106
6 Laboratory and Statistical Astronomy
a direction opposite to the rotation, so that the velocity relative to the Sun is
almost 300 km/s. Only the later discovered Barnard’s star, discovered in 1916
by American astronomer Edward Emerson Barnard (1857–1923), has a larger
proper motion, but it is only 6 light years away.
It has recently been suggested that the Kapteyn’s star is part of a group of
stars that may have escaped from the large globular cluster ω Centauri. This is
probably not a ‘real’ globular cluster, in the sense of representative of the halopopulation, but a remnant of a small stellar system that has been ‘swallowed’
by our Milky Way System, during which the outer stars were detached from it.
Very recently, two planets have also been found around Kapteyn’s star; this was
not expected because the star, and hence the gas from which it was formed,
contained almost no chemical elements except hydrogen and helium.
Another ‘excursion’ of Kapteyn concerned Nova Persei 1901. This star suddenly appeared in February 1901 and became one of the brightest in the
firmament. We now know that it is an example of a late stage in the evolution
of a star; it is a binary star, one component of which is a white dwarf. In such
so-called cataclysmic binaries, gas flows at great velocity to the white dwarf
when the other component in its evolution becomes a red giant and expands;
this releases energy. Of course this was not known at the time; novae had been
seen before, but never so bright. So Nova Persei was probably relatively close.
It slowly became fainter until in 1904 it could only be seen with the largest
telescopes. After a year, a nebula was discovered around the star and a little
later it was discovered that this nebula was expanding (Fig. 6.10).
The expansion was so rapid that it had to indicate a small distance. For the
following we have to realize that today we all grew up with Albert Einsteins
(1879–1955) notion that nothing can move faster than light; however, the
special theory of relativity only dates back to 1905. In the case of Nova Persei,
it was clear that its expansion rate had to be greater than the speed of light
unless the star was very close. But why this occurred was not clear at all. Kapteyn
suggested in an article that what we see is a long, thin string of matter and dust,
more or less pointing away from the star, along which we see the expanding
light-front resulting from the sudden increase and peak in brightness due to
the short burst. And that motion then had to be with the speed of light as
the light-front traveled along the filament. That was a clever idea and gave a
distance of the nova of 91 parsec (Kapteyn did not use that term and spoke of
a parallax of 0 .011).
We now know that the distance to Nova Persei is no less than 460 pc, so the
apparent expansion must be faster than the speed of light after all. How is this
possible? Well, according to Einstein, material things cannot move faster than
light. However, this does not apply to a flare of light. Imagine a lighthouse
6 Laboratory and Statistical Astronomy
a direction opposite to the rotation, so that the velocity relative to the Sun is
almost 300 km/s. Only the later discovered Barnard’s star, discovered in 1916
by American astronomer Edward Emerson Barnard (1857–1923), has a larger
proper motion, but it is only 6 light years away.
It has recently been suggested that the Kapteyn’s star is part of a group of
stars that may have escaped from the large globular cluster ω Centauri. This is
probably not a ‘real’ globular cluster, in the sense of representative of the halopopulation, but a remnant of a small stellar system that has been ‘swallowed’
by our Milky Way System, during which the outer stars were detached from it.
Very recently, two planets have also been found around Kapteyn’s star; this was
not expected because the star, and hence the gas from which it was formed,
contained almost no chemical elements except hydrogen and helium.
Another ‘excursion’ of Kapteyn concerned Nova Persei 1901. This star suddenly appeared in February 1901 and became one of the brightest in the
firmament. We now know that it is an example of a late stage in the evolution
of a star; it is a binary star, one component of which is a white dwarf. In such
so-called cataclysmic binaries, gas flows at great velocity to the white dwarf
when the other component in its evolution becomes a red giant and expands;
this releases energy. Of course this was not known at the time; novae had been
seen before, but never so bright. So Nova Persei was probably relatively close.
It slowly became fainter until in 1904 it could only be seen with the largest
telescopes. After a year, a nebula was discovered around the star and a little
later it was discovered that this nebula was expanding (Fig. 6.10).
The expansion was so rapid that it had to indicate a small distance. For the
following we have to realize that today we all grew up with Albert Einsteins
(1879–1955) notion that nothing can move faster than light; however, the
special theory of relativity only dates back to 1905. In the case of Nova Persei,
it was clear that its expansion rate had to be greater than the speed of light
unless the star was very close. But why this occurred was not clear at all. Kapteyn
suggested in an article that what we see is a long, thin string of matter and dust,
more or less pointing away from the star, along which we see the expanding
light-front resulting from the sudden increase and peak in brightness due to
the short burst. And that motion then had to be with the speed of light as
the light-front traveled along the filament. That was a clever idea and gave a
distance of the nova of 91 parsec (Kapteyn did not use that term and spoke of
a parallax of 0 .011).
We now know that the distance to Nova Persei is no less than 460 pc, so the
apparent expansion must be faster than the speed of light after all. How is this
possible? Well, according to Einstein, material things cannot move faster than
light. However, this does not apply to a flare of light. Imagine a lighthouse