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Starts With A Bang

This Is How Eta Carinae Survived A Near-Supernova Eruption

The most famous ‘supernova impostor’ of all could have died back in the 1840s. Here’s what we think kept it alive.

In all of astronomy, no stellar event releases more energy than a supernova.

An animation sequence of the 17th century supernova in the constellation of Cassiopeia. This explosion, despite occurring in the Milky Way and about 60–70 years after 1604, could not be seen with the naked eye due to the intervening dust. Surrounding material plus continued emission of EM radiation both play a role in the remnant’s continued illumination. A supernova is the typical fate for a star greater than about 10 solar masses, although there are some exceptions. (NASA, ESA, AND THE HUBBLE HERITAGE STSCI/AURA)-ESA/HUBBLE COLLABORATION. ACKNOWLEDGEMENT: ROBERT A. FESEN (DARTMOUTH COLLEGE, USA) AND JAMES LONG (ESA/HUBBLE))

Humanity hasn’t witnessed a naked-eye supernova within our galaxy since 1604, but Eta Carinae came close.

In 1843, previously modest star Eta Carinae brightened to become the 2nd brightest object in the sky, ahead of Canopus (shown here) and trailing only Sirius. Gradually, over the next 13 years or so, it faded until it finally became visible in telescopes only. (CELESTIA USER HENRYKUS)

In 1843, it brightened to become the second brightest star in the sky, gradually fading away by 1857.

This infrared light image showcases the large Carina nebula, which houses Eta Carinae at the lower left. The gas and dust loops visible arise not only from material blown off from Eta Carinae itself, but also from the material of the larger star-forming region that spawned it millions of years ago. Other new, nearby stars are also visible, showcasing the power of this star-forming region. (ESO / VERY LARGE TELESCOPE / T. PREIBISCH ET AL.)

Almost as much energy was released as in standard supernovae, but Eta Carinae remained intact.

The ‘supernova impostor’ of the 19th century precipitated a gigantic eruption, spewing many Suns’ worth of material into the interstellar medium from Eta Carinae. High mass stars like this within metal-rich galaxies, like our own, eject large fractions of mass in a way that stars within smaller, lower-metallicity galaxies do not. Eta Carinae might be over 100 times the mass of our Sun and is found in the Carina Nebula, but it is not among the most massive stars in the Universe, nor is it alone. (NATHAN SMITH (UNIVERSITY OF CALIFORNIA, BERKELEY), AND NASA)

Even today, in 2019, its heaviest star reaches over 100 solar masses.

This multi-panel image shows Eta Carinae in the same field of view using three different types of light: X-ray, optical, and infrared. X-rays reveal an outer horseshoe-shaped ring, a hot inner core, and a hot central source. The optical image shows two giant bubbles expanding away from the center of the system at over a million miles per hour. The infrared data reveal that Eta Carinae is one of the most luminous systems in the Milky Way, as the rapidly expanding cloud of dust absorbs direct radiation from the central star and re-radiates it in the infrared. (OPTICAL: NASA/STSCI, NEAR-INFRARED: 2MASS/UMASS/IPAC-CALTECH/NASA/NSF)

The star’s surroundings reveal the remnants of this recent ejection, with 10–20 solar masses expelled at speeds from 400–3,200 km/s.

The Carina Nebula, with Eta Carina, the brightest star inside it, on the left. What appears to be a single star was identified as a binary back in 2005, and it’s led some to theorize that a third companion was responsible for triggering the supernova impostor event. (ESO/IDA/DANISH 1.5 M/R.GENDLER, J-E. OVALDSEN, C. THÖNE, AND C. FERON)

In 2005, observations revealed that Eta Carinae isn’t a single star, but a binary system in a ~5.5 year mutual orbit.

The 5.5 year binary orbit of the Eta Carinae system consists of a hydrogen-rich star of approximately 100 solar masses in orbit with a hydrogen-free star of 30 solar masses. This type of mass inversion, where the less massive star has lost its hydrogen, suggests a mass transfer that’s difficult to explain. (NASA’S GODDARD SPACE FLIGHT CENTER HOMUNCULUS NEBULA IMAGE COURTESY OF NASA/ESA/HUBBLE SM4 ERO TEAM)

The Universe gave us a delayed replay, as some of the centuries-old emitted light rebounded off of nearby gas, creating light-echoes.

The Variable Star RS Puppis, with its light echoes shining through the interstellar clouds. Although this has nothing to do with Eta Carinae, it’s a fabulous illustration of a light echo: where the light emitted from a specific event (like the pulsing of a variable star, here, or a supernova impostor like Eta Carinae) is reflected off of the surrounding gas, enabling astronomers to watch the event on a sort of cosmic ‘instant replay.’ (NASA, ESA, AND THE HUBBLE HERITAGE TEAM)

It’s possible that a specific cataclysm triggered this outburst: the devouring of a third star.

This six-panel graphic illustrates a scenario for Eta Carinae’s 1843 outburst where a triple-star system has one member enter the giant phase, loses its outer layers to its nearest companion, which drives the donor star farther away, kicking the outer companion inwards, causing an eventual merger that led to the supernova impostor event. (NASA, ESA, AND A. FEILD (STSCI))

This type of mass exchange could explain:

  • the outburst,
  • the most massive star’s survival,
  • and the absence of hydrogen in its 30 solar mass companion.
A number of light echoes have been observed for Eta Carinae’s 1843 outburst, including in 2003, 2010, and 2011. Additional echoes are expected to continue through the late 2020s at least, and could reveal additional details about the explosion that we wouldn’t be able to uncover otherwise. (NASA, NOAO, AND ARMIN REST (STSCI) ET AL.)

With Eta Carinae still shining, future light-echoes could hold the key to solving this mystery.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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