Scholars Online Astronomy - Chapter 18: The Birth of Stars
Reading: Astronomy, Chapter 18: The Birth of Stars
- Section 1: Observations of stellar spectra allow us to determine chemical composition of stars, their masses, radii, and energy output. Astronomers have combined their understanding of gravitational forces, gas behavior, thermonuclear fusion, and how energy flows from a stellar core through the outer layers of the star by conduction, convection, and radiation, to put together models of stellar evolution.
- Section 2: The most widely accepted theory of stellar origin concludes that stars form from the collapse of the gas and dust found in the plane of the Milky Way galaxy:
- Particulate matter not part of stars, planets or other concentrations of mass make up the interstellar medium; collections of such particles bound by common gravitational field makeup clouds or nebula
- Emission nebula heated by nearby stars become hot enough on their own to radiate energy. Protons and electrons form hydrogen atoms (a proton plus an electron) and clouds of unionized hydrogen are HI regions. When protons remain ionized without electrons as H+ atoms, they can still capture electrons, which then cascade to lower energy levels by emitting light. These HII regions can be recognized by a emission of hydrogen alpha (Hα) photons at 656nm in the red visible region when the newly captured electron falls from level 3 to level 2.
- Dark nebula are composed of dust grains which effectively block any light coming from more distant stars. Fine grains of dust and low concentration act as reflection nebula.
- Interstellar dust and gas absorb light over long distances at an average rate, called interstellar extinction. Because blue light is scattered more, red light can penetrate dust clouds more easily. What light from distant galaxies reaches us after passing through the interstellar medium is both dimmer and redder than we would otherwise expect. Do not confuse interstellar reddening with the Doppler effect. Interstellar reddening filters blue light out of the light; the Doppler shift changes the perceived wavelength of all waves of light.
- Section 3: Proto-stars formed during the initial gravitational collapse of dense, cold gas and dust clouds. Two forms of collapsing clouds have been observed: Barnard objects contain several thousand solar masses of gas and dust; Bok globules are smaller and denser and may be the cores of Barnard objects. When the internal pressure increases, temperature increases, and the globule begins to glow. However, observation may be difficult due to the surrounding dark nebula or cocoon nebula, which may itself absorb enough energy to glow. Proto-stars are best observed using infrared telescopes.
- Section 4: The progression of the proto-star along its evolutionary track is dictated by its initial mass. In smaller stars, increasing pressure and temperature releases energy at the core, which flows outward slowly through convection in the cooler layers near the surface. Energy trapped in a core increases the temperature to the point where thermonuclear reactions begin converting hydrogen to helium. In higher mass stars, energy flows by radiation throughout the lower layers, and hydrogen fusion begins earlier. In smaller stars, heat flows by convection. The collapsing protostar becomes a true star ("on the Main Sequence") when thermonuclear fusion is stable and the primary source of energy in the core. Protostar collapse for a massive stars relatively brief: 20 000 years for a 15 M⊕ star; 20 000 000 years and a for a 1 M⊕ star, compared to 5-10 billion years on the Main Sequence.
- Section 5: Not all of the collapsing material winds up in the Main Sequence star. In T Tauri stars (with masses less than 3 M⊕), hot gases ejected during the collapse phase render the final start much smaller than its original dust cloud mass. Proto-stars (and Main Sequence stars) also may eject mass from their poles (bipolar outflow), creating dynamic regions an of high-energy emissions (Herbig-Haro objects). Matter not collapsing into the start itself will collapse to a circumstellar accretion disk where local collapsing protoplanetary discs can eventually form planets for the new star.
- Section 6: Because dust clouds contain hundreds to thousands of solar masses of material, they tend to collapse into multiple proto-stars, forming loose stellar associations or gravitationally-bound open clusters. Studies of clusters of new stars like the Pleiades allow us to see the differences in stellar evolution due to advances in mass. Higher mass stars develop more rapidly than low mass stars, and to begin fusion and mass ejection sooner. Stellar winds from the higher mass stars may interact with the cocoon nebula of neaby low mass stars.
- Section 7: Cool hydrogen clouds (with no significant photon emissions) may be detected from the presence of carbon monoxide immediate radiation at 2.6 mm. In the Orion region, CO emission indicating regions of giant molecular clouds maps to regions of stellar formation. Carbon dioxide mapping on a large scale indicates most giant molecular clouds lie along the Galactic arms. Gravitational shock waves through these arms create denser regions and initiate gravitational collapse that produces O and B type stars, making the arms "bright" and easy to detect.
- Section 8: A supernova also sends out shock waves that can initiate gravitational collapse in nearby nebula. Heavy elements formed in the supernova become part of the new star and its planetary material.
Key Formulae to Know
- There are no key formulae for this chapter.
Read the following weblecture before chat: How stars form
Planetarium Program: Use your planetarium program to explore deep space nebulae objects.
- If possible, turn on only "Dark Nebuala" labels. How are these star-forming regions distributed?
- Observe at least two emission nebulae, such as M8, M20, NGC 3372, M42, and NGC 7000. Describe the detailed structure of each. Observe emission nebulae in M101 (the Pinwheel Galaxy). Where are these nebula within the galaxy? What might that tell us about the distribution of emission nebula in the Milky Way?
- Make close observations of the Pleiades, and use your planetarium program's ability to make an HR diagram of the cluster, or make one of your own based on the cluster stars magnitudes and stellar type. What does the plot of the cluster tell you about the age of the cluster?
Website of the Week: Read about the birth of new stars at the University of Tennesee Astronomy 162 course site.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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