What are the stars?

A clear night sky is a thing of beauty and wonder. Thousands of scattered stars twinkle in the darkness. What are the stars? How do they live and die? Could an exploding star cause disaster on Earth?

Stars of NGC 3603. This is a “starburst region” : a cosmic factory where stars form frantically from a nebula’s extended clouds of gas and dust. Located 22 000 light-years away from the Sun, this is the closest region of this kind known in our galaxy. (Image credit: ESO)

Stars of NGC 3603. This is a “starburst region” : a cosmic factory where stars form frantically from a nebula’s extended clouds of gas and dust. Located 22 000 light-years away from the Sun, this is the closest region of this kind known in our galaxy. (Image credit: ESO)

Over the past century or so astronomers have by observation and calculation established the true nature of these gleaming specks. Now we know that stars are basically enormous balls of very hot gas, mainly hydrogen, with some helium and small amounts of a few other things. Inside the star huge quantities of energy are generated by a nuclear process called fusion where light atoms bash together building bigger atoms with the release of energy as a by-product. Planets, in contrast are balls of rock, metal and gas without any internal nuclear energy source. Earth is a planet, while the Sun is a star.

Although all stars are made of essentially the same stuff they are not all the same. Stars come in different brightnesses, sizes and colours. Look out at the sky tonight and you will see how stars vary in brightness. Some are bright because they are relatively close to us, while others are bright because they are thousands of times brighter than our own nearby Sun. See the three stars of the Summer Triangle; these appear to be roughly as bright in the sky but while Vega and Altair are within 25 light years of Earth,  Deneb is more than a thousand light years distant. This implies that Deneb must be thousands of times as intrinsically bright as Vega and Deneb (and incidentally tens of thousands of times as bright as the Sun).

To bring order to star catalogues, astronomers classify stars into what are called spectral types. This means stars are listed and placed in an order based on the temperature and brightness of their surfaces and the characteristics of the spec­trum of light they shine out. The first attempt to do this was made in the 1860s; since then we have learned more and more so the original classification system has been modified, refined and reworked several times; eventually ending up with the system we use today.  In order of brightest to dimmest star, the sequence runs OBAFGKM. (Most people can tell you that our Sun is classed as a G type star). Here is the classification sequence

•          O- blue-white, brightest and hottest (30 000°C or more).

•          B- white,very bright stars.

•          A- bluish white stars.

•          F- yellowish white stars.

•          G- yellow stars (about 5500°C).

•          K- orange stars, not as bright as the Sun.

•          M- red stars (about 3500°C).

On average, very bright stars are rare in space but they are the most common stars listed in star catalogues because they are easy to find and see. Although the vast majority of stars we see in the night sky are brighter than the Sun, actually small and dim M type stars called red dwarfs (stars physically smaller than our Sun are classed as dwarf stars) seem to be the most common stars in space, at least in our part of the Milky Way galaxy. The fact that the majority of stars we see in the sky are brighter than the Sun, also means that most stars in the galaxy are too dim for us to see!

Stars come in different sizes, different colours and different brightnesses. Can we tie all this together?

Image of HR diagram

The most important diagram in astronomy (Image credit: ESO)

When we gather together lots of information on stars it is possible to plot a graph of stars’ spectral type against their actual brightnesses. Scientists call this the Hertzsprung-Russell diagram and it is the most important diagram in astronomy. It shows that stars are divided into several groups. The most important group is the ‘main sequence’. Our Sun is a main sequence star. These are stars in their prime of life, steadily converting hydrogen into helium in their hot, seething centres and producing heat and light as a by-product. The red giants are another group. These are stars which have run out of hydrogen in their cores and are instead generating energy using other fuels, swelling enormously into huge bloated stars with reddish surfaces, cool by stellar standards. In the distant future our Sun is fated to become one of these red giants. A third group are the white dwarfs. These are essentially the cores of dead stars. Red giant stars are believed to experience huge internal eruptions, blasting off great concentric shells of gas in the process until eventually all the star’s outer layers have been blown away into space. The tiny shrunken core, about the size of the Earth, remains as a white dwarf. White dwarf stars no longer produce light by nuclear fusion, merely continuing to glow like dying embers until they have slowly cooled to cold black balls of dense matter not much bigger than a planet. This will be the ultimate fate of our Sun.

Stars larger than ours have more spectacular ends: their red giant stages terminate in cataclysmic explosions called supernovae (this is the fate awaiting Betelgeuse). A supernova is a giant star tearing itself to pieces in nature’s most destructive detonation. Debris from the explosion forms a great cloud or nebula. At the centre remains a tiny shrunken remnant composed of strange superdense matter. Such a stellar corpse is called a neutron star. Larger still stars will explode in a supernova but leave behind the strangest object of all; a black hole, matter crushed so tightly that not even light escapes its gravitational grip.

Image of epsilon indi B

Epsilon Indi A (the bright star at far right) and its brown dwarf companion Epsilon Indi B (circled). Twelve light years away, this is the closest known brown dwarf to the Sun. (Image credit: ESO/R.-D.Scholz et al. (AIP))

In the past couple of decades astronomers have added a new member to this stellar zoo: brown dwarf stars. These are too small to be classed as proper stars. A star must produce its own heat and light. These objects are either very dim or even black when looked at in visible light. The little energy they radiate is mainly infra-red light. Brown dwarfs can be thought of as failed stars; much bigger than a planet but just not big enough to make it as a star. Perhaps millions of these tiny brown dwarfs lurk undiscovered in the space between the stars. None can be seen with the naked eye.

Studying the great cosmic drama of stellar life and death is fascinating, but most marvelous of all is to step into the night to look up wonder at this great show above our heads.