PH222 - Astrophysical Concepts PDF

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 We've been talking about distinct 'things' stars and planets star clusters and galaxies molecular clouds What about the `stuff' in betwee...

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 We've been talking about distinct 'things' stars and planets star clusters and galaxies molecular clouds What about the `stuff' in between? The Interstellar medium 2 'Baryonic mass' - means matter as we know it 3  4 This means different populations of stars exist based on their time of origin This means there are different stellar populations The mass fraction is defined as + + = 1.0 - where is the mass fraction of , the mass fraction of , and 'everything else' 5 6 7 So what's out there in the ISM (apart from the stars)? 8 Between us and Centauri... Different forms of matter and energy... 9 10 Matter as Dust... dust grains absorb em radiation & re-radiate as they become thermalised... dust grains can also provide a substrate for chemical reactions to take place... 11 Matter as Gas.. 12 The Milky Way Galaxy from above... Note how the HI & H gas tracks the spiral arm structure... 13 Collective effects of matter at low densities... 14 Measuring the 21cm Hydrogen line... Allows us to reconstruct the Milky Way's spiral arm structure 15 The Milky Way Transit Authority 16 Collective effects of matter at high densities... amino acids...! 17 RNA Precusors detected in molecular clouds... Using the IRAM 30-meter telescope, astronomers detected for the first time the molecule hydroxylamine (NH OH) towards the molecular cloud G+0.693-0.027 18 So why does every astronomer care about the ISM? 19 Because we need to see through it...! ISM opacity is a function of we observe at... Optical astronomers in particular observe at set filter bands Understanding the effect the ISM (and so distance) has on a given star's apparent brightness in these bands can allow us to accurately determine it's stellar class 20 Because we need to see through it...! ISM opacity is a function of we observe at... Optical astronomers in particular observe at set filter bands Understanding the effect the ISM (and so distance) has on a given star's apparent brightness in these bands can allow us to accurately determine it's stellar class 21 An example 'filter wheel'... 22 23 24 How Colour Indices are linked to different blackbody spectra For T = 5000 K, the difference in the B and V band fluxes is very different to what you would expect to see from a T = 4000 K star. 25 Linking colour indices to spectral class The idea is very straightforward - if I can measure a star's apparent magnitude in two color bands, this simple look-up table will allow me to determine the spectral class/temperature. 26 The HR Diagram using colour-indices 27 We already this trick be used to figure out how far away a cluster of stars were... 28 Whiteboard time... 29 A B5V star (M = -0.9) has an apparent magnitude of 6.0. Determine the distance to the star. Can use the (same) formula we first saw earlier in the course when understanding the link between apparant and absolute magnitudes = -5 +5 5 = - +5 = 10 = 10 = 10 = 240 pc 30 Interstellar Extinction The panel right shows the molecular cloud Barnard 68 as observed at differing wavelengths. We can see that as the wavelength increases, it becomes less opaque - we can see background stars. So the effects of extinction are a function of density of material along the line of sight 31 Whiteboard time... 32  is the number of magnitudes of extinction produced when light passes through a cloud - the observed magnitude is given by = + - show that = , the optical depth. Some hints on how to figure this one out we're going to be using magnitudes magnitudes can be related back to flux... flux can be related back to intensity... intensity observed flux due to (i) distance (ii) opacity... the drop in intensity is the optical depth... so that's how we make the connection. 33  is the number of magnitudes of extinction produced when light passes through a cloud - the observed magnitude is given by = + - show that = , the optical depth. Let's start with re-arranging the equation we've been given = - trick 1 We've already know that - = -2.5 In optical astronomy, the reference star corresponding to = 0 is Vega ( Lyrae) set =1 So we can define all other magnitudes in units of 'Vega'... = -2.5 ( ) 34 where is in units of Vega intensity...  is the number of magnitudes of extinction produced when light passes through a cloud - the observed magnitude is given by = + - show that = , the optical depth. Let's go back to this: = - We can substitute for our new expression for as follows = -2.5 - (-2.5 ) = 2.5 trick 2 Recall that = = = 35  is the number of magnitudes of extinction produced when light passes through a cloud - the observed magnitude is given by = + - show that = , the optical depth. So, = 2.5 = 2.5 = (2.5) = (2.5) (0.434) = 1.09 Good to 10%! 36 Wrap up... introduction to the 'stuff' in between things we observe the interstellar medium implications for stellar populations the ISM as gas mapping the galaxy the ISM as dust effects on light spectroscopic parallax interstellar extinction 37