Panorama%20of%20the%20Milky%20Way%20showing%20millions%20of%20faint%20stars,%20star%20clusters,%20gaseous%20nebulae%20and%20dark%20clouds%20in%20a%20great%20circle%20around%20the%20galactic%20equator.%20Galileo%20in%201610%20first%20used%20a%20telescope%20to%20resolve%20the%20Milky%20Way%20into%20a%20myriad%20of%20faint%20stars.

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Panorama of the Milky Way showing millions of
faint stars, star clusters, gaseous nebulae and
dark clouds in a great circle around the galactic
equator. Galileo in 1610 first used a telescope
to resolve the Milky Way into a myriad of faint
stars.
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The Origin of the Milky Way by Jacopo
Tintoretto c. 1575 Jupiter, wishing to
immortalize his infant Hercules (whose mother
was the mortal Alcmene) held him to the breasts
of the sleeping goddess Juno. The milk spilt
and spurted upwards to form the Milky Way
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Historical introductionto the Milky WaySome
key landmarks in galactic research
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• Historical introduction
• William Herschels Galaxy of 1785

Herschels Galaxy cross-section was based on star
counts. Fainter stars, he reasoned, indicated a
greater distance to the edge of the star system
that comprised the Milky Way.
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• Jacobus Kapteyns Galaxy, 1922
• (Kapteyns universe)

J. Kapteyn (1851-1922)
The Sun is in the centre of Kapteyns universe,
and the contours show locations of equal star
density
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• Harlow Shapley and the distance to the centre
• of the Galaxy, 1918

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The globular cluster 47 Tucanae, one of the
nearer ones to us. It contains over 105 stars.
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Harlow Shapley and globular clusters
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• Dust clouds in the Galaxy (E.E.Barnard,
• M.Wolf), 1913-27

l Barnard photo of Milky Way centre Dark cloud
Barnard 86 r Edward Emerson Barnard (1857-1923)
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• Work of Robert Trumpler on interstellar dust
• absorption, 1930

Robert Trumpler in 1930 compared the distances of
star clusters from angular size with those from
photometry. He showed space cannot be
transparent by the discrepancy found.
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• Rotation of the Galaxy (Strömberg, 1924
• Oort 1926)

Jan Oort (1900-92) Dutch astronomer
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• Discovery of galactic spiral arms, 1951

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Young open star clusters and OB associations near
the Sun show parts of three spiral arms,
called the Perseus, Orion and Sagittarius
arms (from outside inwards).
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• Spectroscopic analysis of gaseous nebulae by
• William Huggins, 1864, who showed they are
• gas clouds of hot low density gas from their
• emission line spectrum

left Orion nebula right Sir William
Huggins (1824-1910)
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• Discovery of 21-cm radio radiation from cold
• atomic hydrogen gas clouds in disk and spiral
• arms of Galaxy, 1951 (21-cm radiation was
• predicted theoretically in 1944).

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• Discovery of spiral structure from HI 21-cm map
• of Galaxy (Oort et al. 1958)

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• Discovery of interstellar
• organic molecules,
• especially in microwave
• region, from 1963.
• The molecules include
• OH, CO, H2O, H2CO
• and C2H5OH etc.

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End of lecture 1