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Orion’s Belt is one of the highlights of winter nights. Right now it’s in good view in the east-southeast at nightfall, with its three stars standing straight up from the horizon.
The star at the top of the belt is Mintaka. That point of light actually consists of two individual stars, each of which is far bigger, brighter, and heavier than the Sun.
Those stars provided the background for an important discovery — that the space between stars is filled with a thin mixture of gas and dust. The discovery was made by Johannes Hartmann, a German astronomer who was born 150 years ago today.
Hartmann earned his PhD at the University of Leipzig, then moved to the Potsdam Observatory, where he used a spectrograph to the study the stars. This instrument breaks a star’s light into its individual wavelengths, producing a rainbow of colors. The chemical elements in the star’s outer atmosphere imprint patterns of dark lines against the rainbow.
Because Mintaka is a binary star, the lines in its spectrum move back and forth as the stars orbit each other. But Hartmann found that a line produced by calcium remained stationary. There was no way for the moving stars to produce a stationary line, so Hartmann realized that the calcium had to lie between Earth and Mintaka.
Hartmann’s discovery proved that the space between the stars isn’t empty — it’s filled with a thin mixture of atoms and molecules that’s known today as the interstellar medium.