by Lucie Green
Stars go through different phases of fusion for reasons of supply and temperature. When an element that has been fusing within a star is completely used up, the star contracts because it is no longer exerting sufficient thermal pressure outwards to counteract gravity. But the collapse raises the internal temperature and eventually a point can be reached where a different element will start to fuse. The larger the star, the higher the temperature that can be reached, and this means that a succession of elements are used in a sequence of fusion phases.
The insides of these massive stars near the ends of their lives resemble a giant stellar onion with the heavier elements sinking towards the core. But there is a limit. Fusion can occur as far up the periodic table as the element iron; for iron and beyond, energy needs to be absorbed, rather than emitted, in order for particles to be fused together. Therefore the most massive stars can end up having iron cores. I can’t help thinking back to those early studies of the composition of the Sun previously described – even Eddington had thought the Sun was mostly made of iron. Our Sun isn’t massive enough to forge this element, but there are stars out there that do.
Once an iron core has been created the fusion stops. This is the end of the line for the star. And with no fusion to release energy to counteract gravity, the core collapses. There is an unimaginable release of energy and a rebound that blasts the layers of plasma surrounding the core out at thousands of kilometres per second, leaving behind a remnant known as a ‘neutron star’. The name gives away what this star is made of – neutrons, created when the protons and electrons in the collapsing core were squeezed together. These types of star are around the size of a city and so dense that just one teaspoon of their material would weigh a billion tonnes. And the collapse leaves the neutron star spinning at a phenomenal rate – perhaps thirty times a second!
In the case of even more massive stars the neutron star itself can collapse yet further and turn into a black hole: a region of space where gravity is so strong that not even light can escape. And R136a1? Well, this star is so massive that it will live a short life and have plenty of hydrogen and helium when it dies. Rather than fusion proceeding in a controlled way until the stocks are depleted, R136a1 is likely to enter a phase when runaway nuclear fusion creates a thermonuclear explosion that cannot be contained and it will completely blow itself up. Nothing will be left behind.
And our Sun?
Once it enters the phase of helium burning, our Sun will swell to become a red giant. The Sun as we have known it will be gone. And so, too, will we. The Sun will probably inflate in size to the point where it has expanded beyond the orbit of the Earth. And it will start to shed its outer layers in fits and starts and form a planetary nebula. With fusion finished, the core that is left behind collapses, unable to fight against gravity, and shrinks down to become an object that is the size of the Earth but has a temperature of around 100,000 Kelvin – our Sun will then be known as a ‘white dwarf’. Without a power source the white dwarf simply cools down over the course of the next billion years or so, fading from sight. A rather graceful ending.
ONE OF A KIND
I still believe that our Sun is special: for what it is in its own right but also because it created an environment that allowed life to form and thrive. As far as we know, the Earth is the only place in the Galaxy, and maybe the Universe, where life exists. And as of 2015, even though water has been detected on Mars, no extant or extinct life has been found there. Meanwhile, on Earth, ancient rocks and microfossils suggest that life emerged after only a billion years, and the Sun’s magnetic field may well have helped this happen.
The solar magnetic field extends out into the Solar System, creating an invisible shield around us which deflects high-energy cosmic rays coming from our Galaxy. We met these particles earlier because their tremendous speeds and energies mean that if they reach us they can penetrate the human body. When this happened to some of the Apollo astronauts, the particles revealed themselves by causing flashes of light in their eyes. But if the astronauts hadn’t been within the Sun’s magnetic envelope they would have seen an awful lot more flashes. The Sun’s magnetic field provides a protective layer, without which the bombardment of galactic cosmic rays could have damaged early living cells and torn DNA apart. The consequences could have included cell mutations – taking evolution down a different path.
The reason why you are able to read this book is because life did manage to thrive and eventually become intelligent life. We took around 3.8 billion years of evolution to arrive and have spent just the last few hundred years studying the science of the Sun. Perhaps the existence of human intelligence makes the Sun special.
Using just the one example we have, we could conclude that stars need to live for billions of years for intelligent life to form. So extrapolating wildly would mean that the most massive stars, that only live for millions or tens of millions of years, will not last long enough for intelligent life to emerge on any planets that have managed to form around them. And that’s ignoring the impact that their searing surface temperatures and harsh radiation might have on any life that does manage to develop.
So let’s only consider stars that live for over a billion years, i.e. stars slightly more massive than our Sun down to red dwarfs. The formation of life anywhere in the Universe is thought to need water. Liquid water is such a good solvent for chemicals that it is thought to be where chemistry turned into biology and life began. And for water to exist in liquid form needs the right conditions for it to have the right temperature. We know from experience that being on a planet orbiting a Sun-like star can provide these conditions. Can these conditions exist elsewhere?
The radiation from any star spreads out in all directions, so the amount of energy that a planet receives depends on how bright the star is and how far the planet is from it. Doing this calculation for the Earth predicts that our planet should have an average temperature of 7 degrees Celsius – not bad in terms of what we see, and definitely the right temperature for liquid water. In reality the Earth’s temperature is warmer than our simple calculation predicts because we have an atmosphere that traps heat from the Sun. And the different natural environments around the globe and variations in the amount of energy that falls on particular parts of the planet create a temperature that varies from place to place. But averaging out across these diverse features gives a temperature of around 16 degrees Celsius.
A planet in orbit around a cooler red dwarf would have to be much, much closer in to its parent star than we are to be warm enough to have liquid water – perhaps as close in as the planet Mercury is in our Solar System (about one third of the distance between the Sun and the Earth). So even though it looks unlikely that a planet in an Earth-like orbit around a red dwarf would be warm enough for liquid water to exist, the presence of an atmosphere would be a help. An atmosphere with greenhouse gases, such as carbon dioxide or methane, would trap heat and raise the planet’s temperature. Possibly to the point where any water ice melts.
ONLY CHILD
The modern view of star formation is that when vast gas clouds give birth to stars it isn’t on a one-to-one basis. They spawn several, perhaps hundreds, of new stars. Perhaps our Sun is unique in being an only child? To be able to answer this question needs more than a stellar line-up. A solar sibling doesn’t need to have the same mass, size or surface temperature as the Sun. But it does need to be made of the same material; this is the forensic evidence needed to show that two stars share a common origin. The cosmic DNA is found in the stellar spectra, which show the chemical composition of the star – and after a study of nearby stars a solar sibling eventually turned u
p!
The Sun’s sibling was found because it has the same composition as the Sun but also because it is moving along a path that makes it look like it came from the same part of space. The star is known as HD162826 (HD standing for the Henry Draper catalogue) and it’s about 110 light years from the Sun and 15 per cent more massive. This is an exciting find. HD162826 has had plenty of time to wander off into the Galaxy and potentially be lost from our view. It’s a fairly bright star that remained fairly close by and so, if you have a good telescope, you may be able to find it too, in the constellation of Hercules.
But even after the discovery that the Sun is part of a nuclear family, it’s still my favourite.
AS SIMPLE AS A STAR
It is an incredible time to be alive and working in solar physics. The Sun is going from a period of unusually high activity to one that may be unusually quiet – and right at a time when we have the technology both on Earth and orbiting through space to watch it all happen. Every morning when I get to work I know that there are literally terabytes of new data about the Sun that I can look through. And with Voyager 1 at the edge of the heliosphere moving out and Solar Orbiter and Solar Probe Plus about to be launched to see the Sun from close up, we’re continuing to widen our view of this amazing star. We are only going to have more and more solar revelations in the years to come.
Before Eddington focused our attention on understanding something as simple as a star, the discoveries about the Sun were coming from all directions from every field of science. There was no such thing as solar physics – there was just scientific thinking applied to the Sun. And today that is still true: when I look at the range of scientists and engineers I call my colleagues, I realize that there is no such thing as a ‘solar physicist’ (despite what my business cards say). Our community contains all sorts of experts who happen to be interested in the same celestial body.
And the work carries on. We still have fundamental questions about the physical processes that govern the solar plasma and magnetic field. We are desperate to have telescopes that can see more detail, so that the tiny regions where magnetic reconnection occurs might eventually come into view. Magnetic reconnection is a fundamental process in the solar atmosphere but so far we have never been able to see the plasma involved directly. And at the opposite end of the size-scale there is much to learn about the solar dynamo, crucial in understanding the heartbeat of the Sun and knowing what it will do in the future.
So despite the Sun being the most studied and understood of all stars, a detailed physical description of many processes remains to be developed. What we ultimately hope for is a grand unified picture: a systems approach to understanding the Sun in which no one part of the Sun is considered in isolation. And to achieve this we need observations that give a seamless coverage throughout the atmospheric layers – something that hasn’t yet been achieved but might become possible with future generations of satellites and telescopes.
If Eddington were alive today I think there could be no question but that he would be impressed by just how far we have come, and perhaps excited that a whole new set of questions has been raised for future generations to work on. We aren’t yet at the point where we fully understand something as simple as a star. But we have learnt an awful lot in trying.
1. A glass plate from the Harvard College Observatory archive which has tiny stellar spectra recorded on it. An eyeglass is needed to magnify and interpret the spectra.
2. The rainbow colours of the so-called continuous spectrum (top row) contrasted with the emission line spectra of the thin energized gases sodium, hydrogen and calcium, which show individual lines at certain wavelengths (© 21 May 2015 OpenStax College).
3. Image of photospheric granulation taken by the Hinode satellite (JAXA/NASA/UKSA).
4. Miso convection. Miso soup, that is.
5. A diagram showing how fast the plasma inside the Sun is spinning. There is an obvious change in rotation rate between the bottom of the convective zone and the top of the radiative zone (GONG and SOHO/MDI consortia).
6. Image of the Hale–Bopp comet, which was visible in 1997. The dust tail stretches out to the right, while the bright blue ion tail is pointing almost directly away from the Sun (ESO/E. Slawik).
7. Extreme ultraviolet light image of the Sun’s corona taken by NASA’s Solar Dynamics Observatory satellite. Active regions appear bright whereas coronal holes, where plasma escapes the Sun as the fast solar wind, appear dark. Superimposed on the image is a model of the Sun’s atmospheric magnetic field. White lines indicate magnetic arches whereas brown lines show the ‘open’ field lines that extend out into the Solar System. These are the magnetic super-highways (NASA/SDO, AIA science team and LMSAL).
8. An image of the solar corona in extreme ultraviolet light. This image is a so-called Doppler image where the plasma flows are shown (towards us in blue and away from us in red). Superimposed in green and orange are the magnetic field lines from a computer model (Dave Brooks/JAXA/Hinode EIS team/Nature).
9. Another image of the solar corona in extreme ultraviolet light. This is an image of the Sun showing in red, orange and green the regions where the solar wind escapes (Dave Brooks/JAXA/Hinode EIS team/Nature).
10. Skylab view of the corona in X-rays in 1973. These images revealed that the corona is highly structured (NASA).
11. Skylab debris at the Esperance Museum in Western Australia. In front of me is an oxygen tank and above my head a copy of the cheque from a US radio station that paid for the littering fine issued to NASA!
12. A glorious image of the corona glowing in X-rays taken by the Soft X-ray Telescope on the Japanese Yohkoh satellite in May 1992 (JAXA/ National Astronomical Observatory of Japan/University of Tokyo/LMSAL).
13. A solar flare ‘slinky’ seen in extreme ultraviolet radiation. This image was taken by NASA’s TRACE satellite in April 2002 (NASA/Goddard Space Flight Center Scientific Visualization Studio).
14. Extreme ultraviolet image of the Sun showing a solar flare on the left side. Image taken by the AIA telescope on NASA’s Solar Dynamics Observatory (NASA/SDO and AIA science team).
15. The concentric rings of the first sunquake to be detected. The rings are produced as sound waves ripple up to the photosphere. The data were taken by the MDI instrument on the SOHO spacecraft (ESA/NASA/Alexander Kosovichev/Valentina Zharkova).
16. The ‘light bulb’ coronal mass ejections as seen by the SOHO spacecraft. The size of the Sun is shown by the small white circle (ESA/NASA and the LASCO consortium).
17. This image shows a simulated magnetic flux rope that has partially emerged through the photosphere. The magnetic field lines are shown in blue. This simulation has been created by Vasilis Archontis of the University of St Andrews – a centre of excellence for modelling the solar magnetic field.
18. A variety of so-called ‘sigmoids’ seen in X-ray images of the corona (JAXA and the SXT consortium/Lucie Green).
Appendix: How to Safely Observe the Sun
There is a wonderfully simple way to view the Sun which is safe and doesn’t involve anything other than a small hole in a piece of card – no mirrors or lenses are needed. The device is called a pinhole camera and to make one get a couple of pieces of card and some aluminium foil. Cut a hole about 2 centimetres square out of the centre of your sheet of card. Place the aluminium foil over this opening and tape it down at the edges. Using a pin, make a small hole (1 millimetre) in the centre of the foil. Hold up the pinhole to a bright object with the second piece of card (which acts as the screen) behind it and you have a pinhole camera! You may want to test the camera first because getting the distance
between the pinhole and the screen is a bit tricky. To test the camera, hold it a few centimetres away from a lit candle with the piece of white paper on the other side of the card to the candle. You should see an image of the flame projected onto the piece of paper. The image of the flame will be upside down.
Once you have mastered the use of a pinhole camera you are ready to use it to view the Sun. Outside on a cloud-free day place the screen on the floor and hold the pinhole above it. Move the piece of card with the pinhole up and down over the screen in the direction of the Sun to bring the image into focus. To get a projected image of the Sun that is about 1 centimetre in diameter, the other sheet of paper will need to be about 1 metre away. But experiment. You’ll see the disc of the Sun projected through the pinhole and onto the screen.
Using a telescope to observe the Sun is much more risky. The aim of a telescope is to magnify the image of an object, but it also gathers more photons than the eye can by itself because it has a larger opening than our eyes. More photons equal more eye damage. So when it comes to studying the Sun, you’ll need either to let the light pass through the telescope so that it falls onto a flat surface and creates a projected image of the Sun, or to use a purpose-made filter from a specialist supplier. My advice is that, before purchasing this equipment yourself, you should get some support and information from your local astronomical society. They will have telescopes to view the Sun and can show you how to do this safely. To find your local society visit http://fedastro.org.uk/fas. Or you can join the Society for Popular Astronomy and get support all year long: http://www.popastro.com.
