The Shape of the Galaxy

The Shape of the Galaxy (1900-2000s)

Whilst Einstein was devising his theory of general relativity, and Hubble was helping prove the big bang theory, others were discovering our place in the universe. In 1917, American astronomer Heber Curtis argued that Andromeda was in fact an 'island universe' as it contained more nova than the Milky Way. A year later, American astronomer Harlow Shapley measured the size of the Milky Way, using the period-luminosity relation of Cepheid variable stars to measure the distance to globular star clusters. This method was devised by American astronomer Henrietta Swan Leavitt in 1908. Shapley concluded that Andromeda is over one hundred light years away. He did not accept that this could be true and argued that Andromeda must be part of the Milky Way. This accumulated with the 'great debate' of 1920 when Curtis and Shapley both presented evidence for their claims.

In the next few years, Hubble showed that Andromeda and several other galaxies are far outside of the Milky Way. In 1922, Dutch astronomer Jacobus Kapteyn described the Milky Way as a lens-shaped 'island universe' which increases in density towards the centre. Kapteyn estimated that the Milky Way is forty thousand light years in diameter, with the Sun situated two thousand light years from the centre. We now know that the galactic disc is about almost twice as wide and the Solar System is just under twenty five thousand light years from the centre.

Within two years of Kapteyn's prediction, Dutch astronomer Jan Oort discovered that a spherical halo of globular star clusters orbit beyond the galactic disc, these contain the oldest stars in the Galaxy. In 1944, German-American astronomer Walter Baade showed that most star formation occurs in the disc of the galaxy, in open star clusters in the spiral arms.

Kapteyn had noted that stars appear to move in one of only two directions and, in 1926, Swedish astronomer Bertil Lindblad popularised Herschel 's suggestion that the Galaxy rotates. This was confirmed by Oort in 1927. Lindblad showed that the Milky Way is a spiral galaxy and predicted that the Sun orbits at just over two hundred kilometres per second, which is now considered correct.

Welsh physicist Robert d'Escourt Atkinson and Dutch-Austrian-German physicist Fritz Houtermans had first suggested that a large amount of energy could be released by fusing small nuclei together in 1929. A decade later German-American physicist Hans Bethe and Indian-American physicist Subrahmanyan Chandrasekhar showed how stars are fuelled using nuclear fusion, solving one of the main scientific problems of the 19th century (Bethe, pp.434-456). In 1954, English astronomer Fred Hoyle showed how stars can synthesise all of the elements up to iron, after which they explode in a supernova which creates even heavier elements.

The existence of dark matter had been predicted by Swiss astrophysicist Fritz Zwicky in 1933, when he found that the Coma cluster must be four hundred times more massive than the sum of the mass of its stars in order to remain gravitationally bound (Zwicky, 1933, pp.110-127 and Zwicky, 1937, pp.217). In 1970, American astronomer Vera Rubin showed that spiral galaxies must contain a massive halo of dark matter in order for the edge of the disc to rotate as fast as it does whilst remaining gravitationally bound (Rubin and Ford, pp.379).

Some dark matter is composed of objects like black holes and massive planets which do not emit light. The acronym MACHO was devised by American physicist Kim Greist in 1991 in order to describe these objects. MACHO stands for massive astrophysical compact halo objects. Although a large amount of dark matter could be composed of MACHOs, this is not enough to account for it all. Most dark matter is thought to be comprised of hypothetical massive particles which do not interact with the electromagnetic force. These are known as WIMPs, which stands for weakly interacting massive particles, an acronym coined by American astronomers Gary Steigman and Michael Turner in 1985. The most popular candidate for WIMPs are neutralinos.

In 1997, the Hubble Space Telescope was used to show that most galaxies contain supermassive black holes at their centre. The supermassive black hole within our own galaxy was discovered by Danish physicist Rainer Schodel in 2002.

In 2005, NASA's Spitzer Space Telescope was used to show that the Milky Way is a barred spiral galaxy. That same year the first hyper velocity stars were discovered. Hyper velocity stars are thought to have once been part of a binary system which broke apart as it approached the supermassive black hole in the centre of the Galaxy. Whilst one star was captured, the other was catapulted away at a velocity exceeding the escape velocity of the Milky Way. Hyper velocity stars challenge the idea that intergalactic space is empty and by studying the path they have taken, information can be obtained about the distribution of dark matter within the Galaxy.

References

Bethe, H.A., 1939, 'Energy Production in Stars', The Physical Review, Vol.55

Rubin, V. and Ford, W. K. Jr, 1970, 'Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions', Astrophysical Journal, Vol.159

Zwicky, F., 1933, 'Die Rotverschiebung von extragalaktischen Nebeln', Helvetica Physica Acta, Vol.6

Zwicky, F., 1937, 'On the Masses of Nebulae and of Clusters of Nebulae', Astrophysical Journal, Vol.86