Spiral galaxies
By Renske Smit & Marcel van Daalen
Contents
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Classification and types
Characteristics
About the spirals
Theories on the formation of the structure of
spiral galaxies
Summary
Hubble diagram
“Normal” types
Sa type
(M81)
Sb type
(M51)
Sc type
(NGC 2997)
Edge-on galaxy
Sub-types
Grand-design
Flocculent
Barred types
SBa type
(NGC 3992)
SBb
type
(NGC 1433)
SBc type
(NGC 1300)
Facts
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Around 77% of all observed galaxies are
spiral galaxies
On average 100 billion stars and a width of
100.000 light years
A lot of gas, leading to star formation
Arms consist mostly of OB stars
Younger stars in the arms
M81 as observed in IR
M81 as observed in UV
Solid body construction
Leading theories
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Stochastic self-propagating star formation
theory (or chain reaction theory)
Star formation triggering star formation
„ Supernovae expand the star forming region
„ Newly lit areas become the spiral arms
„ These disappear as older stars die off
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Most popular theory
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Quasistatic density wave theory
C.C. Lin, Frank Shu
Density waves
Density waves
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Formation of OB stars in arms
H II regions due to ionizing UV radiation
Density waves
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How are those density
waves formed?
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Rotating ellipses
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Merging of galaxies
Density waves
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How long do the density waves persist?
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Long enough for the number of spiral
galaxies we observe
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Every once in a while a new merging might
take place to renew the density waves
Barred spiral galaxies
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How do you explain barred galaxies?
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Bars form spontaneously in spiral galaxies
due to its own gravitation
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Bars are destroyed due to gas accretion
Barred spiral galaxies
Barred spiral galaxies
The Hubble sequence
Discussion
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Chain reaction theory
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Explains only flocculent and chaotic galaxies
Density wave theory most popular
Does not explain flocculent galaxies
„ Does explain a lot of other things
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A completely new theory?
Summary
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How does the structure come to be?
Winding problem
Chain reaction theory
Density wave theory
Formation wave due to merging
„ Persistence of waves uncertain
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Bars form, are destroyed, and reform
Bibliography
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Spiral Structure (burro.astr.cwru.edu/Academics/
Astr222/Galaxies/Spiral/spiral.html)
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Galaxy types (physics.csustan.edu/Ana/galaxtypes.htm)
Dissipation in barred galaxies (Pfenniger &
Norman)
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Gas accretion on spiral galaxies (Bournaud &
Combes)
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Modern Astrophysics (Carroll & Ostlie)