HR Diagram

Ejnar Hetrzsprung and Henry Norris Russell independently made an interesting observation in 1912 when they attempted to correlate the spectral class of a star with its absolute magnitude. They discovered that 90% of the stars in the Universe fall within a narrow band called "The Main Sequence." They noticed that as a star's surface temperature increased, so does the absolute magnitude. Bluish stars shine brightly, and reddish stars shine more dimly. To put thing into perspective, consider that to light our sky to the current level with a low end M class star, we would have to be 1000 times closer than we presently are to the Sun, or a mere 150,000 km. At the high end of the O class, we would have to be 200 times farther away than we are now, or at a distance 5 times greater than that of Pluto. This relationship is shown in the chart below, and is called the HR Diagram. It is THE most important diagram in all of Astronomy for it holds the clues to the evolution of all stars, including our own.


The bottom of the chart shows the spectral classes with their corresponding surface temperatures. The right hand Y axis reveals the Absolute Magnitudes, ascending from the very dim +15 to the exceedingly bright -10. The left Y axis reveals the Luminosity. A star's luminosity depends on two things; the temperature at the surface and also the radius. The surface temperature determines only the amount of energy radiated per unit area, as well as the spectral color. But the more square meters of surface area, the brighter the star itself will be. There is a relationship between Temperature and Radius such that if we can know the Luminosity and Spectral Class discerned Temperature, we can find the radius. To know the Luminosity, one must take into account that stars shine in all parts of the Electromagnetic Spectrum, so we must factor in the ultraviolet and infrared radiation as well as the visual, which is possible with the proper telescope devices.


From this simple diagram, astronomers have been able to deduce much of the lifecycles of stars, including our very own, and this is the subject of the next section in this course.


Although temperature reigns supreme in defining the spectrum of a star, the density of the gas in the region where the absorption lines are formed plays a role too. Giant and supergiant stars are so large that the densities in their outer regions are low, which subtly changes the appearance of the stellar spectrum. For example, the hydrogen lines are quite broad in main sequence stars as a result of the disturbance of the hydrogen atoms caused by collisions. In the huge distended supergiants, however, lower density leads to lowered collision rates, and as a result the hydrogen lines are narrow. In K-type giants, the dark bands of the CN (cyanogen) molecule are stronger than they are in class K main sequence stars. Each spectral class in fact has its own set of criteria. As a result, once we know what these criteria are, we can tell if a star is a giant, supergiant, or of any other category, from its spectrum alone. Roman numerals are used to indicate size and luminosity, "I" for supergiants, "II" for bright giants, "III" for giants, "IV" for "subgiants" (stars that are developing into giants), and "V" for the main sequence. The result is the "MKK class" of the star, named after the 1940s developers of this system, W. W. Morgan, P. C. Keenan, and E. Kellman. Vega is an A0 V star, Polaris is F7 I or II, and Aldebaran is K5 III. The Sun is a G2 V star. White dwarfs are just called "white dwarfs," or "D."

Mass Luminosity

Based on many years of painstaking observations to deduce the masses of stars through observations of binary star systems and observations (distance determinations) to find the luminosities of stars, a Mass-Radius Relationship for Main Sequence stars has been deduced. The obvious point from the Mass-Luminosity relationship is that the more massive a star the more luminous is the star, as seen in the image above and left. Roughly speaking, we have that:

L/L(Sun) ~ [M/M(Sun)]^3.5

The above exponent is just an estimate. The luminosity of a star is thus a very strong function of its mass. Using the Mass-Luminosity relation, we discover that the least massive stars are at the lower right hand part of the Main Sequence and that the most massive stars are at the upper left hand part of the Main Sequence. The Sun, a G2V star sits around the middle of the Main Sequence, seen in the image above and right.

New Spectral Classes

Recently, astronomers have added three spectral classes to the X-axis of the HR Diagram, now including T stars, L stars, and most recently D stars. Only the main sequence runs through all the spectral classes, OBAFGKMLT. There are no giants, subgiants, or supergiants of classes L and T, both of which contain only low mass dwarfs and brown dwarfs that are insufficiently massive (below 0.08 solar mass) and too cool inside to run full hydrogen fusion. Class L is a mixture of real dwarfs and brown dwarfs, while class T consists entirely of brown dwarfs. The new comparison of stars, spectra, and temperature are in the chart below:

ionized and neutral helium, weakened hydrogen
31,000-49,000 K
neutral helium, stronger hydrogen
10,000-31,000 K
strong hydrogen, ionized metals
7400-10,000 K
weaker hydrogen, ionized metals
6000-7400 K
still weaker hydrogen, ionized and neutral metals
5300-6000 K
weak hydrogen, neutral metals
3900-5300 K
little or no hydrogen, neutral metals, molecules
2200-3900 K
no hydrogen, metallic hydrides, alkalai metals
1200-2200 K
methane bands
under 1200 K

This HR Diagram may help show where all the classes are a bit more readily. Notice how the Main Sequence is pushed over toward the left of the diagram, giving more space to the right where the T and L classes are. Although they are not labeled such on this diagram, Jupiters are considered a type of Brown Dwarf.




The classic spectral sequence is illustrated above by the spectra of real stars in a historic image published in 1901. The strong lines in class A (here, the star Sirius) are hydrogen. Neutral helium appears along with hydrogen in class B (Alnilam, Epsilon Orionis), while ionized helium is strong in class O (Naos, Zeta Puppis), the hydrogen lines nearly gone. Hydrogen weakens downward too, toward lower temperature, nearly disappearing by class M2 (Betelgeuse). The strong lines to the left in classes F (Canopus), G (Capella), and K (Arcturus) are those of ionized calcium. The other lines in these cooler classes are those of other metals. At the bottom, in class M7 (the long-period variable star Mira), we see bands of absorption produced by the titanium oxide molecule. Annals of the Harvard College Observatory, vol. 23, 1901, and copied from James Kahler, whose books I have on my shelf :)

One more look at the HR Diagram ... it is, afterall, very important:

Okay, so now it is your turn. Go to CalBerkeley's Astronomy Webpages. There is an interactive page on where you can learn more about the HR Diagram and even take a little quiz... its pretty cool!

Please move forward now to Sun Lifecycle, or return to Sun Introduction, or to the Syllabus.

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