The Birth and Death of Stars

In more massive stars nuclear fusion continues. As these stars possess more mass and therefore more potential gravitational energy, the carbon core contracts and creates oxygen by fusing nuclei. If the sun still weighs more than 6 times the mass of our own sun, the forces of pressure become strong enough to form iron atoms. Elements of a higher ordinal number can only be created by catching neutrons. At the end of the fusion process, the pressure inside of the star diminishes, the extinguishing sun's core shrinks, the remaining gas envelope extends. Depending on the star's final mass, its ultimate state will differ.

A red giant's structure

As already mentioned, the gas envelope parts from the core and escapes into interstellar space. Later on, the envelope can take part in the creation of new suns. Stars may also be created by crashes of interstellar matter.

The remaining naked core is in many cases a white dwarf, a small sun. It is the size of the earth, but is several trillion times heavier than our home planet. The density mass within a certain volume) of a white dwarf is extremely high and can rise to up to 2 tons per cubic centimeter. Such a white dwarf does not gain its energy by nuclear fusion, but survives only by consuming its warmth energy. The warmth is transported by radiation from the interior to the exterior of the star.

White Dwarves

As white dwarfs are very hot indeed, but not very bright, it has been possible to discover roughly 800 of them within 300 light years at the present time. This is a very low value in contrast to the observed 100 billion sparkling suns situated in the Milky Way, our galaxy. However, these 800 are only observered white dwarfs, and statistics indicate that there must be a billion of them in our galaxy, as many as there are neutron stars.

Neutron stars can form if more than 1.4 solar masses are left after the completion of nuclear fusion inside a sun. This famous value, known as Chandrasekhar limit, was calculated by an Indian physicist as early as in 1932 (the same year the neutron was actually discovered) by referring to physical laws and equations. In contrast to white dwarfs, neutron stars consist - believe it or not - mostly of neutrons. Gravitation and density of matter inside these objects are so high that the electrons no longer occupy any of the empty space between the atoms. Hence, they are simply pressed into the nuclei by the acting forces. This is how protons and electrons are fused to form neutrons. The density of the newly formed neutron pulp is relatively high. This density is equal to the density inside a nucleus, 10¹² to 10¹⁴ grams per cubic centimeter within a diameter of only 10 to 30 kilometers! In comparison the earth is only x kilometers in diameter. Neutron stars can be best compared to massively oversized nuclei.

Although there has been an extensive theory of neutron stars since 1938, it was possible only as late as 1967 to prove the existence of a neutron star by observation. When Susan Bell-Burnham and Antony Hewish had received recurring radio signals from outer space at Mullard Radioastronomy Observatory in Cambridge, they considered them at first as being signs for intelligent alien life. But rapidly these ideas seemed more as simple disillusionment. The signals were actually radio pulses originating from a neutron star. This so-called synchrotron radiation is created by electrons being slowed down in a magnetic field and hence emitting radiation in the range of meter-long wave lengths. Similar to the beam of a lighthouse, the radiation originating from the neutron star touches a certain area of space. If earth is hit by such a beam, the signal is detected as a pulse because of the high angular momentum (1 to 1000 spins per second). This early state of a neutron star is therefore called "pulsar".

The angular momentum and the powerful magnetic field of a sun become more intense when it collapses. The force of the magnetic field of a pulsar is 10⁸ to 10¹¹ times more intense than that of a horseshoe magnet. There are pulsars which rotate only 1.3 times a second, others can do up to 600 spins. Theoretically the limitis reached at 1000 spins per second. Then, centrifugal forces are actually so big that the pulsar tears up itself. As time passes, pulsars give off their rotational energy to their environment and the cosmic lightning extinguishes. The transition to the state of a neutron star is complete now.

Comparable to a sun collapsing into a white dwarf, neutron stars are created during the separation of a star's envelope from its core. Supernova explosions are responsible for the creation of neutron stars and pulsars. These explosions catapult the outer envelopes of a star and force the core to contract to a neutron star immediately. The emitted energy effects an intense flare-up of the star's core and drives the envelope outwards at an extremely high velocity.

If a neutron star weighs more than about 3 solar masses, it won't remain stable for long: the mass's attraction makes it contract further and further. As energy and mass are proportional to each other (as stated in Einstein's Special Theory of Relativity: E=mc²), the energy of the rising inner pressure affects an increase of the star's mass which strengthens gravitation. Its diameter diminishes, attraction increases, hence it contracts further, a vicious circle. In white dwarfs and lightweight neutron stars, this process can be halted because of the existing pressure in the interior, the degeneracy pressure. Yet in heavier neutron stars force of attraction exceeds degeneracy pressure, and the star shrinks more and more. This brings us to and end of the life-cycle of a star. ..Or does it? Read the article on black holes for details on the final state of the most massive stars.