| Summary: | This thesis contains several complementary sets of observations of the starburst in the nucleus of NGC 253. The observations probe the interface between ionised and molecular gas, where UV photons from young stars dominate the heating and chemistry of the gas. The thesis describes how observations of the emission from these photon-dominated regions (PDRs), particularly in the near-IR to millimetre range of the electromagnetic spectrum, can be used to constrain the dominant energy inputs, chemistry and geometry of the star-formation process in a starburst. The near-IR shows several lines of H<SUB>2</SUB>. Observations of excited H<SUB>2</SUB> in Galactic PDRs indicate that the ortho to para (o/p) ratio of H<SUB>2</SUB> is ~2 whereas in shocked regions the o/p ratio is observed to be 3. Towards NGC 253, the o/p ratio of H<SUB>2</SUB> is observed to be ~2 across the entire starburst and so this is direct evidence that PDRs produce the bulk of the H<SUB>2</SUB> emission in the starburst region. Furthermore, the ratio of Brγ/1-0S(1) shows a maximum on the nucleus. With the knowledge that the H<SUB>2</SUB> emission arises in PDRs, the most plausible way to explain the observed Brγ/1-0S(1) ratio is for a large fraction of the O & B stars to be clustered into groups. Away from the nucleus, it appears that the H<SUB>2</SUB> emission is arising from PDRs that are bathed by a relatively diffuse FUV radiation field. There seems to be a clear difference between the geometry of OB stars and PDRs in the starburst to that of the geometry away from the starburst. Observations of isotopic CO show that the bulk of <SUP>13</SUP>CO emission arises from warm gas whereas the bulk of C<SUP>18</SUP>O emission appears to arise from cold gas. It thus appears that some process is at work in NGC 253 that removes C<SUP>18</SUP>O from the warm gas associated with PDRs. A proposed mechanism is selective photodissociation of CO and its isotopomers.
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