An observational Hertzsprung–Russell diagram with 22,000 stars plotted from theHipparcos Catalogue and 1,000 from theGliese Catalogue of nearby stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called themain sequence. In the lower-left is wherewhite dwarfs are found, and above the main sequence are thesubgiants,giants andsupergiants. TheSun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and B−Vcolor index 0.66 (temperature 5780 K,spectral type G2V).
In the nineteenth century large-scale photographic spectroscopic surveys of stars were performed atHarvard College Observatory, producing spectral classifications for tens of thousands of stars, culminating ultimately in theHenry Draper Catalogue. In one segment of this workAntonia Maury included divisions of the stars by the width of theirspectral lines.[1] Hertzsprung noted that stars described with narrow lines tended to have smallerproper motions than the others of the same spectral classification. He took this as an indication of greater luminosity for the narrow-line stars, and computedsecular parallaxes for several groups of these, allowing him to estimate their absolute magnitude.[2]
In 1910Hans Oswald Rosenberg published a diagram plotting the apparent magnitude of stars in the Pleiades cluster against the strengths of thecalcium K line and twohydrogenBalmer lines.[3] These spectral lines serve as a proxy for the temperature of the star, an early form of spectral classification. The apparent magnitude of stars in the same cluster is equivalent to their absolute magnitude and so this early diagram was effectively a plot of luminosity against temperature. The same type of diagram is still used today as a means of showing the stars in clusters without having to initially know their distance and luminosity.[4] Hertzsprung had already been working with this type of diagram, but his first publications showing it were not until 1911. This was also the form of the diagram using apparent magnitudes of a cluster of stars all at the same distance.[5]
Russell's early (1913) versions of the diagram included Maury's giant stars identified by Hertzsprung, those nearby stars with parallaxes measured at the time, stars from theHyades (a nearbyopen cluster), and severalmoving groups, for which themoving cluster method could be used to derive distances and thereby obtain absolute magnitudes for those stars.[6]
There are several forms of the Hertzsprung–Russell diagram, and thenomenclature is not very well defined. All forms share the same general layout: stars of greater luminosity are toward the top of the diagram, and stars with higher surface temperature are toward the left side of the diagram.
The original diagram displayed the spectral type of stars on the horizontal axis and theabsolute visual magnitude on the vertical axis. The spectral type is not a numerical quantity, but the sequence of spectral types is amonotonic series that reflects the stellar surface temperature. Modern observational versions of the chart replace spectral type by acolor index (in diagrams made in the middle of the 20th Century, most often theB-V color) of the stars. This type of diagram is what is often called an observational Hertzsprung–Russell diagram, or specifically a color–magnitude diagram (CMD), and it is often used by observers.[7] In cases where the stars are known to be at identical distances such as within a star cluster, a color–magnitude diagram is often used to describe the stars of the cluster with a plot in which the vertical axis is theapparent magnitude of the stars. For cluster members, by assumption there is a single additive constant difference between their apparent and absolute magnitudes, called thedistance modulus, for all of that cluster of stars. Early studies of nearby open clusters (like the Hyades andPleiades) by Hertzsprung and Rosenberg produced the first CMDs, a few years before Russell's influential synthesis of the diagram collecting data for all stars for which absolute magnitudes could be determined.[3][5]
Another form of the diagram plots theeffective surface temperature of the star on one axis and the luminosity of the star on the other, almost invariably in alog-log plot. Theoretical calculations ofstellar structure and theevolution of stars produce plots that match those from observations. This type of diagram could be calledtemperature-luminosity diagram, but this term is hardly ever used; when the distinction is made, this form is called thetheoretical Hertzsprung–Russell diagram instead. A peculiar characteristic of this form of the H–R diagram is that the temperatures are plotted from high temperature to low temperature, which aids in comparing this form of the H–R diagram with the observational form.
Although the two types of diagrams are similar, astronomers make a sharp distinction between the two. The reason for this distinction is that the exact transformation from one to the other is not trivial. To go between effective temperature and color requires acolor–temperature relation, and constructing that is difficult; it is known to be a function ofstellar composition and can be affected by other factors likestellar rotation. When converting luminosity or absolutebolometric magnitude to apparent or absolute visual magnitude, one requires abolometric correction, which may or may not come from the same source as the color–temperature relation. One also needs to know the distance to the observed objects (i.e., the distance modulus) and the effects ofinterstellar obscuration, both in the color (reddening) and in the apparent magnitude (where the effect is called "extinction"). Color distortion (including reddening) and extinction (obscuration) are also apparent in stars having significantcircumstellar dust. The ideal of direct comparison of theoretical predictions of stellar evolution to observations thus has additional uncertainties incurred in the conversions between theoretical quantities and observations.
An HR diagram with theinstability strip and its components highlighted
Most of the stars occupy the region in the diagram along the line called themain sequence, with low-mass stars at the cooler and less luminous end of the sequence, and more massive stars towards the hotter and more luminous end. During the stage of their lives in which stars are found on the main sequence line, they arefusing hydrogen in their cores. A prominent group of cool stars is found at higher luminosities, and larger sizes, than main-sequence stars. These are known asred giants and include: stars fusing hydrogen around an inert helium core, the red giant branch; stars fusing helium in their cores, thehorizontal branch; and stars fusing helium and hydrogen in shells around a largely-inert core, theasymptotic giant branch. The red giants are separated from the main sequence by theHertzsprung gap, populated bysubgiants and located in the region between A5 and G0 spectral type and between +1 and −3 absolute magnitudes.
Other groups of stars distinguished in the HR diagram are:supergiants, rare evolved massive stars prominent because of their high luminosity;white dwarfs, very common but with very low luminosities; andbrown dwarfs, very cool and with very low luminosities.RR Lyrae variable stars are horizontal-branch stars in a section of the diagram called theinstability strip.Cepheid variables also fall on the instability strip, at higher luminosities.
The H-R diagram can be used by scientists to roughly measure how far away astar cluster orgalaxy 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 thedistance modulus and is a direct measure for the distance (ignoringextinction). This technique is known asmain sequence fitting and is a type ofspectroscopic parallax. Not only the turn-off in the main sequence can be used, but also the tip of the red giant branch stars.[8][9]
Part of the diagram from ESA'sGaia. The dark line likely represents the transition from partly convective to fully convectivered dwarfs
ESA'sGaia mission showed several features in the diagram that were either not known or that were suspected to exist. It found a gap in the main sequence that appears forM-dwarfs and that is explained with the transition from a partly convective core to a fully convective core.[10][11] Forwhite dwarfs the diagram shows several features. Two main concentrations appear in this diagram following the cooling sequence of white dwarfs that are explained with the atmospheric composition of white dwarfs, especiallyhydrogen versushelium dominated atmospheres of white dwarfs.[12] A third concentration is explained with core crystallization of the white dwarfs interior. This releases energy and delays the cooling of white dwarfs.[13][14]
Contemplation of the diagram led astronomers to speculate that it might demonstratestellar evolution, the 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. Stars were thought therefore to radiate energy by converting gravitational energy into radiation through theKelvin–Helmholtz mechanism. This mechanism resulted in an age for the Sun of only tens of millions of years, creating a conflict over the age of the Solar System between astronomers, and biologists and geologists who had evidence that the Earth was far older than that. This conflict was only resolved in the 1930s when nuclear fusion was identified as the source of stellar energy.
Following Russell's presentation of the diagram to a meeting of theRoyal Astronomical Society in 1912,Arthur Eddington was inspired to use it as a basis for developing ideas onstellar physics. In 1926, in his bookThe Internal Constitution of the Stars he explained the physics of how stars fit on the diagram.[15] The paper anticipated the later discovery ofnuclear fusion and correctly proposed that the star's source of power was the combination of hydrogen into helium, liberating enormous energy. This was a particularly remarkable intuitive leap, since at that time the source of a star's energy was still unknown,thermonuclear energy had not been proven to exist, and even that stars are largely composed ofhydrogen (seemetallicity), had not yet been discovered. Eddington managed to sidestep this problem by concentrating on thethermodynamics ofradiative transport of energy in stellar interiors.[16] 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, came an evidence-backed theory of evolution to red giants following which were speculated cases of explosion and implosion of the remnants to white dwarfs. The termsupernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a pre-supernova star, a concept put forth byFred Hoyle in 1954.[17] The pure mathematicalquantum mechanics and classical mechanical models of stellar processes enable the Hertzsprung–Russell diagram to be annotated with known conventional paths known as stellar sequences—there continue to be added rarer and more anomalous examples as more stars are analysed and mathematical models considered.
^A.C. Maury; E.C. Pickering (1897). "Spectra of bright stars photographed with the 11-inch Draper Telescope as part of the Henry Draper Memorial".Annals of Harvard College Observatory.28:1–128.Bibcode:1897AnHar..28....1M.
^Vandenberg, D. A.; Brogaard, K.; Leaman, R.; Casagrande, L. (2013). "The Ages of 95 Globular Clusters as Determined Using an Improved Method Along with Color-Magnitude Diagram Constraints, and Their Implications for Broader Issues".The Astrophysical Journal.775 (2): 134.arXiv:1308.2257.Bibcode:2013ApJ...775..134V.doi:10.1088/0004-637X/775/2/134.S2CID117065283.
^abHertzsprung, E., 1911, Uber die Verwendung Photographischer Effektiver Wellenlaengen zur Bestimmung von Farbenaequivalenten, Publikationen des Astrophysikalischen Observatoriums zu Potsdam, 22. Bd., 1. Stuck = Nr.63 Hertzsprung, E. (1911). "On the Use of Photographic Effective Wavelengths for the Determination of Color Equivalents".Publications of the Astrophysical Observatory in Potsdam. 1.22 (63).
^Russell, Henry Norris (1914). "Relations Between the Spectra and Other Characteristics of the Stars".Popular Astronomy.22:275–294.Bibcode:1914PA.....22..275R.
^Palma, Christopher (2016)."The Hertzsprung-Russell Diagram".ASTRO 801: Planets, Stars, Galaxies, and the Universe. John A. Dutton e-Education Institute: College of Earth and Mineral Sciences: The Pennsylvania State University. Retrieved2017-01-29.The quantities that are easiest to measure... are color and magnitude, so most observers ... refer to the diagram as a 'Color–Magnitude diagram' or 'CMD' rather than an HR diagram.
^Tremblay, Pier-Emmanuel; Fontaine, Gilles; Fusillo, Nicola Pietro Gentile; Dunlap, Bart H.; Gänsicke, Boris T.; Hollands, Mark A.; Hermes, J. J.; Marsh, Thomas R.; Cukanovaite, Elena; Cunningham, Tim (January 2019). "Core crystallization and pile-up in the cooling sequence of evolving white dwarfs".Nature.565 (7738):202–205.arXiv:1908.00370.Bibcode:2019Natur.565..202T.doi:10.1038/s41586-018-0791-x.ISSN0028-0836.PMID30626942.S2CID58004893.
^Hoyle, F. (1954). "On Nuclear Reactions Occurring in Very Hot Stars. I. the Synthesis of Elements from Carbon to Nickel".Astrophysical Journal Supplement.1: 121.Bibcode:1954ApJS....1..121H.doi:10.1086/190005.
