In the field ofstellar evolution, ablue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of theevolutionary track on aHertzsprung–Russell diagram which forms a loop towards the blue (i.e. hotter) side of the diagram, to a place called the blue giant branch.^[1]

Blue loops can occur forred supergiants,red-giant branch stars, orasymptotic giant branch stars. Some stars may undergo more than one blue loop. Manypulsating variable stars such asCepheids are blue loop stars. Stars on thehorizontal branch are not generally referred to as on a blue loop even though they are temporarily hotter than on the red giant or asymptotic giant branches. Loops occur far too slowly to be observed for individual stars, but are inferred from theory and from the properties and distribution of stars in the H–R diagram.

Most stars on the red-giant branch (RGB) have an inert helium core and remain on the RGB until ahelium flash moves them to the horizontal branch. However, stars more massive than about 2.3 M_☉ do not have an inert core. They smoothly ignite helium before reaching thetip of the red-giant branch and become hotter while they burn helium in their cores. More massive stars become hotter during this phase and stars from about 5 M_☉ upwards are generally treated as experiencing a blue loop, which lasts on the order of a million years. This type of blue loop occurs only once in the lifetime of a star.^[2][3][4]

Stars on the asymptotic giant branch (AGB) have largely inert cores of carbon and oxygen, and alternately fuse hydrogen and helium in concentric shells around the core. The onset of helium shell burning causes athermal pulse and in some cases this will cause the star to temporarily increase itstemperature and execute a blue loop. Many thermal pulses may occur as the shells alternately switch on and off, and multiple blue loops can occur in the same star.^[5]

Red supergiants are massive stars that have left themain sequence and greatly expanded and cooled. Their high luminosity and lowsurface gravity means they are rapidly losing mass. The most luminous red supergiants can lose mass quickly enough that they become hotter and smaller. In the most massive stars, this can result in the star evolving permanently away from the red supergiant stage to become a blue supergiant, but in some cases the star will execute a blue loop and return to being a red supergiant.^[6][7] Extremered hypergiantsVY Canis Majoris andNML Cygni are two such known candidates for a star within a second red supergiant phase.^[8][9]

Stars which are executing blue loops cross the yellow portion of the H–R diagram above the main sequence, so that many of them cross a region called the instability strip because the outer layers of stars in that region are unstable and pulsate. Stars from the asymptotic giant branch that cross the instability strip during a blue loop are thought to becomeW Virginis variables. More massive stars, crossing the instability strip during a blue loop from the red-giant branch, are thought to make up theδ Cephei variables. Both types of star have luminous and unstablephotospheres at this stage of their lives and often have the spectra ofsupergiants, although most are not massive enough to ever fuse carbon or reach asupernova.^[5][10][11]

Notable examples of stars believed to be in a blue loop phase include:

• Canopus
• Arneb^[12]
• Eta Leonis^[13]
• 4 Lacertae^[1]

• Stellar evolution
• Hertzsprung–Russell classifications

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