1. PHYS1142 39 Black Holes If the mass of the core is more than about 3 solar masses the neutron degeneracy is overwhelmed and the core goes on collapsing. When the core diameter reaches the Schwarzschild radius (9 km for a 3 solar mass remnant) nothing, not even light, can escape. It becomes a “black hole”. We can’t see a black hole directly but we can see X-rays coming from hot material which is falling into it. The binary system Cygnus X-1 appears to contain a black hole which is sucking material in from its blue supergiant companion star. Figs. Z17.36 & K15-21 or 15-23

2. PHYS1142 40 Interstellar Matter - ISM Gas: mostly hydrogen and helium but heavier elements present. Exists as molecules, atoms or ions depending on temperature (lecture 2). Nebulae: e.g. the Orion Nebula - gas heated by ultraviolet light from young stars, shows emission lines. Can also find absorption lines due to cold interstellar gas clouds showing sodium, potassium, calcium, iron etc. Molecules: infrared spectra show presence of: H 2, carbon monoxide, H 2 O, ammonia, methane, methanol, ethanol, NH 2 CH 2 COOH an amino acid - a building block for proteins. Exist in cool (10 K), dark, clouds of about a million million molecules per cubic metre (as compared to ten million million million million in a lecture theatre).

3. PHYS1142 41 Interstellar Matter - ISM DUST: 1 grain per cubic football pitch Temperature about 100 K; emits in infrared Grain size about a millionth of a metre (1  m) Core: silicates, iron, graphite Mantle: icy solids of carbon dioxide, water, methane, ammonia Surface layer: large molecules, tars etc. Sticky surface of dust grains may help molecules to form. Dust obscures the centre of nebulae - photograph using infrared light which suffers less scattering by the dust (wavelength of infrared light longer than size of dust grains). Figs. Z15.15 & K13-12

4. PHYS1142 42 Recipe for a Planetary System Take 10,000 solar masses of molecular gas chilled to 10 K. Sprinkle liberally with carbon and silicate dust seasoned with metals. Stir well. Hammer to a lumpy consistency. Stars will form and bake. In other words... Gas and dust cloud is compressed by shock wave from a supernova, gravity takes over so cloud condenses, getting hotter and smaller. Cloud becomes a T Tauri star, lowish mass, red, buried in ISM. Original cloud was almost certainly rotating. As it gets smaller the rotation rate increases (ice skater) - problem...

5. PHYS1142 43 Stellar Spurts …too much angular momentum - eventually the star would tear itself apart. Why doesn’t it? Clues are in the spectrum. Red line characteristic of hydrogen (H alpha ). Normally see an absorption line due to hydrogen in the opaque stellar atmosphere. T Tauri stars have both absorption and emission. Emission line comes from a wind emerging from the star. Sometimes spectral line is blue shifted (Doppler effect) - indicates material coming towards us. Must be a large mass loss, but these stars are supposed to be forming and accreting mass from the surrounding dust cloud.

6. PHYS1142 44 Stellar Spurts Sometimes see red shifted absorption lines - due to material falling inwards to make a growing star. Also get ultraviolet radiation due to matter raining down with great energy onto the new star’s surface. So stars are growing due to accretion and losing mass at the same time. What is going on? HH Objects: Herbig and Haro discovered bright fuzzy blobs of matter without an apparent energy source. Complex objects associated with gas clouds. Similar spectra to supernova remnants. Measurements on HH1 and HH2 showed they were coming from a common source. In between was a T Tauri star, buried in the ISM, visible only in the infrared.

7. PHYS1142 45 Stellar Spurts and Accretion Discs Leads to a model of an accretion disc surrounding a star with bipolar jets. Figs. Z15.25 & K13-16 The jets, channeled by magnetic fields, get rid of the excess angular momentum. We end up with a T Tauri star plus an accretion disc plus bipolar lobes. The star is not yet on the Main Sequence of the HR diagram as it has still to begin fusing hydrogen to helium. The bipolar lobes are relatively short- lived. Is there evidence for stars with discs? The Hubble Space Telescope has found many such. Famous examples are in the Orion Nebula.