![Evolutionary track 5m](https://upload.wikimedia.org/wikipedia/commons/3/3b/Evolutionary_track_5m.svg)
In the field of
stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
, a blue 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 the
evolutionary track
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...
on a
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...
which forms a loop towards the blue (i.e. hotter) side of the diagram.
Blue loops can occur for
red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Anta ...
s,
red-giant branch
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...
stars, or
asymptotic giant branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
stars. Some stars may undergo more than one blue loop. Many
pulsating variable star
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...
s such as
Cepheids
A Cepheid variable () is a type of star that Instability strip, pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable frequency, period and amplitude.
A strong direct period-l ...
are blue loop stars. Stars on the
horizontal 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.
Red giants
![CoRoT HR](https://upload.wikimedia.org/wikipedia/commons/6/67/CoRoT_HR.jpg)
Most stars on the red-giant branch (RGB) have an inert helium core and remain on the RGB until a
helium flash moves them to the horizontal branch. However, stars more massive than about do not have an inert core. They smoothly ignite helium before reaching the
tip of the red-giant branch
Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conjun ...
and become hotter while they burn helium in their cores. More massive stars become hotter during this phase and stars from about 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.
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Asymptotic giant branch
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 a thermal pulse
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) l ...
and in some cases this will cause the star to temporarily increase its temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
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.[
]
Red supergiants
Red supergiants are massive stars that have left the main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
and greatly expanded and cooled. Their high luminosity and low surface gravity
The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...
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.[
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Instability strip
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 become W Virginis variable
W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days,Wallerstein, G."The Cepheids of Population II and Related Stars" ''Publications of the Astronomical Society of the Pacific'', 114 p.689– ...
s. More massive stars, crossing the instability strip during a blue loop from the red-giant branch, are thought to make up the δ Cephei variables. Both types of star have luminous and unstable photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
s at this stage of their lives and often have the spectra of supergiant
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...
s, although most are not massive enough to ever fuse carbon or reach a supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
.[
]
References
{{Star
Stellar evolution
Hertzsprung–Russell classifications