LOW MASS STARS
LOW MASS STARS
Stars make energy by fusing light elements into heavier elements.* When a star is on the main sequence it is fusing H into He in its core.
During the fusion process a very small amount of the element’s mass is converted into an enormous amount of energy according to Einstein’s famous equation:�e = mc2
LOW MASS STARS
The rate of fusion depends on the pressure in the core of the star.
More pressure = faster rate of fusion.
Less pressure = slower rate of fusion.
LOW MASS STARS
Due to their greater gravity, high mass stars produce much greater pressure in their cores and so run out of fuel much more quickly even though they started out with much more fuel to begin with.
LOW MASS STARS
Due to their lesser gravity, low mass stars produce much less pressure in their cores and so run out of fuel much more slowly.
LOW MASS STARS
Low mass stars live much longer than high mass stars.
A very low mass red dwarf star can live as long as 1 trillion years.
A red dwarf star is between 0.075 and 0.3 solar masses and will end as a cold lump of solid He.
LOW MASS STARS
Our Sun (1 solar mass) is also a low mass star known as a yellow dwarf star. Yellow dwarf stars range in mass from 0.8 solar masses to 8 solar masses.
LOW MASS STARS
There is an intermediate class of star known as an Orange Dwarf with a mass between 0.3 and 0.8 solar masses.
LOW MASS STARS
The Sun also fuses H into He while on the main sequence. Like smaller stars the H in the core is eventually used up and fusion stops. But for a 1 solar mass star this causes 2 things:
LOW MASS STARS
a) The core collapses and gets hot enough to fuse He into C and a few other elements.
This releases lots more energy, making the star expand.
The outer layers get cooler and redder. Now it is a red giant star.
LOW MASS STARS
b) Due to additional energy from the core, the H outside the core will get hot enough to start fusing H into He. This makes the Sun expand even more. Its outmost layers will become even cooler and therefor redder.
OUTER LAYERS
H to He fusion
He to C/O fusion
LOW MASS STARS
During its red giant stage Mercury, Venus, and maybe the Earth will be burned up.
The planets from Mars outward may no longer be held in the diminished gravity.
Our Sun is 4.5 billion years old. Its end stages are estimated to begin at ~11-12 billion years old.
LOW MASS STARS
During the Red Giant stage of its life the Sun’s outer layers gradually disassociate from the Sun and drift outward forming a planetary nebula.
LOW MASS STARS
When the He has been fused into Carbon and Oxygen fusion stops. The Sun is not massive enough to create the internal pressure needed to fuse Carbon and Oxygen into heavier elements. The sun is now a white dwarf, a hot ball of mostly carbon which will cool over time.
LOW MASS STARS
White dwarf stars are very hot; on the order of 20,000 to 40,000 degrees Celsius. Because they are so hot they radiate strongly in the ultraviolet range of the EM spectrum.
LOW MASS STARS
It is estimated that it takes White Dwarf Stars on the order of 20 to 30 billion years to loose most of their energy. When they do they are know as black dwarf stars.