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STARS add beauty to the sky and are the building blocks of our Galaxy, the Milky Way. But they are also essential for life on Earth. Many of the atoms in our bodies were forged inside stars, and one star, the Sun, sustains us all. The Sun looks different from other stars simply because it is so much closer. Sunlight is really just very bright starlight, and if our star stopped shining, all life on Earth would perish.

In addition to the Sun, our Galaxy harbours hundreds of billions of other stars. But like snowflakes, no two stars are the same. Some stars are bright, others faint; some are blue, others white, yellow, orange or red; some are enormous, others tiny; some are young, while others are old and dying.

To make sense of the tremendous diversity of the stars, astronomers use the Hertzsprung-Russell diagram, a method of studying the differences between stars which was first published by the Danish astronomer Ejnar Hertzsprung in 1911 and independently by the American astronomer Henry Norris Russell in 1913. The H-R diagram is central to the study of stars and stellar evolution. Just as the periodic table allows chemists to sort the elements by their fundamental characters, so the H-R diagram allows us to distinguish stars by their main features.

Luminosity v. colour

A powerful diagram

THE H-R diagram is so powerful for astronomers because it plots two basic stellar properties: luminosity and colour. Stellar luminosity is the amount of light that a star emits. The most luminous stars are a million million times more luminous than the least luminous. This means that the most luminous star in the Galaxy sends out more light in a single second than the least luminous does in a hundred centuries. If the most luminous star in the Galaxy replaced the Sun, the Earth would become so hot that its oceans would boil and its rocks melt. Conversely, if the least luminous star replaced the Sun, daytime would be darker than a moonlit night and Earth’s oceans would freeze.

The Sun’s luminosity falls midway between these two extremes. But the Sun is by no means just an “average star”, because most stars are in fact much dimmer.

On the H-R diagram, the most luminous stars appear at the top and the least luminous at the bottom. Because its luminosity is in the middle of this range, the Sun appears about half-way down on the H-R diagram.

Colour is the other stellar property used in the H-R diagram. To the untrained eye, all stars may look white or yellow. But in fact, stars range in colour from blue and white to yellow, orange and red. The Sun is yellow.

Colour tells us how hot or cool a star is. The blue and the white stars are hot (most of them are between 7500 °C and 50 000 °C), yellow stars are warm (between 5000 °C and 7500 °C), and orange and red stars are cool (between 2000 °C and 5000 °C). In the same way, a poker placed in a fire will first glow red and then, as it becomes hotter, and hotter, will glow orange, yellow and white.

On the H-R diagram, the hot blue stars appear on the left hand side, the warm yellow stars in the middle and the cool red stars on the right. Because the Sun is yellow, it again lies near the middle of the diagram.

Strength in numbers

The main sequence

WHEN Hertzsprung and Russell first plotted the H-R diagram, they were astonished to find that stars did not scatter randomly over it. Instead, most stars lay in a band that stretched diagonally from the upper left (bright and blue) to the lower right (faint and red). This diagonal band included the Sun. Astronomers now know that 90 per cent of all stars fall in this band. It is, therefore, called the main sequence.

Main-sequence stars obey a law: the brighter the star, the bluer it is. This law arises because every main-sequence star generates energy the same way. Every such star fuses hydrogen nuclei into helium nuclei at its centre.

The reason the luminosities and colours of main-sequence stars differ is because the stars differ in mass – the total quantity of material in the star. The greater the mass of a main-sequence star, the hotter the star’s centre and the faster the hydrogen fuses, so the hotter and brighter the star.

Because of this dependence on mass, the main sequence is really a mass sequence. Blue and white main-sequence stars have more mass than the Sun; yellow main-sequence stars have about the same mass as the Sun; and orange and red main-sequence stars have less mass than the Sun. The most massive stars have about 100 times the Sun’s mass, whereas the least massive main-sequence stars have only 0.08 solar masses.

But not all main-sequence stars are equally common. Massive stars are rare, both because few such stars are formed and because they do not live for long. Consequently, only one star in more than a thousand is a blue main-sequence star. Yet many such stars are visible to the naked eye, because they are so luminous that they can be seen from great distances.

In contrast, less massive stars abound but are hard to see. The most abundant main-sequence stars are the red ones, which appear at the bottom right of the H-R diagram and are called red dwarfs. They outnumber all other stars put together, accounting for 70 per cent of the stars in the Galaxy. Yet they are so faint that not a single one is visible to the naked eye.

If a star has even less mass than a red dwarf, it never becomes hot enough to ignite its hydrogen and so never joins the main sequence. These stars, which astronomers have not yet definitely detected, are called brown dwarfs. The name is misleading, however, because if these stars exist, they will be faint and red and look like red dwarfs.

A star’s mass dictates how long the star will live. Although stars with more mass have more hydrogen fuel, they “burn” it much faster and die sooner -just as a millionaire who spent a million pounds a day would go broke long before a poor person with a sensible budget.

The most massive stars use up the hydrogen fuel at their centres just a few million years after they are born. In contrast, the Sun has been burning hydrogen for 4.6 billion years and will continue to do so for billions of years more. Red dwarfs burn their fuel so slowly that some will remain on the main sequence for thousands of billions of years.

Nevertheless, every main-sequence star will someday use up the hydrogen at its centre. When that happens, the star begins to burn hydrogen outside its centre and so is no longer a main-sequence star. The star expands and cools, moving to the right on the H-R diagram, and it may also get brighter, in which case it moves up on the diagram. The star is now a giant or supergiant.

Big and bright

Giants and supergiants

AS THEIR name implies, giants and supergiants are much bigger than the stars on the main sequence. Aldebaran, an orange giant, is 40 times bigger than the Sun. If put in the Sun’s place, Aldebaran would stretch halfway to the innermost planet Mercury. Supergiants are even bigger. Betelgeuse, a red supergiant, is so large that if it replaced the Sun it would engulf Mercury, Venus, Earth, Mars and part of the asteroid belt. The largest red supergiants would stretch all the way to the planet Saturn.

Most giants and supergiants are warm or cool, being yellow, orange, or red, so on the H-R diagram they appear at the upper right (bright and red). A few are blue or white, such as Rigel, a blue supergiant, and Deneb, a white supergiant.

In general, supergiants have evolved from the hottest and bluest main-sequence stars, whereas giants have evolved from less massive main-sequence stars. Red dwarfs are so long-lived that none has yet had enough time to evolve to become a giant.

Because they are so big, giants and supergiants have a low average density, since their mass is spread throughout a large volume. The outer parts of a giant or supergiant are so tenuous that they would pass for a perfect vacuum here on Earth, so a rocket ship could fly through most of such a star unimpeded.

Because of their large size, giants and supergiants are luminous and outshine the Sun. When our Sun becomes a giant, it will be about 100 times brighter than it is now. Supergiants are even brighter, outshining the present Sun thousands of times over.

Despite their impressive size and luminosity, giants and supergiants are rare. In fact, they account for less than 1 per cent of all stars. They are prominent in the night sky simply because they are luminous and easy to see.

Giants and supergiants are rare because they don’t last long. Stars with more than eight times the mass of the Sun have a dramatic but brief life after becoming supergiants. Having consumed all the hydrogen at their core, they soon start fusing helium, carbon, neon, oxygen, silicon and sulphur, the last two of which fuse into iron. But iron does not fuse into heavier elements, and at that point the star is doomed and explodes as a supernova.

Normally, the supernova will be classified type II, which means that hydrogen appears in the exploding star’s spectrum. The hydrogen comes from the star’s outer atmosphere. However, a massive star may lose this hydrogen atmosphere before exploding. If so, the star becomes a type Ib supernova, and no hydrogen appears in the spectrum. A star may even lose both its hydrogen and its helium – which lies deeper inside the star – in which case neither element appears in the exploding star’s spectrum and the supernova is classified as type Ic.

After the supernova, the remains of the star collapse into a small but dense sphere. This may be a neutron star, so called because the star’s protons and electrons smash together and become neutrons. A neutron star is the size of a large city, but typically has a mass 1.5 times that of the Sun. Or the collapsed star may become a black hole, which has such intense gravity that nothing – not even light, the fastest thing in the Universe – can escape it. Neither neutron stars nor black holes appear on the conventional H-R diagram, because they are dimmer than even the dimmest stars.

But most stars never go through the ordeal of a supernova, because most are born with less than eight solar masses. When such a star becomes a giant, it eventually burns helium into carbon and oxygen. During this time, instabilities develop in the star’s outer atmosphere, which gets ejected into space. The star’s hot core emerges and makes the ejected atmosphere glow. This glowing gas is a planetary nebula, not because it has anything to do with planets but because through a small telescope it may look like a planet’s disc.

In only a few tens of thousands of years, the planetary nebula expands so much that it disappears from view. All that is left is a small but extremely hot star. The star is so hot that it does not appear on the conventional H-R diagram, since the star would be far left of even the hottest and bluest main-sequence stars.

But the star soon cools and fades, reappearing on the H-R diagram below the main sequence, because the star is fainter than a main-sequence star of the same colour. The star is now a white dwarf.

Fading stars

White dwarfs

THE FORMER core of a once-living star, a white dwarf is hot and dense. A typical white dwarf is little larger than the Earth but contains about 60 per cent of the mass of the Sun. A teaspoonful of white dwarf matter would weigh more than a tonne.

Because so many stars become white dwarfs, these objects are common, making up 10 per cent of all stars in the Galaxy. But they are so faint that all are invisible to the naked eye. A typical white dwarf leads a boring life. It no longer burns fuel; it shines simply because it contains a store of heat. As it radiates energy into space, the star fades and cools over billions of years. As it does so, it changes colour, so despite their name, white dwarfs can, in fact, be any colour. The newest members are hot and blue, while those that have been around a long time and lost most of their energy are orange or red. So, on the H-R diagram, white dwarfs form a sequence that is parallel to the main sequence. If enough time elapses, the white dwarf will fade completely and become a black dwarf. But no black dwarfs exist, because the Universe is not old enough for any white dwarfs to have faded from view.

On rare occasions, white dwarfs can create spectacles. If another star orbits the white dwarf and dumps material onto it, the material can explode. Astronomers then see a nova. During a nova, the star increases in brightness some 100 000 times. Violent though it may be, the explosion does not destroy either star.

However, if a companion star transfers too much mass, then the white dwarf can annihilate itself in a supernova. This is because the most mass that a white dwarf can have is 1.4 times that of the Sun: anything more than that and the star blows up.

This is now known as the Chandrasekhar limit, so named after the astronomer who discovered it in the 1930s. If a companion star forces the white dwarf over this limit, the white dwarf explodes. This supernova is classified as type Ia; it is the only type of supernova that does not arise from a massive star, the way that types Ib, Ic and II supernovae do.

Supernovae cast heavy elements, such as oxygen and iron, into the Galaxy. These elements were formed both during the star’s life and in the explosion itself. This material, together with debris from planetary nebulae, eventually gathers in star-breeding areas of the Milky Way, where it will give birth to new stars and planets, some of which may one day support life. This is how the Sun and Earth formed 4.6 billion years ago. We are part of this legacy: our bodies contain atoms that were created by the stars.

1: Magnitude, distance and luminosity

A GLANCE at the night sky reveals that some stars look bright and others faint. To quantify this impression, astronomers have for a long time used the concept of apparent magnitude. In about 120 BC, Hipparchus classified the stars into six groups by calling the brightest stars those of first magnitude down to the faintest stars of the sixth magnitude. The system was calibrated more exactly in the 1850s, and put on a logarithmic scale so that each magnitude corresponds to a factor of 2.5 in brightness. A star with an apparent magnitude of 1.00 is 2.5 times brighter than a star with an apparent magnitude of 2.00 and so on. Most of the brightest stars in the night sky are of the first magnitude; the faintest stars that the naked eye can see are sixth magnitude and look only about 1 per cent as bright as first magnitude stars.

Telescopes penetrate to fainter apparent magnitudes. The largest telescope on Earth can detect stars as faint as 30th apparent magnitude. This is 4 billion times fainter than the naked eye can see.

But apparent magnitude alone does not reveal a star’s luminosity. To calculate that, astronomers must also know the star’s distance. A star that looks faint may have a low luminosity and be nearby, or it may be luminous and lie far away. For stars near the Sun, astronomers can measure the distance because they view the star from slightly different perspectives as the Earth circles the Sun. For example, the Earth is on one side of the Sun in January and on the opposite side in July. So, from January to July, every star’s apparent position in the sky shifts slightly. The larger this shift, or parallax, the closer the star. Only nearby stars have parallaxes that are large enough for astronomers to measure.

Distances are often given in light years. One light year is the distance that light travels in a year, or 9.5 million million kilometres. This is an enormous distance: there are as many Earth-Sun distances in a light year as there are inches in a mile (more than 60 000). Yet even the nearest star to the Sun is 4.3 light years away, and most stars that you can see in the night sky are a few hundred light years away. Another unit of distance that astronomers use is the parsec. A parsec is the distance off a star whose parallax is one arcsecond or 1/3600th of a degree. One parsec is also 3.26 light years. The nearest star is, therefore, 1.3 parsecs away.

Once they know a star’s distance, astronomers can calculate its luminosity from its apparent magnitude. They express this luminosity in terms of absolute magnitude. This is the apparent magnitude the star would have if it were 10 parsecs (32.6 light years) from Earth. For example, the Sun has an absolute magnitude of 4.83, which means that if we viewed it from a distance of 32.6 light years, the Sun would have an apparent magnitude of 4.83, barely visible to the naked eye.

But the absolute magnitudes of other stars run the gamut from bright to dim. The most luminous stars have absolute magnitudes around −10, which means that if such a star were 32.6 light years away it would be incredibly bright – about a tenth as bright as the full Moon. In contrast, the dimmest have absolute magnitudes around +20, which means that such a star 32.6 light years from Earth would look hundreds of times fainter than Pluto. The Sun, with an absolute magnitude around +5, is right in the middle of this range.

2: Stellar colour and spectral type

COLOUR is such a basic property of stars that astronomers have developed a system for classifying it. Because stars of different colour have different temperature, and because temperature affects different types of atoms differently, astronomers can classify a star by studying the atoms and molecules that mark its spectrum. For example, white stars have strong spectral lines due to hydrogen, whereas yellow stars, like the Sun, have strong lines due to calcium.

Astronomers have used stellar spectra to divide the stars into seven main types, which can best be remembered with the mnemonic “Oh, Be A Fine Guy/Girl, Kiss Me!”

O: These stars are the hottest and bluest of all. But O stars are rare, and few appear in the night sky.

B: Somewhat cooler than O-type stars, stars of spectral type B are nevertheless hot and blue. Many bright stars in the night sky, such as Spica, Regulus and Rigel, are B-type stars.

A: A-type stars are white and contribute much to the light of our Galaxy. They include among their ranks Sirius, Vega and Altair, which are all main-sequence stars, and Deneb, which is a white supergiant.

F: The F stars are a bit hotter than the Sun and are yellow-white. Viewed from Earth, the two brightest F-type stars are Canopus and Procyon. Polaris, the Pole Star, is also an F star.

G: The Sun is a member of this spectral class. Like the Sun, G-type stars are warm and yellow. Other G stars are Alpha Centauri A, which is a main-sequence star, and Capella, which is a giant.

K: Cooler than the Sun are the orange K stars, which include giant stars, such as Arcturus and Aldebaran, and fainter main-sequence stars, such as Epsilon Eridani. K-type main-sequence stars are called orange dwarfs.

M: These stars are cool and red. Some M stars, such as Betelgeuse and Antares, are supergiants that shine thousands of times more brightly than the Sun, but most M stars are faint stars on the main sequence – red dwarfs.

3: The Sun: something special in the sky

TEXTBOOKS often malign our star, the Sun, by calling it “average”. But it isn’t, not by a long shot. The truth is, the Sun is much more luminous than most other stars.

Seventy per cent of all stars are red dwarfs. These stars are much dimmer than the Sun, and most emit less than 1 per cent of its light. Anorher 15 per cent of stars are orange dwarfs, which are also less luminous than the Sun. And another 10 per cent of stars are white dwarfs, which again are less luminous than the Sun. Thus, 95 per cent of the Galaxy’s stars are fainter than the Sun. Or, to put it the other way, our supposedly “average” Sun is in the most luminous 5 per cent of all stars in the Milky Way.

The night sky is deceptive, however, as dim stars can’t be seen with the naked eye, whereas luminous ones can be seen even if they are hundreds or thousands of light years away. Therefore, nearly all the stars visible to the naked eye are more luminous than the Sun. If you judged the Galaxy by these, you’d be fooled into thinking the Sun was a dim star. Although it looks as if luminous stars outnumber dim ones, the reality is just the reverse.