What will eventually happen to our Sun, which is an ordinary medium sized star of spectral class G? Its age is about five billion years. The luminosity of a star is not constant. Our Sun is gaining size steadily as do all stars and becoming brighter in the process. At the end of its life time (which is expected to be approx. another 5 billion years from now) the Sun will become bigger and bigger and shift in colour to red. Its size will continue to increase, becoming a Red Giant and will explode at the end. This happens when the hydrogen fuel is exhausted, bringing to a halt the fusion process. All atoms of hydrogen inside have been converted into helium and other (heavier) elements. After a certain period of time (relatively short compared with its life time as a whole) the Sun will collapse and unhorse its mantle in an explosion. A so called Planetary Nebula, formed from the smoke of the explosion and the remnants of the fusion process, will appear. This mantle contains not just helium but also the other, more heavier elements. The remainder of the star will concentrate and condense into a small core called a White Dwarf. When the Sun becomes a White Dwarf, it will cool and shrink., eventually being only a few thousands miles in diameter. No further nuclear reactions can take place and the faint star radiates its residual heat into space becoming cold and dark – a Black Dwarf . That really is the end.
It‘s all over. Really? We must accept that there is an end for stars but not for the elementary particles of which they are composed.
The solar life cycle of our Sun (Wikimedia)
There is no "end" for interstellar matter, in the proper sense of the word because the fundamental particles will exist in perpetuity.
It is crucial what mass the stars had at the outset of their life. The starting mass is a criterion for its brightness during life, its duration of life and also for its destiny. Stars having the mass of the Sun, will become Red Giants and form a colourful planetary nebula when life is over (see illustration on that page). Stars greater than our Sun is will form a Red Super Giant and finish in a big explosion called a Nova or Super Nova. This is a stellar explosion which contains more energy than the star could be expected to emit during its entire life span. The explosion expels much or all of a star‘s material for days or even weeks. Nova means "new" in Latin, referring to what appears to be a very bright, new star shining in the celestial sphere. The expanding shock waves from Supernova explosions can trigger the formation of new stars and produce Neutron Stars or Black Holes at the centre. So far we have learned that a black hole is part of the final stage in the lifetime of huge stars called Red Giants.
Smaller Suns form a Planetary Nebula:
Smaller stars like our Sun will form a planetary nebula at a certain phase of their life-time. They consist of an expanding glowing shell of ionized gas, ejected from Red Giants late in their lives. The word nebula is Latin and means mist or cloud and describes what can be seen at certain points in the celestial sky. Time exposures show marvellous colours and bizarre forms from which they get their names. Famous planetary nebulae are the so-called Cat‘s Eye Nebula, Helix Nebula or Eskimo Nebula. For astronomers, they number the most watched objects in the celestial sky. They require a dark sky and large telescopes with high magnification in order for the faint objects to be seen. Sometimes a small star can be seen in the centre, a legacy called a White Dwarf. One very famous planetary nebula is the Ring Nebula M57 in the star configuration Lyra. Sometimes in good viewing conditions the centre star also can be seen.
The famous Ring Nebula (M57) in the star constellation Lyra
(Picture taken by Martin Elsässer of the VSW Munich,
52x2min using ISO 800)
The first to recognise these celestial objects was Wilhelm Herschel, a British-German astronomer of more then 200 years ago. He looked for the planet Uranus, predicted but not yet found. Herschel recognised many objects looking through his big telescope in Datchet in Berkshire (UK). Herschel thought the objects were stars surrounded by material that was condensing into planets. That was not so. Planetary Nebulae have no connection with planets, as we have seen previously. However, Herschels General Catalogue of Nebulae and Clusters from 1864 is still in use, supplemented with discoveries from many others and published 1888 under the name "New General Catalogue" containing thousands of deep sky objects. Most of these objects are of relevance to black holes.
Charles Messier actually was a comet hunter and a contemporary of Herschel. Making his observations in Paris, he also noticed many remarkable misty objects in the celestial sky which he felt couldn´t be comets. Unlike comets, this nebula did not move through the night sky. He noted the positions of 103 objects, which was the beginning of a famous list to which he gave his name, the Messier catalogue. His final version published in 1781, contained planetary nebulae, open and closed star clusters and remains of supernova explosions.
For a many years, the catalogues of Messier and Herschel were a boon for astronomers. Late in the 19th Century, finally the secrets of these celestial objects were beginning to be revealed. Nobody had yet reached the conclusion, that there must be some connection between the existence of massive stars having immense gravity (as John Michell had predicted) and those misty objects in the night sky.
Some Stars end as a Supernovae:
Since Schwarzschild, Chandrasekhar et al. it became clear, that the process of transformation of big stars will be different to that of our (small) Sun. They will form red giants which explode at the end of their life time, becoming supernovae, the pre-stage of black holes. Almost every year one appears in a Galaxy. The first characteristic is the sudden appearance of a new bright spot in the sky, never seen before. The first supernova noted was by Chinese astronomers and also by a monk in Belgium, in 1054, within the constellation, Taurus. This was a very bright event, possibly visible even in day light. The pale afterglow of the event is still to be seen in that star configuration and looks like a crab giving the name "Crab Nebula". It was number ONE in Messier´s
"Catalogue of Nebulae and Star Clusters" starting with M1 and ending with M103.
First appearance of a Supernova reported by Chinese astronomers in 1054, visible as M1 as the so called Crab Nebula in the constellation Taurus (artist´s impression)
There are supernova explosions so strong that they can even be seen in other galaxies. This happened for instance, in January 2014 when a supernova appeared suddenly in the galaxy M82, see photo below. It was taken by Herr Elsner. He was one of the first reporting to the IAU immediately.
Superb picture of a brand-new Supernova in the M82-Galaxy
(picture taken by Ronny Elsner, using a 4" Astrograf,
10x30sec, ISO400, EOS1000Da)
If there were to be any supernova appearing in our own galaxy (the Milky Way) such an event could even jeopardize life on earth. Supernovae radiate not exclusively visible light but also in the entire, invisible spectrum of electromagnetism, e.g. X-rays and Gamma rays, ranking amongst the most dangerous radiation of all.
As it is generally known, Dinosaurs became extinct about 65 million years ago. Before that, they had lived for more than 200 million years and were the dominant species on earth. How could that suddenly come to an end? Were they killed by a Supernova explosion or by the impact of a big meteor strike?
The standard theory is that it was a either a meteor strike or an asteroid strike which caused them to disappear from the face of the earth, as well as the many other species which also suffered extinction. Some smaller animals plus animals living in the sea tended to survive, e.g. crocodiles. The water protected them. Another plausible explanation would be the explosion of a supernova close to earth. There is no shock wave emanating from a supernova explosion but pure energy in the form of highly energetic electromagnetic radiation, mostly invisible, endangering flora and fauna. It is Gamma-and X-rays caused by super massive Black Holes which might wipe out life. Fortunately such Gamma Ray Bursts (GRB) do not happen so often in outer space. Just once in a hundred years in every Galaxy is there such an event. On the whole, it is not a rare event, because of the are billions of Galaxies in the Universe.
The Neutron Star:
Even such a remarkable event as the explosion of a supernova, will come to an end, and surprisingly quickly. After the diminution of light and other forms of electromagnetic radiation, there will be nothing more to be seen, within months or years. Not very much is left of the former Red Giant. When the star‘s fusion process has ceased and it has collapsed after a big explosion, the remaining matter will be reflected by its own mantle, returning to the core. The direction of radiation is reversed, resulting in a small and dense core of highly compressed matter, the size of just a few kilometers. In such a way, a new strange stellar object is born, a fast rotating neutron star. It is one of the celestial objects with extraordinary properties because of the high concentration of matter in that tiny core. The density is so immense that a spoonful of the matter from a Neutron star would have the weight of approx. 10 million tons. It consists solely of neutrons. Hence its name. Such Neutron stars also have so called "jets" at their poles, radiating Gamma- and X-rays.
The conditions within the neutron stars are quiet abnormal – having a state which just does not exist on earth. Nor can scientists create such conditions, in a lab As opposed to black holes, Neutron stars can be viewed in the heavens by astronomers.
The space photo on this page was taken in 2013, showing a star cluster in our Milky Way, 47 Tucanae. Star clusters are an accumulation of stars which are the oldest objects in the Celestial sky. They can even be detected in other Galaxies having the same age as the rest of the Universe, approx. 13 billion years. Because star clusters are so old, many of their stars have already completed their life time and become neutron stars. They have been identified within 47 Tucanae but in addition, black holes are confirmed in other star clusters (see page 55 "Blue Stragglers‘‘ ).
This star cluster 47 Tucanae emitting X-rays, verified to have countless Neutron Stars
(By courtesy of NASA/CXC/Michigan State/A.Steiner)
Research into a special Neutron star within the star cluster 47 Tucanae, recently carried out by astronomers revealed and confirmed that there is a relationship between the size and mass of a neutron star, just as predicted by Chandrasekhar (see page 28). The understanding of the neutron stars is a pre-requisite for the understanding of black holes. Bear in mind that if the neutron star were marginally larger in mass, it would itself have become a black hole with quite different physical qualities.
Furthermore, scientists have demonstrated that the average neutron star within a constellation has about one and a half times the mass of the Sun. In contrast to the Sun, its diameter is only 12 km. That is the size of a minor city. Because the density of the core matter is so high, an ideal sphere, which has pressure ten million to a billion times higher than that pressure needed to form diamonds on Earth, would have "mountains" on its surface not exceeding 5 millimeters.
Neutron stars also radiate other parts of the electromagnetic spectrum i.e. simply radio waves. 1967 was the year when this effect was verified by a team with a Prof. Hewish in Cambridge. He monitored 82 MHz (a frequency close to the VHF FM radio band) and his Doctorial candidate, Jocelyn Bell heard for the first time, a typical rhythm of the noise from a neutron star. The apparatus involved 2048 Dipole antennae, arrayed in a complex configuration. They occupied the area of a football field and all antennae fed one highly sensitive receiver.
The received signal from an unknown source was heard three times per second. It was so regular that the scientists thought that this transmission was a message from aliens in outer space. The newspapers assumed that they had detected "little green men", quickly assigned the letters LGMs. But soon other signals were found, having a different pattern of breaks in their rhythm and located at other spots in the sky. Obviously, these were celestial objects and were given the name Pulsar. Later it was recognized that these signals came from neutron stars.
The evolution of stars, from birth to burn-out
(depicted by the author)
On the upper (simplified) overview, the life cycle of stars is illustrated. From left to right depicts, clouds of hydrogen forming smaller and larger suns under the influence of gravitational forces.
Smaller stars like our Sun become Red Giants after a lifetime of about 10 billion years. After collapsing, their remnants form White dwarfs garlanded by a ring of smog which forms a planetary nebula, nice and colourful (see path A). Stars greater than ours will become Red Super Giants which end as Supernovae (path B and C) in a big explosion. The remainder will become Neutron stars or even form black holes.
Another possible fate for Red Super Giants – a Black Hole:
When the period of stellar fusion is complete and all hydrogen is converted into helium and other elements, the star will collapse, ending in a big explosion. It is the same procedure which happens when forming a neutron star: the remaining matter will be reflected by its own mantle and a shock wave will return to the core. The direction of radiation will be reversed and a small but dense core of highly compressed matter results. A Supernova will appear in the night sky but with a difference: the reflected matter will not stop but the process of collapse continues. Even when the Schwarzschild radius is reached, the process of mass concentration continues unstoppable. A singularity will appear. Because of the immense pressure and gravity, light is unable escape, imprisoned in a gaol only kilometers in diameter – a new black hole is born.
A Black Hole – an artist´s impression (Wikipedia)
Because of the gravitational lense effect, the Galaxy in the background seems to be warped and distorted
As we have already seen, the size of a black hole depends upon the spectral class of the Red Giant from which it emerged, mostly class O. Nothing can prevent them from becoming even bigger. Absorbing more and more matter over time, they may be like a vaccum cleaner in space. There are still copious amounts of clouds in space containing the dust of hydrogen which will be attracted when straying too close to the monster (see the picture on the cover). So far, we know from Science that in the centre of every galaxy, a black hole is located. Some galaxies contain black holes with a mass of more than a million or sometimes (in rare cases) billions of stars. Such black holes are called Supermassive Black Holes. The transformation of an ordinary black hole to a supermassive black hole is spectacular. This extraordinary physical process came to the attention of researchers in recent years. Galaxies rotate. Hence there will usually be accretion discs formed, orbiting their centre. Because of friction of inter-stellar matter, radiation which can be detected on Earth, is generated. Putting it a bit dramatically, it is the death scream of matter, plunging into the Black Hole, emitting all forms of electromagnetic radiations: X-rays and Gamma-rays appearing at the event horizon. But the core of the black hole remains invisible.
Radiation coming from accretion discs of supermassive black holes in the centre of Galaxies, is so bright that it is easy detectable even at distance. Having such extreme brilliance, it can readily be seen on earth. These kinds of interstellar objects also called Quasar, standing for "Quasi Stellar Object" or "Looking like a star". It is the understanding of Physicists today, that quasars are the core of Galaxies consisting of supermassive black holes. Emitting an immensely high level of energy in all parts of the electromagnetic spectrum, they appear very brightly in the Universe. The huge brightness in the centre is so intense that it outshines the rest of the Galaxy. Within a short period of time, more energy is released than was released during the entire previous lifetime of that Galaxy.
Artist´s depiction of a Quasar (Wiki)
That process leading to quasars has similaties to the one creating supernovae but is not yet fully understood. The energy level in quasars is much higher compared to the supernovae. It is all related to supermassive black holes.