HertzsprungRussell HR diagrams A star is born from

Hertzsprung-Russell (H-R) diagrams

A star is born from clouds of gas in a star nursery or Nebula. Areas of the nebula become dense and contract due to gravity. This continually happens until a protostar forms and the central temperature rises to 15 million °C. The massive gravitational inward force overcomes the outward thermal pressure causing nuclear fusion and the star begins to emit heat and light. At this stage the star is called a main sequence star. Our Sun is a main sequence star.

To understand the different types of stars better we need a method to separate categories of star type. Modern stellar classification uses the following table:

Modern stellar classification Spectral type O B A F G K M Temperature (K) Examples > 30, 000– 10, 000– 7, 500– 6000– 5000– 3500 Orion’s Belt stars Rigel Sirius Polaris Sun Arcturus Proxima Centauri, Betelgeuse <3500 Colour Blue-white White Yellow-white Yellow Orange Red The order of the spectral classes has been remembered by generations of astronomy students using the mnemonic ‘Oh, Be A Fine Girl/Guy, Kiss Me’. Notice the table runs from hottest to coolest and that hot stars are blue and red stars are cool.

A Hertzsprung-Russell, H-R, diagram takes this idea further and classifies a star by plotting it’s luminosity (y-axis) and temperature (x-axis).

Notice that the x-axis shows the kelvin temperature in reverse order and that the y-axis shows luminosity as a logarithmic scale. The chart can be broken down into four sections: Main sequence - Diagonal line from Top left to bottom right Red Giant - Top right Supergiant - Top central White Dwarf - Bottom left

2015 specimen paper

The mass of the star both governs how long they live and the fate of the star at the end of its life cycle. Main sequence stars (like our sun) Once a star has formed and established hydrogen fusion it will occupy a position somewhere along the main sequence line. The more massive top left - the less massive bottom right. A star remains “main sequence” as the inward gravitational forces balance the outward thermal pressure due to fusion.

Giants These are formed when fusion of hydrogen in the core stops. The core contracts and heats up under the weight of gravity. Some fusion still occurs in the upper layers and the contracting core heats up these upper layers, producing increased outward thermal pressure causing them to expand. As the outer layers expand, the radius of the star will increase until the point when the outward thermal pressure is once more balanced by the inward gravitational pull and it will become a red giant with a cooler surface temperature and significant increase in radius and volume.

White Dwarf As a red giant continues to expand it eventually reaches a size where the outer layers of the star can no longer be retained by gravitational attraction and the outer layers steadily disperse to form what is called a planetary nebula. (Planetary nebula is a historical term - does not imply planet formation. ) This leaves behind the stellar core. Since fusion no longer occurs in this dead core it steadily cools to become a white dwarf.

Super giants The heaviest of the stars (many times the mass of our sun) live for only a few million years before swelling into super giants and exploding as supernovae. In the resulting explosion the outer layer of the star is blown off. If the mass of the remnant is 1. 4 times or more of our Sun’s mass it will collapse and become a neutron star. If the mass of the remnant is >=3 times our solar mass it will collapse and form a black hole. Students do a flow diagram of star evolution

As stars go through the various stages of their life cycles the position they have on the H-R diagram will change and as a consequence they move around on the chart because temperature and luminosity will be affected due to the changing amounts of fusion occurring in the star.

note: the variety of axis labels and the nonlinear scales