Henry Norris Russell

Today, we celebrate the 140th anniversary of the birth of one of America’s greatest astrophysicists: Henry Norris Russell (1877-1957).  Called the “Dean of American Astronomers”, he is perhaps best remembered for his discovery of the relationship between the luminosity (absolute brightness) of a star and its color.  We call any plot of luminosity vs. color for a group of stars an H-R diagram, named after Russell and Danish astronomer Ejnar Hertzsprung (1873-1967) who independently discovered this relationship.

Russell noticed that cool (relative to other stars) red stars come in two varieties: those that are dim, and others that are very bright.  The only way a cool, red star could be so bright would be if the star were very, very large1.  In this way, Russell discovered that there are red giants and red dwarfs, but no medium-sized red stars.  Further studies by Russell and others led to the use of the H-R diagram as a tool in understanding the life cycles of stars.  Red giants, it turns out, are one of the final stages in the life of an ordinary star (like the Sun, for example).  Red dwarfs are low-mass stars that change very little throughout their lives.

After famously rejecting the revolutionary conclusion (in 1925) by Cecilia Payne-Gaposchkin (1900-1979) establishing that hydrogen is the primary constituent of the Sun and other stars, Henry Russell concluded four years later that Payne-Gaposchkin was correct, and acknowledged her significant contribution.  Moreover, he surmised that the main physical characteristics of stars are determined by just two basic parameters: mass and chemical composition.  This idea is known as the Vogt-Russell theorem, named after Russell and German astronomer Heinrich Vogt (1890-1968), who independently came up with the same idea.

An interesting sidenote.  Early in his stellar career, when he was just 24 years of age, Henry Russell wrote an interesting article published in the May 1902 issue of Popular Astronomy and dated March 24, 1902: “Shadows Cast by Starlight”.  It is a fascinating read—all the more special because it was written at a time (now over 115 years ago) when light pollution had not yet destroyed our nocturnal environment.

1Here we are comparing stars at comparable distances, such as in a star cluster.

Eugène Delporte and the Constellation Jigsaw

Belgian astronomer Eugène Joseph Delporte (1882-1955) discovered 66 asteroids from 1925 to 1942, but he is best remembered for determining the official boundaries of the 88 constellations, work he completed in 1928 and published in 1930.  The constellation boundaries have remained unchanged since then.

The International Astronomical Union (IAU), founded, incidentally, in Brussels, Belgium in 1919, established the number of constellations at 88—the same number, coincidentally, as the keys on a piano—in 1922 under the guidance of American astronomer Henry Norris Russell (1877-1957).  The IAU officially adopted Delporte’s constellation boundaries in 1928.

All the constellation boundaries lie along lines of constant right ascension and declination—as they existed in the year 1875. Why 1875 and not 1900, 1925, or 1930? American astronomer Benjamin Gould (1824-1896) had already drawn up southern constellation boundaries for epoch 1875, and Delporte built upon Gould’s earlier work.

As the direction of the Earth’s polar axis slowly changes due to precession, the constellation boundaries gradually tilt so that they no longer fall upon lines of constant right ascension and declination. Eventually, the tilt of the constellation boundaries will become large enough that the boundaries will probably be redefined to line up with the equatorial coordinate grid for some future epoch. When that happens, some borderline stars will move into an adjacent constellation. Even now, every year some stars change constellations because proper motion causes them to move across a constellation boundary. For faint stars, this happens frequently, but for bright stars such a constellation switch is exceedingly rare.