7.3 Star Streams
131
the notations NP and SP to indicate where in this representation the celestial
poles lie, and the Milky Way with a thick, curved line.
What one now would expect is to see the pattern of Fig. 7.5 Q would
appear in each of the ten areas, pointing away from the Apex and displaying
the corresponding symmetry in the black and light sectors. The patterns of
course point away from the Apex (that is how it was found in the first place),
but the symmetry is totally missing. The Sun moves through space in the
direction Apex-Antapex, but apart from that there is absolutely no sign of
a full symmetry in the distribution of the motions of the stars. So Kapteyn
showed that there are one or more preferred directions and that was a very
remarkable result.
Further analysis by Kapteyn showed what the situation is: the stars move in
general terms in two separate groups, which Kapteyn called Star Streams. The
directions of the two streams were about 125 ◦ apart. But if he then corrected
those directions of the two streams for the motion of the Sun through space,
then they came to lie more or less in the Milky Way and roughly diametrically
opposite from each other. In Fig. 7.6 these are indicated with the symbols ‘V’,
one at the far right and the other to the bottom left from the letter M for the
Milky Way. So the conclusion was that there are two opposite streams in the
Milky Way.
You need to know the average distance of the stars involved to estimate how
fast those Star Streams move. At St. Louis, Kapteyn could not go beyond the
rough estimate that that was about 10 to 20 km/s. Later he found that the
streaming velocities in space were about 20 km/s, so they had a velocity of
some 40 km/s relative to each other.
This discovery of the Star Streams, which had not been announced in
advance, must have made an enormous impression at the time. The publication of the lectures during the congress in St. Louis lasted until 1908, but
the news went around like wildfire. It has been said that it was the best-known
discovery, everyone had heard of well before it appeared in print. Kapteyn’s collected papers do not even contain a final reprint of this article 2 —apparently he
did not have one in his possession, but there are at least three copies in Groningen as stencils on thin, translucent paper, with the figures inscribed with a pen.
Two of them are part of the volumes with articles in which Kapteyn had collected his papers (see Fig. 0.3), and one is in the archives of his successor Pieter
van Rhijn. It is a fact that in 1905 Kapteyn also presented the concept of Star
Streams at a congress in Cape Town, but from that only a, not very detailed,
summary appeared in print.
What certainly helped an almost immediate acceptance in a broad circle of
the concept of Star Streams, was that independent confirmation soon came.
2 It is available electronically via [70], beginning on page 396.
131
the notations NP and SP to indicate where in this representation the celestial
poles lie, and the Milky Way with a thick, curved line.
What one now would expect is to see the pattern of Fig. 7.5 Q would
appear in each of the ten areas, pointing away from the Apex and displaying
the corresponding symmetry in the black and light sectors. The patterns of
course point away from the Apex (that is how it was found in the first place),
but the symmetry is totally missing. The Sun moves through space in the
direction Apex-Antapex, but apart from that there is absolutely no sign of
a full symmetry in the distribution of the motions of the stars. So Kapteyn
showed that there are one or more preferred directions and that was a very
remarkable result.
Further analysis by Kapteyn showed what the situation is: the stars move in
general terms in two separate groups, which Kapteyn called Star Streams. The
directions of the two streams were about 125 ◦ apart. But if he then corrected
those directions of the two streams for the motion of the Sun through space,
then they came to lie more or less in the Milky Way and roughly diametrically
opposite from each other. In Fig. 7.6 these are indicated with the symbols ‘V’,
one at the far right and the other to the bottom left from the letter M for the
Milky Way. So the conclusion was that there are two opposite streams in the
Milky Way.
You need to know the average distance of the stars involved to estimate how
fast those Star Streams move. At St. Louis, Kapteyn could not go beyond the
rough estimate that that was about 10 to 20 km/s. Later he found that the
streaming velocities in space were about 20 km/s, so they had a velocity of
some 40 km/s relative to each other.
This discovery of the Star Streams, which had not been announced in
advance, must have made an enormous impression at the time. The publication of the lectures during the congress in St. Louis lasted until 1908, but
the news went around like wildfire. It has been said that it was the best-known
discovery, everyone had heard of well before it appeared in print. Kapteyn’s collected papers do not even contain a final reprint of this article 2 —apparently he
did not have one in his possession, but there are at least three copies in Groningen as stencils on thin, translucent paper, with the figures inscribed with a pen.
Two of them are part of the volumes with articles in which Kapteyn had collected his papers (see Fig. 0.3), and one is in the archives of his successor Pieter
van Rhijn. It is a fact that in 1905 Kapteyn also presented the concept of Star
Streams at a congress in Cape Town, but from that only a, not very detailed,
summary appeared in print.
What certainly helped an almost immediate acceptance in a broad circle of
the concept of Star Streams, was that independent confirmation soon came.
2 It is available electronically via [70], beginning on page 396.