6.2 Stars and Galaxies
97
As far as stars are concerned, one has to remember that these occur in a great
variety. First of all, they can have different intrinsic luminosities. I usually use
the word ‘luminosity’ for that, but also sometimes absolute magnitude. This
is the apparent magnitude a star would have at a standard distance. We saw
above that the parallax is measured using the projection in the sky of the Earth’s
orbit around the Sun. This generally gives an ellipse, of which the semi-major
axis is called the parallax. If this is 1 second of arc, the distance is defined as
1 parsec (pc). The absolute magnitude is then the apparent magnitude a star
would have if the distance were 10 pc (and the parallax 0 .1).
Stars can also have very different masses, ranging from about a tenth to
several tens of that of the Sun. A star spends most of its life as a so-called Main
Sequence star. This sequence was first noticed in the Hertzsprung–Russell diagram (see Fig. 6.4), which Ejnar Hertzsprung, the later son-in-law of Kapteyn,
and the American Henri Norris Russell (1877–1957) independently devised
around 1910 (for more background, see Appendix A.5). The diagram shows
that for the vast majority of stars there is a fundamental relationship between
their luminosity (the vertical axis of the diagram) and the temperature at the
surface (the horizontal axis). This feature has been called the Main Sequence.
The temperature at the surface of a star includes relatively much blue light
when the star is hot and red light when relatively cool. For the horizontal axis
of the diagram the spectral type can also be used. In cool stars we see different dark spectral lines of elements than we do in hotter stars. These lines are
created by the absorption of light from the star by atoms in the outer layers
of that star. Particularly the temperature, but also other physical conditions in
these external parts determine which chemical elements and associated spectral
lines will be visible. It turns out that along the Main Sequence the mass of the
stars also changes systematically: massive stars are blue and hot and emit much
light; light stars are red, cool and faint. Our Sun is in the middle of the Main
Sequence (Fig. 6.4).
During this Main Sequence phase of its life, in which our Sun is now, the
temperature in the central parts of a star is so high that nuclear reactions can
occur, because the energy in the motions of the atoms is so great that they can
overcome the repulsive force of electrically positively charged nuclei. During
these reactions hydrogen is converted into helium and energy is released. This
longest phase in the life of a star lasts for different times according to the mass of
a star. For a star like the Sun, it lasts about ten billion years and the temperature
at the surface (as deep as we can see from outside into the Sun) is about 5800
Kelvin. A star that is much heavier than the Sun is much more wasteful with its
hydrogen; the process is then much faster and the temperature at the surface
much higher. A star with a mass of one tenth of the Sun stays on the Main
97
As far as stars are concerned, one has to remember that these occur in a great
variety. First of all, they can have different intrinsic luminosities. I usually use
the word ‘luminosity’ for that, but also sometimes absolute magnitude. This
is the apparent magnitude a star would have at a standard distance. We saw
above that the parallax is measured using the projection in the sky of the Earth’s
orbit around the Sun. This generally gives an ellipse, of which the semi-major
axis is called the parallax. If this is 1 second of arc, the distance is defined as
1 parsec (pc). The absolute magnitude is then the apparent magnitude a star
would have if the distance were 10 pc (and the parallax 0 .1).
Stars can also have very different masses, ranging from about a tenth to
several tens of that of the Sun. A star spends most of its life as a so-called Main
Sequence star. This sequence was first noticed in the Hertzsprung–Russell diagram (see Fig. 6.4), which Ejnar Hertzsprung, the later son-in-law of Kapteyn,
and the American Henri Norris Russell (1877–1957) independently devised
around 1910 (for more background, see Appendix A.5). The diagram shows
that for the vast majority of stars there is a fundamental relationship between
their luminosity (the vertical axis of the diagram) and the temperature at the
surface (the horizontal axis). This feature has been called the Main Sequence.
The temperature at the surface of a star includes relatively much blue light
when the star is hot and red light when relatively cool. For the horizontal axis
of the diagram the spectral type can also be used. In cool stars we see different dark spectral lines of elements than we do in hotter stars. These lines are
created by the absorption of light from the star by atoms in the outer layers
of that star. Particularly the temperature, but also other physical conditions in
these external parts determine which chemical elements and associated spectral
lines will be visible. It turns out that along the Main Sequence the mass of the
stars also changes systematically: massive stars are blue and hot and emit much
light; light stars are red, cool and faint. Our Sun is in the middle of the Main
Sequence (Fig. 6.4).
During this Main Sequence phase of its life, in which our Sun is now, the
temperature in the central parts of a star is so high that nuclear reactions can
occur, because the energy in the motions of the atoms is so great that they can
overcome the repulsive force of electrically positively charged nuclei. During
these reactions hydrogen is converted into helium and energy is released. This
longest phase in the life of a star lasts for different times according to the mass of
a star. For a star like the Sun, it lasts about ten billion years and the temperature
at the surface (as deep as we can see from outside into the Sun) is about 5800
Kelvin. A star that is much heavier than the Sun is much more wasteful with its
hydrogen; the process is then much faster and the temperature at the surface
much higher. A star with a mass of one tenth of the Sun stays on the Main