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 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.
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:-
In 1896, German scientist Wilhelm Wien derived a formula relating the Temperature of an object to the peak Wavelength from experimental observations:-
λPeak T = 2.898x10-3 mK
λPeak = Peak Wavelength (m)
T = Temperature of Object (K)
Note - the Unit in the above formula (mK) is not Milli-Kelvin. It is actually Meters-Kelvin, a unit which has no actual meaning.
Example 1 -
Rigel and Betelgeuse are two stars found within the constellation of Orion, and is visible in the winter months of the northern hemisphere.
Rigel is a visibly blue coloured star with a peak Wavelength of 263 nm. Betelgeuse is a visibly red coloured star with a surface Temperature of 3,500 K.
1. The surface Temperature of Rigel.
2. The peak Wavelength of Betelgeuse.
λPeak T = 2.898x10-3
T = 2.898x10-3 / 263x10-9
T = 11,019 K
λPeak T = 2.898x10-3
λPeak = 2.898x10-3 / 3500
λPeak = 828x10-9 m
λPeak = 828 nm
Note - Betelgeuse and Rigel are two of the clearest examples in the night sky that are obviously non-white stars. However, as the calculation has shown above, neither star has a peak Wavelength within the visible spectrum. This is an example of how stars do not emit solely at peak Wavelength, but with a range of Wavelengths.
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 catagorised into three main categories:-
1. Main Sequence - The vast majority of stars are found in this section, this section shows stars fusing Hydrogen.
2. Giant Branch - Once Hydrogen fusion ends, stars swell and cool to form Red Giant stars, fusing Helium and other heavier elements.
3. 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 Supergiant 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:-
1. 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.
The video below shows an in-depth summary of the life cycle of stars:-