Lecture 6: Colours & Interstellar Extinction

1. Lecture 6: Colours & Interstellar Extinction • Spectra contain lot of information (T, abundances, etc) • but are more difficult to obtain than just flux measurements •  obtain indication of T from colours of stars • modern astronomical detectors v. efficient, wideλ response •  need flux measurements through different filters •  must specify filter pass-bands carefully  standardised • measure stellar magnitudes, difference  colour • main advantage: can observe fainter stars Objectives: to describe: what astronomers mean by colour relation between colour and T importance of colour for measuring extinction PHYS1005 – 2003/4

2. Examples of astronomical colour images: A2219 all are composite images formed from B&W images through different colour filters (e.g. B, I) M37 PHYS1005 – 2003/4

3. Standard Filters are Johnson UBVRIJHK: • must be standardised to allow comparison of observations • Johnson set standards in 50s and 60s • based on glass and detector properties; in wide use today • key parameters: • central λ and bandwidth (= width at half-max) • designated by λcene.g. U = ultra-violet, B = blue • JHK cover IR, not listed here PHYS1005 – 2003/4

4. Stellar Colours • m1 – m2 = -2.5 log10 (F1/F2) • magnitude measured through filter written: • mV, or usually V • e.g. Sirius has visual magnitude of V = -1.4 • now measure B, V of a star  colour = B –V • N.B. this is ratio of fluxes through each filter • e.g. Antares has B – V = +1.8  it is 1.8 mags fainter (i.e. factor 5) through B compared to V • Vega is defined to have zero magnitude through all filters, hence B – V = 0 •  colours are relative to Vega • hence colour  T Shorter λwritten first e.g. B-V or V-R; but neverV - B N.B. positive colours  RED PHYS1005 – 2003/4

5. Temperatures and Colour • U – R colour of B-B curve:- • clearly colour related to T: • as T → 0 , colour → ∞ • as T → ∞ , colour → constant Interstellar Extinction • important use of colour to measure interstellar extinction • due to gas and dust (~ smoke) which absorbs and scatters light • e.g. Coalsack Nebula (AAO image) PHYS1005 – 2003/4

6. Effect of Extinction: • extinction very patchy! • averages ≈ 1.9 mags/kpc in plane of Galaxy in V • > 30 mags towards Galactic Centre (what is the factor!) • effect stronger at short λ objects appear red, hence reddening (e.g. Sunset) • N.B. 10 mags of extinction in V (factor 104) ≡ only 1.1 mags in K (IR) • hence studies of Galactic Centre only performed in IR and beyond • absorption weakens again at hard X-ray and γ-ray wavelengths • can use reddening to estimate amount of interstellar extinction: • e.g. star has spectral type known to have B – V = +0.2, but is observed to have B – V = +3.0; what is visual extinction (in mags) to the star? • Answer: Essential to account for extinction and reddening! PHYS1005 – 2003/4