Ejnar Hertzsprung

Who Found Giants Among The Dwarfs · Astronomer & Engineer · 1873–1967

The photographic chemist who found that two stars of exactly the same spectral class can differ a thousandfold in brightness — so one of them must be enormous — and plotted the diagram that has been on every classroom wall since.

Who was Hertzsprung?

Ejnar Hertzsprung was born at Frederiksberg in 1873 and trained as a chemical engineer, working in photographic chemistry before turning to astronomy — which is why he trusted a photographic plate and understood its errors better than most astronomers of his generation. Between 1905 and 1907 he published, in a photographic journal that almost no astronomer read, the observation that stars of the same spectral class come in two radically different kinds: some stars of class K or M are intrinsically faint, while others of exactly the same class are thousands of times more luminous than the Sun. Since the spectral class fixes the surface temperature, and since a surface at a given temperature radiates a fixed amount from each unit of its area, a star far brighter at the same temperature can only be far larger. He named the two kinds giants and dwarfs. To make the comparison honest he turned to star clusters, where every member is at effectively the same distance, so that apparent brightnesses may be compared directly; plotted against spectral class, the members did not scatter but fell along a band, with a separate group of luminous stars off to one side. Henry Norris Russell arrived at the same relation independently in America and published it far more visibly in 1913, and the diagram carries both names. Hertzsprung also calibrated Henrietta Swan Leavitt's period–luminosity relation for Cepheid variables, giving the first real distance to the Small Magellanic Cloud, and directed the Leiden Observatory. He worked into his eighties.

Major achievements

• Discovered that stars of one spectral class divide into giants and dwarfs
• Reasoned from equal surface temperature and unequal brightness to enormously unequal size
• Plotted cluster members against spectral class, producing the diagram now named after him and Russell
• Calibrated Leavitt's Cepheid period–luminosity relation, giving the first distance to the Small Magellanic Cloud
• Made major advances in photographic photometry and its systematic errors

Life in brief

• 1873 — Born at Frederiksberg: Trained as a chemical engineer.
• 1905 — Giants and dwarfs: Published in a photographic journal almost nobody read.
• 1909 — Göttingen, then Potsdam: Brought out of obscurity by Schwarzschild.
• 1913 — The Cepheid calibration: The first real distance to the Small Magellanic Cloud.
• 1913 — Russell publishes the diagram: Independently, and far more visibly.
• 1919 — Leiden Observatory: Later its director.
• 1967 — Dies in Denmark: At ninety-four, having worked into his eighties.

Explore the life of Hertzsprung — five chapters

1. A Chemical Engineer Who Trusted a Photographic Plate — Copenhagen, photographic chemistry, and an amateur observatory
2. Giants and Dwarfs — Same colour, wildly different brightness
3. Use a Cluster — How to compare stars honestly
4. Calibrating the Cepheids — The first real distance to another galaxy
5. Leiden, and Ninety-Four Years — A self-effacing man and a diagram that outlasted everything

Read the five-chapter life of Ejnar Hertzsprung →

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