Behemoths

EtaCarinae

(a malestrom of energy and power)

All stars are immense nuclear furnances, that will burn for millions, billions or even trillions of years. Their influence upon their local environment is all-encompasing. As mentioned in The Sun, our local star drives all the key processes here on earth – but other stars have also impacted us.

The picture above is of a binary star system. That means there are two stars that orbit each other – what you don’t know is that the stars of the Eta Carinae system dwarf our own sun. The picture is not of the stars themselves, but rather of the nebula the primary is in the process of creating through violent outbursts.

A star’s fate is determined by how much mass it has when it first switches on. Our sun has existed for several billion years and will exist for several billion more. This is not the destiny of massive stars. They have so much mass that they burn through their hydrogen far more quickly, and they meet a violent end.

Eta Carinae A is a true giant – a hypergiant in fact, with a mass between 100 and 200 times that of our sun. It is anywhere from 60 to 800 times the diameter of the sun. It is far hotter than our star and much more luminous. You be wondering why some of the numbers are ambiguous – the problem is that Eta Carinae A (and B for that matter) is obscured by the huge dusty nebula you see in the picture, and combined with the nature of the system as a binary, this makes it difficult for observers and scientists to pin down the system’s exact characteristics.

Nevertheless, even at the lower estimates, Eta Carinae A is a monster, and an appropriate case study when considering stellar behemoths.

Eta Carinae B is in fact a giant in its own right. It weights in at between 30 to 80 times the mass of the sun, and a diameter between 14 and 23 times that of the sun. It has a powerful stellar wind of its own, that crashes into the wind produced by the primary. This is a dynamic and chaotic region of space, one we should be thankful is far away.

Both stars are going to explode in spectacular fashion one day. Eta Carinae A will be the first to go, given its much greater mass, and it is already exhibiting signs of instability. It is thought to straddle the Eddington Limit (the point where the fusion reaction at the core is so strong it overcomes the force of gravity pushing down), and as a result, struggles to contain mass. It’s believed Eta Carinae A has already lost several solar masses since it formed, and continues to loss mass at a fast rate. The star is also prone to huge outbursts (these are partially responsible for the plumes in the picture), and these explosions render the star somewhat unstable. They are precursor of what’s to come.

Supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope more than 400 years ago, is no stranger to the NASA/ESA Hubble Space Telescope. The observatory has been on the frontline of studies into this brilliant dying star since its launch in 1990, three years after the supernova exploded on 23 February 1987. This image of Hubble’s old friend, retreived from the telescope’s data archive, may be the best ever of this object, and reminds us of the many mysteries still surrounding it. Dominating this picture are two glowing loops of stellar material and a very bright ring surrounding the dying star at the centre of the frame. Although Hubble has provided important clues on the nature of these structures, their origin is still largely unknown. Another mystery is that of the missing neutron star. The violent death of a high-mass star, such as SN 1987A, leaves behind a stellar remnant — a neutron star or a black hole. Astronomers expect to find a neutron star in the remnants of this supernova, but they have not yet been able to peer through the dense dust to confirm it is there. The supernova belongs to the Large Magellanic Cloud, a nearby galaxy about 168 000 light-years away. Even though the stellar explosion took place around 166 000 BC, its light arrived here less than 25 years ago. This picture is based on observations done with the High Resolution Channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 25 by 25 arcseconds.

The above image is of SN1987A, the supernova remnant of a massive star that exploded 29 years ago. This was one of the closest core collapse supernovae to earth for hundreds of years, providing astronomers with a remarkable chance to observe such an event up close. The star in question was comparatively small when measured up to Eta Carinae A, yet produced an explosion bright to view with the naked eye, here on earth, despite being 168,000 light years away. It was a core collapse supernova, commonly referred to as a Type II (though a little confusingly, core collapse supernovae also include Type Ib and Ic). To put it in a straight-forward fashion, a core collapse supernova begins with the same process that will one day afflict our sun – hydrogen runs out at the core, and the core contracts until helium fusion can begin. Helium fusion is not as efficient, and produces both oxygen and carbon. Eventually there is no helium left to burn, and carbon fusion begins. This in turn leads to neon burning, oxygen and then silicon.

Adding to this is the process of hydrogen, helium and other stages of fusion igniting in shells around the core as temperatures and pressures within the star reach the point where they can begin again. This means the star is undergoing multiple reactions at once, which renders it unstable. Then comes the final blow.

The byproduct of silicon fusion is nickel, which quickly decays into iron. Whereas the fusion process is possible with lighter elements, Iron represents the point where no energy can be derived from fusion, and there is no way for fission to begin the conditions don’t allow it. As a result, a core of iron builds up within the star, which comes under increasing pressure. Only the intense density of electrons prevents core collapse – but this cannot last forever.

What happens next is far too complicated for me to explain in detail (because frankly, I don’t understand it myself), but basically, the core reaches a mass of 1.4 solar masses, and collapses. This happens fast, at something like 23% the speed of light, and the energy released from this event tears the star apart in what we see as a supernova.

What happens afterward depends on the mass of the star. If it had under 20 solar masses, then what remains would be a neutron star – these dense objects cram the mass of our sun into a ball with the diameter of around 12-15 miles. They very powerful and emit huge levels of radiation, as well as possessing powerful magnetic fields. They rotate at an astonishing speed (sometimes dozens of times per second). They eventually cool and fade away in the same fashion as white dwarfs.

In stars with over 20 solar masses, it’s expected that a black hole will form. More on these here.

So, back to Eta Carinae.

EtaCarinae2The primary star has more than enough mass to collapse into a black hole. It is also theorised that it will produce a particularly energetic supernova, more than 100 times as powerful in fact, earning the title ‘hypernova’. Alternatively, it may undergo a direct collapse into a black hole with virtually no visible explosion.

The fate of Eta Carinae B in this scenario is likely to be mass loss in the event of a supernova, and possibly even being ejected from the system. The smaller star may even influence the supernova – as Eta Carinae A expands during its process of new fusion reactions, the smaller, denser companion may acrete mass from the primary.

There are bigger monsters out there

It’s worth noting that Eta Carinae A, whilst massive, is not the biggest, either in terms of mass or size. Those awards go to (drum roll please)… R136a1 (catchy) and UY Scuti respectively. UY Scuti actually has relatively little mass (between 7-10 solar masses – the margin of error is due to the difficulty in measuring stars that are huge and puffy), and a diameter of 1,708 suns, whereas R136a1 has a mass of 315 solar masses, but a radius no more than 35 times that of our sun. It’s hard to comprehend objects like this – placed within our solar system, their solar wind alone would be enough to catastrophically affect the earth, and in the case of UY Scuti, it would easily engulf our planet.

These stars, and others like them, are among the most fascinating objects in the universe. My take on them is by no means comprehensive, and should not be taken as a scientific reference, but if this and my other pages inspire you to look into this field in more detail, then my work is done.

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