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The two acronyms in the title are a pair I am currently using in my work and often confuse due to typing errors. BHB stands for Blue Horizontal Branch and HBB is for Hot Bottom Burning – doesn’t sound so comfortable does it? ;)

The horizontal branch is a part of the colour-magnitude diagram (CMD), also known as Hertzsprung-Russell or HR Diagram. Simply put it is a plot of stars’ colour (related to its temperature) vs. the stars’ magnitude (luminosity). This is a major tool for understanding stellar evolution and a stars life can be traced along the CMD as it evolves by moving into the different parts of the plot. Now the horizontal branch is called so because the stars at that stage form a horizontal line on the CMD plot! The stars that lie in the bluer (i.e. which are hotter) part of this horizontal line are called Blue Horizontal Branch stars (BHBs).

Hot Bottom Burning is something else – it refers to a nucleosynthesis mechanism that takes place within certain stars. Stars “synthesis” or create much of the elements we see today. Different elements are produced at different stages of stars and at different locations within the stars. The details of the various reaction chains are too much to go into detail in here, lets see if I can explain the basics to the non-scientist….

Hydrogen “burning” is the main power source in stars. The basic burning here is converting Hydrogen to Helium. So as burning continues the star’s interior is sets into layer – like an onion. H burning stars first at the core – where it is hotter, and the outer envelope is cooler. It progress upwards once all the core H is finished and burning of H in the shell of gas above the core starts. Later He burning starts and the reactions go on and different materials are produced. So at some stage you get H burning in an inner shell and an convective envelope layer above. When the bottom of the envelope reaches into the top of the hydrogen shell, this thin layer at the bottom of the envelope gets very hot, enough to start burning. This is creatively referred to as hot bottom burning!!!

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