Our 'Island Universe' Transcript Date: Thursday, 30 October 2008 - 12:00AM OUR 'ISLAND UNIVERSE' Professor Ian Morison The Milky Way On a dark night with transparent skies, we can see a band of light across the sky that we call the Milky Way. (This comes from the Latin - Via Lactea.) The light comes from the myriads of stars packed so closely together that our eyes fail to resolve them into individual points of light. This is our view of our own galaxy, called the Milky Way Galaxy or often "the Galaxy" for short. It shows considerable structure due to obscuration by intervening dust clouds. The band of light is not uniform; the brightness and extent is greatest towards the constellation Sagittarius suggesting that in that direction we are looking towards the Galactic Centre. However, due to the dust, we are only able to see about one tenth of the way towards it. In the opposite direction in the sky the Milky Way is less apparent implying that we live out towards one side. Finally, the fact that we see a band of light tells us that the stars, gas and dust that make up the galaxy are in the form of a flat disc. Figure 1 An all-sky view of the Milky Way. The major visible constituent of the Galaxy, about 96%, is made up of stars, with the remaining 4% split between gas ~ 3% and dust ~ 1%. Here "visible" means that we can detect them by electromagnetic radiation; visible, infrared or radio. As we will discuss in detail in the next lecture, "The Invisible Universe", we suspect that there is a further component of the Galaxy that we cannot directly detect called "dark matter". Figure 2 Cross Section of our galaxy. Open Star Clusters Amongst the general star background we can see close groupings of stars that are called clusters. These are of two types; open clusters and globular clusters. Open clusters are a consequence of the formation of a group of stars in a giant cloud of dust and gas and are thus naturally found along the plane of the Milky Way - the disc of our galaxy. Over time the stars will tend to drift apart but, whilst they are young, we will see the stars relatively closely packed together. Prime examples observable in the northern hemisphere are the Hyades and Pleiades clusters in Taurus and the Double Cluster in Perseus. Figure 3: The Pleiades Open Cluster Globular Clusters The globular clusters are, in contrast, very old stars in tight spherical concentrations (~ 200 light-years across) of 20,000 to 1 million stars. In the northern hemisphere the most spectacular is M13 in Hercules, whilst Omega Centauri is a jewel of the southern hemisphere. They date from the origin of the Galaxy and were formed in the initial star formation period of our galaxy but their precise origin and role in the evolution of the galaxy is still unclear. Globular clusters orbit the centre of our galaxy and form a roughly spherical distribution helping to form what is known as the galactic halo. We know of 150 globular clusters associated with our Galaxy and perhaps a further 20 may be present but obscured by dust. Their spherical shape is due to the fact that they are very tightly bound by gravity, a further consequence of which is that the stars near their centers are very tightly packed. Figure 4: M13, the globular cluster in Hercules The interstellar medium and emission nebulae Together the gas and dust make up what is called the interstellar medium or ISM. Most of the ISM is not apparent to our eyes but in some regions we can see either "emission nebulae" where the gas glows or "dark nebulae" where a dust cloud appears in silhouette against a bright region of the galaxy. Perhaps the most spectacular example of an emission nebula is the Great Nebula in Orion, or more simply the Orion Nebula - a region of star formation where the hydrogen gas is being excited by the ultraviolet light emitted by the very hot young stars - forming the "Trapezium" - at its heart. This type of emission nebula is called an HII ("H-two") region as it contains ionized hydrogen where the electrons have been split off from the protons by the ultraviolet photons emitted by very hot stars. The protons and electrons can then recombine to form neutral hydrogen atoms (HI - "H- one") and the electrons drop through the allowed energy levels to the lowest energy state emitting photons of various wavelengths as they do so. One of these transitions gives rise to a bright red emission line at 6563 Angstroms, so in photographs these regions look a lovely pinky-red colour. Figure 5: The Orion Nebula, M42. An example of a dark nebula is the "Coal Sack", seen against the background of the Milky Way close to the Southern Cross. Often the two types are seen together, as in the Eagle Nebula in Serpens and the Horsehead Nebula in Orion, where the dark pillars of dust and the "horse's head" respectively are seen against the bright glow of excited gas clouds. Figure 6: The Eagle Nebula (left) and Horsehead Nebula (right). Size, shape and structure of the Milky Way The size of the galaxy was first measured by Harlow Shapley who measured the distances to 100 of the globular clusters associated with our galaxy. He found that they formed a spherical distribution, whose centre should logically be the centre of the galaxy, and so deduced that our Sun was ~30,000 light-years distant from it and the diameter was about 100,000 light years across. Figure 6 shows a cross section of the galaxy with the positions of the globular clusters observed by Harlow Shapley. Figure 7: The distribution of globular clusters as observed by Harlow Shapley. We now believe that the Sun is 27.7 light-years from the galactic centre and, using spectroscopic measurements to observe its motion relative to the globular clusters, we can calculate that the Sun is moving around the centre of the galaxy at about 230 km/sec, taking ~ 220 million years to travel once around it. It appears that the central parts of the galaxy rotate like a solid body so that the rotation speed increases as one moves out from the centre. Measurements of the speed at which stars and gas rotate around the centre of the galaxy as a function of distance produce what is called the "galactic rotation curve". But what of its structure? Neutral hydrogen (HI) emits a radio spectral line with a wavelength of 21 cm. Radio observations of this line along the plane of the Milky Way show that the gas in the disc is not uniformly dense but is concentrated into clouds whose velocity away from or towards us can be determined using the Doppler shift in its observed wavelength. These data can be used to plot out the positions of the gas clouds and when this is done so a pattern of spiral arms emerges - indicating that we live in a typical spiral galaxy thought to be quite similar to the nearby Andromeda Galaxy. Observations of the Hydrogen Line Let us consider this in more detail. A neutral hydrogen atom consists of a proton and an electron. As well as their motion in orbit around each other, the proton and electron also have a property called spin. This is actually a quantum mechanical concept, but is analogous to the spin of the Earth and the Sun about their rotation axes. Again using this analogy, the spin may be clockwise or anticlockwise. Thus their two spins may be oriented in either the same direction or in opposite directions. In a magnetic field (as exists in the galaxy) the state in which the spins of the electron and proton are aligned in the same direction has slightly more energy than one where the spins of the electron and proton are in opposite directions. Very rarely (with a probability of 2.9×10-15 s-1) a single isolated atom of neutral hydrogen will undergo a transition between the two states and emit a radio spectral line at a frequency of 1420.40575 MHz. This frequency has a wavelength of ~ 21 cm so that it is often called the 21-cm line. A single hydrogen atom will only make such a transition in a timescale of order 10 million years but, as there are a very large number of neutral hydrogen atoms in the interstellar medium, it can be easily observed by radio telescopes. This radio spectral line was first detected by Professor Edward Purcell and his graduate student, Harold Ewen, at Harvard University in 1951 using a simple horn antenna. Let us consider what we would see if out Galaxy had a number of spiral arms. As well as delineating concentrations of stars and dust, they will also contain a higher concentration of hydrogen gas. As all the material in the galaxy is rotating around its centre, when we look in different directions along the galactic plane the material in the spiral arms - in some directions we might see several - will have differing velocities away from or towards us. The hydrogen line from these different arms will thus be Doppler shifted to higher or lower frequencies. Due to motion of the gas within an element of an arm that we observe, any given arm will give a line profile that is approximately a Gaussian as shown in (a) of figure 8. This shows the gas close to us in our local spiral arm so its average velocity relative to us is zero.