262
12 Coda
that not long afterwards the government approved the naming of Kapteyn’s
laboratory, so that it formally became the Astronomical Laboratory ‘Kapteyn’.
12.4 Kapteyn and Shapley
Kapteyn published his ‘Kapteyn Universe’ (Fig. 12.6) in 1922. Before that,
however, there was already a discussion going on about the size of the Milky Way
as a stellar system and its relation to the spiral nebulae, such as the Andromeda
Nebula and NGC 891 in Fig. 6.7. In April 1920, the American Academy held
a debate on the issue of The scale of the Universe between Harlow Shapley and
Heber Doust Curtis (1872–1942), which came into the history book as the
‘Great Debate’. Shapley defended the view that the Galaxy was large and that
nebulae were part of it, while Curtis thought that the Milky Way was a smaller
galaxy (comparable in size to the Kapteyn Universe published not much later)
and that the spiral galaxies had similar internal structures. As we now know,
Shapley was right about the first, Curtis was right about the second.
Shapley based his position on his determinations of the distances of globular
clusters. As we saw earlier, these are spherical clusters of old stars in a more or less
spherical volume (Sect. 6.2 and see Fig. 9.15); Shapley had determined from
one of these globular clusters (M13) that the interstellar absorption by dust
was negligible. In 1912 Henrietta Swan Leavitt (1868–1921) had discovered
the existence of a class of variable stars, called Cepheids, whose period in a
regular variation of brightness was a measure of its intrinsic luminosity. One
can use that then to determine the distance. The variation is caused by the
fact that these are somewhat unstable stars are in a late stage of evolution,
pulsating periodically. Hertzsprung was the first to apply Leavitt’s discovery in
Fig. 12.6 The Kapteyn Universe. Kapteyn Astronomical Institute, University of Groningen
12 Coda
that not long afterwards the government approved the naming of Kapteyn’s
laboratory, so that it formally became the Astronomical Laboratory ‘Kapteyn’.
12.4 Kapteyn and Shapley
Kapteyn published his ‘Kapteyn Universe’ (Fig. 12.6) in 1922. Before that,
however, there was already a discussion going on about the size of the Milky Way
as a stellar system and its relation to the spiral nebulae, such as the Andromeda
Nebula and NGC 891 in Fig. 6.7. In April 1920, the American Academy held
a debate on the issue of The scale of the Universe between Harlow Shapley and
Heber Doust Curtis (1872–1942), which came into the history book as the
‘Great Debate’. Shapley defended the view that the Galaxy was large and that
nebulae were part of it, while Curtis thought that the Milky Way was a smaller
galaxy (comparable in size to the Kapteyn Universe published not much later)
and that the spiral galaxies had similar internal structures. As we now know,
Shapley was right about the first, Curtis was right about the second.
Shapley based his position on his determinations of the distances of globular
clusters. As we saw earlier, these are spherical clusters of old stars in a more or less
spherical volume (Sect. 6.2 and see Fig. 9.15); Shapley had determined from
one of these globular clusters (M13) that the interstellar absorption by dust
was negligible. In 1912 Henrietta Swan Leavitt (1868–1921) had discovered
the existence of a class of variable stars, called Cepheids, whose period in a
regular variation of brightness was a measure of its intrinsic luminosity. One
can use that then to determine the distance. The variation is caused by the
fact that these are somewhat unstable stars are in a late stage of evolution,
pulsating periodically. Hertzsprung was the first to apply Leavitt’s discovery in
Fig. 12.6 The Kapteyn Universe. Kapteyn Astronomical Institute, University of Groningen