Galileo found nebulae but was unable to resolve them. Halley listed six nebulae in the Royal Society’s Philosophical Transactions (1716). Thomas Wright, Immanuel Kant and Johann Heinrich Lambert (1728-77) speculated that there were other systems similar to but distant from the Milky Way. Messier published a catalogue of 103 nebulae to reduce the chance of them being mistaken for comets. Lacaille discovered 24 new nebulae in the southern skies. William Herschel studied many of the nebulae catalogued by Messier and reported that he could see stars in 29 of them. Herschel is credited with discovering more than 500 nebulae. Cleveland Abbe thought that the smaller and fainter nebulae were separate galaxies.
In 1845 William Parsons (1800-67), the third Earl of Rosse, completed a 72-inch (183-cm) reflector in the grounds of his family seat at Birr Castle in County Offaly in Ireland. It was by far the largest telescope in the world and with it he detected the spiral structure of the Whirlpool Galaxy (M51). He went on to study other nebulae and was able to resolve some of them into star clusters.
In 1898 James Edward Keeler (1857-1900) at the Lick Observatory (est.1888) of the University of California (est.1868) began a programme of photography that revealed that over the entire sky the number of nebulae visible to the technology was in excess of 100,000 and that spiral nebula greatly outnumbered all the other hazy objects detectable in the sky.
Because of their distance, the stars in the Hyades cluster appear to be moving to a point of convergence. Identifying that point gives enough information to translate each star’s apparent motion into a real space velocity. The distance to a star in the cluster can be deduced by comparing its actual speed with how fast it appears to be moving. This is called the moving cluster method. The validity of this method can be checked because Hyades is close enough for its proper motion and therefore its distance to be measured. Comparison of the brightness of its stars with those in other clusters enables the distances to those clusters to be determined.
In 1906 Jacobus Cornelius Kapteyn (1851-1922) began to map the size and shape of the Galaxy. He finally measured it as 30,000 light years across, 6000 light years thick and put the Solar System at the centre. This is smaller than the true dimensions, but larger than anyone had previously imagined.
Harlow Shapley (1885-1972) began working at the Mount Wilson Observatory (est.1904) in 1914. He discovered cepheids in globular clusters (densely packed balls of stars, containing hundreds of thousands or even millions of stars) and used Leavitt’s period-luminosity law to measure their relative distances from us and from each other. He found that they were randomly distributed and appeared to be concentrated in a smallish area in the direction of Sagittarius. He argued that this would make sense if the Galaxy had the shape of a flattened disc with the clusters grouped around the galactic centre. This required the Solar System to be displaced by a considerable distance from its accepted central position. Shapley had found the general shape of the Galaxy but not its size.
Suppose a random selection is made of a group of stars which seem to be at roughly the same distance and close enough for their proper motions to be measured. If the Doppler Effect is used to measure the line-of-sight velocities of the stars and the average velocity of the group found, then this average velocity can be assumed to be the same in any direction, including across the line-of-sight, and this makes it possible to assign an average distance to all the stars by comparing this guesstimate with their proper motions. This technique is called statistical parallax.
Shapley examined the proper motion of a number of nearby cepheids and used statistical parallax to estimate their average distance. With this he was able to translate apparent magnitudes into absolute magnitudes and to calibrate Leavitt’s period-luminosity relationship. His results showed the Galaxy to be much larger (this was an overestimate) than Kapteyn’s, with the Sun someway from the centre.
Heber Doust Curtis (1872-1942), continuing Keeler’s work at the Lick Observatory, found that spiral nebulae edge-on to us had a peripheral band of obscuring matter and he thought that similar obscuring matter in our own galaxy prevents us from seeing spiral nebulae close to the plane of the Galaxy. In 1920 in the Great Debate he argued with Shapley that these were separate galaxies. Shapley, who sided with Adriaan Van Maanen (1884-1946), propounded (wrongly) that the spiral nebulae were relatively small and nearby.
In 1913 Vesto Melvin Slipher (1875-1969) discovered reflection nebulae. By 1922 Edwin Powell Hubble (1889-1953) had confirmed that non-spiral nebulae shine by light reflected from stars within or near them, or they absorb enough energy from nearby stars to cause the hot gas of which they are composed to glow. Hubble’s study confirmed his suspicions that these nebulae are part of the Milky Way and that others were probably more distant objects, beyond the Milky Way.
In 1930 Robert Julius Trumpler (1886-1956) proved that obscuration dims the light from distant objects in the galactic plane. Obscuration had made Shapley’s galactic cepheids appear more distant than they really were and so his diameter for the Galaxy fell from 300,000 to 100,000 light years.
Kapteyn in a programme measuring proper motions had discovered two directions of motions (star streams). Bertil Lindblad (1895-1965) said that this suggested that the stars in the Galaxy were orbiting a remote galactic centre. In 1927 Jan Hendrik Oort (1900-1992), from his observations and calculations, showed that the Sun is some 30,000 light-years from the centre of the Galaxy and takes about 225 million years to complete its orbit.
The spins of the electron and proton in a hydrogen atom can align or oppose each other. When they are aligned the total energy of the atom is slightly more than when they are opposed. The atom prefers to be in the lower energy state and when it flips over it emits a low-energy photon at a wave-length of 21.11 cm. This is a very rare event in space, but because there are so many hydrogen atoms in interstellar space, 21-cm radiation can be detected with radio telescopes. In 1944 Hendrik Christoffel van de Hulst (1918-2000), while a student of Oort, showed how hydrogen should emit 21-cm radiation. In 1951 Edward Mills Purcell (1912-1997) and Harold Irving Ewen (1922-2015) detected 21-cm emission from interstellar hydrogen clouds. The same year, van de Hulst and Oort began to use Doppler shifts in hydrogen emission to examine the Galaxy’s structure.
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