Difference between revisions of "Hertzsprung-Russell diagram" - New World Encyclopedia
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| − | The '''Hertzsprung-Russell diagram''' (usually referred to by the abbreviation '''H-R diagram''' or '''HRD'''), also known as a '''color-magnitude diagram''' (or '''CMD'''), is a diagram that shows various [[stellar classification|classes]] of [[star]]s in the context of properties such as their [[luminosity]], [[absolute magnitude]], [[color]], and effective [[temperature]]. Created around 1910 by [[Ejnar Hertzsprung]] and [[Henry Norris Russell]], the diagram provided a huge leap forward in understanding [[stellar evolution]], or the "lives of stars." | + | The '''Hertzsprung-Russell diagram''' (usually referred to by the abbreviation '''H-R diagram''' or '''HRD'''), also known as a '''color-magnitude diagram''' (or '''CMD'''), is a diagram that shows various [[stellar classification|classes]] of [[star]]s in the context of properties such as their [[luminosity]], [[absolute magnitude]], [[color]], and effective [[temperature]]. Created around 1910 by [[Ejnar Hertzsprung]] and [[Henry Norris Russell]], the diagram provided a huge leap forward in understanding [[stellar evolution]], or the "lives of stars." Scientists may also use the diagram to obtain a rough measure of the distance of a star cluster from [[Earth]]. |
==Diagram== | ==Diagram== | ||
| Line 14: | Line 14: | ||
In another form of the diagram, the [[effective temperature]] of the star is plotted on one axis, and the [[luminosity]] of the star is plotted on the other. This is what theoreticians calculate using computer models that describe the [[Stellar evolution|evolution of stars]]. This type of diagram should probably be called ''temperature-luminosity diagram'', but this term is hardly ever used, and the term ''Hertzsprung-Russell diagram'' is preferred instead. Despite some confusion regarding the nomenclature, astrophysicists make strict distinctions between these types of diagrams. | In another form of the diagram, the [[effective temperature]] of the star is plotted on one axis, and the [[luminosity]] of the star is plotted on the other. This is what theoreticians calculate using computer models that describe the [[Stellar evolution|evolution of stars]]. This type of diagram should probably be called ''temperature-luminosity diagram'', but this term is hardly ever used, and the term ''Hertzsprung-Russell diagram'' is preferred instead. Despite some confusion regarding the nomenclature, astrophysicists make strict distinctions between these types of diagrams. | ||
| − | The reason for this distinction is that the exact transformation from one to the other is not trivial, and it depends on the stellar-atmosphere model being used and its parameters (such as [[metallicity|composition]] and [[pressure]], apart from temperature and luminosity). Also, one needs to know the distances to the observed objects and the [[interstellar reddening]]. Empirical transformation between various color indices and [[effective temperature]] are available in the literature (Sekiguchi 2000, Casagrande 2006). | + | The reason for this distinction is that the exact transformation from one to the other is not trivial, and it depends on the stellar-atmosphere model being used and its parameters (such as [[metallicity|composition]] and [[pressure]], apart from temperature and luminosity). Also, one needs to know the distances to the observed objects and the [[interstellar reddening]]. Empirical transformation between various color indices and [[effective temperature]] are available in the literature (Sekiguchi and Fukugita 2000, Casagrande et al. 2006). |
The H-R diagram can be used to define different types of stars and to match theoretical predictions of [[stellar evolution]] using computer models with observations of actual stars. It is then necessary to convert either the calculated quantities to observables, or the other way around, thus introducing an extra uncertainty. | The H-R diagram can be used to define different types of stars and to match theoretical predictions of [[stellar evolution]] using computer models with observations of actual stars. It is then necessary to convert either the calculated quantities to observables, or the other way around, thus introducing an extra uncertainty. | ||
==Interpretation== | ==Interpretation== | ||
| + | |||
Most of the stars occupy a region in the diagram along a line called the [[Main Sequence]]. During that stage, stars are [[Proton-proton chain reaction|fusing hydrogen]] in their cores. The next concentration of stars is on the [[horizontal branch]], corresponding to [[helium fusion]] in the core and hydrogen burning in a shell surrounding the core. | Most of the stars occupy a region in the diagram along a line called the [[Main Sequence]]. During that stage, stars are [[Proton-proton chain reaction|fusing hydrogen]] in their cores. The next concentration of stars is on the [[horizontal branch]], corresponding to [[helium fusion]] in the core and hydrogen burning in a shell surrounding the core. | ||
Another prominent feature is known as the [[Hertzsprung gap]], located in the region between A5 and G0 [[spectral type]]s and between +1 and −3 [[absolute magnitude]]s — that is, between the top of the main sequence and the [[Giant star|giants]] in the horizontal branch. [[RR Lyrae variable|RR Lyrae]] stars can be found to the left of this gap. [[Cepheid variable]]s reside in the upper section of the [[instability strip]]. | Another prominent feature is known as the [[Hertzsprung gap]], located in the region between A5 and G0 [[spectral type]]s and between +1 and −3 [[absolute magnitude]]s — that is, between the top of the main sequence and the [[Giant star|giants]] in the horizontal branch. [[RR Lyrae variable|RR Lyrae]] stars can be found to the left of this gap. [[Cepheid variable]]s reside in the upper section of the [[instability strip]]. | ||
| − | The | + | ==The diagram's role in the development of stellar physics== |
| − | + | Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolution, a main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes. However, following Russell's presentation of the diagram to a meeting of the [[Royal Astronomical Society]] in 1912, [[Arthur Eddington]] was inspired to use it as a basis for developing ideas on [[stellar physics]] (Porter, 2003). In 1926, in his book ''The Internal Constitution of the Stars'', he explained the physics of how stars fit on the diagram. This was a particularly remarkable development because at that time the major problem of stellar theory — namely, the source of a star's energy — was still unsolved. [[Nuclear fusion|Thermonuclear energy]], and even that stars are largely composed of [[hydrogen]], had yet to be discovered. Eddington managed to sidestep this problem by concentrating on the [[thermodynamics]] of [[thermal radiation|radiative transport]] of energy in stellar interiors (Smith, 1995). On that basis, Eddington predicted that dwarf stars remain in an essentially static position on the main sequence for most of their lives. In the 1930s and 1940s, with an understanding of hydrogen fusion, scientists formulated a physically based theory of evolution to red giants and white dwarfs. By this time, study of the Hertzsprung-Russell diagram did not drive such developments but merely allowed stellar evolution to be presented graphically. | |
| − | Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolution, a main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes. However, following Russell's presentation of the diagram to a meeting of the [[Royal Astronomical Society]] in 1912, [[Arthur Eddington]] was inspired to use it as a basis for developing ideas on [[stellar physics]] (Porter, 2003). In 1926, in his book ''The Internal Constitution of the Stars'' he explained the physics of how stars fit on the diagram. This was a particularly remarkable development | + | |
| + | == Measurement of distance of a star cluster == | ||
| + | |||
| + | The H-R diagram may be used by scientists to roughly measure how far away a [[star cluster]] is from Earth. This can be done by comparing the apparent magnitudes of the stars in the cluster to the absolute magnitudes of stars with known distances (or of model stars). The observed group is then shifted in the vertical direction, until the two main sequences overlap. The difference in magnitude that was bridged in order to match the two groups is called the [[distance modulus]] and is a direct measure for the distance. This technique is known as ''main-sequence fitting'', or, confusingly, as the ''spectroscopic parallax''. | ||
== See also == | == See also == | ||
Revision as of 00:02, 22 December 2008
The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD), also known as a color-magnitude diagram (or CMD), is a diagram that shows various classes of stars in the context of properties such as their luminosity, absolute magnitude, color, and effective temperature. Created around 1910 by Ejnar Hertzsprung and Henry Norris Russell, the diagram provided a huge leap forward in understanding stellar evolution, or the "lives of stars." Scientists may also use the diagram to obtain a rough measure of the distance of a star cluster from Earth.
Diagram
One version of the Hertzsprung-Russell diagram is shown below.
Forms of the diagram
There are several forms of the Hertzsprung-Russell diagram, and the nomenclature is not very well defined. The original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical axis. The first quantity (namely, spectral type) is difficult to determine unambiguously and is therefore often replaced by the B-V color index of the stars. This type of Hertzsprung-Russell diagram is called a color-magnitude diagram, and it is often used by observers. However, a color-magnitude diagram is also used in some cases to describe a plot with the vertical axis depicting the apparent magnitude, rather than the absolute magnitude.
In another form of the diagram, the effective temperature of the star is plotted on one axis, and the luminosity of the star is plotted on the other. This is what theoreticians calculate using computer models that describe the evolution of stars. This type of diagram should probably be called temperature-luminosity diagram, but this term is hardly ever used, and the term Hertzsprung-Russell diagram is preferred instead. Despite some confusion regarding the nomenclature, astrophysicists make strict distinctions between these types of diagrams.
The reason for this distinction is that the exact transformation from one to the other is not trivial, and it depends on the stellar-atmosphere model being used and its parameters (such as composition and pressure, apart from temperature and luminosity). Also, one needs to know the distances to the observed objects and the interstellar reddening. Empirical transformation between various color indices and effective temperature are available in the literature (Sekiguchi and Fukugita 2000, Casagrande et al. 2006).
The H-R diagram can be used to define different types of stars and to match theoretical predictions of stellar evolution using computer models with observations of actual stars. It is then necessary to convert either the calculated quantities to observables, or the other way around, thus introducing an extra uncertainty.
Interpretation
Most of the stars occupy a region in the diagram along a line called the Main Sequence. During that stage, stars are fusing hydrogen in their cores. The next concentration of stars is on the horizontal branch, corresponding to helium fusion in the core and hydrogen burning in a shell surrounding the core.
Another prominent feature is known as the Hertzsprung gap, located in the region between A5 and G0 spectral types and between +1 and −3 absolute magnitudes — that is, between the top of the main sequence and the giants in the horizontal branch. RR Lyrae stars can be found to the left of this gap. Cepheid variables reside in the upper section of the instability strip.
The diagram's role in the development of stellar physics
Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolution, a main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes. However, following Russell's presentation of the diagram to a meeting of the Royal Astronomical Society in 1912, Arthur Eddington was inspired to use it as a basis for developing ideas on stellar physics (Porter, 2003). In 1926, in his book The Internal Constitution of the Stars, he explained the physics of how stars fit on the diagram. This was a particularly remarkable development because at that time the major problem of stellar theory — namely, the source of a star's energy — was still unsolved. Thermonuclear energy, and even that stars are largely composed of hydrogen, had yet to be discovered. Eddington managed to sidestep this problem by concentrating on the thermodynamics of radiative transport of energy in stellar interiors (Smith, 1995). On that basis, Eddington predicted that dwarf stars remain in an essentially static position on the main sequence for most of their lives. In the 1930s and 1940s, with an understanding of hydrogen fusion, scientists formulated a physically based theory of evolution to red giants and white dwarfs. By this time, study of the Hertzsprung-Russell diagram did not drive such developments but merely allowed stellar evolution to be presented graphically.
Measurement of distance of a star cluster
The H-R diagram may be used by scientists to roughly measure how far away a star cluster is from Earth. This can be done by comparing the apparent magnitudes of the stars in the cluster to the absolute magnitudes of stars with known distances (or of model stars). The observed group is then shifted in the vertical direction, until the two main sequences overlap. The difference in magnitude that was bridged in order to match the two groups is called the distance modulus and is a direct measure for the distance. This technique is known as main-sequence fitting, or, confusingly, as the spectroscopic parallax.
See also
- Galaxy
- Giant
- Star
- Sun
- Supergiant
- White dwarf
ReferencesISBN links support NWE through referral fees
- Casagrande, L., L. Portinari, and C. Flynn. 2006. Accurate fundamental parameters for lower main-sequence stars. (Abstract). MNRAS. 373(1):13–44. Retrieved December 13, 2008.
- Porter, Roy. 2003. The Cambridge History of Science. Cambridge, UK: Cambridge University Press. ISBN 9780521572439.
- Sekiguchi, Maki, and Masataka Fukugita. 2000. A Study of the B-V Color-Temperature Relation. The Astronomical Journal. 120(2):1072–1084. Retrieved December 13, 2008.
- Smith, Robert. 1995. Observational Astrophysics. Cambridge, UK: Cambridge University Press. ISBN 978-0521278348.
External links
- JavaHRD. (An interactive Hertzsprung-Russell diagram as a Java applet.) Retrieved December 13, 2008.
- BaSTI: A Bag of Stellar Tracks and Isochrones. (Simulations with FRANEC code by Teramo Astronomical Observatory.) Retrieved December 13, 2008.
- The first Hertzsprung-Russell diagram. Leos Ondra. Retrieved December 13, 2008.
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- ↑ The Hertzsprung-Russell Diagram. Richard Powell. Retrieved December 21, 2008.
