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abstraction > hypothesis > theory > astronomy theory > galaxy theory

galaxy theory
subject			fact

galaxy theory	has domain extragalactic astronomy
	is a kind of astronomy theory
theory	has validity correct or incorrect with caveats
	has author or reasearch group
	has date or a range of dates for which the theory was active

Kinds of galaxy theory :

• Bahcall-Soneira Model (2 facts) - A model for the Galaxy first published by John Bahcall and Raymond Soneira in 1980. In its original form, it sought to reproduce star counts in different parts of the sky by employing only a (thin) disk and a halo; it had no thick disk.
• biased galaxy formation (2 facts) - The theory that bright galaxies form preferentially from anomalously overdense perturbations in the early universe.
• bottom-up scenario (3 facts) - A galaxy-formation scenario in which small galaxies form first. Larger and larger structures are then formed in due course., The idea that small structures, perhaps galaxies or even smaller substructures, form first in the universe, followed later by larger structures.
• density wave theory (4 facts) - Spiral structure is modelled as a small-amplitude wave propagating with fixed angular velocity, as the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
• explosive galaxy formation (2 facts) - A theory of galaxy formation wherein the explosion of a large number of stars creates a giant shock wave that travels outward and compresses the surrounding gas. Galaxies form in the regions of high-density gas.
• local hypothesis of quasars (2 facts) - The hypothesis that quasars are not at the distances inferred from their redshifts.
• spiral density wave (2 facts) - A wave, due to a local increase in the gravitational field, that produces a series of alternate compressions and rarefactions as it propagates with fixed angular velocity in a rotating galaxy. The compression also acts on interstellar gas in the galaxy, which is triggered to form stars on the leading edges of the spiral arms. The large-scale structure of spiral galaxies can be understood in this way.
• top-down scenario (2 facts) - A scenario of galaxy formation in which large structures form first and then fragment to become galaxies.
• Tully-Fisher relation (2 facts) - An observed relation between the intrinsic luminosity of a spiral galaxy and the rotational speed of its stars. More luminous galaxies have stars that are moving faster.
• twin-exhaust model (2 facts) - A theoretical model for radio galaxies in which a compact source in the galactic nucleus is assumed to emit twin beams of rapidly moving plasma that traverse hundreds of thousands of light-years, eventually splattering to a halt in the ambient intergalactic gas, where the resulting dissipation energizes the radio lobes.