Hertzsprung-Russell Diagram
Hertzsprung-Russell Diagram
Stellar classification and the HR diagram
The HR diagram and stellar evolution
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 star’s atmosphere. They are called absorption lines because they appear 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 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 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 (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). 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 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 V—the long, narrow strip running diagonally across the diagram—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 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 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.
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 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 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
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.