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Stars - Birth, Life & Death

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Stars are alive. They are born, they live, they die. Some are big, others are small, and their size effects how long they live. Some may finally die after more than 10 billion years; others survive for a mere 100 million years.

If one thing is true, it is that stars cannot be bought! READ MORE ON NAMING STARS.

Birth of a Star

Interstellar matter is distributed irregularly in space, being concentrated here and there. For a number of reasons, for example, due to a blast wave from a nearby supernova explosion, such a concentration might start to compress. Due to gravity, the compression would continue, finally resulting in a protostar, which is mainly composed of hydrogen molecules.

As the compression continues, the temperature increases, causing the hydrogen molecules to disintegrate into hydrogen atoms and finally electrons in hydrogen atoms separate from protons, or ionize. This is called a plasma.

When the temperature increases still, the helium atoms also ionize. Now the cloud of interstellar matter has compressed to approximately 1/400 the size of the original cloud. As the compression continues, the centre becomes so hot that nuclear fusions begin. A star is born. The expanding force of the fusion reactions and the compressing force of gravity keep the star in balance.


A main sequence star in the H-R diagram, for example, our own Sun. Dwarfs are in the luminosity class V. Stars below dwarfs are divided into red dwarfs, subdwarfs, and white dwarfs. Above are giants.

Luminosity class describes the absolute brightness, or magnitude, of a star. The classes are marked with Roman numerals I - VII.

Luminosity classes:

    I includes supergiants
    II includes bright giants
    III includes giants
    IV includes subgiants
    V includes main sequence stars (dwarfs)
    VI includes subdwarfs
    VII includes white dwarfs

When stars are classified by their properties, the spectral class is given first, followed by the luminosity class. This is called the Yerkes classification.


A supergiant is the largest of all stars. In fact, they are so big that the process that powers them (fusion), occurs so quickly that many supergiants will live only for 50 million years!


A red giant is an enormous star that has been through its main sequence phase (on the HR diagram). Its diameter is 10 - 1,000 times that of our Sun (if supergiants are included as giants). The surface temperature of a red giant is usually under 5,000 degrees, causing its colour, and giving rise to its popular name of red giant. Our Sun will also become a red giant when it stops fusing hydrogen. It will expand so much that our planet will be swallowed up.

  • Surface of Betelgeuse seen for first time!

    The giant phase of a star lasts for a considerably shorter time than its main sequence phase. After the giant phase a star turns into a white dwarf, a neutron star or a black hole, according to its size. The absolute magnitude of a giant is always below zero.

    The Sun's giant phase will not be in the near future. It will take about 5 billion years, before the Sun swallows Earth. After the giant phase the Sun will turn into a white dwarf. That is because of the Sun's small size.

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    When a sufficiently large star, at least 8 times more massive than the Sun, has burned up its fusion energy fuels and is in the final stages of life, it loses its inner balance and explodes. To an outside observer, the star will suddenly appear to powerfully brighten, then to fade out slowly. The explosion is called a supernova.

    Without past supernova explosions, Earth and its natural life would not exist. In our Galaxy, an estimated two or three supernovas occur each century.

    This Hubble Space Telescope picture shows three rings of glowing gas encircling the site of supernova 1987A, a star which exploded in February 1987. The supernova is 169,000 light years away, and lies in our neighbouring galaxy called the Large Magellanic Cloud.


    When a star of a certain size (less than 8 solar masses) has used up its fuels for fusion reactions, it goes through a giant phase and then ends up as a dense star about the size of the Earth, weighing about the mass of the Sun or less. A star like this is called a white dwarf. The name white dwarf is misleading, as the star's colour changes with age, sometimes ending as a black sphere, called a black dwarf.

    There are an estimated ten billion white dwarfs in our Galaxy. The more mass a dwarf has, the smaller its diameter and greater its density. The density of a typical white dwarf is three tons per cubic centimetres. In other words one litre of white dwarf matter would weigh as much as a medium size ship.


    A singularity in spacetime.

    A celestial object, whose escape velocity is larger than the speed of light so that even light (which can be thought of as particles) cannot break free from a black hole due to its massive gravitation. This is why black holes are invisible. Matter that falls in and heats up is the only clue to a black hole; it can be detected as faint radiation.

    A black hole is born when a star with great mass explodes and becomes a supernova after running out the fuel which kept its nuclear fusions going. The remains of the matter form a black hole. The existence of black holes has been difficult to prove due to their invisibility, but there are several candidates. Huge black holes, with millions of times the mass of the Sun might be found in the centres of distant galaxies, forming quasars. It is suspected that a black hole is located also in the centre of our own Galaxy.


    Brown dwarfs are intriguing objects, intermediate between stars and planets. Often picturesquely described as 'failed stars', they are more massive than Jupiter, the largest planet in the solar system, but they fall short of the minimum mass a true star needs -- 8% of the Sun's mass. Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter.

    Hundreds of young brown dwarfs are now known to exist in the Sun's neighbourhood. They have surface temperatures that range down from about 3,500 K (3,200 °C) to 1,500 K (1,200 °C). Over most of this range their appearances are similar to cool stars of the same temperature. However, as the surface of a brown dwarf cools below 1,500 K, a dramatic chemical change takes place: large amounts of methane form, considerably altering the appearance of the brown dwarf.

    Three newly discovered brown dwarfs bridge the gap between the young, warmer group and the cooler methane group. They are not identical, but form a sequence linking the warmer more star-like and the cooler more planet-like types.

    Star Maps of The Northern Hemisphere


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