STARS

Stars are - as our Sun - huge glowing balls of gas. Some stars are bigger, but most of them smaller as the Sun, which itself has a mass 332 830 times more than the Earth.

At the beginning of their life stars contain about two thirds hydrogen and one third helium. Heavier elements are less than one percent. During the birth of a star a cloud of gas collapses until in the center pressure and temperature are high enough to start the nuclear fusion. The cloud begins to shine, first due to the set free gravitational energy, later because of the nuclear fusion. A star is born.
In its center the star now fusions at several million degrees hydrogen to helium. This it does for the longest part of its life as a main sequence star. The more massive a star is indeed the more fuel it has got, but it fusions it much quicker than a small star. The more luminous it is then.

If in a small star the hydrogen in its center is spent, it can't continue with the fusion. But the universe with its maybe 14 billion years is still too young as that small stars could already have spent their fuel. When this time comes, these stars will simply slowly become cooler and dimmer.

Bigger stars continue by fusioning hydrogen in their outer layers. Therefore the hull heats up and is driven out into space. The star expands and increases its brightness massively. It leaves the main sequence, a red giant is evolved. This happens to all stars with up to three times the mass of the Sun (except the very small ones). The hull is driven further away and we can see them in telescopes often as a very beautiful planetary nebula (it looks like a planet but it has nothing to do with it). The core is left behind as an inactive white dwarfwith about the size of Earth (but much more massive), which slowly fades.
Stars with three to eight solar masses behave similar, but create further elements like carbon, oxygen, nitrogen and neon, at first in their core, later in the outer layers.

Massive stars with more than 8-10 solar masses fusion even more heavy elements, up to iron. The star becomes bigger and bigger and becomes unstable. It pulsates and can erupt heavily. This last phase is relatively short compared to the main sequence phase of the star.
When the star has created an iron core then it has reached a dead end. It can't produce any more energy. The core with more than 1.44 solar masses cools down and therefore can't handle its own gravitation any more. It collapses in one fell swoop to an only some kilometers big neutron star or a black hole. The thereby arising shockwave pushes against the hydrogen and helium hull, which is about to crash inside. The complete hull now fusions immediately - a supernova explosion (of type Ib or II).
Only a few stars have the required mass for a supernova. During the explosion the star shines for a few days several billion times brighter and then leaves a nebula and a tiny spot in its center - a neutron star or a black hole.


Stars very rarely appear single without a companion. This is because the cloud from which a star evolves rotates slowly. By contracting the rotation becomes quicker (pirouette effect) and a single star can't handle the angular momentum. Therefore often a multiple system evolves or a single star with planets.
So called exoplanets, planets of other stars, aren't easy to discover for us and even more difficult to see, because they don't shine by their own. But there have been discovered already more than a hundred exoplanets because of the proper motion of their mother stars, most of them huge gaseous planets likeJupiter.

About Stars: Measured Values

An explanation of the values given on the star pages:

Constellation: Tells where the star is in the sky. There are 88 different constellations.
Age: Is difficult to specify and often can only be approximated. Very big stars only live a few million years whereas small stars can have more than 100 billion years of life-span. Admittedly the universe itself is only 13.7 billion years old. Our Sun has an age of 4.6 billion years.
Distance: Our galaxy (Milky Way) has a diameter of circa 100 000 light-years. Our Sun is 8 light-minutes away from earth, the next other star, Alpha Centauri, 4.3 light-years. One light-year is about 9.5 trillion kilometers.
Spectral class: Tells the color (wavelength) and therefore the surface temperature. The designations span the stectrum in the order O B A F G K M R S C whereas O and B is blue, A and F is white, G is yellow, K orange and M - C red. R, S and C are stars with a special frequency of chemical elements. L and T is forbrown dwarfs. Furthermore there are some special classes like W for Wolf-Rayet stars.
For an exacter definition the letters are followed by a number between 0 (shorter wavelength) and 9 (longer wavelenght). According to this the temperature is between 50 000 kelvin (O3 stars) and 2000 kelvin (M9 stars), beside extreme exceptions. O3 is the highest spectral class.
Visual magnitude: The brightness as seen from us. The smaller the value the brighter the star is. Sirius for example is with a negative value extremely bright. Up to 6.0 stars are just about to see with the naked eye at optimal conditions. 5 magnitudes make a difference of 100 times.
Luminosity: The absolute luminosity compared to our Sun (in units of solar luminosities). Generally it refers to the whole spectrum and not only to the interval of visual light.
Mass: In units of solar masses. The mass of the Sun is 1.9884 * 10³⁰ kilogramm
Diameter: In units of the diameter of the Sun. This is 1 392 000 kilometers
Radial velocity: The movement of the star to us (positive value) or away from us (negative value). The horizontal speed is much more difficult to measure and isn't indicated.

The distance within a stellar system is given in AU, Astronomical Units. 1 AU accords 149 597 871 kilometers. This is the average distance of Earth and Sun.

From time to time a temperature is mentioned. This is, if not explicitly noted otherwise, always the surface temperature. It is measured in kelvin. To get degrees centigrade 273.15 must be substracted. To get it in fahrenheit please use the astronomical calculator as for other conversions as well.

About Stars: Cosmology

Cosmology is the lore of the origin and the evolution of the universe. I try to resume this naturally very extensive subject in a generally understandable way. The statements refer to the momentary state of knowledge and aren't completely indisputable. But they are the probably most correct available models of this universe in which we live.

The universe forms the frontier of our possible knowledge. Outside or before the universe is and was nothing which we could detect or which could influence us in any way. We live in a part of the universe which isn't significantly different from other parts. The laws of nature are the same everywhere in the universe. The universe is finite in space, but has no borders, like the surface of a sphere, but with one more dimension. Therefore it doesn't have a center.

It emerged in the Big Bang 13.7 billion years ago. In the beginning it was infinitely small, infinitely hot and infinitely dense. Since then it expands.

After the Planck time, 5.391*10^-44 seconds, the shortest possible time span, the universe had the tiny extent of 1.6 * 10^-33 cm and a temperature of 10³² kelvin. Gravity separated from the unification of the other forces. From here on it makes sense to speak of time. Space, time and matter exist now separately, the particles weren't distinguishable then.

After 10^-37 seconds cosmic strings emerged, one-dimensional structures, which today because of their gravitational force influence the motion of galaxies and galaxy clusters and order them. Supposedly they are responsible for the formations visible in the picture to the right.

From 10^-35 seconds to 10^-33 seconds there was the inflationary epoch, in which the universe expanded from a radius of a hundred billionth of an atomic core to one billion light-years. Why the universe did something like that we don't know, but there is much evidence that this phase really happened.

After 10^-33 seconds at 10²⁷ kelvin the strong nuclear force separated. This force is responsible for the cohesion of elementary particles and atoms. From there on matter isn't unified any more. It can be distinguished now between quarks, leptons (e.g. electrons) and their antiparticles.

After 10^-12 seconds at 10¹⁵ kelvin the weak nuclear force separated. Charged and uncharged particles from now on differ.

Until 10^-7 seconds and 10¹³ kelvin was the quark epoch. This was followed by the hadron epoch, during which the quarks combined to hadrons (protons, neutrons and their antiparticles). Furthermore there emerged myones, electrons, positrons, neutrinos and photons. Particles and antiparticles destroyed themselves after their appearance at once and nearly completely and so created more photons. But because there was slightly more matter than antimatter we today still have matter in the universe, but (nearly) no antimatter.
The hadrons were complete after about 10^-4 seconds, the leptons not until 10 seconds. All elementary particles that still exist in the universe were formed then and the antimatter was destroyed completely.

Until 200 seconds the primordial nucleosynthesis (nucleon epoch) took place. Protons and neutrons fusioned to bigger cores. Deuterium, tritium and helium 3 for the most part fusioned to helium 4. In only very, very small amounts lithium formed. Heavier elements weren't produced at all. At the end of this phase for every 12 hydrogen cores (protons) was one helium 4 core. The amounts of deuterium, helium 3 and lithium were minuscule. Neutrons and tritium are radioactive and decay in a short time.

For the next 300 000 years (photon epoch) the universe was intransparent, because electrons and atomic cores were separated from each other (plasma) and interacted with the light particles, the photons. At the end of this era the universe had cooled down from 1 billion to 3000 kelvin.

Then began the epoch of matter. Atomic cores and electrons joined to become atoms, the universe became transparent. This is the earliest stage which we theoretically could observe with our instruments.
Because matter wasn't distributed evenly in the early universe there were chunks of matter, which grew due to their gravitation. Cosmic strings probably played a role there. These chunks have been the progenitors of the superclusters, which again inside formed smaller chunks, galaxy clusters, galaxies and finally stars.

The first galaxies appeared not sooner than several million years after the Big Bang. Again later the first stars formed, population III stars. These were (at least mostly) all tremendous stars with up to 1000 solar masses. The lack of heavy elements prevented the formation of smaller stars like these we have today. Those early giants had an accordingly short span of life. They fusioned with an enormous speed and an unimaginable luminosity elements up to iron and soon exploded. Thereby they spread the first heavy elements into the interstellar medium.

So the universe evolved in which we find ourselves today. Space still expands. The further a galaxy is, the faster it moves away from us.
If the universe will expand forever or if the expansion will invert some time we don't know yet. This depends on the mass the universe contains. Here dark matter and dark energy play a leading role. Those make about 90% of our universe. Sadly we know almost nothing about both. It is supposed that the mass of the universe is exactly on the frontier between what is needed for an eternal expansion and a future collaps.