Stellar Life Cycle

As was seen in previous units, distances within space are so huge that actually going to a star and observing it in location is impossible. Our knowledge of the Universe is entirely based on observations of emitted Electromagnetic radiation. 

Note - As of May 2016, a second method to observe the Universe was discovered. Scientists have now proven that it is possible to observe the collision of two Black Holes by detecting the Gravitational waves the collision emits. 

The Electromagnetic radiation that can be observed can, however, give a great deal of information about a star. 

By simply observing the colour of a star, information about the Temperature (and therefore the composition/size) of a star can be found. 

The diagram below shows the colour of some notable stars visible in the northern night sky :-

The colour of a star is directly related to its surface temperature, with red stars being relatively cool ( ~3,000 K ) and blue stars being relatively hot ( ~ 18,000 K ). 

The image below is a time-lapse image of the constellation of Orion, in which the camera has been continually brought out of focus to emphasise the stars' colours  :- 

Black-Body Radiation

Any object with a Temperature above absolute zero will emit E-M Radiation. The Wavelength of this emitted radiation depends upon the Temperature. The higher the Temperature, the shorter the Wavelength of the peak emitted Wavelength. 

In order to describe qualitatively how the Temperature affects peak Wavelength, the concept of a class of object called a Black-Body object. 

A Black-Body object is a hypothetical object that is a perfect absorber and radiator. This means that absolutely no E-M radiation is reflected from it. 

A Black-Body will emit radiation over a range of Frequencies, with a peak Wavelength which is dependant on to the Temperature of the object. The diagram shows a composite image of four Black Body radiation curves, each at a different Temperature:-

As can be seen in the above diagram, as the Temperature of the object increases, the peak Wavelength moves towards the shorter Wavelengths. 

The video below shows a steel nut being heated by an induction coil :-

Note -  As the Temperature of the nut increases, the colour of the peak emitted Wavelength shortens, giving the changing colour. 

The diagram below shows how observing the colour of a star, the peak Wavelength can be identified and ( if we assume the star acts as a Black-Body ), its Temperature can be found :- 

Hertzsprung-Russell Diagrams

If all the visible stars have their Temperature measured using the above method, as well as their Luminosity ( see Unit 3 ) , the following diagram can be plotted :-

The above diagram is called a Hertzsprung-Russell ( H-R )  diagram, named after the two scientists who created it. The H-R Diagram shows that all stars can be categorised into three main categories :-

Main Sequence - The vast majority of stars are found in this section, this section shows stars fusing Hydrogen. 

Giant Branch - Once Hydrogen fusion ends, stars swell and cool to form red giant stars, fusing Helium and other heavier elements.  

White Dwarfs - When all fusion ends, stars shrink and cool, becoming White Dwarfs stars, radiating thermal Energy out into Space

Note - There is a fourth Section of the H-R Diagram, the super-giant stars. These stars all have a very high Mass, and after the giant phase do not end their lives as White Dwarfs. These stars collapse under their own Gravity to form either a Neutron Star or Black Hole (again depending on initial stellar Mass) . 

Stellar Life Cycles

The life cycle of a star can range from Millions to 1000's of billions of years. Due to this, there is no way we could observe the full life cycle of a star. However, by observing the hundreds of thousands of stars visible from Earth ( each giving a snapshot in time ), we can piece together the full life cycle :-

The life cycle of a star can be split into three main sections, all based around Hydrogen fusion :-

Proto-Star -  Cloud of dust and gas collapsing under Gravity, increasing in Temperature and Pressure (No Fusion) .

Main Sequence Star - Star held in equilibrium, inwards Gravitational Force balanced by the outwards thermal Pressure due to Hydrogen fusion. 

Post Main Sequence Star - Hydrogen fusion ends, final fate of the star dependent on its original Mass. 

1 Solar Mass Star Life Cycle

The flash animation below shows the life cycle of a 1 Solar Mass star as well as how its position changes across the HR Diagram. 

Summary

The video below shows an in-depth summary of the life cycle of stars :-