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Download Lecture 10: The Milky Way
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X. The Milky Way h"p://sgoodwin.staff.shef.ac.uk/phy111.html 0. Our home galaxy Most stars visible from the Earth are concentrated in the band of the Milky Way. This is our view from the inside of our Milky Way galaxy (or ‘the Galaxy’) which contains some 100-400 billion stars. 1. A disc galaxy That most stars lie in a band around the sky from the Earth suggests we live in a disc galaxy. But how big is it, and where are we in it? 1. The disc Most stars locally are concentrated in the disc, but are fairly smoothly distributed within the disc. The disc is around 1kpc thick, but can be divided into two populations: The thin disc is concentrated near the mid-plane of the disc and is mostly made of young (<8Gyr), metal-rich stars. The thick disc is more diffuse and extends out to 1kpc and is made of older, more metal-poor stars. Typically the stellar density in the disc is about 0.1 star pc-3 (typical separations between stars of about 1pc). 1. Metallicity The metal (ie. everything apart from H and He) content of stars is usually measured as a log relative to the Sun. [Fe/H] is the iron-to-hydrogen ratio of a star relative to the Sun. [Fe/H]=-1 means there is 0.1 of the Sun’s Fe/H. [O/H]=+0.3 means there is 2x the Sun’s O/H. [Fe/O]=0 means there is the same (x1) Fe/O as the Sun. Typically [Fe/H] is used as it is easy to measure from optical spectra (Fe has lots of lines in the visible). 1. Distances We only have parallax distance measurements out to about 100pc (Gaia will take us much further in the next few years). This gives us the absolute luminosities of low-mass stars, and using binary systems we can calibrate our models to true masses and radii (see earlier). The trouble is that within 100pc we have no massive stars and only 4 giants – how do we calibrate these? To get distances to objects further away we use a series of steps on the ‘distance ladder’ 2. Star clusters A few per cent of stars are found in star clusters: ~1pc radius gravitationally bound groups. Spectroscopy tells us that all the stars in a star cluster have almost exactly the same chemical composition – which suggests they were born together at the same time. 2. Star clusters The HR diagrams of star clusters look very ‘clean’ Because all of the stars are the same age all stars above a particular mass have ‘turned off’ the MS onto the giant branches. By using our knowledge of local MS stars we can find the distance to the cluster, and also find the mass of the turnoff. 2. Star clusters We understand MS lifetimes pretty well, so knowing the turnoff mass tells us the age of the cluster. We now have the age and distance of all the stars in the cluster. The higher the turnoff mass – the younger the cluster. This is the Pleiades – the turnoff is at about 90L From lecture 5 this is a mass of about 6M, and from lecture 6 this gives a MS lifetime of ~100Myr. 2. Variable stars Some star clusters contain types of unstable AGB stars that have regular pulsations (they lie on the ‘instability strip’). The most important of these variables are Cepheid variables – with luminosities of up to 104 L they can be seen in distant galaxies (Polaris is a nearby Cepheid variable). Their period is related to their luminosity – so if you observe one pulsating, you can calculate its absolute luminosity and so its distance. These become very important later, but are calibrated locally when found in clusters. 2. Open clusters When we look at the region that contains most of the stars in the Milky Way (the disc) we find that clusters tend to be a) fairly small (103-104 M), b) generally young (<1 Gyr), c) have similar levels of metals to the Sun. We call these clusters open clusters. 2. Globular clusters When we look away from the main Milky Way we find that clusters tend to be a) Massive (104-106 M), b) Very old (~12 Gyr), c) Have very few heavy elements. We call these clusters globular clusters. 3. The size of the Milky Way If we plot the distribution of globular clusters we find they lie in a sphere whose centre is about 8kpc from the Sun. Most globular clusters lie within about 10kpc of the centre, but some lie as far as 100kpc away. This spherical distribution of very old stars and star clusters is known as the halo. 3. The bulge When we look at the centre of the Milky Way as found by the globular clusters we find the spherical bulge (only visible from the south). This is about 4kpc in radius, and mostly old, metal-poor stars, but does have some gas and young stars as well. 4. The structure of the MW We live in a galaxy that has three major components of different ages and metallicities. Disc (thin+thick) – about 25kpc in radius, only about 1kpc thick. Most of the stars are young (0-8 Gyr), and have about the same metal content as the Sun. Total stellar mass of about 6x1010M. Bulge – a mostly old (10 Gyr), metal-poor population in a ~4kpc sphere, but does contain some young stars. Total stellar mass of about 1010M. Halo – a very old, very metal-poor spherical population out to >100 kpc. Total stellar mass of about 0.3x1010M. 4. The structure of the MW Summary The Milky Way is a spiral galaxy. It has disc, bulge and halo components with different masses, ages, and metallicities. We live around 8kpc from the Galactic centre near the midplane of the disc. Key points To describe the structure and make-up of the Milky Way both in words and diagramatically. To describe how we determine the size of the Milky Way and our distance from the centre. Example short questions A cluster is observed to be 100 Myr old. In what component of the Galaxy would you expect it to be located and why? A cluster is observed to be 12 Gyr old. In what component of the Galaxy would you expect it to be located and why? Sketch the structure of the Milky Way, naming and indicating the main stellar components. A star is observed with a metallicity of 10-3 of that of the Sun. In what component of the Galaxy would you expect it to be located? Would you expect a core collapse supernova to occur in the halo of the Milky Way? Briefly justify your answer.