by Lucie Green
The lessons learnt from making predictions about solar cycle 24 and seeing how things played out in reality tell us a great deal about what we do and don’t yet know about our local star. It’s been a thrilling time to take a step back and reflect on the bigger picture; to focus not just on what the Sun is doing now but also on what it might do in the years, decades and centuries ahead. There is another way to expand our horizons though, and that is to compare the Sun to the other hundred billion or so stars in our Galaxy, the Milky Way. This is the Sun’s extended family. What can we learn from considering our Sun as a star among so many billions of others?
Conclusion: Our Special Sun
As our local star, the Sun holds a special place for those of us on Earth. Not just because it makes life here possible in the first place, but also because its proximity means it absolutely dominates the sky: literally outshining all other stars. It is easy to become obsessed with our local star and forget that it is part of a much wider nuclear family.
But how does our local star fare on the galactic scene? Sure, it rules over our Solar System, but how does it compare to the hundreds of billions of other stars in our Galaxy? Scientifically, we can use our knowledge of the Sun to help us understand other stars, and indeed surveying so many other stars can give us insight into, and context for, the Sun. Personally, I’m quite keen to find some validation for my belief that my favourite star is still special among all the others in the Universe.
I first realized I had an innate want to champion our local star (in the same way you would support a local football team) when I read about star R136a1. Admittedly, it hasn’t got a very catchy name. With billions of stars out there, astronomers don’t try to give them all individual names like Betelgeuse or Alpha Centauri, so new discoveries are given a catalogue number. The letters tell you which catalogue a star is in (based on which observatory or project discovered or logged it; the R in R136a1 tells us it was discovered by the Radcliffe Observatory in Pretoria, South Africa) and the number is its position in that catalogue. Anything after that number, in this case ‘a’, tells you that when astronomers looked again later, using better telescopes and techniques, at what they assumed was one star, they realized it was actually a few stars huddled together, which are then given the suffix a, b, c, etc. And when that happens again, R136a is sub-divided and the stars are labelled R136a1, etc.
R136a1 lives in a cluster of stars about 165,000 light years from Earth. For a start it is much hotter than our Sun: whereas the Sun’s photosphere is around 6000 Kelvin, R136a1 is about 50,000 Kelvin. And it is much younger, with an age of only 1.5 million years compared to the Sun’s 4.6 billion (the Sun is 3000 times older, in other words). But what really caught my attention is how massive R136a1 is: it has a mass 265 times greater than that of our Sun. It helps to look at other stars when trying to understand our own.
THE MAIN SEQUENCE
Before we look more closely at R136a1 though, we need to take a moment to orientate ourselves. When it comes to looking at the range of stars in the Galaxy it’s useful to refer to stars that are collectively known as ‘main sequence’ stars. These are the stars that Annie Jump Cannon classified at that incredible rate of three a minute. Broadly speaking, main-sequence stars can be thought of as stars that are all being powered the same way: they are stars in whose interiors hydrogen is fusing into helium.
As you will recall from chapter 3, different types of star within the main sequence are labelled with the letters O, B, A, F, G, K and M in that odd order. O-type stars are the hottest and M-type the coolest, so R136a1’s blistering-hot surface makes it an extreme example of an O-type star. The Sun’s roughly 6000 Kelvin surface makes it a G-type star, not far off the middle of the OBAFGKM sequence: so in terms of temperature the Sun looks pretty average.
It’s not really fair to compare the mass of the Sun to the mass of R136a1 though. R136a1 isn’t your normal star. After it had first been spotted, a study led by Paul Crowther, Professor of Astrophysics at the University of Sheffield, worked out that as well as currently being 265 times more massive than the Sun, it would have started out with much more material than that. Since it formed over 1.5 million years ago, strong winds from the star’s atmosphere have been continually eroding its outer layer. At birth R136a1 is thought to have been 320 times more massive than the Sun and this makes it a record-breaker: it’s currently the most massive star that we know of in the entire Universe. But what really makes it remarkable is that it is twice as massive as was previously thought possible for a star.
The reason why there should be an upper limit to a star’s mass comes down to how stars form: from the accumulation of material in a shrinking cloud of gas and dust called a nebula – the same basic process which produced our Sun and that we visited in chapter 3. And logically it would follow that if more material falls in during this process, the growing star will become more massive.
But nebulae are finite in size, and when the supply of material stops, the star ceases to grow any further – you can’t keep building a house if you run out of bricks. But construction can stop for other reasons before you run out of raw materials, and likewise there’s another mechanism that can prevent a star endlessly accumulating mass, and which derives from the star itself and how it interacts with the surrounding nebula: at some point the star starts to push back against the material that gravity is pulling in and so there is an upper limit to how massive a star can be.
During the formation of the Sun, the pressure of the high-temperature plasma works against gravity. Since the two became balanced, the Sun has kept at a constant size. But there is another way to push back on the material, and that’s by using light. Our Sun isn’t creating enough radiation to have a significant effect so only a small outward pressure comes from the photons that are working their way out and into the Solar System. But the most massive stars in the main sequence are also the most luminous. They do produce enough light to exert a pressure on the surrounding material of the nebula, so that it starts to push the cloud back.
It was thought that this effect becomes important for stars that are much more massive than our Sun, perhaps around 150 times as massive, so this would set the upper limit on the mass. Or at least that was what was thought until R136a1 was discovered: it extended the range of masses that we know stars to have. Scientists are now re-evaluating their theories to try and find a more accurate understanding of how stars form and interact with the nebula that birthed them, one that fits the stars we see. This is how science advances: theories are revised in the light of new evidence. And when I spoke to Paul Crowther about R136a1 he told me that, although it is still a record-holder, scientists have since found another ten stars in the R136 cluster that have around 100 times the mass of the Sun.
With this in mind, let’s put aside extreme examples like R136a1 and look to see whether the Sun’s mass is special or not. We know it is certainly not a high-mass star, but how does it compare to the lowest-mass stars? The minimum mass a star can have is determined by the conditions needed to switch a star on. And it’s been known for some time that there is a lower limit to the mass that a star can have.
Stars need a sufficiently high temperature and pressure in the core for fusion to occur. And to get to a high temperature and pressure requires enough mass to have been pulled in under gravity. Taking this into account means that the smallest stars will have around 8 per cent of the mass of the Sun. Any less than this and the temperature and pressure will not be sufficient for hydrogen fusion to take place. These small stars are called ‘red dwarfs’. They are less massive and cooler than our Sun – they have a surface temperature in the range
2500–4000 Kelvin – giving them a red glow.
So we have giants like R136a1 at the high-mass end that are over 260 times more massive than our Sun, and dwarfs at the low-mass end which are twelve times less massive than the Sun. From this I would say that when compared against all main-sequence stars the Sun is a very light flyweight – it’s puny and not impressive at all.
But that’s just the range of star masses. I’ve been very careful so far to say ‘massive’ and not large. There is a big difference between how much mass a star has and how much space it actually takes up. And it’s perhaps easier to conceptualize and compare stars in terms of their physical size. How does the Sun rate in this measure?
If the sequence of stars were placed side by side in a line-up, ordered by their size, the smallest red dwarfs would be around 10 per cent of the width of the Sun; R136a1 on the other hand, despite its vast mass, would be only about thirty times wider: if R136a1 replaced the Sun it would extend less than halfway to Mercury. So, in a size line-up, the Sun looks neither particularly small nor large – it seems very average.
Size is actually a slight distraction: it’s the mass that really matters because it sets the age that the star will reach. Our Sun is currently halfway through its nine-billion-year lifetime. The lowest-mass stars, red dwarfs, are the coolest and use up their hydrogen supply very gradually: fusion runs slowly in their interiors. So red dwarfs aren’t terribly bright, but they do live out long lives. So long, in fact, that the lowest-mass red dwarfs could have a lifetime that is longer than the current age of the Universe – the Universe is 13.8 billion years old. Stars more massive than our Sun have higher central temperatures and pressures, which means they fuse hydrogen more rapidly, shine more brightly and run out of their supplies much more quickly. An O-type star that is forty times the mass of the Sun typically lives for only 5 million years. They burn bright and die young. In this sense the average nature of the Sun in terms of its lifetime makes it a Goldilocks star. It has just the right amount of mass.
But what has always made the Sun special to me and the generations before me, since the era of Hale and the Mount Wilson Observatory, is its magnetic field. This is what turns an otherwise bland ball of plasma into a dynamic and active star. Our Sun starts to look much more interesting when we know that its magnetic field powers solar flares and coronal mass ejections. And that the magnetic field stretches across the Solar System, extended out through the gusty flow of the solar wind. From space weather on Earth to the edge of the heliosphere, the Sun exerts its presence because of its magnetic field. Do other stars have magnetic fields and do they have magnetic activity too?
STELLAR MAGNETISM
For me the discovery of the Sun’s magnetic field by George Ellery Hale in 1908 marked the birth of solar physics and, but for scientists of that era, it would have made the Sun unique, because it became the first star known to have a magnetic field. It was indeed the first time any magnetic field was detected beyond the Earth. But the Sun is fundamentally no different to other stars in the main sequence – they are all spheres of plasma fusing hydrogen nuclei to helium nuclei in their cores – which raises the question: do other stars have magnetic fields too? It turns out that answering this question is a very challenging task.
The Sun has a relatively easy magnetic field to detect because of its proximity, which allows us to measure the photospheric features like sunspots. Other stars are just dots of light; there are exceptions – like Betelgeuse, which is so big and relatively close in astronomical terms that its size and shape can be made out with a very powerful telescope. But in most cases stars are points of light and this makes it incredibly hard to find out whether they harbour a magnetic field.
As Hale and others realized, magnetic fields reveal themselves in a star’s light. A north magnetic pole imprints a certain signature in the spectral lines and the south pole imprints a different one. On the Sun the north and south poles can be resolved, but for other stars the light from these different magnetic field patches is merged together – and the imprint of the magnetic field can be lost. There are other things that can affect the spectral lines too, such as the rotation of the star and the effect of the temperature of the emitting plasma. With so many unknowns it is hard to pull out only the information about the magnetic field. So it was a major triumph when, in 1980, a magnetic field was detected on another star. It had been widely assumed that some stars would have magnetic fields, but detecting such a field made this assumption a scientific certainty. And the observations of stellar magnetic fields that followed means today magnetic fields on other stars are seen as commonplace and are fundamental to studies of them. Our Sun is not a special case.
BLOOD, SWEAT AND FLARES
Perhaps our Sun stands out because of the activity that the magnetic fields create? The use of energy stored in the magnetic field produces the most magnificent flares and coronal mass ejections. And this storage and release is all part of the solar cycle. Curiously, it was possible to detect a cycle on other stars before a stellar magnetic field was first detected. This was because there are proxies that can be used to infer that a star has a magnetic field (without detecting the magnetic field directly) and whether it varies. This technique uses light from a star’s chromosphere, the amount of which changes if the star’s magnetic field changes. According to data gathered at the Mount Wilson Observatory, some stars showed no regularity in their inferred magnetic activity, but about 60 per cent of the stars observed did behave in a cyclic way which indicated they had a magnetic field and that it was cycling too. Meaning that our Sun isn’t alone in having a magnetic cycle.
Given that there are magnetic cycles on other stars, do their magnetic fields power stellar eruptions and flares? The magnetic cycles of other stars are peppered with bursts of activity that look very much like solar flares. We observe flares on the Sun, using wavelengths across the electromagnetic spectrum; creating images that can see the details of the flare, we can infer the changing configuration of the magnetic field, and we monitor the total output of light to make a so-called light curve that allows us to track the intensity of the radiation. Other stars cannot be spatially resolved, but the amount of radiation they emit across the electromagnetic spectrum can be measured and used to produce light curves for them too. When these light curves are studied I am sorry to report that they show the same transient increases in brightness that the Sun shows when a solar flare occurs. Our Sun is not unique in this way either.
But there are ways in which our Sun might be more interesting. Data collected by a satellite that was actually designed and launched to study planets outside our Solar System suggest that we may not have seen all that the Sun can do.
NASA’s Kepler satellite was launched in 2009 to keep an eye on 100,000 stars in the constellations of Cygnus and Lyra. That vast number provides a big pool to dip into when looking for the almost imperceptible dips in light as planets pass in front of a star. But in this pool are many stars different in age to our Sun but similar in terms of their surface temperature and rotation rate. They provide a way of investigating what our Sun might have looked like in the past or might look like in the future. And looking at the light emitted by these Sun-like stars shows that they have flares just like our Sun does. While this was known about before Kepler, Kepler provided a larger number of stars to study and it showed that we might not yet have witnessed the full potential of what the Sun can do.
The Kepler data show that flares on these Sun-like stars can be 10–1000 times more energetic than those seen on our Sun. The activity on our Sun seems dramatic to us, but is dwarfed by these so-called super-flares. But such l
arge flares are thought to be rare, happening perhaps once in 1000–5000 years. I am glad that our Sun doesn’t regularly produce super-flares. This is probably the reason why life was able to evolve. If this makes it boring, then good! There is no point having an exciting local star if you can’t evolve to enjoy it.
Amongst the main-sequence stars, all types produce flares, from the most massive stars to the least massive. Flares have even been observed on very young stars that haven’t yet started fusing hydrogen nuclei in their cores and made it to the main-sequence phase. It also doesn’t matter if the star is one of a multitude of stars or even in a binary system of two stars orbiting each other – they produce flares. Our Sun is not unique when it comes to having a magnetic field, a magnetic cycle or magnetic activity like flaring.
END OF THE SUN
Maybe the Sun will be special when it dies?
The word ‘supernova’ is widely known to describe the explosive and near-catastrophic end to a star’s life. But whether or not this happens depends on how massive a star is.
When most high-mass stars come to the end of their lives they first become a red supergiant star. Betelgeuse – in the constellation of Orion and a star that I’ve mentioned before – is the best-known example of a red supergiant. It’s moved beyond the hydrogen-burning phase of its life and is currently fusing helium into carbon in its core. It has over ten times the mass of the Sun but it has swollen in size to become around 1000 times bigger as well. If you replaced the Sun with Betelgeuse it would engulf all the planets as far out as Jupiter.
