The Meerkat of Science: The Death of Giants

In the previous episode of The Meerkat of Science, I wrote of our own star, and how it is destined to one day expand into a red giant, consume the earth, and then become a white dwarf. This is the fate of many stars, but there are some that have different prospects. These stars, with a mass more than eight times that of our Sun, will not go out with a relative whimper, but with a powerful, spectacular bang.

With all stars, there is a relationship between its mass and its lifespan. The more massive the star, the more quickly it burns through the hydrogen fuel at its core. For stars up to eight solar masses, the process is the same as the Sun’s; the cores contract until helium fusion begins, and the star will expand and contract until eventually it becomes a white dwarf. For stars of more than eight solar masses, the heat and pressure within the core can become so great that new fusion reactions take place.

The Birth of the Elements

These massive stars will begin to fuse carbon into other elements, such as neon, sodium, magnesium and aluminium. Once carbon fuel runs out, gravity once again compresses the core, until temperatures rise enough to fuse neon into oxygen. Once neon fuel is exhausted, the core collapses once more, until conditions are right to fuse oxygen into silicon. This is where matters become very dicey for the star.

Silicon fuses into nickel, which decays to become iron. Because each new stage of fusion becomes increasingly inefficient, and because it becomes harder and harder to produce energy via fusion with heavier atoms, there is eventually a tipping point, and that point is iron. Iron atoms have binding energy too great for a fusion reaction to take place, and as a result, an iron-nickel core begins to grow. Fusion reactions that push against gravitational forces cease, and now, gravity wins. The outward layers of the core collapse at some 23% the speed of light, rapidly heating the core, which shrinks as well. High-energy gamma rays create conditions that lead to the production of neutrinos, which normally don’t interact with other matter, and these begin to escape the core, carrying tremendous energy with them.

The core’s collapse detaches it from the star’s outer layers, and some neutrinos are then absorbed by the outer layers, beginning what we see as a supernova explosion. All of this happens in the span of seconds, and the supernova ejects material across the stellar medium at high speeds, enriching local nebulae with newly-formed elements. In one sense, we owe our existence to this process (among others).

What Remains?

The supernova explosion is not the end of a massive star’s story. Depending upon the star’s initial mass, one of two stellar bodies remain. If the star has less than 20 solar masses, a neutron star is born. These super-dense, highly energetic stars initially spin extremely fast, and they also give off intense radiation. Whereas a sugar cube-sized piece of a white dwarf would weigh around one tonne (roughly the equivalent of car), a sugar cube-sized piece of a neutron star weighs as much as Mount Everest, just to give you an idea as to the density. Another way to look at it is that a white dwarf crams around half the mass of the Sun into a ball the size of the earth; a neutron star crams the mass of the Sun into an object around 10 KM wide.

Neutron stars are fascinating, but there remains a lot unknown about them. They are – unsurprisingly – composed of neutrons, packed so tight that they almost defy our understanding of physics. However, even neutron stars cannot compare to the fate of stars that are even more massive.

Hearts of Darkness

Neutron stars are the bulwark against further gravitational collapse, but with massive enough stars, the crushing force of gravity can squeeze even these dense balls into a singularity, a point of infinite density, where gravity becomes so powerful that nothing, not even light, can escape its grasp. We know these to be black holes, and we now know they exist in all sorts of sizes.

Stellar black holes – the focus of this post – could well be found across the Milky Way. In fact, scientists theorise that there are millions of them. Their very nature makes them hard to detect, since they give off no light, or emissions of any kind. Instead, their presence is observed in how they affect their surroundings, with some black holes having accretion disks of superheated matter spiralling around them. They are the ultimate triumph of gravity.

Famous Examples

Core collapse supernovae are rare, but scientists and astronomers have seen a number of examples across several galaxies. The most famous of recent times is SN1987A, of which there is a photo above. Supernovae are usually named after the year they are spotted, hence 1987A was seen in 1987! It has the distinction of being the closest supernova to us for nearly a century, and because of its relative closeness, SN1987A has become of the most intensively-studied objects in the sky. There is growing evidence for the presence of a neutron star remnant (predicted to form in some core collapse models) buried within the rubble and debris of the explosion.

Possible Prospects

The red supergiant star Betelgeuse is the top-left star of the famous constellation Orion. It is also a star that most observers believe is destined to erupt as a supernova in the astronomically near future, possibly within the next 100,000 years. Eta Carinae is a massive star, and known to be unstable. It will almost certainly produce a powerful supernova in the astronomically near future, and this could be complicated by the presence of a companion that that’s quite massive in its own right. The chances are that the next observed supernova will come from a different galaxy to our own, and the odds are even better than it won’t be a core collapse supernova, but more on that in another post.

For now, watch the skies above us, and we’ll all have to wait and see what stellar fireworks await us!

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