle of the 19th century from South Africa,” he stated. “Now, over 150 years later we can look back in time, thanks to these light echoes, and unveil the secrets of this supernova wannabe using the modern instrumentation on Gemini to analyze the light in ways Hershel couldn’t have even imagined!

Eta Carinae is a kind of unstable star called a Luminous Blue Variable (LBV), located nearly 7500 light years from Earth in a young star-forming nebula discovered in the southern constellation of Carinae. Being one of the innately brightest in the Milky Way galaxy, the star shines about five million times brighter compared to the Sun with a mass almost 100 times more. Stars like the Eta Carinae have the highest mass-loss rates before experiencing supernova explosions; however, the quantity of mass expelled in the 19th-century Great Eruption of Eta Carinae exceeds any others known.

Eta Carinae will possibly experience a real supernova explosion any time within the next half-million years at most, but probably even sooner. Certain types of supernovae have been observed to undergo eruptive blasts similar to that of Eta Carinae in only the few years or decades prior to their final explosion; hence, some astronomers propose that Eta Carinae might explode sooner and not later.

The Gemini Observations were performed with the help of the Gemini Multi-Object Spectrograph on the Gemini South telescope in Chile and used a robust technique known as Nod and Shuffle that allows greatly enhanced spectroscopic measurements of very faint sources by minimizing the contaminating impacts of the night sky. The new outcomes have been reported in two papers accepted for publication in the Monthly Notices of the Royal Astronomical Society.

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