A Down to Earth Guide to the Cosmos

Leavitt’s discovery was of profound importance and turned out to be key in understanding the scale of the Universe. The Danish astronomer Ejnar Hertzsprung was the first to realize the significance of this. If one of these Cepheid Variable stars was identified from its light curve (a graph showing the way its brightness changes with time), a quick measure of the brightness with which it appeared in the sky would allow its distance to be calculated. The American astronomer Harlow Shapley was the first to apply this technique to measure distances in space and was able to use it along with other observations to calculate the size of our own galaxy, the Milky Way, and even to calculate our approximate position within it. The next leap in understanding came in 1924, when Edwin Hubble studied the Andromeda Galaxy and the spiral galaxy in the constellation of Triangulum and identified Cepheid Variables using the 2.5m telescope on Mount Wilson. From this, he calculated their distances as 900,000 light years and 850,000 light years respectively, and although we now know they are somewhat further away than this, it did conclusively show them to be objects well outside our own galaxy.

Cepheid Variables have been successfully used to measure distances in the Universe many times now, but this technique is limited to galaxies where individual stars can be discerned. A similar approach can be applied to the type 1a supernova, which is a white dwarf star that slowly collects matter from a neighbouring star. Over time, the increasing pressure from the matter slowly heats the core of the white dwarf star until a runaway fusion process starts, rapidly converting the star’s material into heavier elements. As this continues, the temperature increases violently until the star rips itself apart. This brings with it a dramatic increase in brightness that can be seen across the vast distances of space.

Because the fundamental process that causes the type 1a supernova is dependent on a certain critical mass, the brightness that accompanies the final explosion is always the same. Not only is the same brightness observed but so too are characteristic features in the spectrum. With these reliable properties it is possible to identify a type 1a supernova and, just as with a Cepheid Variable, comparing its apparent brightness with how bright it really is can determine its distance. This method can be used over much greater distances than the Cepheid Variables approach.

To understand the final method for determining distances in space it is necessary to understand a little about the characteristics of light. If you put white light through a prism or a device called a diffraction grating, then the white light is split into its component parts and you see all the colours of the rainbow. The reason this happens is because the different wavelengths are bent or diffracted by varying amounts, so the individual coloured light is spread out. Extending further out either side would be the other types of radiation that were discussed in Chapter 4. Using specialist devices called spectroscopes, astronomers can look at the light coming from stars and separate it into its component parts, revealing its spectrum. When the German optician Joseph von Fraunhofer studied the solar spectrum in the first half of the nineteenth century he noticed strange dark lines superimposed on the rainbow of colours, but it was not until 1859 that the German chemist Robert Bunsen discovered their origins. He realized that tiny particles inside the atom, called electrons, were absorbing very specific wavelengths of light and stopping them from coming through to the observer. In the case of the solar spectrum, the electrons present in certain gases in the Sun, but also in the Earth’s atmosphere, were responsible for absorbing some of the incoming light. This effect is of great use to astronomers as different gases produce specific patterns of lines, called absorption lines. They appear in very particular positions, and identifying these lines in the spectrum of a distant object allows its chemical make-up to be determined.

The absorption lines can be seen in the spectrum of stars and galaxies alike, and it is the latter that allows us to measure the vast distances to the edge of the observable universe. By studying the position of absorption lines in the spectrum of a star in 1848, the French physicist Hippolyte Fizeau noticed the expected pattern of lines had shifted very slightly from where they should have been and he put this down to a phenomenon known as the Doppler effect. The effect is named after the Austrian mathematician and physicist Christian Doppler, who explained that sound or light emanating from a moving object would change its frequency as the object moved. A popular example of the effect is a moving emergency vehicle: as it approaches, the pitch of its siren seems to go up, and as it moves away, the pitch goes down. The reality is that sound waves emitted by the siren are ‘squashed up’ as the vehicle approaches you, and you receive a higher-frequency noise (the pitch goes up), and the waves are stretched out as it recedes, leading to a lower frequency being received (the pitch goes down).

Just as Fizeau discovered, the effect is not restricted to sound waves but is seen in light too. It manifests itself in light as a shift in position of the absorption lines against the background spectrum. Vesto Slipher noticed in 1912 that most spiral nebulae seemed to show quite a significant jump in the position of the lines, all of which seemed to shift towards the red end of the spectrum. From measuring the apparent red shift it is possible to calculate the speed at which the object is moving away from the observer. Edwin Hubble took this one step further and realized there was a mathematical relationship between the speed the galaxies were moving away at and how far away they were. He had found the final piece of the mathematical puzzle (now called Hubble’s Law) in measuring the incredible distances in intergalactic space, and with it the vast scale of the Universe was finally revealed.

Interestingly, the relationship between speed of movement and distance is related to something that goes back nearly 14 billion years, the Big Bang. Before this explosive event, the entire Universe, everything, was compressed into a tiny point called a singularity, and after the Big Bang the whole Universe, space included, was catapulted into violent expansion. It is important to realize that it is not the galaxies themselves that are shooting out in all directions but space itself, with the galaxies being dragged along with it. The red shift phenomenon is seen because space itself has grown since the light left its origin and, with the intervening expansion of space, the spectrum has been stretched, shifting the absorption lines.

It is difficult to really visualize the enormous dimensions of the Universe so it is sometimes useful to look at scaled-down representations, but even then to consider the entire Universe on one scale is almost impossible. For example, if the Earth were represented as a grain of sand, with the Sun about one metre away, then Proxima Centauri, the nearest star, would be 354km away, and the nearest galaxy just over 2 million kilometres away (that is, roughly six times the distance to the Moon).

It is not just the distances in space that are hard to imagine; the size of objects varies enormously too. The structure, evolution and even appearance of the Universe are dictated by the very small, so in considering the size of objects we should start at the atom. In the same way that it is hard to imagine the size of the very big, it is hard to imagine the size of the very small and atoms definitely come into this category. If an atom were enlarged to the size of a football, then in comparison a small coin would become the size of the Earth! There are even smaller components making up the atom: the neutrons and protons in the nucleus and the electrons in orbit around it. While this may seem the stuff of science fiction – after all, no one has actually ‘seen’ an atom – it is their influence that determines stellar evolution, the crushing power inside black holes and even the features in spectra that enable us to probe the nature of distant objects.

A little easier to visualize are the bits we can actually see, like the particles inside the glowing clouds called nebulae where stars form, through to the dust particles making up the rings of Saturn. The most common, comprising the interstellar dust that pervades the majority of galaxies, are tiny fractions, often millionths, of a millimetre in size. Moving up a little on the scale takes us to the average pieces of interplanetary dust, which we often see ‘burning up’ when they fall through our atmosphere as the streaks of light known as meteors. They range from the size of a grain of sand up to a few millimetres. This is in contrast to the size of the particles in Saturn’s rings, which are mostly water ice, and vary from a centimetre up to several metres.

It is not just tiny pieces of dust and small chunks of ice that join the planets in the Solar System. The bodies making up the asteroids and comet nuclei are a little larger but they differ in composition, comets generally being more icy than rocky. The smallest of the asteroids and cometary nuclei are tens of metres across, while the largest asteroid, Ceres, measures 975km.

Ceres is actually considered a dwarf planet like Pluto, whose diameter is 2306km, making it the second-largest of the dwarf planets after Eris, which also goes round the Sun beyond the orbit of Neptune. Dwarf planets differ from planets because they have not ‘cleared their neighbouring region of other objects’ and it is because of its relatively large companion, Charon, that Pluto was demoted from its planetary status. The remaining ‘major’ planets vary in size considerably, from the smallest, Mercury, at 4879km across, to the mighty Jupiter at 142,984km. In fact, Jupiter is so large that you could fit all the other planets inside it and still have room to spare.

Surprisingly, even some stars are of comparable size to the planets, such as the red dwarf Proxima Centauri, which is thought to be around 200,000km in diameter (compared to Jupiter’s 142,984km). There are even some stars like Sirius B, the companion star of the much brighter star Sirius, at just 12,000km, that are smaller than the Earth. It belongs to a class of star known as white dwarfs, which are remnants of massive stars that have blown most of their layers off into space, revealing the cool and dying core. There is a giant leap in magnitude between the smallest stars and the largest ones, with VY Canis Majoris measuring a staggering 2.7 billion kilometres. It is so large that you could fit just over 218,000 planet Earths across its diameter!

VY Canis Majoris is around 2000 times the size of the Sun, and if you were to put it in the Sun’s position then it would be so large it would swallow up the orbits of Mercury, Venus, Earth, Mars and Jupiter, but stop just short of the orbit of Saturn. There is no doubt that VY Canis Majoris is a monster among stars yet even it is dwarfed by the Solar System. Defining the size of the Solar System is more about identifying its edge but this is no mean feat. The edge can only really be defined by the point at which it is difficult to differentiate between the influence of the Sun and the influence of the nearest star, or the interstellar medium. This distance is known as the heliopause and it is expected to lie somewhere between 8 billion and 14 billion kilometres from the Sun. In the next five to ten years Voyagers 1 and 2 are expected to confirm its exact location as they become the first manmade spacecraft to leave the Solar System.

The stars and their systems of planets (over 700 planets have been discovered around other stars) make up the vast proportion of objects found inside galaxies. Our own galaxy, the Milky Way, measures around 100,000 light years from side to side, which is on a completely different scale to the familiar size of the planets, but as galaxies go, even the Milky Way is dwarfed by IC1101. This supergiant elliptical galaxy lies 1.07 billion light years away in the constellation of Serpens and is a member of the Abel 2029 galaxy cluster. Measuring 5.5 million light years across, it is the largest known galaxy in the Universe and is fifty-five times the diameter of the Milky Way.

Whether it is the sizes or distances in the Universe being considered, it is incredibly difficult to demonstrate either of these on a single scale as the range from smallest to largest is so immense. From the size of the atom to the monstrous elliptical galaxies, or from the relative proximity of the Moon to the far edge of the observable Universe, the extremes are truly astronomical. It is unfortunately a limitation of the human brain that we cannot comprehend such gigantic numbers, but while they may well be beyond the reach of our imagination, the quest to understand them will continue.

May: Northern Hemisphere Sky

Arcturus is unmistakable in the southern part of the northern hemisphere sky during May. It is by far the brightest star in the region and with its orange glow is easy to identify. Shining at magnitude 0.15 it is a great place to start this month’s guide to the night sky. Stretching to the north and slightly to the east is the rest of the constellation of Boötes, home to Arcturus and several other moderately bright stars. Pick out the second-brightest star in the constellation, Izar, which is a beautiful binary star to the north-west of Arcturus and almost 200 light years more distant. The brighter of the two stars can be seen by the naked eye and is an orange giant star with a surface temperature of 4500 degrees. A telescope with an aperture of at least 75mm is needed to detect the fainter companion star, which is a white star shining at magnitude 5.12. Through a telescope the system looks stunning and is well worth seeking out. The rest of the stars trace out the shape of a kite to the north-west of Arcturus, which is supposed to represent a herdsman.

Just to the north-west of Arcturus is the familiar sight of Ursa Major and to the south of its tail is the rather less conspicuous constellation of Canes Venatici. The two brightest stars form a line which runs parallel to the tail, and between its easternmost star, Cor Caroli, and Arcturus is one of the real treasures of the northern night sky, the globular star cluster M3.

M3 was discovered by Charles Messier in 1764 and is just visible to the unaided eye from a dark site. Binoculars show a rather more hazy patch but a telescope with a diameter of at least 20cm is needed to reveal individual stars, although in reality there are around half a million stars in the cluster. The galactic centre which is seen in the direction of Sagittarius is closer than M3, which lies at a distance of just under 34,000 light years.

Another great example of a globular cluster can be found fractionally to the north of the celestial equator, using Arcturus as a guide to spot it. Look to the south-east of Arcturus and find Zeta Boötis, which is fainter than Arcturus and about 10 degrees away (a clenched fist at arm’s length measures around 10 degrees against the sky). Follow the line these two stars make towards the celestial equator and, at just over twice the same distance again, the faint fuzzy-looking star is the cluster M5. At 165 light years in diameter it is one of the largest of its kind.

Just a couple of degrees to the north of M5 is IC1101, the largest known galaxy in the Universe, although it is so faint that it takes a telescope about 35cm in diameter to pick it up. To the north-east of M5 is the brightest star in the constellation of Serpens, Alpha Serpentis, and it is easily recognized as a moderately bright orange star. It is nearing the end of its life and is burning helium in its core, producing carbon and oxygen, a phase that our Sun will go through in a few billion years’ time.

Directly to the north of M5 and Alpha Serpentis is a small constellation called Corona Borealis, which looks like a curve of stars, open to the north and with its brightest star, Alphecca, to its south-west corner. This star is one member of a pair of stars known as an eclipsing binary, like Spica in Virgo, which means they appear along the same line of sight from Earth and seem to keep eclipsing each other.

To the north-west of Corona Borealis is the northern end of Boötes and directly to the north of Nekar, the second-brightest star in the constellation, lies Edasich in Draco. This is one of the largest constellations and extends out to the east, taking it north of Hercules, and to the west by Ursa Major. Edasich, or Iota Draconis, is an orange supergiant star with a surface temperature of around 4500 degrees, about 1000 degrees cooler than the Sun. With a mass just a little greater than the Sun’s, Iota Draconis is thought to be a little ahead of the Sun in its evolutionary cycle and gives us a glimpse of the Sun’s future. In 2001 a planet was discovered in orbit around this giant star, and by studying its movement an estimate could be made of its mass at around nine times that of Jupiter. It has a highly elliptical orbit and the planet, now called Iota Draconis B, moves from its furthest point 2.1 times the Earth–Sun distance to its nearest point of 0.4 times. This means that it is very unlikely for any smaller rocky Earth-type planets to exist within the system.

A little further to the north is another orange giant star similar to Iota Draconis but much larger. At a distance of just under 130 light years Kochab in Ursa Minor shines with a brightness equivalent to around 500 Suns, yet it is giving off a lot less light and energy than Polaris to its north. Known as the Pole Star, Polaris lies at a point in the sky directly above the Earth’s axis of rotation, so all objects in the sky seem to rotate around it over a 24-hour period. It is the brightest star in the constellation of Ursa Minor, the Small or Lesser Bear, with Kochab just slightly fainter. The reality is that Polaris gives off about five times as much light as Kochab, but at a distance of around 430 light years it appears at a similar brightness in the sky. As well as having two fainter companion stars, Polaris is a Cepheid Variable and, as we have seen, it was stars like this that allowed us to map some of the distances in space. Between them, these two stars mark the far ends of Ursa Minor, which looks like a smaller version of the Plough, with Polaris remaining almost motionless at the end of the handle and Kochab representing the end of the pan along with Gamma Ursae Minoris.