Thousands of stars twinkle in the sky – but take a closer look: not all stars are the same. There are brighter and darker ones, shining red or rather blue ones. Some stars are gigantic giants, others are small dwarfs. Some are hot, others rather cool.
There are ancient stars and very young ones. And stars go through an evolution – many are born as dwarfs, sometimes grow into giants, gradually burn up or end in a spectacular explosion.
Over 90 per cent of all stars are dwarf stars – (like our sun): Their diameters range from 0.1 to 25 solar diameters, they have 0.07 to 50 solar masses, and their luminosity ranges from -6 mag to 16 mag absolute brightness.
Thus they are rather faint stars of luminosity class V. Because they form the sharply defined main branch in the Hertzsprung-Russell diagram, they are called main-sequence stars.
This branch reaches from the red, faint M-stars – the red dwarfs – over the yellow dwarfs to the bright, blue shining and very hot O-stars – the blue dwarfs.
So all spectral types are represented. In the evolution of a star, its existence as a main-sequence star is the long, stable resting phase of hydrogen burning: in the core of the star, hydrogen atoms are fused to helium by enormous pressure and heat.
The star is very stable and shines with constant brightness and colour. But when the hydrogen in the core runs out, the quiet star existence is over:
The star expands into a gigantic giant or supergiant, and helium or heavier elements are fused inside.
Blue dwarfs are the largest, brightest and hottest main sequence stars, top left in the Hertzsprung-Russell diagram.
They reach fifty solar masses and shine 10,000 times brighter than our sun.
Thus they resemble blue giants but are not as large as these. But their large mass creates enormous pressure in the core, which accelerates nuclear fusion.
Blue dwarfs burn up quickly, developing high surface temperatures. Therefore they glow in blue and ultraviolet light – they belong to spectral type O.
In a few million years, they fuse all the hydrogen in their interior into helium – a rather short life span for a star. Blue dwarfs are often the child stage of a later giant. Since they only exist for a short time, they are rare:
Only about one in 10 million stars is a blue dwarf. But you can look at one of them yourself: In the double star Almaak (γ And) in the constellation Andromeda, the larger bright component is a red giant, the smaller star is a blue dwarf with an apparent brightness of about 5 mag.
Much more common in the main sequence are the yellow dwarfs – cosy stars like our Sun.
Yellow dwarfs are an important type of star for us: Our Sun is a yellow dwarf. Smaller stars of spectral type G, because they shine in yellow light.
They are the average stars of the main sequence: Cozy suns that gradually and quietly fuse the hydrogen inside to form helium.
They are about the size and mass of our Sun – rather small compared to other stars.
Their surface has moderate temperatures of about 5,000 degrees. For us, this is only good – in the region of such a star, it is easy to live on a planet. Moreover, these suns have a long life expectancy:
Our sun has been around for about four and a half billion years – and it has only used about half of its hydrogen. But after that, it will also start to burn helium and our star will expand into a red giant.
One of our neighbouring stars, which you can see in the southern sky, is also a G2V star like the Sun: Alpha Centauri in the constellation Centauri. But much more common are stars like Proxima Centauri, the closest star to us: Red dwarfs, which you cannot see, although they are the most common stars.
Proxima Centauri is closer than any other star (except our Sun) – and yet it is not visible to you. It is a red dwarf like two-thirds of all stars – the invisible majority in the sky.
They are the smallest stars still active. The nuclear fusion in their interior is running at low speeds since they have only about one to seven-tenths of the mass of our Sun.
This is how they produce little energy: the surface of these M-stars is rather cool at below 4,000 degrees, and their reddish light is very faint.
On the other hand, they become incredibly old: Their hydrogen often lasts for a hundred billion years – while the universe is only 14 billion years old.
Once they have fused all the hydrogen into helium, red dwarfs do not have the necessary mass to burn helium.
The nucleus collapses in on itself. For a while, the hydrogen in the shells still fuses in shell-burning, which causes them to expand temporarily.
But red dwarfs never become a giant star, but shrink to a white dwarf after the end of the shell burning. At even lower mass, the star completely lacks the central nuclear fusion process – it is a brown dwarf.
A brown dwarf is hardly a star anymore – it is more of a cosmic loser: It has less than 10 percent of the mass of the Sun and thus not enough pressure and heat inside to ever initiate the fusion of hydrogen into helium.
But this means that it lacks the crucial process by which stars generate their energy and shine.
One could think of them as an intermediate stage between stars and very massive gaseous planets, but brown dwarfs are formed just like other stars – only the mass is too small from the outset.
In the hottest case, the surface of a young brown dwarf reaches temperatures of around 3,000 degrees – as spectral type M or L.
But certainly not for long: even while they are being formed, brown dwarfs cool down, because the “oven” inside them has never been ignited. Their luminosity is ten thousand times less than that of our sun.
No wonder that the first brown dwarf was discovered only in 1995. But since then so many have been discovered and explored that it is assumed that brown dwarfs could be the vast majority of stars in space.
Above all, they are found in the galactic halo – where the mysterious dark matter is also located.
Giants & Supergiants
In addition to the main sequence stars, there are giants and supergiants – stars that are characterized by their unusual size.
Their diameter is ten to a thousand times larger than that of our sun. A larger diameter also means a much larger surface area (at ten times the diameter, the surface area increases to a hundred times).
A larger surface provides a star with a much higher luminosity at the same surface temperature. So a red giant has about the same colour and surface temperature as a red dwarf but is many times brighter.
Therefore, they form their own giant branches in the Hertzsprung-Russell diagram.
They are found as red giants in the cool spectral classes of M stars, and as blue giants in the hot O stars.
Depending on their brightness, the normal, most common giants with luminosity class III are distinguished from subgiants (IV), bright giants (II) and supergiants (I).
While red giants are a late stage of development of earlier dwarf stars, blue giants are already born as giants.
When they outgrow their childhood shoes, they become red supergiants.
A red giant is the later stage of development of an earlier dwarf star like our Sun:
When the hydrogen in the star’s core runs out, helium-burning starts when there is enough mass and heavy elements like carbon start to breed. Shortly before this, a jolt is sent through the star:
For a moment the nuclear fusion stops and the core collapses. At the same time, hydrogen shell burning begins in the outer shells and releases so much energy that the shells expand enormously. Red giants are radiantly bright giants – like Betelgeuse in Orion.
It is 800 times larger than the sun and umpteen thousand times brighter. Red giants appear stable for a long time. But the hulls lose density as they expand and cool down as the core becomes denser and heats up.
The star becomes unstable. It may even lose its hulls – which we find again in the sky as planetary nebulae.
If the red giant has more than eight solar masses, further fusion processes begin at the end of the helium burn until it eventually explodes as a spectacular supernova. If its mass is smaller, the red giant shrinks to a white dwarf.
Blue giants are the shining giants in the starry sky: Like red giants they extend to a hundred solar diameters. But blue giants are not bloated former dwarf stars, but are born as giants.
Therefore, although they are of similar size to red giants, they have much more mass: up to one hundred and fifty solar masses are pressed onto the core of the star.
A star can hardly have more mass. This creates enormous pressure inside the star and fuels nuclear fusion: Similar to blue dwarfs, blue giants have only a short life because they burn themselves up at enormous speed.
In just a few million years, these giant reactors have fused their hydrogen into helium.
They develop a lot of energy and high surface temperatures of up to 30,000 degrees – hot, blue glowing giants of spectral type O or B.
Their enormous size makes them very bright – they often belong to the highest luminosity classes I or II. Rigel, the foot of Orion, is a shining example.
At the end of its short life, when helium burning begins, the hulls of the blue giant expand massively – it becomes a red supergiant.
Red supergiants are the radiant final stages of massive stars. They are formed like red giants when the outer hulls of the star inflate after hydrogen-burning: the radiative pressure inflates the star like a balloon, while the core is compressed so strongly that helium fusion begins.
In the case of red supergiants, however, fifty solar masses and more make for gigantic dimensions.
They were often giant stars in their youth. The hulls can expand to a thousand solar diameters and then produce extreme luminosity with their large surface, although they themselves are rather cool.
Antares, for example, the red giant star in Scorpio, has a surface temperature of less than 4,000 degrees, so it is significantly cooler than our sun.
But because its diameter is almost five hundred times larger, its luminosity is around 40,000 times higher.
It is one of the brightest stars in the firmament – and will shine again in a few million years: Because even when a red supergiant loses its extremely unstable hulls, it usually still has so much mass that it ends up exploding as a giant supernova.
Special Types of Stars
Stars spend most of their time as main-sequence stars, which very gradually fuse hydrogen into helium in their core and radiate the energy thus gained as heat and light.
But everything has an end – even the hydrogen of a star. When it runs out, the star undergoes a dramatic change.
Which one depends on its mass. Stars with less than half the mass of the sun, such as red and brown dwarfs, live an inconspicuous existence:
The core goes out and after the burning of the shell the hulls cool down gradually until the star contracts under its own weight to a white dwarf and ends up as a black dwarf.
Somewhat heavier stars inflate before that to a red giant but also end as a white dwarf. But very heavy stars – from about eight solar masses upwards – end in a very big bang: a supernova.
The hulls are blown off, leaving an extremely compressed stellar remnant – a neutron star or a black hole.
Often the end of a stellar life is a spectacular, brightly shining farewell – and at the end remains an invisible but very extreme stellar remnant.
A star “lives” only as long as the nuclear fusion in its interior is active – this energy makes it glow. Massive stars go over to helium fusion when their hydrogen ends.
Very massive stars fuse even heavier elements like carbon after helium. However, if the fusion ends in the core, the only thing that happens is shell burning – the star expands into a red giant.
But without an igniting spark inside, all stars with at most 1.46 solar masses collapse in the end.
Their own weight presses them approximately on earth size – a white dwarf. White dwarfs often have very high temperatures of up to 100,000 degrees.
But because they are so tiny, they achieve only a low luminosity. Instead, the repelled shells often shine around this core as planetary nebulae.
Despite their high density, there are no fusion processes in the white dwarf.
They only give off residual heat until one day they cool down completely and a black dwarf remains.
Unless another star gives them some mass: Then they shine again as novae – or even become a supernova.
A supernova is actually not a type of star, but only a short moment at the end of very massive stars.
After hydrogen and helium, stars with more than eight solar masses also fuse the increasingly heavy, hatched elements – until at the end only iron is left in the core.
Now every nuclear fusion in the interior stops abruptly. The iron core has an enormous density (the famous electric locomotive pressed into a thimble), but is very unstable and collapses immediately: the electrons are pressed into the atomic nuclei.
The density of the nucleus increases to one million electric locomotives in a thimble!
The outer hulls of the star, which have long since been enriched with heavy elements, now collapse inwards without the fusion process inside but are reflected by the dense, hard core with enormous force and hurled into space in a gigantic explosion: the supernova.
For a short time, the dying star shines as brightly as ten billion stars. Its exploded shells will continue to shine as colourful nebulae for around 100,000 years. But the core of the star has become invisible – a neutron star or a black hole.
When, at the moment of a supernova, the hatched iron core of very massive stars collapses, something completely new is created: the electrons are pressed into the atomic nuclei and combine with the protons to form neutrons.
This nucleus is packed as densely as possible – as densely as particles can be packed together. One and a half to three solar masses now form a body of about twenty kilometres in diameter.
An extreme object in which nuclear fusion no longer takes place – a neutron star.
It hardly emits any light any more, and no processes take place in its interior. But it is now extremely stable. A tiny, enormously heavy and dense core – almost a stellar body.
Some neutron stars nevertheless emit signals: pulsars are neutron stars with extreme magnetic fields that rotate up to a thousand times per second.
Due to the strong magnetic fields, pulsars emit electromagnetic waves exactly in time with their rotation, which can be measured as radio waves.
If more than three solar masses remain in the iron core after a supernova, an even more extreme object is created: a black hole.
If the iron core of a supernova has more than three solar masses, it does not simply collapse into a neutron star, but into a far more extreme object: a black hole.
So far, this has been a rather model-like object, because black holes cannot be observed and studied directly.
They are objects of such high weight, density and gravitational force that all matter in their vicinity plunges into this massive object – including photons of light.
So one thing can be said with certainty about black holes: they are absolutely black, no light escapes them.
The space around them is swept clean by the enormous gravitational field. Not even electromagnetic waves, which we still receive from pulsars, leave a black hole.
For such a gravitational force, our sun would have to be compressed to a diameter of about six kilometres.
For stargazers, black holes are pure theory: They can only be observed indirectly – for example, by their influence on stars in their vicinity.
Researchers also suspect a black hole in the centre of the Milky Way.
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