Cyberphysics: The Life Cycle of a Star

The Life Cycle of a Star

Star Birth

Stars form from massive clouds of interstellar dust and gas in space called nebulae (the singular of nebulae is nebula - latin for 'cloud').

The gas in the nebula is made up of mainly hydrogen and helium gases which were in the early universe. The dust is the product of supernova - dramatic explosions of massive stars that fling the atoms made within the stars out into space.

Gravity is the force that pulls matter together - it is an attractive force that acts on masses. It acts on the dust and gas together. It pulls in gas and dust from a vast area - steadily building up the mass of the 'star'. This is called accretion. (Accretion is a process in which the size of something gradually increases by steady addition of smaller parts}.

As the cloud continues to contract there is conversion of gravitational energy of the particles to thermal kinetic energy. So a s the matter is pulled together it swirls ever faster (like water going down a plug hole!). This results in the matter getting even hotter and the pressure building up. It also results in the gas and dust forming a disc shape with a concentration of mass in the middle or core of the protostar.

Some of the material from the disk is ejected outward in a bipolar jet. Jets of fast moving particles can be detected coming out at right angles to the accretion disc.

It gives out radiation from atoms that are excited due to their high kinetic energy collisions - but at this stage there is no fusion going on.

The outer parts of the disc might clump together to form planets and the inner core will gather most of the matter into the central star (or even stars - binary systems develop two from the central core). At last the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main sequence star

Stable State - the Main Phase or Sequence

When the core of the protostar becomes hot enough and their is sufficient pressure on the atoms inside the process of nuclear fusion begins. This is what makes stars shine. There is a lot of energy released when fusion occurs and this is sent out into the Universe in the form of electromagnetic radiation. We can only see the visible light emitted from stars and only feel the infra red emitted from our star - the Sun - but electromagnetic radiation from the full spectrum is emitted from a star in its main phase. We can observe this radiation using telescopes that detect the part of the EM Spectrum. that the telescope is made to detect. This information is then translated into visible images by a computer so that we can have a visual representation of what the star is emitting in that region of the spectrum.

So during most of the lifetime of a star, the interior heat and radiation is provided by nuclear reactions near the center; this is phase of the star's life is called the main sequence and is the stable phase of the star.

The force of gravity is continually trying to cause the star to collapse but this is counteracted by the pressure of hot gas and/or radiation in the star's interior. The two processes create a dynamic equilibrium. This is called hydrostatic support and results from the pressure inwards being balanced by the pressure outward. Our Sun is almost halfway through its 10 billion year stable phase.For more detail on this look at this NASA page

Star Death

In about 4 to 5 billion years from now the Sun will will begin to exhaust its supply of hydrogen fuel. Then the Sun's core will collapse under its own gravity because the outward pressure of the core will decrease (because fusion reactions deep within the core are less prevelant). This will make the core hotter Its atmosphere (the outer atoms) will become unstable because of the increased pressure from the hotter core and start to expand. This will transform the Sun into a huge red giant star. At the bloated surface, the increased amount of energy will be spread out over a larger area so each square metre will be cooler. The surface will have a red color because it is so musch cooler than it was at the yellow stage and will be much further from the center than during the main sequence.Our sun will swell in size to encompass the inner planets (perhaps us included) when this happens!

Despite its cooler surface temperature, a red giant is very luminous because of its enormous surface area; so even if the Earth is not swallowed up when our Sun becomes a red giant, conditions onthe planet will become impossible for life to exist. The surface temperature would be so high that the oceans and atmosphere would evaporate away.

Finally, when the fuel runs out completely the crushing force of gravity makes the star collapse under the force of its own weight; if it is a small star, it collapses gently and remains collapsed. Such a collapsed star, at its life's end, is called a white dwarf. The Sun will probably end its life in this way. It stills shines as radiation is still given out from electron promotion/demotion transitions until the kinetic energy of atoms diminishes enough to stop radiation emission - it is then a black dwarf - a cold mass moving through space.

A different fate awaits a large star. Its final collapse generates a violent explosion, blowing the innards of the star out into space - this is called a supernova. This is a really violent explosion and is seen as a star increasing in brightness to very bright star. During this supernova event fusion of nuclei cause large nuclei to form - larger one than would form in a small star. The materials of the exploded star are flung out into space and mix with the primeval hydrogen of the universe as dust. Later in the history of the galaxy, other stars are formed out of such clouds of dust and gas - nebulae (see above) and the whole cycle begins again.

After the supernovas has blown all of the atoms from the outer star into space the dense core becomes a neutron star or even a black hole.

Our Sun and solar system contains the debris of countless other stars that exploded before the Sun was born. The heavier elements that are in our solar system - and you! - are star dust. As the 60s hit says - 'We are stardust!'

Here is a link to a 'rap' that a pupil of mine wrote as a homework exercise... it goes through the life of a star.

This question sheet asks questions about circular motion, satellites and stellar evolution. It is at UK GCSE standard (age 15 years). Here is a link to the solutions to the questions.

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Cyberphysics.co.uk is written and maintained by a teacher who has more than 20 years experience of teaching in a British selective girls' Grammar School