At the end of part one of this article we left our galactic explorers uncovering the first hints of the existence of spiral structure within our Milky Way. In part two we see how the new field of radio astronomy opened up the Galaxy for viewing to our explorers.
A century ago astronomers were limited to the visual wavebands for their exploration of the heavens. Unknown to them, interstellar dust was restricting their view of the Galaxy. Tiny particles of dust – specks of sand covered by coatings of ice – are spread throughout the plane of the Galaxy. Typically less than a micron in diameter, they are well-matched in size to the wavelength of visible light, and so block its passage through the interstellar medium. At longer wavelengths, such as the infrared, radiation is little impeded by the tiny dust particles. The full distribution of stars in the Galaxy can be seen. However it was not until the latter part of the 20th century that the technology of infrared detection was developed for astronomy. Another waveband was opened first which enabled the structure of the Galaxy to be probed – that of the radio.
Emerging largely as a technology spin-off from radar developments of World War 2, radio astronomy offers a very different view of the universe than does the optical, one that shows us the gas rather than the stars. Radio waves pass right through the Galaxy, little affected by the environments they encounter. Moreover, they can be used to measure the spectral line emission from the hydrogen atom, rather than the continuum radiation that arises from stars. This has the very great advantage that motions in the gas can be directly measured, due to the Doppler effect arising from the relative motion between source (i.e. the gas cloud we are studying) and observer (i.e. the telescope we are using to do so). The shift in wavelength that results provides us the velocity of the gas along the line of sight, and so tells us how fast it is moving towards or away from us.
The first maps of the radio Galaxy combined the results of pioneering radio telescopes on opposite sides of the globe, in Leiden in the Netherlands and in Sydney in Australia, brought together by the great Dutch astronomer Jan Oort. The Leiden telescope was a converted 7.5m German Würzburg radar reflector used during World War 2, the Sydney telescope a 36 foot diameter steerable dish constructed at Potts Hill in the city’s western suburbs (it is interesting to note the different measuring units then in use by scientists on opposite sides of the globe!).
In the 1950’s both these telescopes were mapping the distribution of the hydrogen atom in interstellar space through its newly discovered spectral line, emitted at 21cm wavelength in the radio bands. This line arises from a change in orientation of the spin of the electron around its host proton in this, the very simplest of atoms. This is an extraordinarily rare event, occurring, on average, just once every 10 million years for any individual atom. But so large is the number of hydrogen atoms in space that it results in a strong emission signature, one whose presence can readily be measured right across the Galaxy, even with the first radio telescopes ever to be built.
To turn these maps of the hydrogen intensity in the sky into maps of the structure of the Galaxy requires another crucial piece of information, however. We need to also understand the internal motions of the stars and gas within the Galaxy. We call this the rotation curve – the speed of rotation of any part of the Galaxy about its centre. By then measuring the actual motion along many sight lines, through the Doppler shift of the hydrogen line, trigonometry allows the astronomer to infer the distance to the emitting gas cloud. A picture of the 3D structure of the Galaxy can be built up.
Knowledge of the rotation curve is not easy to obtain, however! For the Solar System the motions of the planets around the Sun are well understood – the Keplerian orbits named after the famous German astronomer who empirically derived their orbital motions in the 17th century, then later explained by Newton’s law of gravity. For the Galaxy, however, there is no single object whose gravitational pull dominates the orbital motions . They are the result of the collective gravitational potential of all the stars and gas within the Galaxy.
Oort had, however, cleverly applied the techniques of optical spectroscopy to infer the orbital motions of stars relatively nearby to the Sun. He measured their relative speeds in different directions in the sky, under the assumption they were travelling in circular orbits about the centre of the Galaxy. The inference he reached was remarkable. Outside of the inner portion of the Galaxy, all the stars appear to be orbiting about the centre in a plane, in the same direction, and at roughly the same speed. That speed is about 220 km/s, and the distance to the centre about 25,000 light years. The implications of this result are profound, and form the basis for the subsequent deduction of the existence of vast amounts of dark matter need to explain these motions – but that is another tale. For our story here, this rotation curve allows the map of the hydrogen intensity on the sky to be turned into a map of the structure of the Galaxy.
The map below is what Oort and his colleagues produced in 1958, the first true map of the Galaxy. The right half of the map comes from the Leiden telescope, the left half mostly from the Sydney telescope (which of course looks at a different part of the sky). The missing wedge in the middle (i.e. towards and away from the centre of the Galaxy) is because the galactic rotation curve technique cannot be applied in these directions, as the orbits are largely perpendicular to the sight line. Several irregular filamentary structures are evident, tracing arcs around the Galactic centre. These are the spiral arms, clearly seen for the first time, and their extent is vast. The most distant structures seen are over 60,000 light years from the Sun.
This was a spectacular leap forward in the understanding of our place in the cosmos. Not that Oort’s map is without its flaws, indeed this is not the distribution of the gas as we understand it today. If we could look down on our Galaxy from above we would expect to see a view like that seen of the Whirlpool Galaxy ). It still remains a challenging problem to turn the image of hydrogen distribution in our own Galaxy into a map revealing its structure, even with the vastly better resolution and sensitivity of modern telescopes. The rotation is not perfectly circular, significant deviations occur. The hydrogen gas is also so extensive, and the line profiles so wide, that individual features are blended. It is hard to discern the separate spiral arms from one another. The hydrogen also turns out to be extended with respect to the arms. A better tracer is needed, one that can now provided by molecular gas.
Part three of this article will talk about the mapping of the molecular gas in our Galaxy, and the latest survey being undertaken by the Mopra radio telescope in Australia, producing the highest fidelity map we have of the molecular clouds, the most active component of the interstellar gas and the site for the formation of stars.
Article by Michael G. Burton, Director of Teaching in the School of Physics at the University of New South Wales. Director and CEO of Armagh Observatory and Planetarium (Commencing 1 August 2016)