How interstellar matter is detected 8.1.1 Absorption of starlight

1. How interstellar matter is detected8.1.1 Absorption of starlight Byeon Jae Gyu

2. The Interstellar Media of Galaxies • The space between star • Rarefied gas, dust particles, a magnetic field • Relativistically moving electrons, protons and other atomic nuclei • Interstellar medium, Interstellar matter of ISM • Fundamental differences between galaxies of different Hubble types • Until the early 1980s late-type galaxies had ISM • Advances in X-ray and microwave astronomy habe now demonstrated that many early-type galaxies also have rich ISM

3. Absorption of Starlight • Dust absorbs and scatters blue light more than red. • Interstellar dust is always associated with interstellar gas • Giving rise to sharp absorption lines in stellar spectra • Figure 8.1 Absorption by interstellar Na atoms • About different V-shaped features each about 0.01nm wide • Various features are due to difference of ~10kms-1 in the line-of-sight velocities of the clouds • Calcium atoms (Ca+) gives to a similar phenomenon • Understanding of interstellar space took a great stride forward during World War II • W.S Adams studies Ca+ absorption of about 300 star

4. Absorption of Starlight • Calcium and sodium measure interstellar absorption features in optical spectra, but their line do not provide reliable of the density and temperature of the ISM • Hydrogen and helium dominate the ISM • Only a small fraction of all Na and Ca atoms are expected to be in the ionization states • Ultraviolet spectra are required to resolve this problem. • Copernicus obtained ultraviolet spectra for over 100 stars • International Ultraviolet Explorer (IUE) studied fainter object

5. Absorption of Starlight • Figure 8.2 Curve of growth • Important tool that astronomers use to determine the value of N and thus the abundances of elements in stellar atmospheres • Plot (equivalent width) log(W) vs log(N) Column density N of absorbing atoms • Using curve of growth and a measured equivalent width, we can obtain the number of absorbing atoms • The Boltzmann and Saha equation are the used to convert this value in to the total number of atoms of that element lying above the photosphere

6. Absorption of Starlight • The Curve of growth cannot be used to determine N when a line is strongly saturated • The general idea is to focus attention on the ‘wings’ of the line • Lyα transition of neutral hydrogen is possible • Two factors determine how far from line-center a line will extend • Doppler broadening • The line’s natural width

7. Absorption of Starlight • Doppler broadening • Atoms moving at different speeds absorb photons of different energies. • The velocity spread of hydrogen atoms is of order 10kms-1 • Wavelength spread Δv ≈ 0.004 nm. • The line’s natural width, Δn • The energy of the excited state is ill-determined, the wavelengths of the photons that excite the transition are uncertain too. • |λ – λ0| < Δv , determined by Doppler brodening. • |λ – λ0| > Δv , follow the readily-calculated Lorentzian profile