Stars have their own life cycle: they are born, change and die, often bringing to life another star. And so it goes on…
Although this text is devoted to the evolution of stars, the first explanation is not related to astronomy but semantics. This is necessary because the concept of ‘evolution’ means something different than in the context of Darwin’s theory. Biological evolution refers to changes occurring within a certain species over the course of thousands and millions of generations. The evolution of stars is different, because it refers to the life cycle of a single star – from birth until death. Stars have no DNA and are not regulated by the fundamental principle of the animate world, namely that ‘like generates like’. They do not inherit, fight for existence or partake in the ‘survival of the fittest’. The evolution of stars is a term that has been accepted in astrophysics, but it would probably be more accurate to employ the more poetic term ‘life of stars,’ or, to borrow straight from Ovid, ‘metamorphoses of stars’.
There is a second introductory remark that needs to be made. How do we even know how it all works if the lifespan of stars so greatly exceeds the temporal scope of our studies? We do not observe the evolution of stars directly, not to mention the fact that they cannot be recreated in a laboratory. How then? Thanks to statistics! There are 10²² to 10²⁴ stars in the universe, which means that despite the narrow window for making observations, we can witness various stages in the evolution of many stars at the same time.
Let’s begin with mass
The key factor in the fate of any star is its mass: the bigger they are, the shorter they live. One might assume that larger stars have more fuel and would thus shine longer, but it’s actually the other way round. However, in order to explain this, we need to consider what stars, in fact, are in general.
As you might know, stars are gigantic orbs made of gases, with nuclear synthesis taking place inside them (allow me to simplify this as the ‘burning’ of hydrogen or other elements). This process generates energy, which is then radiated outside. One important question is: why do stars not collapse under their own weight? Well, this is because their own gravitational pull (‘weight’) is balanced by internal pressure (generated by temperature and radiation). The bigger the star, the brighter it shines (and thus burns more) and the more nuclear fuel it has at its disposal. However, along with the increase of mass, brightness (or fuel consumption) increases exponentially (the exponent ranging from 2.5 to 4), while the available fuel increases only linearly. Consequently, in the case of bigger stars there is more fuel available, but it burns even quicker. This is the reason why bigger stars live shorter lives.
What kind of scale are we talking about? The lower limit for the mass of a star equals the minimum mass that can sustain hydrogen synthesis at the core. This limit is around 0.08 the mass of the sun (stellar astronomy is decidedly sun-centric, because all values are expressed in terms of the mass of our sun, i.e. 1.99 × 10³⁰ kilograms). Objects below the above limit are called brown dwarfs, although the boundary between them and large gaseous planets is blurred and still not researched enough.
Stars above this limit, on the other hand, are called red dwarfs. They shine weakly, but more steadily and longer. As the most popular kind of star in our galaxy, they constitute as many as three-quarters of its total population. Weighing 0.1 the mass of the sun, their predicted life expectancy can amount to even several trillion years. This is only an estimate and not an observed fact, because the universe itself is ‘only’ 13.8 trillion years old, which means that none of these smaller stars could go through their entire life cycle. Stars the size of the sun – i.e. light welterweight – live for around 10 billion years (the sun is thus halfway through its lifespan). The bigger the mass, the shorter the lifespan: stars three time the mass of the sun live only 400 million years, while those 30 times the weight of the sun live for 10 million years.
The star’s mass determines not only its lifespan, but also its fate when it runs out of nuclear fuel, because its shortage disrupts the aforementioned balance between gravitation and pressure.
When they run out of hydrogen, red dwarfs slowly shrink into white dwarfs. What are these? They are a very common yet highly curious kind of star – or rather the remnant of a star – inside which nuclear reactions no longer occur. They shine very weakly, slowly cooling off and radiating out the heat accumulated in the past. Most importantly, however, white dwarfs can have a mass equal to 0.2-1.3 the mass of the sun, but in terms of size they are closer to Earth. This means that their matter is incredibly dense, impossibly so in terms of our everyday experience. On Earth, one spoon of matter from a white dwarf would weigh as much as five tonnes! What happens later to white dwarfs? Nothing special – cooling off, they emit thermal energy and gradually become black dwarfs, which are only different in that they no longer emit visible light. In short, such stars are cold, shrunken, and about to go out.
The path of evolution leading to white dwarfs and then further on to black ones is important, because it is followed by stars that are not massive enough to become neutron stars or black holes. This concerns around 97% of all stars.
Stars bigger than 0.3 the mass of the sun are large enough to burn hydrogen around them after its supply in the core runs out. This causes them to ‘swell’ and reach a diameter of tens or even hundreds of billions of kilometres, which explains their name: red giants. Examples include Aldebaran and Arcturus, some of the brightest stars on our sky. This will also be the future of our sun. Many have heard that in the distant future it will swell and swallow up Mercury, Venus, and perhaps even Earth. This is what the stage of red giant means. Still, the sun will enter it only in five trillion years, so there is nothing to worry about yet.
The life of a red giant ends pretty spectacularly – by ejecting huge amounts of matter into space. This is how planetary nebulae are born. The term is actually a misnomer because they have nothing to do with planets, except for eliciting such associations among astronomers in former times (just like jellyfish, which are not really fish, or peanuts, which are not really nuts). The best known planetary nebula is probably the Ring Nebula (M57) in the constellation of Lyra. They are often spectacularly beautiful and play an important role in the galactic ecosystem (more about this later). The central part of the star, which was not ejected into space, finally turns into a white dwarf, following the course of the events described above.
When the core collapses
However, stars that are eight or more times bigger than the sun have a different future in store. Such colossal celestial bodies are capable of generating much higher temperatures in the core and can thus burn heavier elements. When they run out of hydrogen, they begin to burn helium, then carbon, and so on. So far, so good (relatively speaking). But then we hit upon a mystery.
Sometimes we hear about nuclear fusion (the joining of atomic nuclei – as in the sun or in the hydrogen bomb) and sometimes about fission (as in the atomic bomb or in a nuclear power plant). Energy is released in both situations. Then we might ask the question: How does it all really work? Is energy generated by joining nuclei into bigger wholes, or by splitting them into smaller ones?
In fact, both answers are correct, because the binding energy in the atomic nucleus changes in two different ways depending on the part of the periodic table we are looking at. Nucleosynthesis (the joining of nuclei) produces energy only in the range from hydrogen to iron. Beyond iron, it would be necessary to provide energy from outside in order to create even heavier nuclei. If a star is massive enough to reach the iron stage, a borderline situation is created. The star has a lot of iron in the core, but is unable to burn it. This means it cannot produce energy and therefore balance its own gravitation. In this case, the star’s core collapses. What emerges then is a supernova, a neutron star, or a black hole.
Supernovas are explosions of hugely massive stars that have nothing left to burn. This peak is short-lasting (weeks or months), but releases immense amounts of energy. The brightness of a supernova is trillions of times greater than that of the sun. Despite huge distances, they may be so bright that we can see them on our sky in the daytime (which happened in the years 1006, 1054 and 1604), while a possible explosion of a supernova near Earth would entail huge problems for our civilization and the entire biosphere. The explosion is so powerful that the star is destroyed, while the matter it was made of is sent flying through space.
Still, it is not always the case that an entire star is disintegrated. Whatever was not jettisoned during the explosion can become a neutron star or a black hole. Let’s start with the former – a truly bizarre entity. Its physics and properties resemble nothing we know from school or daily life. This is because normal stars that shine by synthesizing helium (or other elements) do not collapse under their own weight thanks to the kind of pressure that we know from physics classes in high school. However, in the case of white dwarfs we are dealing with quantum physics, because their situation falls under the Pauli exclusion principle, which says that two electrons in the same quantum states cannot exist in a single atom. In any ‘regular’ gas this principle does not play an important role, but it does so for white dwarfs. Electrons refuse to be ‘compressed’ further and when forced to do so, they generate very high pressure. Neutron stars thus take it to another level. What comes into play in their case is the pressure of neutrons, which is three times higher than that of electrons, which creates overall much higher pressure.
As a result of this, neutron stars have absurd parameters. Although their mass amounts to one or two times the mass of the sun, they only have about 20 kilometres in diameter. Neutron stars are like atomic nuclei the size of a planetoid. They have the density of nuclear matter and one spoon would weigh on Earth as much as a large mountain. It is also worth noting that the gravitational acceleration on the surface of such stars amounts to around 200 trillion g (on Earth we have 1 g, while 100 g is a deadly g-force in a car accident). Indeed, these objects are truly out of this world.
However, neutron stars are still objects of sorts, which cannot be said about black holes – the last stage in the evolution of even bigger stars. Black holes are formed when stars have nothing left to burn, but are still so massive that neither the pressure of electrons nor that of neutrons – nor anything else – can prevent gravitational collapse. Black holes are stars that have collapsed beyond physics, as it were. In accordance with predictions made by the general theory of relativity, in the case of black holes, gravitation bends time-space so much that nothing, no matter or even light, can escape outside. More precisely, black holes are no longer stars because they do not emit radiation. They cannot be directly observed and have no tangible surface. In fact, they lack many properties and appear to be naked beings of sorts. If it is true that ‘black holes have no hair’, they would have only three parameters: mass, electric charge, and angular momentum. From the perspective of the evolution of stars, it is the final product and simultaneously the biggest challenge for both theoreticians and observers.
It was mentioned at the outset that the evolution of stars does not resemble biological evolution. Nevertheless, it needs to be mentioned that the death of one star can lead to the creation of others. Quite literally, a supernova shockwave can cause gas clouds to contract, resulting in the formation of new stars in the future. Primarily, however, massive stars that have a short lifespan produce inside them elements that could not be created anywhere else. These elements then disperse across the galaxy thanks to expanding planetary nebulae and supernovas. As a result, the galactic ‘ecosystem’ is enriched with heavier elements that give rise to new stars and planetary systems.
There is a popular poetic idea that we are all ‘stardust’. It can be understood quite literally (although it would be more accurate to say that we are all ‘star ash’). In the young universe, there could not have been life as we know it, because it lacked elements other than hydrogen, helium and possibly lithium. Heavier elements that facilitate life – e.g. carbon, oxygen, nitrogen, phosphorus and iron – were created in heavy, massive stars that burnt out and exploded long before the solar system was formed. Indeed, every atom in our DNA and every atom of iron in our blood was once created inside the core of a huge star that shone and went out trillions of years ago. Marcus Aurelius would be content to know that in this case, too, everything proves to be interconnected.
Translated from the Polish by Grzegorz Czemiel
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