Hertzsprung–Russell diagram

The Hertzsprung–Russell diagram (also called an H–R diagram, HR diagram, or HRD) is a scatter plot that shows how stars’ absolute magnitudes or luminosities relate to their stellar classifications or effective temperatures. It is sometimes referred to as a color–magnitude diagram. The diagram was created independently by Ejnar Hertzsprung in 1911 and by Henry Norris Russell in 1913, and it marked a major advance in understanding how stars evolve.

In the late 1800s, large-scale photographic spectroscopic surveys at Harvard College Observatory produced spectral classifications for tens of thousands of stars, culminating in the Henry Draper Catalogue. Antonia Maury, in one part of this work, divided stars by the width of their spectral lines. Hertzsprung noticed that stars with narrow lines tended to have smaller proper motions than others of the same spectral type, which he interpreted as a sign of greater luminosity. He then computed secular parallaxes for several groups of these stars to estimate their absolute magnitudes.

In 1910, Hans Oswald Rosenberg published a diagram that plotted the apparent magnitudes of stars in the Pleiades cluster against the strengths of the calcium K line and two hydrogen Balmer lines. These spectral lines acted as a proxy for temperature, an early form of spectral classification. Because all stars in a cluster are at roughly the same distance, their apparent magnitudes are equivalent to their absolute magnitudes, so this early diagram was essentially a plot of luminosity against temperature. This type of diagram is still used today to show stars in clusters without needing to know their distances and luminosities in advance. Hertzsprung had already been working with such diagrams, but his first publications showing them appeared in 1911. Russell’s early versions from 1913 included Maury’s giant stars identified by Hertzsprung, nearby stars with measured parallaxes, stars from the Hyades cluster, and several moving groups, for which the moving cluster method gave distances and absolute magnitudes.

There are several forms of the Hertzsprung–Russell diagram, and the terminology is not strictly defined. All forms share the same basic layout: more luminous stars are placed toward the top, and stars with higher surface temperatures are toward the left. The original diagram had spectral type on the horizontal axis and absolute visual magnitude on the vertical axis. Spectral type is not a numerical value, but the sequence of types is a monotonic series that reflects surface temperature. Modern observational versions replace spectral type with a color index (often B-V in mid-20th-century diagrams). This is called an observational Hertzsprung–Russell diagram, or specifically a color–magnitude diagram (CMD), and is commonly used by observers. For stars known to be at the same distance, such as in a cluster, a CMD plots apparent magnitude on the vertical axis, since the difference between apparent and absolute magnitude—the distance modulus—is the same for all cluster members. Early studies of nearby open clusters like the Hyades and Pleiades by Hertzsprung and Rosenberg produced the first CMDs, a few years before Russell’s synthesis of data for all stars with known absolute magnitudes.

Another form of the diagram plots effective surface temperature on one axis and luminosity on the other, almost always on a log-log scale. Theoretical calculations of stellar structure and evolution produce plots that match observations. This type is sometimes called a theoretical Hertzsprung–Russell diagram, though the term “temperature-luminosity diagram” is rarely used. A peculiar feature of this form is that temperatures are plotted from high to low, which helps when comparing it to the observational form.

Although the two types of diagrams are similar, astronomers make a sharp distinction between them. The exact transformation from one to the other is not straightforward. Converting effective temperature to color requires a color–temperature relation, which is difficult to construct and depends on stellar composition and factors like rotation. Converting luminosity or absolute bolometric magnitude to apparent or absolute visual magnitude requires a bolometric correction, which may come from a different source than the color–temperature relation. One also needs the distance to the observed objects (the distance modulus) and must account for interstellar obscuration, which affects both color (reddening) and apparent magnitude (extinction). Color distortion and extinction also appear in stars with significant circumstellar dust. These complications add uncertainty to direct comparisons between theoretical predictions of stellar evolution and observations.