The Formation and Structure of Stars Chapter 9 The Formation and Structure of Stars
The Interstellar Medium (ISM) The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the interstellar medium because: a) Dense interstellar clouds are the birth place of stars. b) Dark clouds alter and absorb the light from stars behind them.
Three kinds of nebulae 1) Emission Nebulae (HII Regions) 1) Emission Nebulae (HII Regions) A hot star illuminates a gas cloud; excites and/or ionizes the gas (electrons kicked into higher energy states); electrons recombining, falling back to ground state produce emission lines The Fox Fur Nebula The Trifid Nebula NGC 2246
2) Reflection Nebulae Star illuminates gas and dust cloud Star illuminates gas and dust cloud star light is reflected by the dust reflection nebula appears blue because blue light is scattered by larger angles than red light same phenomenon makes the day sky appear blue (if it’s not cloudy)
Emission and Reflection Nebulae
3) Dark Nebulae Dense clouds of gas and dust absorb the light from the stars behind; appear dark in front of the brighter background Barnard 86 Horsehead Nebula
Hubble Telescope Pictures Slide show on internet titled 20 years of Hubble pictures. Many resources like this on the Hubble site Start for www.nasa.gov to find many different resources which can be downloaded.
Cone Nebula - Monoceros
Cats Eye Nebula
Matter ejection from star
Orion Nebula
Butterfly Nebula
Large Gas and Dust cloud
Trifid Nebula
Pillars of Creation nebula
Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar clouds. Red light can more easily penetrate the cloud, but it is still absorbed to some extent. Infrared radiation is hardly absorbed at all. Barnard 68 Interstellar clouds make background stars appear redder. Visible Infrared
Interstellar Absorption Lines The interstellar medium produces absorption lines in the spectra of stars. These can be distinguished from stellar absorption lines through: a) Absorption from wrong ionization states Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple components b) Small line width (too low temperature; too low density) c) Multiple components (several clouds of ISM with different radial velocities)
Structure of the ISM HI clouds: Hot intercloud medium: The ISM occurs in two main types of clouds: HI clouds: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm3); size: ~ 100 pc Hot intercloud medium: Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3); gas can remain ionized because of very low density
Shocks Triggering Star Formation The gas in the ISM needs to be compressed in order to collapse and form stars: Shocks traveling through interstellar space can do this.
Shocks Triggering Star Formation
Compression of the ISM by Winds from Hot Stars
The Contraction of a Protostar
From Protostars to Stars Star emerges from the enshrouding dust cocoon Ignition of H → He fusion processes
Evidence of Star Formation Nebula around S Monocerotis: Contains many massive, very young stars, including T Tauri Stars: strongly variable; bright in the infrared.
T Tauri Stars Very young stars, still in the forming stage Very young stars, still in the forming stage Typically 100,000 – 10 million years old
Protostellar Disks and Jets – Herbig Haro Objects Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects
Contracting to form protostars Globules Bok Globules: ~ 10 – 1000 solar masses; Contracting to form protostars
Globules Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars.
Winds from Hot Stars Very young, hot stars produce massive stellar winds, blowing parts of it away into interstellar space. Eta Carinae