Look up into the sky. That thing shining there – or ‘burning ferociously’ as might be a better description – has been doing its thing for four and a half billion years. Throughout the course of human history, it’s been there the whole time, burning away and giving life to our planet.
4.6 billion years. That’s a number that we can hardly even imagine. Yet, scientists reckon that our Sun is about halfway through its lifecycle.
Give it another four billion years and it will, like all stars do, ultimately ‘die’ – changing from a main sequence star into a supernova or into what is known as a planetary nebula. We ain’t gonna be alive to see any of this happen – and, by we, we mean planet earth.
Here, we’re going to be talking about the life of a star. We’re going to be talking about the forces that bring it together and that force it ultimately apart. And we’re going to be talking about the things that help it keep its shape and size for the unimaginably long time that occurs in between.
We’ll be talking about those things with which you’ve probably heard in passing: the red giant, neutron stars, black holes – and white dwarfs and supernovas.
All of these things make up phases in the life of a star. But let’s take a closer look.
What is a Star?
Now we’re all quite familiar with the Sun – from a distance at least. And, unless you live in the biggest of cities and have never looked up, it is pretty likely that you will have seen some stars in your life too.
But do you know what a star actually is? Apart from the fact that it ‘twinkles’ and is in ‘the sky’.
A star is a massive object in space, held together by gravitational forces, that is distinguished from a planet by its luminosity – or the fact that it produces light.
That’s the short answer. Now to the long answer.
A star is a ball of plasma and gas that radiates energy in the form of heat and light. This radiation is due to the thermonuclear fusion of hydrogen into helium that occurs at its core.
All this wouldn’t happen if stars weren’t so big. But, under the force of gravity, and under all sorts of other molecular forces, atoms are smashed together, and new elements are formed. All of this releases energy. This process, which we call nuclear fusion, is, incidentally, something we hope to replicate on Earth – because of the sheer amount of energy that it produces.
But no, stars aren’t really ‘burning’ or ‘on fire’ or any of these words we use to describe them. Rather, the processes that are happening mean that the sun is much hotter and much more energetic than any fire we’ve ever seen.
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How is a Star Formed?
But why does all this happen? One of the most amazing things about our universe is that there is anything at all in it. As the philosopher Gottfried Wilhelm Leibniz once asked, how come there is something rather than nothing? This question is a little relevant when it comes to thinking that stars produce the very conditions that support life.
Imagine an empty, desperately cold space filled with dust and gases that are the debris of old planets and stars. Star formation begins when, in this intense cold, all of this interstellar dust and gas slowly starts to clump together. Gases reach higher densities in the cold, whilst the atoms bind together.
This is the first step in the life cycle of stars: the planetary nebulae, these molecular clouds that drift across the universe.
As soon as higher densities are reached, the gravitational forces get stronger, meaning that all of the gases and particles in the nebula slowly start coming together. These great big molecular clouds then start collapsing and, as they start moving in on each other, the heat increases.
With all this stuff clumping together, the core becomes what will later be the star – or often even two or three stars known as star clusters. Meanwhile, different parts of the cloud might become planets or might just stay as dust – as in our solar system.
(All of this, by the way, takes about ten million years. As a comparison, humans have been about for only two hundred thousand years.)
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What are the Seven Stages in the Life Cycle of a Star?
So far we have seen how stars are created – from the big messy clouds of dust and gas in the universe. But what these nebulae create are hardly even stars just yet. Rather, they are protostars, which are the very beginning of the star life cycle.
After the initial phase as a nebula, the start of stellar evolution is in the protostar. This is when the star is essentially still growing – when it is still gathering dust and material from the cloud that formed it.
The protostar begins with only one percent of the mass of its future self. But, with all of the mass that is ‘infalling’ due to the core’s gravity, it builds up relatively quickly.
Only when thermonuclear fusion begins at the core does the star stop being a protostar and becomes instead a main sequence star. At this point the star’s mass is stable – as it produces a ‘stellar wind’ that prevents the infall of further mass.
The T-Tauri Phase
As the stellar winds blow, they push away any lingering molecules and gases leaving the newly-formed star spinning rapidly. A full rotation takes only about 10-12 days; compared to the sun's rotation, which takes a full month to execute one complete spin.
At this stage of a star's life, it is still young; only about 10,000 years old. Its temperature is too low; it doesn't generate enough heat for hydrogen fusion so it relies on its gravitational pull to contract into itself.
After about 100 million years, a star will conclude its T-Tauri phase, moving towards its 'main-sequence' phase.
Main sequence stars are identified by their colour and brightness and where they fit on the Herzsprung-Russell Diagram. We'll talk more about this diagram in the next section.
Most of the stars in the universe are main-sequence stars; our sun is one, too.
At this point in a star's life, it has achieved stability: pressures on the star's core due to gravitational collapse of its outer layers is balanced against its internal thermal pressure. This balancing act is called hydrostatic equilibrium and it is what gives stars their shape.
This stage comprises roughly 90% of a star's life, during which it will continuously fuse hydrogen and form helium to feed its core.
Main sequence stars are also called dwarf stars for their relatively small size and low luminosity.
Note: the word 'dwarf' is used to designate small stars; besides the dwarf stars we just explored, we have white and red dwarf stars, as well as...
What Are Brown Dwarfs?
If protostars don’t become big enough – and by that we mean about eight percent of the size of the sun – they never really become stars at all. Instead, they become brown dwarfs, sort of failed stars in which thermonuclear fusion does not take place.
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Understanding the Herzsprung-Russel Diagram
Before moving any further along a star's evolutionary path, we have to understand how its growth, evolution and demise are presented - or, more specifically, how those processes are tracked.
- Ejnar Herzsprung was a Danish chemist/astronomer who worked at the Leiden Observatory.
- Henry Norris Russell was an American astronomer who worked at the Cambridge Observatory and later took a post at the Princeton Observatory
- Mr Russell is also known for his collaboration with Canadian-American physicist Frederick Saunders which resulted in the Russell-Saunders Coupling, also known as the LS Coupling.
Photographic spectroscopic surveys of stars had been ongoing - on a large scale at Harvard College Observatory since the 19th Century. These large-scale depictions showed spectral classifications for thousands of stars, a collection eventually to become known as the Henry Draper Catalogue.
It didn't take too long for astronomers to note the width of the spectral lines the catalogue displayed.
Herzsprung concluded that those with narrower lines have smaller 'proper motions', which he interpreted as being far brighter than wider-lined spectra.
Independent of the Danish scientist's work, Mr Russell plotted a diagram of stars' apparent magnitude against three standard spectrum line emissions to reveal stars' approximate temperature.
The two astronomers' diagrams put together became a plot diagram of luminosity versus temperature of stars.
Today, the Herzsprung-Russell Diagram continues to help astronomers calculate the age of a star and where it is in its life cycle by plotting its relationship between luminosity and temperature.
This diagram is known by many other names: The H-R Diagram, the HR Diagram or simply HRD and, just as there are several names for these diagrams, there are several forms but all roughly follow a specific layout.
The horizontal axis displays the spectral 'type' of stars and values indicating the brightness or visual magnitude on the vertical axis.
Thus we understand that very bright stars will find their place towards the upper left-hand corner of the diagram while older, dimmer stars will be found toward the lower-right side.
Now that we have a rough understanding of how stars' evolution is tracked, let's see what happens after dwarf stars (main-sequence stars) change.
The Red Giant Phase
By the time a star becomes a red giant, it has a large radius and a relatively low temperature. Its outer atmosphere is substantially inflated and weak, unable to resist the core's expansion. Such stars are typically very large and very bright.
Before exploring the life of stars any further, let's review what we've learned so far:
- As previously discussed, stars form in molecular clouds. These clouds consist mainly of hydrogen and helium but also other elements in trace amounts.
- Science defines these elements as anything other than hydrogen and helium; in other words, anything with an atomic number greater than two (2).
- These trace elements are blended in equally throughout the star in part because of the star's gravitational pull and in part because of its rotation.
- When the star's core reaches a temperature hot enough to begin hydrogen fusion, it is said to have attained its main-sequence phase.
This phase lasts as long as the star continues to convert its hydrogen into helium. Once its hydrogen stores are nearly used up, the star's inability to feed the fusion process that would stave off the weight of its outer layers, it loses its hydrostatic balance.
The star's core starts to contract under its own weight, aided by gravity.
Counter-intuitively, the star does not shrink during this process; rather, it undergoes a phase described as 'the mirror principle'.
As the star's core collapses, it creates room to let more hydrogen in. However, the core is very dense at this point so the fusion process starts up in the shell surrounding the core. As this progresses, the outer layers grow in diameter while the core, now under tremendous external pressure, shrinks even further.
This process of simultaneous cooling and expansion is what makes stars at this stage in their life so very bright; this is when they become sub-giant stars.
As the fusion process continues in the shell, pushing the star's outer edges out, those outer reaches become cooler, setting up a convective process - effectively turning the heat of fusion inward. The star stops expanding and gets yet more bright.
Where these red giants fit on the HRD - and what happens at the next stage of their evolution depends on their mass.
If it is not particularly massive - say about twice the mass of our sun, the electrons in its core will degenerate so much that further collapse is prevented. The core will continue heating up, though, until it gets hot enough to fuse helium, a process known as helium-flash.
Stars with more massive cores will degenerate more slowly but will attain temperatures high enough to fuse helium before degeneration is complete. Such massive stars do not undergo helium-flash; their burn is much smoother.
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Does Every Star Become a Red Giant?
Stars with smaller-mass cores are totally convective, meaning they will burn for possibly a trillion years.
They increase in temperature and luminosity just like more massive stars do but because they burn for so long, their temperature only increases by about 50% and the light they emit only increases by a factor of ten.
Such stars can become hotter than our sun but still never achieve that level of brightness, even though they are more luminous at that stage than when they formed.
Over the course of billions of years, their light will dim and they become cooler, eventually earning the classification of 'white dwarf'.
What Happens Next?
It depends on the size and mass of the star.
As we just mentioned, the nature and processes of a star’s life cycle depends on the particular star’s mass. And so we’ll split this here into two separate streams.
There are those stars that have roughly the mass of the Sun – the sun being fairly ‘normal’-sized as far as stars go. Then there are those that are much bigger. The bigger stars are, the quicker they burn. So, whilst Sun-sized stars remain as main sequence stars for about ten billion years, a massive star would not live as long.
As mentioned above, about ninety percent of a star’s life is as a main sequence star – in which it will continuously fuse hydrogen into helium. When the hydrogen in its core runs out, the core will begin to collapse and will get much hotter.
As the core increases in temperature, it pushes the rest of the star outwards, causing its outer edges to cool.
Stars the Size of the Sun – Roughly
The most commonly sized stars are stars the size of the Sun. After about ten billion years, once they have run out of hydrogen, they slowly become white dwarfs.
White dwarfs are cool little things that have perplexed scientists despite their commonness. Imagine the mass of the sun all in the space of the earth and you’ve got yourself a white dwarf. And bizarrely, they are denser the smaller they are – meaning the bigger stars would form the smallest white dwarfs.
They are hugely dense things that keep themselves from collapsing further due to the activity of electrons. However, with no way of producing energy, there is nothing really that keeps them together. So, gradually cooling, they just tend to fade away.
These stars are the smallest and coolest of all. They are also the most common star in our galaxy. They are very hard to see because they are not very bright but one example, in particular, Proxima Centauri, hangs very near to our sun.
So do about 50 other such dwarfs.
You can't see these dwarfs with the naked eye but some astronomers suspect that fully three-fourths of the Milky Way is made up of red dwarfs.
Red and brown dwarf stars - especially massive brown dwarfs with low metallic properties share a few characteristics such as temperature ranges and spectral types. This blending of classification is not an accident; the term 'red dwarf' is a catch-all name for stars that do not have an obvious, definite classification.
In its earliest use, the 'red dwarf' label was used to distinguish between hot, bright 'blue dwarfs' and those stars that are substantially cooler and less luminous. Defining stars in such a manner is a vague way to go about things in such a disciplined field but, when it comes to red dwarfs, vagueness persists to this day.
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Massive stars have a different end in store for them.
If a star is about eight times bigger than the sun, you can expect it to end in a massive explosion known as a supernova.
Remember that the bigger the star the quicker they burn through hydrogen. And when they have run out of hydrogen, they produce iron as the result of a long series of chemical reactions. When that happens, the core collapses in a matter of seconds from five thousand miles across to just twelve.
Temperatures reach a hundred billion degrees and the supernova becomes brighter than a whole galaxy.
What is a Black Hole?
Particularly dense stars produce one of the most fascinating phenomena in the universe when they die. They become black holes.
Rather than exploding outwards, these stars implode, collapsing into themselves to form an object so dense that nothing – not even light – can escape it.
These 'holes' pull everything around them into themselves whilst emitting huge amounts of radiation. The boundary that marks black holes are called the event horizon.
The greatest astrophysicist of our times, Sir Stephen Hawking, was not immune to the pull of black holes. As far back as 1974, he postulated that quantum effects near a black hole horizon must emit radiation - what we identify today as Hawking Radiation.
Although modern-day astrophysicists speculate - indeed, some dedicate their entire career to studying this phenomenon, the hypothesis is not new. As far back as the 18th Century, keen minds grappled with the possibility of objects whose gravitational fields were too strong to permit light to escape.
The postulate merited varying degrees of attention from then until 1967, when astrophysicist Jocelyn Bell Burnell discovered neutron stars - collapsed cores of super-giant stars. Suddenly, gravitationally-collapsed, supremely dense celestial bodies moved from the realm of the possible into probable reality.
Discoveries such as these invariably lead to more profound questions such as: is there life in the universe?