Sigma starred molecular orbitals

If the two atomic orbitals are s-orbitals then the resulting molecular orbitals are called o (sigma) bonding molecular orbitals and a (sigma star) antibonding molecular orbitals (Figure 1.5). 

Figure 2. Molecular orbital energy diagrams for (a) FI2 showing both electrons in the bonding sigma MO and (b) Fle2 in which two of the electrons are in the antibonding sigma-starred MO...

Consider the hydrogen molecule, H2. Two hydrogen atoms, each with an electron in the Is orbital, approach each other. The two Is orbitals overlap and merge to form two new molecular orbitals a (sigma) and O (sigma star) (Figure 14.15). 

Atomic orbitals can also combine in a destructive way, canceling each other. The cancellation is similar to the darkness that results when two light waves cancel each other or to the silence that results when two sound waves cancel each other (Figure 1.3b). The destructive combination of two s atomic orbitals is called a a antibonding molecular orbital. An antibonding orbital is indicated by an asterisk ( ), which chemists read as star. Thus, a is read as sigma star. ... 

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