170 Years Ago, Eta Carinae Erupted Dramatically. Astronomers Now Think They Know Why

Eta Carinae, a double star system located 7,500 light years away in the constellation Carina, has a combined luminosity of more than 5 million Suns – making it one of the brightest stars in the Milky Way galaxy. But 170 years ago, between 1837 and 1858, this star erupted in what appeared to be a massive supernova, temporarily making it the second brightest star in the sky.

Strangely, this blast was not enough to obliterate the star system, which left astronomers wondering what could account for the massive eruption. Thanks to new data, which was the result of some “forensic astronomy” (where leftover light from the explosion was examined after it reflected off of interstellar dust) a team of astronomers now think they have an explanation for what happened.

The studies which describe their findings – titled “Exceptionally fast ejecta seen in light echoes of Eta Carinae’s Great Eruption” and “Light echoes from the plateau in Eta Carinae’s Great Eruption reveal a two-stage shock-powered event” – recently appeared in the Monthly Notices of the Royal Astronomical Society.

Eta Carinae, one of the most massive stars known. Image credit: NASA
Eta Carinae, one of the most massive stars known and one of the brightest in the night sky. Credit: NASA

Both studies were led by Nathan Smith of the University of Arizona’s Steward Observatory, and included members from the Space Telescope Science Institute (STSI), the National Optical Astronomy Observatory (NOAO), the Millennium Institute of Astrophysics, the Harvard-Smithsonian Center for Astrophysics (CfA), the Cerro Tololo Inter-American Observatory and multiple universities.

In their first study, the team indicates how they studied the “light echoes” produced by the explosion, which were reflected off of interstellar dust and are just now visible from Earth. From this, they observed that the eruption resulted in material expanding at speeds that were up to 20 times faster than with any previously-observed supernova.

In the second study, the team studied the evolution of the echo’s light curve, which revealed that it experienced spikes before 1845, then plateaued until 1858 before steadily declining over the next decade. Basically, the observed velocities and light curve were consistent with the blast wave of a supernova explosion rather than the relatively slow and gentle winds expected from massive stars before they die.

The light echoes were first detected in images obtained in 2003 by telescopes at the Cerro Tololo Inter-American Observatory in Chile. For the sake of their study, the team consulted spectroscopic data from the Magellan telescopes at the Las Campanas Observatory and the Gemini South Observatory, both located in Chile. This allowed the team to measure the light and determine the ejecta’s expansion speeds – more than 32 million km/h (20 million mph).

Based on this data, the team hypothesized that the eruption may have been triggered by a prolonged battle between three stars, which destroyed one star and left the other two in a binary system. This battle may have culminated with a violent explosion when Eta Carinae devoured one of its two companions, sending more than 10 Solar masses into space. This ejected mass created the gigantic bipolar nebula (aka. “the Homunculus Nebula”) which is seen today.

As Smith explained in a recent HubbleSite press release:

“We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived. The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds.”

In this scenario, Eta Carinae started out as a trinary system, with two massive stars orbiting close to each other and the third orbiting further away. When the most massive of the binary neared the end of its life, it began to expand and then transfer much of its material onto its slightly smaller companion. This caused the smaller star to accumulate just enough energy to cause it to eject its outer layers, but not enough to completely annihilate it.

The companion star would have then grown to become about 100 times the mass of our Sun and extremely bright. The other star, now weighing only 30 Solar masses, would have been stripped of its hydrogen layers, exposing its hot helium core – which represent an advanced stage of evolution in the lives of massive stars. As Armin Rest – a researcher from the STSI, The John Hopkins University and a co-author on the paper – explained:

“From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes. So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make sense without a transfer of mass.”

The Homunculus Nebula, surrounding Eta Carinae. Credit: ESO, IDA, Danish 1.5 m, R. Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron

This transfer of mass would have altered the gravitational balance of the system, causing the helium-core star to move farther away from its now-massive companion and eventually travel so far that it would interact with the outermost third star. This would cause the third star to move towards the massive star and eventually merge with it, producing an outflow of material.

Initially, the merger caused ejecta that expanded relatively slowly, but as the two stars finally joined together, they produced an explosive event that blasted material off 100 times faster. This material caught up to the slow ejecta, pushing it forward and heating the material until it glowed. This glowing material was the main light source that was viewed by astronomers 170 years ago.

In the end, the smaller helium-core star settled into an elliptical orbit around around its massive counterpart, passing through the star’s outer layers every 5.5 years and generating X-ray shock waves. According to Smith, while this explanation cannot account for everything observed in Eta Carinae, it does explain both the brightening and the fact that the star remains:

“The reason why we suggest that members of a crazy triple system interact with each other is because this is the best explanation for how the present-day companion quickly lost its outer layers before its more massive sibling.”

These studies have provided new clues as to the mystery of how Eta Carinae appeared to explode in a massive supernova, but left behind a massive star and nebula. In addition, a better understanding of the physics behind the Eta Carinae explosion could help astronomers to learn more about the complicated interactions that govern binary and multiple star systems – which are critical to our understanding of the evolution and death of massive stars.

Further Reading: HubbleSite, MNRAS, MNRAS (2)

Twelve New Moons Discovered Around Jupiter, and One of Them is Pretty Odd!

The gas giant Jupiter, which was named in honor of the king of the gods in the Roman pantheon, has always lived up to its name. In addition to being the largest planet in the Solar System – with two and a half times the mass of all the other planets combined – it also has an incredibly powerful magnetic field and the most intense storms of any planet in the Solar System.

What’s more, it is home to some of the largest moons in the Solar System (known as the Galilean Moons), and has more known moons than any other planet. And thanks to a recent survey led by Scott S. Sheppard of the Carnegie Institution of Science, twelve more moons have been discovered. This brings the total number of known moons around Jupiter to 79, and could provide new insight into the history of the Solar System.

The team was led by Scott S. Sheppard and included Dave Tholen (University of Hawaii) and Chad Trujillo (Northern Arizona University). It was this same team that first suggested the existence of a massive planet in the outer reaches of the Solar System (Planet 9 or Planet X) in 2014, based on the unusual behavior of certain populations of extreme Trans-Neptunian Objects (eTNOs).

Artist’s impression of Jupiter’s moons, with the newly-discovered moons indicated in blue and red. Credit: Carnegie Institution of Science/Roberto Molar Candanosa

Curiously enough, it was while Sheppard and his colleagues were hunting for this elusive planet that they spotted the first of these new moons in 2017. As Sheppard explained in a recent Carnegie press release:

“Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant Solar System objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our Solar System.”

The initial discoveries were made using the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. They were then confirmed with the help of the Dark Energy Camera (DECam), which was added to the Blanco telescope as past of the Dark Energy Survey. Additional data was provided by the Carnegie Observatories 6.5-meter Magellan Telescopes.

The orbits of the newly-discovered moons were then calculated by Gareth Williams of the International Astronomical Union’s Minor Planet Center (MPC), based on the team’s observations.  “It takes several observations to confirm an object actually orbits around Jupiter,” he said. “So, the whole process took a year.”

As you can see from the image above, two of the newly-discovered moons (indicated in blue) are part of the inner group that have prograde orbits (i.e. they orbit in the same direction as the planet’s rotation). They complete a single orbit in a little less than a year, and have similar orbital distances and angles of inclination. This is a possible indication that these moons are fragments of a larger moon that was broken apart, possibly due to a collision.

Nine of the new moons (indicated in red) are part of the distant outer group that have retrograde orbits, meaning they orbit in the opposite direction of Jupiter’s rotation. These moons take about two years to complete a single orbit of Jupiter and are grouped into three orbital groups that have similar distances and inclination. As such, they are also thought to be remnants of three larger moons that broke apart due to past collisions.

The team observed one other moon that does not fit into either group, and is unlike any known moon orbiting Jupiter. This “oddball moon” is more distant and more inclined than the prograde moons and takes about one and a half years to orbit Jupiter, which means its orbit crosses the outer retrograde moons. Because of this, head-on collisions are much more likely to occur with the retrograde moons, which are orbiting in the opposite direction.

The orbit of this oddball moon was also confirmed by Bob Jacobson and Marina Brozovic at NASA’s Jet Propulsion Laboratory in 2017. This was motivated in part to ensure that the moon would not be lost before it arrived at the predicted location in its orbit during the recovery observations made in 2018. As Sheppard explained,

“Our other discovery is a real oddball and has an orbit like no other known Jovian moon. It’s also likely Jupiter’s smallest known moon, being less than one kilometer in diameter…This is an unstable situation. Head-on collisions would quickly break apart and grind the objects down to dust.”

Caption: Recovery images of Valetudo from the Magellan telescope in May 2018. The moon can be seen moving relative to the steady state background of distant stars. Jupiter is not in the field but off to the upper left.

Here too, the team thinks that this moon could be the remains of a once-larger moon; in this case, one that had a prograde orbit that formed some of the retrograde moons through past collisions. The oddball moon already has a suggested name for it – Valetudo, after the Jupiter’s great-granddaughter, the goddess of health and hygiene in the Roman pantheon.

In addition to adding to Jupiter’s overall moon count, the study of what shaped these moon’s orbital histories could teach scientists a great deal about the earliest period of the Solar System. For instance, the fact that the smallest moons in Jupiter’s various orbital groups (prograde, retrograde) are still abundant suggests that the collisions that created them occurred after the era of planet formation.

According to the Nebular Hypothesis of Solar System formation, the Sun was still surrounded by a rotating protoplanetary disk at this time – i.e. the gas and dust from which the planets formed. Because of their sizes – 1 to 3 km – these moons would have been more influenced by surrounding gas and dust, which would have placed a drag on their orbits and caused them to fall inwards towards Jupiter.

The fact that these moons still exist shows that they likely formed after this gas and dust dissipated. In this respect, these moons are much like time capsules or geological records, preserving pieces of Jupiter’s (and the Solar Systems) history of formation and evolution.

This research was partially funded by a NASA Planetary Astronomy grant, and was made possible thanks to assistance by multiple observatories. These included the 4-meter Discovery Channel Telescope at Lowell Observatory Arizona, the 8-meter Subaru Telescope and the University of Hawaii’s 2.2 meter telescope, and the 8-meter Gemini Telescope in Hawaii.

Further Reading: Carnegie Institute of Science