THE GALAXY. Spitzer Space Telescope Images & Spectra: 3µm - 170µm

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1 THE GALAXY Composite infrared colour image of Galactic Centre region taken at 1.25, 2.2 and 3.5 microns with COBE/DIRBE instrument (NASA/GSFC). GALAXY: A conglomeration of stars, gas + dust Topics: Star clusters Stellar Populations Stellar evolution: an overview Variable stars and their use as distance indicators Mass loss from stars Binary stars with compact components The interstellar medium Structure and rotation of the Galaxy Spitzer Space Telescope Images & Spectra: 3µm - 170µm RCW49: Giant H II Region GLIMPSE Survey: Galactic Legacy Infrared Midplane Survey Extrordanaire Massive OB Stars: Emission from Ionized Gas, Dust Fluorescence

2 Triggered Star Formation Lecture Notes & Textbook All material for exam covered in lecture notes Notes will be posted on my webpage Textbook: Astronomy A Physical Perspective, Marc Kutner Will use material from: Part III: Stellar Evolution Part IV: The Milky Way Contact me: Kenny Wood, kw25@st-andrews.ac.uk Physics & Astronomy, Room 316

3 Two main themes: An overview of the structure of the Milky Way Galaxy (MWG) how the various components are inter-related and mutually interacting leading to hints about the formation and evolution of galaxies. An understanding of the distance scale in the Universe starting from stars with accurately known parallaxes using properties of star clusters, binary stars, variable stars to build a picture of the MWG + distances to other galaxies. Analysing radiation from stars Spectra Apparent brightness Colours Positions Astrophysical theory Radial velocities Proper motions Classification of stars into different types Distances Space motions Chemical composition or population group 3-D distribution of stars of different types Calibration of types via: nearby stars, stars in clusters, binary stars for : masses, sizes, intrinsic brightness, temperatures, chemical composition.

4 Star clusters and colour-magnitude diagrams CLUSTERS: Congregations of stars held together by their mutual gravitational attraction. Range from loose associations (~ 100 stars) to open clusters (~ 10 3 stars) and globular clusters (~ 10 5 stars). Globular cluster: 47 Tuc Open cluster: The Pleiades (D. Malin) The importance of star clusters Stars in a cluster are all at ~ same distance e.g. Pleiades: diameter 10 pc, distance 126 pc h Persei: diameter 10 pc, distance 2200 pc Hence, relative properties independent of distance. Stars in a cluster had a common origin formed about the same time from same pre-stellar gas, i.e. same chemical composition. Hence direct tests of theoretical models for luminosity (L) and temperature (T) as function of stellar mass (M) and stellar ages (t). Construct H-R diagram and establish main sequence

5 Distances to clusters Parallaxes d(pc) = 1/P(arcsec) Most accurate distance indicator (just geometry) Accurate to distances of several hundred parsecs Moving Clusters Convergent point Need space motions of individual stars Proper motions + radial velocities + direction to convergent point give distance to cluster. e.g. the Hyades star cluster at d = 40 pc. Main-Sequence Fitting Measure apparent magnitudes (V) and colour indices (e.g. B-V) yields calibration of M V vs. (B-V, spectral type), (using V-M V =5 log d - 5 ) Distances: Moving Clusters Group of stars with same space velocity (Fig in book) θ V r θ V V t convergent point Measure radial velocity (Doppler shift of spectral lines) V r = V cos θ Measure angle to convergent point, θ (Textbook, Chapter 13) Now get V, and then V t from V t = V sin θ Convert to convenient units: V t (km/s) = µ (rad/s) d (km) V t = 4.74 µ d V in km/s, d in pc, µ in arcsec/year Gives distance: d = V r 4.74µ tan"

6 Distances: Main-Sequence Fitting Colour-magnitude diagrams Now better, more accurate than moving-cluster method. Determine distances throughout Galaxy Calibrated with stars in solar neighbourhood e.g. ordinary F-type stars (d < 20 to 25 pc) Distances from parallaxes (P = 1/d). HIPPARCOS measured some 10 5 stars within several 100 pc V, (B-V), etc values measured to accuracies < 1%. Also known that interstellar space in solar neighbourhood is free of dust hence no scattering, or extinction of starlight between these local F stars and us Main-Sequence Fitting Calibration: via V - A V - M V = 5 log (d/10) with A V = 0, and d = 1/P can determine accurate M V vs. (B-V) 0 etc relationships for main-sequence F stars -- empirical! Application: Observe a cluster of stars to determine V, (B-V) etc. Correct for interstellar extinction to get A V and (B-V) 0 Compare colour-magnitude diagram for cluster [(B-V) 0 vs. (V-A V )] with (B-V) 0 vs. M V diagram for solarneighbourhood stars. 2.8 M V 4.7 M V (V-A V ) (B-V) 0 Move cluster C-M diagram until the two main sequences overlap. Hence get distance modulus (V - A V - M V ) (B-V) 0

7 Interstellar reddening and extinction Effect of dust in interstellar space is to scatter and absorb starlight Blue light is scattered more easily then red light: I scat " # $4 Total extinction: absorption and scattering: I tot " # $1 Dust makes stars appear fainter A star s colour will also be affected, because of the λ 1 dependence. Light more strongly scattered and absorbed at short (blue) wavelengths. So star appears to be redder (or less blue) than its intrinsic colour. Selective extinction : changes colour of light that reaches us Dust effects also known as reddening Need link between Total (A v ) and Selective extinction (colour excess). Selective and total extinction If a star s intrinsic colour index (e.g. B-V) is Cl 0 and the observed value is CI, then we define the colour excess E(CI) by the equation: E(CI) = CI! CI 0 In general, interstellar reddening gives E(CI) > 0 e.g. for (B-V), we have E(B! V) = (B! V)! (B! V) 0 Many observations show that the wavelengthselective extinction E(B-V) is related to the total extinction A V in the V band by: A V = 3.2E(B "V ) There are equivalent relationships for other colour indices [e.g. (U-B), (V-R), etc.]