188
9 Mount Wilson
Fig. 9.13 An image of a small area in Orion, just below the leftmost star in the belt (see
Fig. 7.11). In this picture the north is to the left, so it is rotated by 90 ◦ anticlockwise.
To the left of the middle the so-called Horsehead Nebula. This is a cloud of small dust
particles that absorbs the light of the bright nebula behind it, which is hot gas like the
Orion Nebula in Fig. 9.11. Space Telescope Science Institute [84]
9.5 Interstellar Extinction
None of this was known to Kapteyn and his contemporaries. Kapteyn wrote
two very important papers on this subject, which he published in 1909. Because of his association with the Mount Wilson Observatory and the Carnegie
Institution, he had begun to publish mostly in English; if it did not concern
publications with many tables and observation data (which he reserved for the
Publications of the Astronomical Laboratory at Groningen), he mainly submitted
his papers to American journals such as the Astronomical Journal and especially
the Astrophysical Journal.
In the first article Kapteyn looked at clues for interstellar material in spectra of stars. This concerned both spectral lines of atoms (or in principle also
molecules), as well as possible effects of a general nature. For example, Antonia
Maury, who we have come to know as one of the authors of the classification
scheme of stars that culminated in the Henry Draper Catalog, had noted that
spectra of stars sometimes appeared to show a general sign of absorption at blue
wavelengths (‘shorter than 4307’, and that must be Ångström 2 ). Kapteyn dis2 Astronomers still often use this old unit for wavelengths, named after Swedish physicist Anders Jonas
Ångström (1814–1874). It is one ten millionth of a millimeter or 10 −10 m. So 10,000 Å is one μm
(micron), and 10 Å a nanometer.
9 Mount Wilson
Fig. 9.13 An image of a small area in Orion, just below the leftmost star in the belt (see
Fig. 7.11). In this picture the north is to the left, so it is rotated by 90 ◦ anticlockwise.
To the left of the middle the so-called Horsehead Nebula. This is a cloud of small dust
particles that absorbs the light of the bright nebula behind it, which is hot gas like the
Orion Nebula in Fig. 9.11. Space Telescope Science Institute [84]
9.5 Interstellar Extinction
None of this was known to Kapteyn and his contemporaries. Kapteyn wrote
two very important papers on this subject, which he published in 1909. Because of his association with the Mount Wilson Observatory and the Carnegie
Institution, he had begun to publish mostly in English; if it did not concern
publications with many tables and observation data (which he reserved for the
Publications of the Astronomical Laboratory at Groningen), he mainly submitted
his papers to American journals such as the Astronomical Journal and especially
the Astrophysical Journal.
In the first article Kapteyn looked at clues for interstellar material in spectra of stars. This concerned both spectral lines of atoms (or in principle also
molecules), as well as possible effects of a general nature. For example, Antonia
Maury, who we have come to know as one of the authors of the classification
scheme of stars that culminated in the Henry Draper Catalog, had noted that
spectra of stars sometimes appeared to show a general sign of absorption at blue
wavelengths (‘shorter than 4307’, and that must be Ångström 2 ). Kapteyn dis2 Astronomers still often use this old unit for wavelengths, named after Swedish physicist Anders Jonas
Ångström (1814–1874). It is one ten millionth of a millimeter or 10 −10 m. So 10,000 Å is one μm
(micron), and 10 Å a nanometer.