Protostar
When a giant molecular cloud collapses down, it forms a protostar. It continues to collect matter and to collapse down for about 100 thousand years for a solar mass 1 star. When this collection of matter ends, the protostar becomes known as a T Tauri star.These stars are too cold for fusion to take place. Their only energy comes from the release of gravitational potential energy is converted to heat as the matter is drawn closer in.
T Tauri Star
Like protostars, T Tauri stars do not have high enough temperature and pressure at their cores for fusion.They continue to contract for about 100 million years before becoming a main sequence star.
Main Sequence Star
A main sequence star is one which has reached an equilibrium between gravity pulling the star inwards and pressure from fusion pushing it outwards. This balance is called hydrostatic equilibrium. It enables the star to keep its form as a sphere.When fusion occurs, hydrogen is converted to helium in the core of the star.
A main sequence star can range in size from 0.08 solar masses (80 times the mass of Jupiter), where fusion starts, to theoretically over 100 solar masses.
Main Sequence stars are split into several categories, labelled by letters, which are listed below from largest, hottest and brightest to smallest. The colours are the standard colour descriptions of the stars.
- O - blue
- B - blue to blue white
- A - white
- F- yellowish white
- G - yellow
- K - orange
- M - red
The largest stars are the shortest lived, and the smallest are the longest. The Sun could stay in this stage for 10 billion years or so.
When the star runs out of hydrogen in its core it moves on to the next stage of its evolution.
Red Giant (Stars of 0.5-10 solar masses)
Without hydrogen to fuel fusion in the core, the fusion stops. This means that the outward pressure which was balancing gravity is removed causing the star to collapse in on itself. This causes the hydrogen around the core to heat and initiates fusion in the shell. The heat generated by this causes the outer layers of the star to expand greatly. Because of this expansion, the surface area increases meaning that the heat from fusion is spread out. This leads to a lower surface temperature and the visible light shifts towards red. The star has entered its red giant phase.Through various processes, depending on size, the red giant phase ends after a few million years. The next phase is the white dwarf.
Red Dwarfs (Stars of 0.075 - 0.5 solar masses)
These stars do not accumulate an inert core of helium and thus may exhaust all of their hydrogen fuel without ever becoming red giants. They are relatively cool stars and so burn very slowly leading to a predicted lifespan greater than that of the universe at present. This means that there are no observations of these stars aging.Supergiants (Stars of 10-70 solar masses)
Instead of forming red giants, this massive stars form supergiants. These can be red supergiants or blue supergiants. This class of stars is the source of super novae. I'll look at this in more detail later.White Dwarf Star
Eventually, through burning and contracting, huge pulsations build up causing a massive release of gravitational potential energy as heat. This blasts the outer layers of the star out into space, perhaps forming a planetary nebula. The core of the star cools to form a small, dense white dwarf.Sources
Wikipedia: ProtostarWikipedia: T Tauri Star
Wikipedia: Main Sequence Star
Wikipedia: Spectral Classification
Wikipedia: Red Giant
Wikipedia: Red Dwarfs
Wikipedia: Supergiants
Wikipedia: Life of a Star
Universe Today: Types of Stars
Universe Today: Star Evolution
Enchanted Learning: Star types
Cosmos: White Dwarf
2nd April 2011 Formation of Elements
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