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Hertzsprung-Russell diagram

Hertzsprung-Russell diagram [for Ejnar Hertzsprung and H. N. Russell], graph showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude, increases upwards on the vertical axis; the temperature (or some temperature-dependent characteristic such as spectral class or color) decreases to the right on the horizontal axis. It is found that the majority of stars lie on a diagonal band that extends from hot stars of high luminosity in the upper left corner to cool stars of low luminosity in the lower right corner. This band is called the main sequence. Stars called white dwarfs lie sparsely scattered in the lower left corner. The giant stars—stars of great luminosity and size (see red giant)—form a thick, approximately horizontal band that joins the main sequence near the middle of the diagonal band. Above the giant stars, there is another sparse horizontal band consisting of the supergiant stars. The stars in the lower right corner of the main sequence are frequently called red dwarfs, and the stars between the main sequence and the giant branch are called subgiants. The significance of the H-R diagram is that stars are concentrated in certain distinct regions instead of being distributed at random. This regularity is an indication that definite laws govern stellar structure and stellar evolution. In population I regions (see stellar populations) like the spiral arms of galaxies or open star clusters, the stars fall almost exclusively on the main sequence. In population II regions like the nuclei of galaxies and globular clusters, the stars are older and have evolved significantly. The most luminous stars have evolved furthest, and an H-R diagram of such a region will show the upper end of the main sequence depopulated and will show a well-developed giant branch. In such a diagram it appears that the main sequence has "burned down" from the top like a candle. Thus, the point at which the main sequence terminates and the giant branch begins is an indication of the age of a star cluster. A modified H-R diagram of the stars in a cluster of unknown distance can be used to determine the absolute magnitude, or luminosity, of the stars. Since the apparent magnitude of a star of given absolute magnitude depends only on the star's distance, the observed apparent magnitude of the stars can be used to calculate the distance to the cluster.

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Hertzsprung-Russell diagram

Hertzsprung-Russell diagram (H-R diagram) Plot of the absolute magnitude of stars against their spectral type; this is equivalent to plotting their luminosity against their surface temperature or colour index. Brightness increases from bottom to top, and temperature increases from right to left. Henry Norris Russell devised the diagram in 1913, independently of Ejnar Hertzsprung, who had the same idea some years before. The H-R diagram reveals a pattern in which most stars lie on a diagonal band, the main sequence.

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Hertzsprung-Russell Diagram

Hertzsprung-Russell Diagram

Stellar classification and the HR diagram

The nature of the HR diagram

The main sequence

Giant stars

The HR diagram and stellar evolution

Resources

A Hertzsprung-Russell diagram, or HR diagram, is a graph of stellar temperatures (plotted on the horizontal axis) and luminosities or brightnesses (plotted on the vertical axis). HR diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an HR diagram for a group of stars, an astronomer can make estimates of many important stellar properties including diameter, mass, age, and evolutionary state. Our understanding of the processes at work in the stars depends on knowing these parameters, so HR diagrams have been essential tools in twentieth-century astronomical research.

Stellar classification and the HR diagram

The nineteenth century saw the development of a powerful technique called spectroscopy. This technique involves the use of an instrument called a spectrograph, which disperses light passing through it into its component colors in the same way that an ordinary prism does. Indeed, many spectrographs in use today have prisms as one or more of their components.

When sunlight or starlight passes through a spectrograph and is dispersed, the resulting spectrum has many narrow, dark lines called absorption lines in it. These occur only at certain wavelengths and are caused by the presence of specific elements in the stars atmosphere. They are called absorption lines because they appear when elements in the stars atmosphere absorb some of the light radiating outward from the stars surface. Less light escapes from the stars atmosphere where there is a line than in other portions of the spectrum, so the line looks dark.

Different stars have different absorption line patterns, and the pattern present in a particular star depends on the its surface temperature. For example, hydrogen, the most common element in stars, produces several very strong absorption lines in the visual

part of the spectrumbut only if the stars temperature is about 10,000K (17,541°F [9,727°C]). If the star is much hotter, say 20,000K (35,541°F [19,727°C]), the hydrogen atoms can no longer absorb as much light in the visual spectrum, so the lines are weaker. Very cool stars also have weaker hydrogen lines.

In the early 1900s, a group of astronomers led by Annie Jump Cannon (1863-1941) at the Harvard Observatory began to classify stellar spectra. They grouped stars into spectral classes, with all the stars in a given spectral class having similar line patterns, just as biologists classify animals into groups such as families and species. Spectral classes are denoted by letters, and the main ones, in order of decreasing surface temperature, are O, B, A, F, G, K, and M. You can remember this by the mnemonic Oh Be A Fine Girl (or Guy), Kiss Me! Because stars have many elements in their atmospheres (hydrogen, helium, calcium, sodium, and iron, to name only a few), their spectra can have thousands of lines. To accommodate this complexity, the spectral classes are each divided into 10 subclasses, denoted by numbers. For example, there are F0 stars, F1 stars, and so on until F9; the next class is G0. The Sun, with a surface temperature of 5,800K (9,981°F [5,527°C]), is a G2 star.

The first HR diagrams were created independently in the early 1900s by the astronomers Ejnar Hertzsprung (1873-1967) and Henry Norris Russell (1877-1957). Russells graph had spectral class plotted along the x-axis and a quantity related to luminosity (or brightness) plotted along the y-axis. Figure 1 shows such a graph.

The nature of the HR diagram

Figure 1 shows the important features of the HR diagram. The stars fall into several relatively narrow strips, denoted by Roman numerals, which W. W. Morgan, another famous classifier of stellar spectra, called luminosity classes.

The main sequence

Luminosity class Vthe long, narrow strip running diagonally across the diagramis called the main sequence. The Sun lies on the main sequence, as do 90% of all stars. Stars on the main sequence are stable and healthy, shining as a result of nuclear fusion reactions in their cores that convert their hydrogen to helium. Stars spend most of their lives on the main sequence, so it is not surprising that most are found there.

The main sequence slopes from upper left to lower right on the HR diagram. Therefore, the hotter main sequence stars are, the brighter they are. Main sequence O stars, or O V stars (using the luminosity class numeral), are extremely hot and blaze away with the brightness of 10,000 or more suns. At the other end of the main sequence are the little M V stars, shining with a dull glow, only 1% as bright as the sun.

For main sequence stars, there are also relationships between surface temperature, radius, mass, and lifetime. Hotter main sequence stars are both larger (greater radius) and more massive than cooler ones. So not only are O V stars brighter than the sun, they are also physically larger and may be 20 or more times as massive. M V stars may be only a tenth as massive as the sun. However, the brilliant O stars have to consume their hydrogen fuel thousands of times faster than their cooler cousins. Therefore, they live for a very short timeno more than a few million years while stars like the sun may remain on the main sequence for 10 billion years. And the tiny, faint M stars, though not very impressive, will remain shining faintly on for hundreds of billions of years.

Giant stars

Main sequence stars are, by definition, normal. The other luminosity classes, of which the main ones are III and I, contain stars that are very different.

Class III stars are fairly cool since they lie near the right side of the HR diagram. But they are also much brighter than any normal K and M star should be perhaps 100 times as luminous as the sun. We know that luminosity depends on temperature. Normally cool stars would not be as bright as hot stars, just as a glowing ember in a campfire gradually gets dimmer as it cools. However, luminosity also depends on the size of an object. Imagine a glowing ember the size of a marble and another one, equally hot, the size of a beach ball. Clearly the larger one will be brighter, simply because there is more of it. Therefore, class III stars must be huge to be so bright and yet so cool.

For this reason, stars in luminosity class III are called giant stars. For example, Aldebaran, a bright K5 III star in the constellation Taurus (the Bull), has a diameter roughly 100 times greater than the suns. Aldebaran and many of the other bright but reddish stars you can see with the unaided eye are giants. If they were small main-sequence stars, they would be too faint to see.

Now consider luminosity class I, lying at the very top of the HR diagram. If red stars 100 times brighter than the Sun are large, red stars 10,000 times brighter must be monstrous indeed. And they are: Antares, the M1 I star in the constellation Scorpio (the Scorpion), is so large that astronomers have been able to measure its

KEY TERMS

Giant A star that has exhausted nearly all of its hydrogen fuel and is using heavier elements as fuel to sustain itself against its own gravity. The processes occurring in its interior have forced it to expand until it is 10 to 100 times the diameter of the Sun.

Luminosity The amount of energy a star emits in a given amount of time. More massive stars are more luminous less massive ones, and they do not live as stable stars for as long.

Luminosity class One of several well-defined bands of stars on the HR diagram. The main luminosity classes are denoted by the Roman numerals I, II, III, IV, and V, and stars belonging to them are called supergiants, bright giants, giants, subgiants, and dwarfs (or main sequence stars), respectively.

Main sequence The narrow strip of stars running from upper left to lower right on the HR diagram. Main sequence stars are those that are shining stabily and without any dramatic changes in their size or surface temperature. About 90% of all stars are main sequence stars, including the sun.

Spectral class A classification category containing stars with similar patterns of absorption lines in their spectra. The spectral classes are denoted by the letters O, B, A, F, G, K, M, and represent a temperature sequence. The hottest stars are type O, while the coolest are type M.

Supergiant A star of extraordinary size and luminosity, belonging to luminosity class I. These are massive stars (five to 30 times as massive as the sun) that have exhausted the hydrogen fuel in their cores and are burning heavier elements like helium and oxygen to sustain themselves.

Turn-off point The upper end of the main sequence in an HR diagram of a star cluster. Since more massive (hotter) stars evolve off the main sequence faster than less massive (cooler) ones, the turnoff point gradually moves down the main sequence as the cluster ages. The location of the turn-off point reveals the current age of the cluster.

diameter directly. Antares, it turns out, is about 400 times larger than the sun. If placed at the center of the solar system, Antares would extend past the orbit of Mars. All four inner planets, including Earth, would be swallowed in a 4,000K (6,741°F [3,727°C]) inferno. Stars like this are called supergiants, and Antares as well as hotter supergiants like Rigel (the foot of Orion, spectral type B8I) are among the largest, most luminous, and most massive stars in the galaxy.

The HR diagram and stellar evolution

One of the most important properties of the HR diagram is that it lets us trace the lives of stars. A ball of gas officially becomes a star at the moment that nuclear fusion reactions begin in its core, converting hydrogen to helium. At the point the star is a brand-new main sequence object, and lies at the lower boundary of the main sequence strip. Sensibly enough, this is called the zero-age main sequence, or ZAMS.

As a star ages, it gradually gets brighter. This means the star moves upward on the HR diagram, because it is getting more luminous. That is why the main sequence is a band and not just a line: different stars of a given spectral type are different ages and have slightly different luminosities.

When a star runs out of hydrogen, bizarre and fascinating things begin to happen. With its hydrogen nearly exhausted, the star begins fusion of heavier elements like helium, carbon, and oxygen to keep its interior furnace going. This causes the surface of the star to expand greatly, and it becomes very luminous, moving to the upper parts of the HR diagram.

Giant stars, therefore, are dying beasts. They are stars that have run out of hydrogen and are now burning heavier elements in their cores. Many giant stars are unstable and pulsate, while others shine so fiercely that matter streams away from them in a stellar wind. All these are important evolutionary states and occur in stars in specific parts of the HR diagram.

Nowhere is stellar evolution more dramatically illustrated than in a star cluster HR diagram. Clusters are large groups of stars that all formed at the same time. Figures 2 and 3 show the HR diagram for two clusters, the Pleiades and M67.

These are only a few of the ways in which HR diagrams reveal stars essential properties. The power and elegance of the HR diagram in improving our understanding of stars and how they evolve has made its invention one of the great advances in twentieth-century astronomy. More importantly, it demonstrates how careful classification, often considered mundane or even boring work, can reveal the beautiful patterns hidden in nature and reward humans with a clearer understanding of the universe of which they are such a small part.

See also Spectral classification of stars; Spectroscopy; Stellar magnitudes; Stellar wind.

Resources

BOOKS

Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.

Meadows, A.J. Stellar Evolution. 2nd ed. Oxford: Pergamon, 1978.

Shu, F. The Physical Universe: An Introduction to Astronomy. Chap 8-9. University Science Books, 1982.

OTHER

University of Utah: Astrophysics Science Project Integrating Research and Education (ASPIRE). Hertzsprung-Russell Diagram <http://sunshine.chpc.utah.edu/labs/star_life/hr_diagram.html> (October 7, 2006).

Jeffrey C. Hall

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Hertzsprung-Russell Diagram

Hertzsprung-Russell diagram

A Hertzsprung-Russell diagram, or H-R diagram, is a graph of stellar temperatures (plotted on the horizontal axis) and luminosities, or brightnesses (plotted on the vertical axis). H-R diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an H-R diagram for a group of stars, an astronomer can make estimates of many important stellar properties including diameter, mass , age, and evolutionary state. Our understanding of the processes at work in the stars depends on knowing these parameters, so H-R diagrams have been essential tools in twentieth-century astronomical research.

Stellar classification and the H-R diagram

The nineteenth century saw the development of a powerful technique called spectroscopy. This technique involves the use of an instrument called a spectrograph, which disperses light passing through it into its component colors in the same way that an ordinary prism does. Indeed, many spectrographs in use today have prisms as one or more of their components.

When sunlight or starlight passes through a spectrograph and is dispersed, the resulting spectrum has many narrow, dark lines in it. These lines are called absorption lines. A line occurs only at a certain wavelength and is caused by the presence of a specific element in the star's atmosphere. They are called absorption lines because they are caused when elements in the star's atmosphere absorb some of the light radiating outward from the star's surface. Less light escapes from the star's atmosphere where there is a line than in other portions of the spectrum, so the line looks dark.

Different stars have different patterns of absorption lines, and the pattern present in a particular star depends on the star's surface temperature . For example, hydrogen , the most common element in stars, produces several very strong absorption lines in the visual part of the spectrum—but only if the star's temperature is about 10,000K (17,541°F [9,727°C]). If the star is much hotter, say 20,000K (35,541°F [19,727°C]), the hydrogen atoms can no longer absorb as much light in the visual spectrum, so the lines are weaker. Very cool stars also have weaker hydrogen lines.

In the early 1900s, a group of astronomers led by Annie Jump Cannon at the Harvard Observatory began to classify stellar spectra. They grouped stars into spectral classes, with all the stars in a given spectral class having similar patterns of lines. This is just like the way that biologists classify animals into groups such as families and species . Spectral classes are denoted by letters, and the main ones, in order of decreasing surface temperature, are O, B, A, F, G, K, and M. You can remember this by the mnemonic "Oh Be A Fine Girl (or Guy), Kiss Me!" Because stars have many elements in their atmospheres (hydrogen, helium, calcium , sodium , and iron , to name only a few), their spectra can have thousands of lines. To accommodate this complexity, the spectral classes are each divided into 10 subclasses, denoted by numbers. For example, there are F0 stars, F1 stars, and so on until F9; the next class is G0. The Sun , with a surface temperature of 5,800K (9,981°F [5,527°C]), is a G2 star.

The first H-R diagrams were created independently in the early 1900s by the astronomers Ejnar Hertzsprung and Henry Norris Russell. Russell's graph had spectral class plotted along the x-axis and a quantity related to luminosity (or brightness) plotted along the y-axis. Figure 1 is such a graph.


The nature of the H-R diagram

Figure 1 shows all the important features of the H-R diagram. The stars fall into several relatively narrow strips which W. W. Morgan, another famous classifier of stellar spectra, called luminosity classes. Luminosity classes are denoted by Roman numerals.


The main sequence

Luminosity class V is the long, narrow strip running diagonally across the diagram, and it is called the main sequence. The Sun lies on the main sequence, as do 90% of all stars. Stars on the main sequence are stable and healthy, shining as a result of nuclear fusion reactions in their cores that convert their hydrogen to helium. Stars spend most of their lives on the main sequence, so it is not surprising that most stars are found there.

The main sequence slopes from upper left to lower right on the H-R diagram. Therefore, the hotter main sequence stars are, the brighter they are. Main sequence O stars, or O V stars (using the luminosity class numeral), are extremely hot and blaze away with the brightness of 10,000 or more Suns. At the other end of the main sequence are the little M V stars, shining with a dull glow, only 1% as bright as the Sun.

For main sequence stars, there are also relationships between surface temperature, radius, mass, and lifetime. Hotter main sequence stars are both larger (greater radius) and more massive than cooler ones. So not only are O V stars brighter than the Sun, they are also physically larger and may be 20 or more times as massive. M V stars may be only a tenth as massive as the Sun. However, the brilliant O stars have to consume their hydrogen fuel thousands of times faster than their cooler cousins. Therefore, they live for a very short time—no more than a few million years—while stars like the Sun may remain on the main sequence for 10 billion years. And the tiny, faint M stars, though not very impressive, will remain shining faintly on for hundreds of billions of years.


Giant stars

Main sequence stars are, by definition, normal. The other luminosity classes, of which the main ones are III and I, contain stars that are very different.

Consider class III stars. They are fairly cool since they lie near the right side of the H-R diagram. But they are also much brighter than any normal K and M star should be—perhaps 100 times as luminous as the Sun. We know that luminosity depends on temperature. Normally cool stars would not be as bright as hot stars, just as a glowing ember in a campfire gradually gets dimmer as it cools off. However, luminosity also depends on the size of an object. Imagine a glowing ember the size of a marble and another one, equally hot, the size of a beach ball. Clearly the larger one will be brighter, simply because there is more of it. Therefore, class III stars must be huge to be so bright and yet so cool.

For this reason, stars in luminosity class III are called giant stars. For example, Aldebaran, a bright K5 III star in the constellation Taurus (the Bull), has a diameter roughly 100 times greater than the Sun's. Aldebaran and many of the other bright but reddish stars you can see with the unaided eye are giants. If they were small main-sequence stars, they would be too faint to see.

Now consider luminosity class I, lying at the very top of the H-R diagram. If red stars 100 times brighter than the Sun are large, red stars 10,000 times brighter must be monstrous indeed. And they are: Antares, the M1 I star in the constellation Scorpio (the Scorpion), is so large that astronomers have been able to measure its diameter directly. Antares, it turns out, is about 400 times larger than the Sun. If placed at the center of the solar system , Antares would extend past the orbit of Mars . All four inner planets, including Earth , would be swallowed in a 4,000K (6,741°F [3,727°C]) inferno. Stars like this are called supergiants, and Antares as well as hotter supergiants like Rigel (the foot of Orion, spectral type B8 I) are among the largest, most luminous, and most massive stars in the galaxy .


The H-R diagram and stellar evolution

One of the most important properties of the H-R diagram is that it lets us trace the lives of the stars. A ball of gas officially becomes a star at the moment that nuclear fusion reactions begin in its core, converting hydrogen to helium. At the point the star is a brand-new main sequence object, and lies at the lower boundary of the main sequence strip. Sensibly enough, this is called the zero-age main sequence, or ZAMS.

As a star ages, it gradually gets brighter. This means the star moves upward on the H-R diagram, because it is getting more luminous. That is why the main sequence is a band and not just a line: different stars of a given spectral type are different ages and have slightly different luminosities.

When a star runs out of hydrogen, many bizarre and fascinating things begin to happen. With its hydrogen nearly exhausted, the star has to begin fusion of heavier elements like helium, carbon , and oxygen to keep its interior furnace going. This causes the surface of the star to expand greatly, and it becomes very luminous, moving to the upper parts of the H-R diagram.

Giant stars, therefore, are dying beasts. They are stars that have run out of hydrogen and are now burning heavier elements in their cores. Many giant stars are unstable and pulsate, while others shine so fiercely that matter streams away from them in a stellar wind. All these are important evolutionary states and occur in stars in specific parts of the H-R diagram.

Nowhere is stellar evolution more dramatically illustrated than in a star cluster H-R diagram. Clusters are large groups of stars that all formed at the same time. Figures 2 and 3 show the H-R diagram for two clusters, the Pleiades and M67.

These are only a few of the ways in which H-R diagrams reveal the essential properties of stars. The power and elegance of the H-R diagram in improving our understanding of stars and how they evolve has made its invention one of the great advances in twentieth-century astronomy . More importantly, it demonstrates how careful classification, often considered mundane or even boring work, can reveal the beautiful patterns hidden in nature and reward humans with a clearer understanding of the universe of which they are such a small part.

See also Spectral classification of stars; Spectroscopy; Stellar magnitudes; Stellar wind.

Resources

books

Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.

Meadows, A.J. Stellar Evolution. 2nd ed. Oxford: Pergamon, 1978.

Shu, F. The Physical Universe: An Introduction to Astronomy. Chap 8-9. University Science Books, 1982.


Jeffrey C. Hall

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Giant

—A star that has exhausted nearly all of its hydrogen fuel and is using heavier elements as fuel to sustain itself against its own gravity. The processes occurring in its interior have forced it to expand until it is 10 to 100 times the diameter of the Sun.

Luminosity

—The amount of energy a star emits in a given amount of time. More massive stars are more luminous less massive ones, and they do not live as stable stars for as long.

Luminosity class

—One of several well-defined bands of stars on the H-R diagram. The main luminosity classes are denoted by the Roman numerals I, II, III, IV, and V, and stars belonging to them are called supergiants, bright giants, giants, subgiants, and dwarfs (or main sequence stars), respectively.

Main sequence

—The narrow strip of stars running from upper left to lower right on the H-R diagram. Main sequence stars are those that are shining stabily and without any dramatic changes in their size or surface temperature. About 90% of all stars are main sequence stars, including the Sun.

Spectral class

—A classification category containing stars with similar patterns of absorption lines in their spectra. The spectral classes are denoted by the letters O, B, A, F, G, K, M, and represent a temperature sequence. The hottest stars are type O, while the coolest are type M.

Supergiant

—A star of extraordinary size and luminosity, belonging to luminosity class I. These are massive stars (five to 30 times as massive as the Sun) that have exhausted the hydrogen fuel in their cores and are burning heavier elements like helium and oxygen to sustain themselves.

Turn-off point

—The upper end of the main sequence in an H-R diagram of a star cluster. Since more massive (hotter) stars evolve off the main sequence faster than less massive (cooler) ones, the turnoff point gradually "moves down" the main sequence as the cluster ages. The location of the turn-off point reveals the current age of the cluster.

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