The Week in Space and Physics: Eta Carinae

On Eta Carinae, the possibility of life on Europa, ice on the Moon and Psyche

Clouds of debris billow out from Eta Carinae, as seen by Hubble. Image credit: Jon Morse (University of Colorado), and NASA

Two centuries ago the star Eta Carinae blew up. It was a dramatic moment, marked by the star - which had hitherto been rather unremarkable - suddenly becoming one of the brightest in the night sky. That caught the attention of astronomers, who watched as the star brightened and faded over the course of a decade.

Unlike most stars that explode, however, Eta Carinae somehow continued to shine. For decades after its initial detonation - now known as the Great Eruption of 1837 - it burned on, though rather more dimly than before. Then, in 1887, the star seemed to blow up a second time, in an event that became known as Eta Carinae’s Lesser Eruption.

Still, despite exploding twice, the star is still around. Over the past century astronomers have seen Eta Carinae gradually brighten in the sky. Today it is once again easily visible to the naked eye, though it lies too far south in the sky to be seen from Europe or North America. 

Eta Carinae, we now know, is a binary star system. Two gigantic stars, both dozens of times bigger than the Sun, are locked in a tight orbit around one another. The larger of the two is one of the biggest stars known, holding perhaps two hundred times the mass of the Sun. Around it whips a hot blue star, itself a supergiant at over thirty solar masses.

More recent observations of the star system revealed the presence of a huge cloud of debris expanding away from the stars. This seems to have come from the Great Eruption, likely being thrown out by whatever caused the detonation. Telescopes have also found a bright ring of X-rays around the stars, probably coming from one of its eruptions.

Given the rarity of a system like Eta Carinae, astronomers have captured several observations of it over the past two decades with the Chandra X-ray telescope. Put together, those images show the ring of X-rays rapidly expanding outwards. By projecting the video backwards in time, astronomers concluded that it formed around the time of the Great Eruption.

A closer examination of those images, however, found a second, fainter shell of X-rays surrounding the stars. The researchers behind this discovery think the shell originated in the shockwave of the Great Eruption. Had that shockwave smashed into an existing cloud of debris, they reason, it would have generated a burst of X-rays. That, in turn, points to an even earlier explosion in Eta Carinae, one that took place up to eight hundred years ago.

All these observations confirm that Eta Carinae is a remarkable star; one that somehow survived an eruption that would have destroyed most other stars. Quite how it did that, and what it might do next, are questions astronomers are eager to answer.

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Europa’s Oceans

In the search for life beyond Earth, Mars has long been the favoured target of American alien hunters. NASA has sent half a dozen rovers to that world, scouring its surface for signs of living creatures - or, at least, the remains of long dead ones.

Despite all this effort, the search has come up mostly empty handed. Mars, we have found, once held plenty of water and was, at least briefly, probably as habitable as Earth was. Yet if there ever was life on Mars, it has almost certainly perished amid the four billion year long drought that has hit the planet.

As the hopes for finding Martians have faded, scientists have started to turn their eyes further afield. The moons of the outer solar system, many now think, could offer conditions ideal for life to thrive. Many of the moons around Jupiter and Saturn, indeed, are large and seem to hold vast, liquid oceans under their icy surfaces.

Attention last week turned to Europa, a world long considered potentially habitable. In the early 2000s the Galileo probe found signs of carbon dioxide ice on the moon - a discovery that excited scientists, since carbon is considered essential for life. But it was unclear where the carbon dioxide was coming from, and whether it had originated on Europa or somehow been transported there.

A recent survey of Europa by the James Webb telescope, as reported in two papers last week, found clear signs of carbon dioxide on the moon. They were particularly strong in regions where water seems to be upwelling from deep inside Europa, suggesting that the carbon dioxide is being produced in Europa’s oceans.

That is good news for alien hunters, though it is far from a clear sign that life is indeed present on Europa. For that researchers may need to wait for the arrival of a set of space probes planned to reach Jupiter in the early 2030s. One, JUICE, was launched earlier this year by the European Space Agency. When it arrives it will take a detailed look at Jupiter’s icy moons, examining them for signs of biology.

Europa will get additional attention with the arrival of NASA’s Europa Clipper, currently planned for a launch next year. This probe will scan the surface of Europe in order to assess its habitability, paying particular attention to plumes of water thought to streak upwards from its surface; and to the chemical makeup of its oceans.

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A Litte Less Ice on the Moon

Many recent lunar missions have focused their attention on the Moon’s polar regions. Much of this has been driven by interest in reserves of lunar ice, which researchers believe lies in the shadows cast by craters around the Moon’s poles.

In those regions, a combination of high crater walls and a Sun that never climbs far into the sky creates permanently shadowed areas - places, effectively, which never see sunlight. Since those areas would be eternally cold, researchers reason that ice could have survived there since the formation of the Moon billions of years ago.

Yet researchers also know that the Moon’s spin has changed over time. When the Moon was young, it may have been far more tilted - thus subjecting the poles to intense sunlight. Regions that are today eternally in shadow, then, may not always have been this way.

To estimate when the Moon’s tilt stabilised, researchers recently looked at the known areas of ice on the Moon. None, they found, is older than about four billion years. That implies that the Moon did not stabilise until later than previously thought, and so lost much of the ice that had formed on its surface.

Overall, they report, the findings suggest there is less water ice on the Moon than once hoped. But on the positive side, the ice that does exist is probably scattered widely in small quantities. The study also allowed researchers to better map out where ice should exist, creating a guide book of kind for future lunar explorers.

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Psyche Prepares for Launch

NASA will soon launch Psyche, a mission to explore a large asteroid. Though the exact launch date depends on weather conditions and other factors, NASA expects lift-off to take place between October 12 and 25. Afterwards, Psyche will spend six years flying towards its target, the asteroid 16 Psyche.

16 Psyche is one of the largest known asteroids in the belt that lies between Mars and Jupiter. It measures some one hundred and forty miles across and is mostly made of metals like iron. That could mean 16 Psyche resembles the core of our own planet which, unlike the surface, is thought to be a metallic mix of nickel and iron. 16 Psyche, indeed, could be a fragment from the core of a long lost planet.

The Psyche spacecraft will probe the asteroid for signs of a magnetic field. If it finds one, mission scientists reckon that will be strong evidence the asteroid really was once part of a larger planet. That, in turn, would mean we have the opportunity to study a rare relic from the early days of the Solar System.