Giant star

Hertzsprung-Russell diagram

Spectral class

Brown
dwarfs

White dwarfs

Red
dwarfs

Sub-dwarfs

Dwarfs

Main series

Subgiants

Giants

Bright giants

Supergiant

Hypergiant

Absolute
brightness
ness
(mag)

A giant star (or just giant ) is a star with an extremely large diameter and extremely high luminosity . It is the second stage of the stellar evolution of sun-like stars, which it enters after a long-lived state of equilibrium (main sequence or “actual” dwarf star ). In the Hertzsprung-Russell diagram (HRD) the giant stars are above the main sequence at the same surface temperature . As a rule, giants have a radius between 10 and 100 solar radii with a brightness that is between 10 and 1000 times that of our sun .

There are five types of giants.

• Lower giants , stars of luminosity class IV. They are located in the HRD between the giant branch and the main row.
• (Normal) giants of luminosity class III. They form the giant branch in the HRD .
• Bright giants , stars of luminosity class II. They are found in the HRD above the normal giants
• Supergiants , stars of luminosity class I. Due to their even higher luminosity, they are still above the bright giants in the HRD.
• Hypergiants , stars of luminosity class 0

In the late spectral classes , the radiation maximum of giants lies in the red spectral range . They are therefore also called red giants or red supergiants at this stage . Correspondingly, giants of medium or earlier spectral classes are called yellow or blue giants .

Internal structure of a sun-like star and a red giant.

Development scenarios

Stars up to 0.25 solar masses

In stars with less than 0.25 solar masses, continuous convection takes place inside for most of their lifetimes, i.e. there is a steady flow of heat within the core, so that the hydrogen merges for a time of more than a trillion (10 ¹² ) years can continue; a period that is much longer than the previous age of the universe . At some point, however, its center will develop into a radiation core, with the result that the hydrogen in the core will be exhausted and hydrogen will begin to burn in a shell around the core. (For stars with a mass of more than 0.16 solar masses, the shell can expand, but this expansion will never be very large.) Shortly afterwards, the supply of hydrogen in such a star will be completely exhausted and it will be collapse into a white dwarf with a helium core .

Stars between 0.25 and 0.5 solar masses

A star that is on the main sequence and whose mass is between about 0.25 and 0.5 solar masses will probably never reach the temperatures required for helium to fuse. A hydrogen-burning red giant will develop from such a star, which will ultimately become a white dwarf with a helium core. ^{, § 4.1, 6.1.} Stars between 0.25 and 0.5 solar masses contract as soon as all of the hydrogen inside them has been consumed by the fusion . Hydrogen is now burned to helium in a shell around the helium-rich core , with the part of the star outside the shell expanding and cooling. During this period of its development such a star is now the subgiants -AST in the Hertzsprung-Russell Diagram belong (HRD). This section contains stellar objects, the luminosity of which remains more or less constant, but their surface temperature decreases. Such a star may also begin to move into the area of ​​the red giants in the HRD . At this point, the surface temperature of the star, which has typically reached the stage of a red giant here, will drastically expand its radius while maintaining an almost constant luminosity. The core will continue to contract, which now leads to a continuous increase in its temperature.

Stars from 0.5 solar masses

In stars with more than 0.5 solar masses, after the hydrogen burn phase, when the core temperature reaches a value of around 100 million (10 ⁸ ) Kelvin , the helium burn begins , with carbon and oxygen being formed in the core through the three-alpha process . ^{, § 5.9, Chapter 6.} The energy that is generated by the nuclear fusion of the helium causes the nucleus to expand. This creates an effect in which the pressure in the vicinity of the hydrogen-burning bowl is reduced, which in turn reduces energy generation. The star's luminosity decreases, its outer shell contracts again and the star leaves the branch of the red giants. Its further development now depends on its mass. If it is not very massive, it will move into a horizontal branch in the HRD or its position will loop through the diagram. ^{, Chapter 6.}

Stars up to 8 solar masses

If the star is not heavier than about 8 solar masses, it will have used up the helium in the core after a while and a helium fusion will begin in a shell around its core. As a result of this, its luminosity will then increase again and it will rise, now as a AGB star , in the asymptotic giant branch of the HR diagram. After the star has lost most of its mass, its core will be left as a white dwarf made up of carbon and oxygen. ^{, § 7.1–7.4.}

Stars from 8 solar masses

For those main sequence stars whose masses are large enough to finally ignite a carbon fusion - this is the case from approx. 8 solar masses ^{, p. 189} - different scenarios can arise. These stars will not increase their brightness much after leaving the main sequence, but they will appear redder. However, they can also evolve into a red or blue supergiant . There is also the possibility that a white dwarf will develop from them, which has a core of oxygen and neon . The formation of a type II supernova is also conceivable , which ultimately leaves a neutron star or even a black hole behind. ^{, § 7.4.4-7.8.}

Examples

Well-known giant stars of different luminous colors are:

• Alkione (η Tauri), a blue-white (B-type) giant, the brightest star in the star cluster of the Pleiades .
• Thuban (α Draconis), a white (A-type) giant in the constellation of Dragon .
• σ Octantis , a white (F-type) giant that is the southern counterpart of the Pole Star .
• Capella , a yellow and white (G-type) giant, the main star in the constellation Carter .
• Pollux (β Geminorum), an orange (K-type) giant of the constellation Gemini .
• Mira (ο Ceti), a red (M-type) giant in the constellation Whale .
• VFTS 102 , the fastest rotating star to date, as a giant star in the Large Magellanic Cloud .

literature

• Hollis R. Johnson: Evolution of peculiar red giant stars. Cambridge University Press , Cambridge 1989, ISBN 0-521-36617-8 .
• Harm J. Habing, Hans Olofsson (Eds.): Asymptotic giant branch stars. Springer , New York 2004, ISBN 0-387-00880-2 .

Web links

Commons : Giant Stars  - Collection of images, videos and audio files

• astronews.com: Outbursts of brightness at birth November 8, 2016
• astronews.com: How a mass star grows November 23, 2016

Individual evidence

1. ↑ Giant Stars. Entry in: Patrick Moore (Ed.): Astronomy Encyclopedia. Oxford University Press , New York 2002, ISBN 0-19-521833-7 .
2. ^ Gregory Laughlin, Peter Bodenheimer, Fred C. Adams: The End of the Main Sequence. In: The Astrophysical Journal . June 10, 1997, No. 482, pp. 420-432.
3. ↑ SO Kepler and PA Bradley: Structure and Evolution of White Dwarfs , Baltic Astronomy 4, pp. 166-220. bibcode : 1995BaltA ... 4..166K , p. 169.
4. ^ Giant, entry In: John Daintith, William Gould: The Facts on File Dictionary of Astronomy. 5th edition, Facts On File, New York 2006, ISBN 0-8160-5998-5 .
5. ↑ ^{a b c d e f g} Evolution of Stars and Stellar Populations , Maurizio Salaris and Santi Cassisi, Chichester, UK: John Wiley & Sons , Ltd., 2005. ISBN 0-470-09219-X ^{, § 5.9.}
6. ↑ Giants and Post-Giants ( Memento of the original from July 20, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF file; 447 kB), class notes, Robin Ciardullo, Astronomy 534, Penn State University . 
7. ↑ Blowing Bubbles in the Cosmos: Astronomical Winds, Jets, and Explosions , TW Hartquist, JE Dyson, and DP Ruffle, New York: Oxford University Press, 2004. ISBN 0-19-513054-5 , pp. 33-35.
8. ↑ Supergiant . In: The Encyclopedia of Astrobiology, Astronomy, and Spaceflight , David Darling, accessed May 15, 2007.
9. ^ Alcyone , entry at SIMBAD , accessed May 16, 2007.
10. ↑ Alcyone ( Memento of the original from July 7, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. at Jim Kaler's STARS, accessed May 16, 2007. 
11. ^ Thuban , entry at SIMBAD, accessed May 16, 2007.
12. ^ Sigma Octantis , entry at SIMBAD, accessed May 16, 2007.
13. ↑ α Aurigae Aa , entry at SIMBAD, accessed May 16, 2007.
14. ^ Pollux , entry at SIMBAD, accessed May 16, 2007.
15. ^ Mira , entry at SIMBAD, accessed May 16, 2007.
16. ↑ http://www.uni-online.de/presse.php?id=454628  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
17. ↑ eso1147 - Science Release: VLT Finds Fastest Rotating Star. 5th Dec 2011
18. ↑ PL Dufton, PR Dunstall et al .: The VLT-FLAMES Tarantula Survey: The fastest rotating O-type star and shortest period LMC pulsar - remnants of a supernova disrupted binary? In: Astrophysical Journal Letters. (astro-ph.SR) December 6, 2011 ( full text / PDF )