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11 The Kapteyn Universe
On this occasion Kapteyn set out his ideas about the structure of the Universe. The title of his lecture was A few recent researches in the area of evolution
of the fixed stars and the Stellar System. He began by explaining which different
types of stars could be distinguished according to their spectra, and he expressed
the suspicion that this would be an evolutionary sequence. According to him,
the helium stars were then stars at a young, early stage and they would then
evolve to a stage where they cooled down and would become similar to the Sun.
That would continue until they finally became relatively red, cool stars (which
we now know as M-stars). This is incorrect according to current insights, but
as far as average ages are concerned, Kapteyn’s view was not so bad. After all,
the O- and B-stars (including the helium stars) are indeed heavy, bright and
hot and burn their hydrogen very fast on the Main Sequence. So they are on
average young, because they are short-lived. Stars like the Sun take much longer
(the Sun takes about ten billion years), and such stars are on average older. The
M-stars live even longer than the present age of the Universe, so they are on
average the oldest. In terms of age, what Kapteyn said is true, except that it
is not an evolutionary sequence at all, but a progression in how old stars can
become.
Kapteyn also expressed the view that stars form out of the material (gas
and dust) of which the nebulae and dark clouds also consist. That should
then mean, that on average they have even smaller relative velocities than the
helium stars. This also corresponds to our current insights. In earlier studies
Kapteyn had extensively investigated whether nebulae had measurable proper
motions, or whether their radial velocity could be measured. This was not
possible in 1911, so it could not be verified. There were also nebulae that were
called planetary nebulae, because they looked like a planet in a telescope with
poor optics. A nice example, which Kapteyn also used to illustrate this, is the
so-called Ring Nebula in the constellation Lyra (see Fig. 11.4 for an image
obtained with the Hubble Space Telescope). Just like in that figure, there is
often a star in the center of the nebula. Kapteyn now speculated that this might
just be a late stage of a star, and that is correct. He probably came up with this
because of his research of Nova Persei and the expanding nebula around it. He
distinguished this from objects like the Orion Nebula (Fig. 9.10, in which stars
are indeed forming) and the nebulae around the stars of the Pleiades (Fig. 9.12,
which concerns reflection of starlight in dust and therefore not directly related
to star formation). Kapteyn’s ideas about the formation and evolution of stars
are out of line with our current understanding, especially his interpretation
that different spectral types are different stages of evolution, but apart from
that he is not even very far off the mark in a general sense.
11 The Kapteyn Universe
On this occasion Kapteyn set out his ideas about the structure of the Universe. The title of his lecture was A few recent researches in the area of evolution
of the fixed stars and the Stellar System. He began by explaining which different
types of stars could be distinguished according to their spectra, and he expressed
the suspicion that this would be an evolutionary sequence. According to him,
the helium stars were then stars at a young, early stage and they would then
evolve to a stage where they cooled down and would become similar to the Sun.
That would continue until they finally became relatively red, cool stars (which
we now know as M-stars). This is incorrect according to current insights, but
as far as average ages are concerned, Kapteyn’s view was not so bad. After all,
the O- and B-stars (including the helium stars) are indeed heavy, bright and
hot and burn their hydrogen very fast on the Main Sequence. So they are on
average young, because they are short-lived. Stars like the Sun take much longer
(the Sun takes about ten billion years), and such stars are on average older. The
M-stars live even longer than the present age of the Universe, so they are on
average the oldest. In terms of age, what Kapteyn said is true, except that it
is not an evolutionary sequence at all, but a progression in how old stars can
become.
Kapteyn also expressed the view that stars form out of the material (gas
and dust) of which the nebulae and dark clouds also consist. That should
then mean, that on average they have even smaller relative velocities than the
helium stars. This also corresponds to our current insights. In earlier studies
Kapteyn had extensively investigated whether nebulae had measurable proper
motions, or whether their radial velocity could be measured. This was not
possible in 1911, so it could not be verified. There were also nebulae that were
called planetary nebulae, because they looked like a planet in a telescope with
poor optics. A nice example, which Kapteyn also used to illustrate this, is the
so-called Ring Nebula in the constellation Lyra (see Fig. 11.4 for an image
obtained with the Hubble Space Telescope). Just like in that figure, there is
often a star in the center of the nebula. Kapteyn now speculated that this might
just be a late stage of a star, and that is correct. He probably came up with this
because of his research of Nova Persei and the expanding nebula around it. He
distinguished this from objects like the Orion Nebula (Fig. 9.10, in which stars
are indeed forming) and the nebulae around the stars of the Pleiades (Fig. 9.12,
which concerns reflection of starlight in dust and therefore not directly related
to star formation). Kapteyn’s ideas about the formation and evolution of stars
are out of line with our current understanding, especially his interpretation
that different spectral types are different stages of evolution, but apart from
that he is not even very far off the mark in a general sense.