Blue loop

In the field of stellar evolution, 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 on a Hertzsprung–Russell diagram which forms a loop towards the blue (i.e. hotter) side of the diagram.

Blue loops can occur for red supergiants red giant branch stars, or asymptotic giant branch stars. Some stars may undergo more than one blue loop. Many pulsating variable stars such as Cepheids 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.

Evolutionary track 5m
Evolutionary track of a 5 M star showing a blue loop

Red giants

CoRoT HR
Stellar evolutionary tracks, some showing blue loops in the more massive red giants

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 2.3 M do not have an inert core. They smoothly ignite helium before reaching the tip 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.[1][2][3]

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 and in some cases this will cause the star to temporarily increase its temperature 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.[4]

Red supergiants

Red supergiants are massive stars that have left the main sequence and greatly expanded and cooled. Their high luminosity and low surface 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.[5][6]

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 variables. 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 photospheres at this stage of their lives and often have the spectra of supergiants, although most are not massive enough to ever fuse carbon or reach a supernova.[4][7][8]

References

  1. ^ "hapter 9: Post-main sequence evolution through helium burning" (PDF). Retrieved 2019-01-17.
  2. ^ Xu, H. Y.; Li, Y. (2004). "Blue loops of intermediate mass stars . I. CNO cycles and blue loops". Astronomy and Astrophysics. 418: 213–224. Bibcode:2004A&A...418..213X. doi:10.1051/0004-6361:20040024.
  3. ^ Halabi, Ghina M.; El Eid, Mounib (2012). "Sensitivity of the blue loops of intermediate-mass stars to nuclear reactions". American Institute of Physics Conference Series. 1498 (1): 334. arXiv:1410.1652. Bibcode:2012AIPC.1498..334H. doi:10.1063/1.4768514.
  4. ^ a b Groenewegen, M. A. T.; Jurkovic, M. I.; Meynet, Georges (2017). "Luminosities and infrared excess in Type II and anomalous Cepheids in the Large and Small Magellanic Clouds". Astronomy and Astrophysics. 603: A70. arXiv:1705.00886. Bibcode:2017A&A...603A..70G. doi:10.1051/0004-6361/201730687.
  5. ^ Meynet, Georges; Georgy, Cyril; Hirschi, Raphael; Maeder, André; Massey, Phil; Przybilla, Norbert; Nieva, M. -Fernanda (2011). "Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: The single massive star perspective". Bulletin de la Societe Royale des Sciences de Liege. 80: 266. arXiv:1101.5873. Bibcode:2011BSRSL..80..266M.
  6. ^ Saio, Hideyuki; Georgy, Cyril; Meynet, Georges (2013). "Evolution of blue supergiants and α Cygni variables: Puzzling CNO surface abundances". Monthly Notices of the Royal Astronomical Society. 433 (2): 1246. arXiv:1305.2474. Bibcode:2013MNRAS.433.1246S. doi:10.1093/mnras/stt796.
  7. ^ Turner, David G.; Abdel-Sabour Abdel-Latif, Mohamed; Berdnikov, Leonid N. (2006). "Rate of Period Change as a Diagnostic of Cepheid Properties". Publications of the Astronomical Society of the Pacific. 118 (841): 410–418. arXiv:astro-ph/0601687. Bibcode:2006PASP..118..410T. doi:10.1086/499501.
  8. ^ Duerbeck, H. W.; Seitter, W. C. (1996). "5.1.2.1 Cepheids - CEP". Stars and Star Clusters. Landolt-Börnstein - Group VI Astronomy and Astrophysics. 3B. pp. 134–139. doi:10.1007/10057805_40. ISBN 978-3-540-56080-7.
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 (0.6–10 solar masses) late in their lives.

Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars.

Canopus

Canopus is the brightest star in the southern constellation of Carina, and is located near the western edge of the constellation around 310 light-years from the Sun. Its proper name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta. Canopus has the Bayer designation α Carinae, which is Latinised to Alpha Carinae and abbreviated Alpha Car or α Car. It is the second-brightest star in the night sky, after Sirius. Canopus' visual apparent magnitude is −0.74, and it has an absolute magnitude of −5.71.

Canopus is an ageing bright giant of spectral type A9 or F0, so it is essentially white when seen with the naked eye. Canopus is undergoing core helium burning and is currently in the so-called blue loop phase of its evolution, having already passed through the red-giant branch after exhausting the hydrogen in its core. Canopus has eight times the mass of the Sun and has expanded to 71 times the Sun's radius. It is radiating over 10,000 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of around 7,000 K. Canopus is an X-ray source, which is likely being emitted from its corona.

The prominent appearance of Canopus means it has been the subject of mythological lore among many ancient peoples. The acronychal rising marked the date of the Ptolemaia festival in Egypt. In Hinduism, it was named Agastya after the revered Vedic sage. For Chinese astronomers, it was known as the Old Man of the South Pole.

Five Weirs Walk

The 8 km long Five Weirs Walk runs alongside the River Don in Sheffield, England. From Lady's Bridge in Sheffield City Centre, it heads downstream, northeast, over the Cobweb Bridge, through Attercliffe to Meadowhall.53.414°N 1.411°W / 53.414; -1.411

As the name of the walk suggests, it passes five weirs. It is now possible to continue the walk along the Don, under the Tinsley Viaduct, to Rotherham. As of 2010, the section of the walk between Sheffield and Meadowhall has been linked with the parallel Sheffield and Tinsley Canal towpath as an 8-mile circular walk known as The Blue Loop. The walk links with the Upper Don Walk at Lady's Bridge.

HD 84810

HD 84810, also known as l Carinae (l Car), is a star in the southern constellation of Carina. Its apparent magnitude varies from about 3.4 to 4.1, making it readily visible to the naked eye and one of the brighter members of Carina. Based upon parallax measurements, it is approximately 1,600 light-years (490 parsecs) from Earth.From the characteristics of its spectrum, l Carinae has a stellar classification of G5 Iab/Ib. This indicates the star has reached a stage in its evolution where it has expanded to become a supergiant with 169 times the radius of the Sun. As this is a massive star with 8–13 times the mass of the Sun, it rapidly burns through its supply of nuclear fuel and has become a supergiant in roughly 17-19 million years, after spending 15–17 million years as a main sequence star.l Carinae is classified as a Cepheid variable star and its brightness varies over an amplitude range of 0.725 in magnitude with a long period of 35.560 days. The radial velocity of the star likewise varies by 39 km/s during each pulsation cycle. Its radius varies by about 40 R☉ as it pulsates, reaching maximum size as its brightness is decreasing towards minimum.It has a compact circumstellar envelope that can be discerned using interferometry. The envelope has been resolved at an infrared wavelength of 10μm, showing a radius of 10–100 AU at a mean temperature of 100 K. The material for this envelope was supplied by mass ejected from the central star.The period of l Carinae is calculated to be slowly increasing and it is thought to be crossing the instability strip for the third time, cooling as it evolves towards a red supergiant after a blue loop.

HD 96919

HD 96919, also known by its Bayer designation of z2 Carinae and the variable star designation of V371 Carinae, is a blue supergiant star in the constellation Carina. It lies near the Carina Nebula and at a comparable distance.

V371 Car is an α Cyg variable, erratically pulsating and changing brightness by a few hundredths of a magnitude. Periods of 10 - 80 days have been identified. It shows unusual emission lines in its spectrum, and high-velocity absorption (HVA) events, temporary spectral features that are thought to indicate localised regions of enhanced mass loss.HD 96919 is a B9 supergiant, possibly located 6,000 light years from Earth. It is considered to be a post red supergiant star, either evolving towards a Wolf-Rayet star or on a blue loop before returning to a cooler temperature.

Infrared dark cloud

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

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 stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.

Red clump

The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g. the Hipparcos stars).

The red clump giants are cool horizontal branch stars, stars originally similar to the Sun which have undergone a helium flash and are now fusing helium in their cores.

Red supergiant star

Red supergiants 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 Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

SV Vulpeculae

SV Vulpeculae is a Classical Cepheid (δ Cepheid) variable star in the constellation Vulpecula. It is a supergiant at a distance of 8,000 light years.

SV Vulpeculae is a δ Cepheid variable whose visual apparent magnitude ranges from 6.72 to 7.79 over 45.0121 days. The light curve is highly asymmetric, with the rise from minimum to maximum taking more less than a third of the time for the fall from maximum to minimum. The period has been decreasing on average by 214 seconds/year.SV Vulpeculae is a yellow bright supergiant around twenty thousand times as luminous as the sun, with a spectral type that varies from late F to early K. It pulsates and varies in temperature from below 5,000 K to above 6,000 K. The radius is 216.5 R☉ at maximum, and varies from 188 R☉ to 238 R☉ as the star pulsates.The mass of SV Vulpeculae is now near 15 M☉, and is estimated to have been about 17 M☉ when it was on the main sequence. The rate of change of the period and the atmospheric abundances show that the star is crossing the instability strip for the second time. The first instability strip crossing occurs rapidly during the transition from the main sequence to becoming a red supergiant. The second crossing occurs during core helium burning when the star executes a blue loop, becoming hotter for a time before returning to the red supergiant stage.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Stellar mass

Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (M☉). Hence, the bright star Sirius has around 2.02 M☉. A star's mass will vary over its lifetime as additional mass becomes accreted, such as from a companion star, or mass is ejected with the stellar wind or pulsational behavior.

V810 Centauri

V810 Centauri is a double star consisting of a yellow supergiant primary (V810 Cen A) and blue giant secondary (V810 Cen B). It is a small amplitude variable star, entirely due to the supergiant primary which is visually over three magnitudes brighter than the secondary.V810 Cen A shows semi-regular variations with several component periods. The dominant mode is around 156 days and corresponds to Cepheid fundamental mode radial pulsation. Without the other pulsation modes it would be considered a Classical Cepheid variable. Other pulsation modes have been detected at 89 to 234 days, with the strongest being a possible non-radial p-mode at 107 days and a possible non-radial g-mode at 185 days.The blue giant secondary has a similar mass and luminosity to the supergiant primary, but is visually much fainter. The primary is expected to have lost around 5 M☉ since it was on the main sequence, and has expanded and cooled so it lies at the blue edge of the Cepheid instability strip. It is expected to get no cooler and may perform a blue loop while slowly increasing in luminosity.V810 Cen was once thought to be a member of the Stock 14 open cluster at 2.6 kpc, but now appears to be more distant. The distance derived from spectrophotometric study is larger than the mean Hipparcos parallax value but within the margin of error.

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

Yellow supergiant star

A yellow supergiant star is a star, generally of spectral type F or G, having a supergiant luminosity class (e.g. Ia or Ib). They are stars that have evolved away from the main sequence, expanding and becoming more luminous.

Yellow supergiants are smaller than red supergiants; naked eye examples include Canopus and Polaris. Many of them are variable stars, mostly pulsating Cepheids such as δ Cephei itself.

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