Hartmann Discovers the First Evidence of Interstellar Matter

Johannes Franz Hartmann discovered the first indications that matter is present between the stars.

Summary of Event

Astronomy changed in the late nineteenth and early twentieth centuries. Rather than studying only the motions and positions of heavenly bodies, astronomers utilized new tools that enabled them to learn about the physical composition of the heavens, from stars to Earth’s galaxy itself. Johannes Franz Hartmann found the first evidence that the Milky Way galaxy Milky Way galaxy contains diffuse interstellar matter as well as the stars and planets, which form discrete objects. The discovery that matter exists between the stars had implications for many areas in astronomy, from the debate over the nature of the Milky Way to the question of the beginnings of the solar system. The work of many astronomers contributed to the knowledge of interstellar matter. Interstellar matter
Astronomy;interstellar matter
[kw]Hartmann Discovers the First Evidence of Interstellar Matter (1904)
[kw]Interstellar Matter, Hartmann Discovers the First Evidence of (1904)
Interstellar matter
Astronomy;interstellar matter
[g]Germany;1904: Hartmann Discovers the First Evidence of Interstellar Matter[00900]
[c]Science and technology;1904: Hartmann Discovers the First Evidence of Interstellar Matter[00900]
[c]Astronomy;1904: Hartmann Discovers the First Evidence of Interstellar Matter[00900]
Hartmann, Johannes Franz
Deslandres, Henri-Alexandre
Barnard, Edward Emerson
Slipher, Vesto Melvin
Frost, Edwin Brant

Spectroscopy Spectroscopy
Astronomy;spectroscopy was among the new tools that were important to astronomy in this time period. The spectroscope is an instrument that breaks starlight down into its component parts (its spectrum) by passing the light through a prism or reflecting it from a finely ruled grating. Sir Isaac Newton Newton, Sir Isaac (1642-1727) was the first to realize that white light is composed of light of many colors, or wavelengths, which blend together and appear white. Astronomers can use the colors and dark bands of an object’s spectrum to determine its chemical composition and many other properties. In particular, the spectrum of a star Stars;spectra reveals its velocity along a line of sight to the observer (its radial velocity) and the velocity with which it turns on its axis (its rotational velocity).

The dark lines appearing in a star’s spectrum reveal the presence of particular elements, each of which produces its own distinctive pattern of lines at particular wavelengths in the spectrum. The atoms of each element can absorb light at several specific wavelengths only, and as light from a radiating object passes through a gas, the atoms of various elements in the gas absorb light at their own peculiar wavelengths. This absorption of light causes the dark bands, and once the wavelength at which a band occurs is measured, the element responsible for the band can be determined. Thus it is possible to identify the elements in a star’s atmosphere or in any other cloud of gas intervening between the observer and the rest of the star.

In 1900, Henri-Alexandre Deslandres discovered that lines in the spectrum of the star Theta Orionis were undergoing rapid changes in their positions in relation to the rest of the spectrum. Hartmann followed up this observation at the Potsdam Astrophysical Observatory, where state-of-the-art spectrographs were well suited for studying this phenomenon. Hartmann found changes in position of spectral lines for the star Delta Orionis. Delta Orionis (star) He found slow shifts, however, rather than the rapid shifts described by Deslandres. He also discovered that some of the lines did not shift in the same way as the rest of the lines, but instead demonstrated a different type of motion relative to the motion of the other spectral lines.

The reason for the motion of all the spectral lines turned out to be the Doppler effect. Doppler effect As an object that is emitting waves (for example, light waves) approaches an observer, the waves appear bunched together, and the wavelength appears shorter than it really is (light appears to have shorter wavelengths and looks bluer; sounds seem to be higher pitched). Conversely, as a wave-emitting object recedes from an observer, the waves appear stretched out and the wavelength appears longer than it really is (light appears to have longer wavelengths and looks redder; sounds seem to be lower pitched). The amount by which the light or sound is shifted in wavelength is related to the velocity of the moving object. This effect, which can appear to shift the lines in a star’s spectrum either redward or blueward, is the tool astronomers use to measure the velocity of an individual star along the line of sight to the star (its radial velocity).

Some double-star systems, such as Delta Orionis, appear edge-on or partially edge-on in the sky, and thus the stars in these systems, as they orbit one another, are moving alternately toward and away from Earth. This motion results in Doppler shifts Doppler shifts alternately toward the redward and the blueward, such as those observed by Hartmann. Antonia Maury Maury, Antonia at Harvard College Observatory Harvard College Observatory later used this information to identify as double stars Double stars
Stars;double those that display the Doppler shifts but visually do not appear double.

The motion of the double stars explained the type of shift (redward and then blueward) observed for most of the spectral lines in Delta Orionis. It did not, however, explain the lines that had a different Doppler motion, one that did not move the lines from redward to blueward and back but moved the lines in the same direction and by the same amount in a constant manner. Hartmann called these lines stationary because they must be caused by matter that is stationary relative to the rest of the double-star system. The obvious explanation was that the lines were caused by a massive component of the double-star system that was so heavy relative to the other components that, for practical purposes, it could be considered the center of mass around which the other stars revolved; at the same time, it remained essentially motionless with respect to the rest of the system.

The unchanging Doppler shift would represent, then, the motion of the entire system with respect to Earth. Hartmann carefully ruled out this explanation, however, as well as the possibility that Earth’s atmosphere was responsible for producing the lines. The possibility remained that an immobile cloud of matter was producing the lines. Because the lines were of the appropriate wavelength for the element calcium, Hartmann concluded that a cloud of calcium gas was causing the lines. He was not sure where the calcium was located, however. He believed it could be situated somewhere between Earth and the double-star system and unconnected with the double stars, or it could be part of the double-star system. Other astronomers addressed and resolved these questions years after Hartmann’s original discovery.

Hartmann had also observed stationary spectral lines in the spectrum of Nova Persei, a nova that occurred in 1901. He suggested that perhaps these lines were the product of clouds of matter that bore some relation to the dark clouds that Edward Emerson Barnard had observed in photographs of the Milky Way. Barnard experienced many years of doubt and confusion before he came to the conclusion finally that the dark spots he saw in his photographs indicated the presence of cold dark matter that blocked out the light of stars behind them, rather than merely gaps in the sky where no stars were to be found.

Barnard’s work was a long chapter in the discovery of interstellar matter; Hartmann’s discovery of stationary lines provided the first direct evidence for clouds of gaseous matter independent of any stars or star systems. Today, several areas of astronomy are aided by knowledge of gaseous and particulate interstellar matter and the ramifications of this matter. Unfortunately, the importance of Hartmann’s work essentially went unrecognized for many years.


Hartmann’s discovery of the stationary lines was only one step in a long story. Vesto Melvin Slipher at the Naval Observatory in Flagstaff, Arizona, and Edwin Brant Frost at Yerkes Observatory in Wisconsin confirmed Hartmann’s work. Slipher made more observations of Delta Orionis and was the first to suggest that it was truly interstellar matter that was responsible for the lines. (Hartmann’s supposition had been that the matter was associated with the double-star system, although not linked to it.) In 1909, Frost observed other stars with spectra that also displayed stationary lines.

Some astronomers disagreed with Hartmann, Slipher, and Frost about the nature of the lines and used several observations to argue that the lines were, in fact, connected with the stars in question rather than interstellar in nature. Reynold Kenneth Young Young, Reynold Kenneth noted in 1920 that stationary lines appeared only in the spectra of relatively young stars and argued from this relationship between stellar type and presence of the lines that there must be some connection between the star and the calcium gas causing the lines. Also, Oliver Justin Lee Lee, Oliver Justin presented a doctoral dissertation at the University of Chicago on the orbit of a double star in which he interpreted some of his data as evidence that the calcium gas was connected with the double-star system and could, in fact, explain some peculiarities in the way the velocities of the stars changed over time. This seemed to indicate that the lines were stellar rather than interstellar in origin.

In the late 1920’s, Sir Arthur Stanley Eddington Eddington, Arthur Stanley and Otto Struve Struve, Otto presented work that finally settled the nature of the stationary lines and resolved the question of the existence of interstellar matter. Several papers that Struve wrote on the calcium lines in stellar spectra stimulated much discussion and led to the acceptance of interstellar matter as the most satisfactory explanation for the stationary lines. Eddington explained why the lines appeared only in relatively young stars by showing that older stars have spectra that would not reveal the presence of the stationary lines easily, even when they were present. Eddington ignored Lee’s work, as no one had been able to confirm his findings.

More than twenty years after Hartmann’s original discovery, astronomers accepted the existence of clouds of interstellar calcium. The presence of interstellar matter had ramifications for various studies in astronomy. For example, when interstellar matter appears in the form of dust, it absorbs some of the light from stars and makes stars appear dimmer (and more distant) than they really are. This knowledge affected the distance scale astronomers devised for the universe. Also, interstellar matter plays an important role in theories of how stars form because it provides the material for star formation. Because these clouds move around the galactic center, their motion, when measured, can give indications of the speed and direction of galactic rotation. Thus Hartmann’s seminal discovery had far-reaching effects on the science of astronomy. Interstellar matter
Astronomy;interstellar matter

Further Reading

  • Asimov, Isaac.“Stellar Evolution.” In The Universe from Flat Earth to Quasar. Rev. ed. New York: Walker, 1971. Concise summary of interstellar matter’s detection and implications for theories of star formation, presented in Asimov’s enjoyable style. Clearly explains Hartmann’s work in the context of stellar formation from interstellar gas and dust. Includes diagram and list of suggested readings.
  • Berendzen, Richard, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. New York: Columbia University Press, 1984. Focuses on early twentieth century discoveries about the nature of the Milky Way and other galaxies. Presents some information on Hartmann’s work, set in the context of galactic studies at the time, as well as the work done by astronomers who followed up on Hartmann’s original discovery. Uses original documents to help bring the events described to life. Includes photographs, drawings, and graphs.
  • Mitton, Simon, ed. The Cambridge Encyclopædia of Astronomy. New York: Crown, 1977. Several chapters of this standard handbook on astronomy discuss interstellar matter, both gaseous and dusty, its detection, and its role in star formation. Explains how absorption lines (such as the stationary lines) are formed. Includes many drawings, charts, and photographs, as well as an appendix containing brief explanations of some of the physical concepts discussed in the text.
  • Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Cowritten by Struve, who participated in some of the events connected with Hartmann’s work. Contains a chapter on interstellar matter and how it was discovered, including Hartmann’s work. One of the best sources for material on the history of twentieth century astronomy. Includes photographs (some of spectrographs in which “stationary lines” appear), graphs, drawings, and bibliography.
  • Verschuur, Gerrit L. Interstellar Matters: Essays on Curiosity and Astronomical Discovery. New York: Springer-Verlag, 1989. Excellent, well-written history of the circuitous path astronomers took to arrive at the present knowledge of interstellar matter, gaseous and dusty. Presents details about Hartmann’s work and how it was interpreted by other astronomers, as well as about many other astronomers’ contributions to the story of interstellar matter. Includes photographs, graphs, and drawings.
  • Zeilik, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Monterey, Calif.: Brooks/Cole, 1997. This introductory text provides a useful overview of general astronomy, including basic spectral issues.

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