Astronomers are able to gather an immense amount of information about stars—their temperatures, their velocities, their distances, their chemical compositions, and also variations in these quantities. Given all of this data, we can construct a physical model for stars that accurately reproduces the observable quantities. The accuracy of our models at reproducing the properties of stars suggests that we understand the internal structure of stars quite well.

The Life of Stars:

Our observations and our theory of stars show us that the stars are not eternal, unchanging objects. Stars follow a lifecycle: they are born, they change slowly as they age, and eventually they die.

As our physical understanding of stars grew, astronomers began to publish more "astrophysical" papers dealing with, for example, the structure of stars, the generation and emission of starlight, and the evolution of stars.

Simplified description:

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 dwarf with 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 like Jupiter.

Measured Values:

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 for brown 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.

Star Ages:

Young Stars:

Stars form in groups of a few up to many thousands. This happens when one of the numerous interstellar nebulae begins to collapse (interstellar = between the stars). It gets denser and denser and breaks up into segments, so called Bok globules. These are the progenitors of the protostars, which can still have a size of some light-years.

The protostars densify more and more because of their own gravitation and begin to shine due to the released gravitational energy. Meanwhile the cloud of the protostar still can break up, depending on its angular momentum, in multiple systems and dust disks, the progenitor of planets. The early days of stars are very turbulent and the star can erupt from time to time.

When finally pressure and temperature in the center of the cloud excess a certain value the star begins its nuclear fusion. First all available deuterium is fusioned into helium 3. Is this done the star shrinks for a last time until the real hydrogen fusion can start. From there on the star is stable and has reached the main sequence, its up to 100 000 years lasting youth is over.

Example: T Tauri

Main Sequence Stars:

A star resides on the main sequence when it fusions hydrogen to helium in its core. This is the case for the most part of its life. It is relatively stable then.

From its color (= surface temperature) it can then be directly concluded to its mass, size and luminosity.

In a diagram (Hertzsprung-Russell diagram) those stars are all in a row, the main sequence.

Red Dwarfs:

Spectral class: M, K

Red dwarfs are the most frequent and most inconspicuous stars in the universe. Red dwarfs have between eight percent and the half of the mass of the Sun and shine in red. The reason for this is their low surface temperature of 2500 - 4000 kelvin.

Red dwarfs live very long, in any case longer than the universe is old now. Therefore they are so numerous. In groups of new born stars they haven't got this frequency, because the bigger stars are all still there.

Many red dwarfs should have planets. But with red light photosynthesis is more difficult and less effective. So life there would have different conditions. Furthermore planets, that are near enough to get sufficient energy, are tidally locked. This means, they show their star always the same side. It is debatable if under such conditions the planet could keep an atmosphere, but if so there would be very strong storms.

Stars aren't quiet on their surface. There are often eruptions, so called flares. Many young red dwarfs are flare stars with frequent and heavy eruptions, increasing their brightness enormously for a short time.

Subdwarfs are very old stars which lie below the main sequence. So they are too dim for their color. The reason for this is that due to their age they contain only very few heavy elements.

Example: Lalande 21185

Orange, Yellow and White Stars:

Spectral class: K, G, F, A

Stars as our Sun. With three quarters solar mass a star shines orange, with three times it is white. The surface temperature varies according to this between 4000 and 10000 kelvin. Our Sun has 5770 kelvin and therefore is yellow.

The life span of such stars ranges from one to several billion years, the bigger the less. Our Sun will live about 10 billion years, 4.6 it has already behind. At the end of their life these stars expand to red giants and finally cast away their hull. A white dwarf is left behind.

Planets in the right distance around these stars would be suitable for life. Meanwhile we are quite good in discovering giant gaseous planets, but there is still a long way to go until we can discover life.

Example: Tau Ceti

Blue Stars:

Spectral class: A, B, O

With more than three and up to 150 solar masses a star shines blue - the bigger and hotter the more intensive. These stars burn their fuel much quicker than their smaller fellows and for this are much more luminous. After a few to a few hundred million years they leave the main sequence and, except the most biggest, become red giants or supergiants. Very big ones of these don't expand so strongly when leaving the main sequence, stay blue, but become unstable.

Blue stars are rather seldom but because of their brightness we can see many of them in the sky.

Blue stars could have planets, but their life span is much too short to evolve life on this planets.

Example: Alnitak

Old Stars:

The life span of a star is determined mainly by its mass, if nothing unusual happens. But every star, except the very largest, stays about 80% of its life on the main sequence. If the hydrogen in the core is exhausted the star inflates and leaves the main sequence. The sometimes uproaring end of its life begins.

Old Stars can be classified as follows:



A white dwarf mostly is an inactive star. At least when left alone. But many are part of a double or multiple system.

When two stars are close together, it is not unusual when they interchange matter. But if one of the partners is a white dwarf, the gas can't fall directly from the other star to its very small surface. It aggregates in a disk around it, before trickling down like in a funnel. Thereby the gas, mostly hydrogen, heats up and is pressed together. Now it can fusion at once in an explosion - a nova.

Nova means 'new', a historically founded wrong expression. A nova radiates for a short time 100 000 times brighter as the star did before. So a 'new' star can be seen in the sky where before was visually nothing. Novae are relatively frequent, several times each century one can be seen with the naked eye.

Despite the explosion the mass of the white dwarf increases. If it had a mass of slightly less than 1.44 solar masses (Chandrasekhar limit), it can happen that it excesses this value now. Now the star can't resist its own gravitation any more and collapses. But it is no iron core like at a type Ib or II supernova of a supergiant, but mostly helium and carbon. This still can fusion and under such circumstances does it immediately. The result is a type Ia supernova, a billion times brighter than the Sun. The white dwarf is disrupted completely, throwing large amounts of heavy elements into space - the basis for new planets like the Earth.

Example: Nova Persei

Brown Dwarfs:

Spectral class: T, L, M

These can be seen as an intermediate between star and planet. Too small zu start the hydrogen fusion yet some fusion processes take place in their core, especially deuterium to helium and the lithium fusion. Below 13 times the mass of Jupiter an object counts as planet, above 80 times as star (red dwarf). In-between are the brown dwarfs whose glowing can be measured only by the best instruments.

Young brown dwarfs gain their energy through the gravitation of the collapsing cloud. When the collapse finishes, they cool down and get dimmer, because the fusion processes provide much less energy. After not more than some hundred million years even their fuel for the nuclear fusion is spent.

Example: DENIS-P J020529.0-115925


Many stars, as our Sun, have planets. But these are difficult to discover for us, because planets don't shine by their own and therefore are very dim and close to a much brighter star. They are more easy to find when certain irregularities in the proper motion of the star or fluctuations of its luminosity caused by an eclipse are discovered. By those one or more partners and their quality can be implied.

The most planets that have been discovered thus are gaseous giants like our Jupiter or Saturn. These are bigger than terrestrial planets as Earth or Mars and therefore easier to find. But which kind of planet is more frequent can't be decided yet.

Planets evolve from the dust disks that surround many young stars. Small, solid particles collide and pack together, first to kilometer sized chunks, so-called planetesimals, later to protoplanets, whose gravitational force even can hold gas around them. So the biggest planets become gaseous giants by drawing in the interplanetary matter, to a large part hydrogen and helium.

Planets outside our solar system, orbiting other stars, are called exoplanets. These, like our planets, could have moons which we cannot discover yet.

Each star has a life zone in which planets would have the right temperature for life as we know. Gaseous giants are unsuitable for life but they could have moons with better conditions.

Example: HD 70642


A planemo is defnined as an object rounded by self-gravity that does not achieve core fusion during its lifetime. Planemo stands for 'planetary mass object'.

Therefore planets and even big moons are planemos. But also objects smaller than brown dwarfs, which orbit no star and didn't have a proper name before, are now called planemos. These nearly only can be discovered when they are very young and do still shrink. Then they glow by theirown due to the set-free gravitational energy. Cold, single planemos are extremely difficult to find.

Even single planemos can have planets. But due to a lack of light life there probably can't evolve.

Example: Cha 110913-773444


The Nearest Stars:

The stars in our neighborhood represent quite good the distribution of stars in our galaxy. The complete missing of giant stars shows their rareness.

Big Stars:

Big stars are scarce, but for their high luminosity easy to find. Most stars that we can see with the naked eye are big blue stars or giants.

Many of those stars change their size and luminosity more or less regularly. The given values here are therefore mean values.

Ordinary Blue Stars don't count as giants, but are very big for main sequence stars.

Special Stars:

Unusual, extreme and notable stars.

Prototype Stars:

Prototypes are called these stars which serve as eponyms for an own class of stars.