A) The protostar stage is very long.
B) They are all so far away that the light hasn't reached us yet.
C) Most of their radiation is in the form of X-rays.
D) They are surrounded by cocoons of gas and dust.
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A) T Tauri stars
B) Bok globules
C) O associations
D) Herbig-Haro objects
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A) O, A, K, M
B) A, B, F, G
C) K, F, B, O
D) B, A, M, G
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A) shock waves from supernovae
B) nearby spectral type G stars
C) the rotation of the cloud
D) the cloud becoming transparent to ultraviolet radiation
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A) Higher mass stars burn through their nuclear fuel faster.
B) Lower mass stars don't get their energy from nuclear fusion like higher mass stars do.
C) Higher mass stars have less hydrogen fuel to burn.
D) Lower mass stars spend a longer time evolving to the main sequence.
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A) ultraviolet radiation emitted by dust
B) narrow emission lines seen in stellar spectra
C) the presence of bright nebulae
D) the blue colour of stars seen near dark regions
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The main sequence is a continuous and di...View Answer
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A) nuclear
B) gravity
C) electric
D) magnetic
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B
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A) extinction of light from distant stars
B) very narrow calcium absorption lines in the spectra of O and B stars
C) reddening of more distant stars
D) molecular absorption lines in the spectra of cool stars
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A) High temperatures increase the ground state energy of the hydrogen atom.
B) High temperatures increase the velocity of the protons so they can overcome the Coulomb barrier.
C) High temperatures lower the density of the gas.
D) High temperatures allow the neutrinos to carry more energy away than the reaction produces.
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A gas cloud, typically composed of hydro...View Answer
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The Orion Nebula, also known as M42, is ...View Answer
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The nuclear reactions in a star are kept...View Answer
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Interstellar absorption lines are narrow compared to the width of lines that originate in the photospheres of stars due to several factors related to the conditions of the interstellar medium and the nature of stellar photospheres.
1. **Temperature and Pressure**: The interstellar medium (ISM) is generally much cooler and less dense than the photospheres of stars. In the photosphere, atoms and ions are subject to higher temperatures and pressures, which lead to broadening of the spectral lines through various mechanisms. One such mechanism is thermal broadening, where higher temperatures increase the velocity of atoms and ions, causing a spread in the energies and wavelengths at which they absorb or emit light. Another is pressure broadening, where the presence of other particles affects the energy levels of the absorbing/emitting atoms, again broadening the lines.
2. **Turbulence and Velocity Fields**: In a stellar photosphere, there are often large-scale motions such as convection currents, pulsations, and rotational effects that can cause Doppler broadening of spectral lines. These effects are less pronounced in the interstellar medium, where the gas is more quiescent and the velocity fields are less turbulent on the scales that affect line broadening.
3. **Homogeneity**: The interstellar medium is relatively homogeneous over small scales compared to the complex and dynamic environment of a stellar photosphere. This homogeneity means that there are fewer local variations in temperature, pressure, and velocity that could lead to broadening of the absorption lines.
4. **Natural and Collisional Broadening**: In the dense environment of a star's photosphere, natural broadening (due to the uncertainty principle) and collisional broadening (due to interactions between particles) also contribute to the width of spectral lines. These effects are minimized in the sparse interstellar medium.
5. **Single vs. Multiple Absorption Events**: The absorption lines we see from the interstellar medium are often the result of a single, relatively simple absorption event as light from a star passes through a cloud of interstellar gas. In contrast, the lines from a star's photosphere are the result of many absorption and re-emission events in a complex, stratified layer of gas, leading to a broadening of the lines.
6. **Chemical Composition**: The interstellar medium is primarily composed of hydrogen and helium with trace amounts of heavier elements, whereas stellar photospheres can have a more complex chemical composition. The simplicity of the interstellar medium can contribute to narrower absorption lines, as there are fewer types of atoms and ions to interact with the light.
In summary, the narrowness of interstellar absorption lines compared to those originating in stellar photospheres is due to the cooler, less dense, and more homogeneous conditions of the interstellar medium, as well as the simpler nature of the absorption events that occur there.
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