index: the death of the starsindex: the death of stars brown dwarf

Stars too are born and die

In the past, stars was considered immutable; only in the XX century was it undestood that stars are born and die, but their life is much longer than human life: the Sun may live about ten billion years.
Stars originate from a slow gravitational collapse in an interstellar gas and dust cloudGlossary (Fig. 1) with molecular hydrogen and helium. The potential energy lost in the collapse is transformed in thermal energy. When the central part of the cloud reaches a temperature of 2000 oK, the hydrogen molecules break into atoms. After that, the central temperature continues to increase).

Fig. 1: A spectacular image of the center of an "Omega Nebula", called M 17; this nebula is 5000 light yearsGlossary from us, in the Sagittarius constellation and is a star formation region.
(Credit: Photo NASA, ESA 24/04/2003)

When the central temperature is greater than half a milion oK, thermonuclear fusion starts  and a protostellar object is born that emits mainly infrared radiationGlossary.
The destiny of a star depends on its initial mass:

  1. If the mass is less than 0.013 solar massGlossaryno thermonuclear reactions start and a planet is produced.

  2. If the mass is between 0,013 and 0,075 solar massGlossary in the core there is a temperature of 1 million Kelvin degrees, the first thermonuclear reactions start but this is not enough to have a steady hydrogen burning: we obtain a brown dwarfs Glossary (BD).

  3. Between 0,075 and 8 solar massGlossary a normal star, like our Sun, is produced; after an initial phase of some billions of years there is the formation of a planetary nebulaGlossary phase with a strong mass loss. The stars become white dwarfsGlossary (WD), like the nebula Helix central star (Fig. 2); if the mass of these stars is between 0.075 and 0.4 solar mass they are called red dwarfsGlossary and have a very long life, but this is not a final stage of a star's life.
    Fig. 2: The Nebula Helix image taken by the Hubble Space TelescopeGlossary. The central small star is the white dwarf.
    (Credit: Photo NASA, ESA, STscI
  4. Stars heavier than 8 solar massGlossary explode like a type II supernovae SN IIGlossary; they emit a cloud and the core becomes a neutron starGlossary (NS) if its initial mass is less than 25 solar mass.

  5. Above this mass value (not a sure value) a stellar black holeGlossary (BH) derives from the SN II explosion.
The Webweavers: Last modified Mon, 11 Oct 2004 10:00:51 GMT