Abekta

The Encyclopédie of CASSA

User Tools

Site Tools

You are here: root » Courses » AST 403: Galaxies and Intergalactic Medium » Galaxy Classification and the Hubble Sequence
courses:ast403:galaxy-classification

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revisionPrevious revision
courses:ast403:galaxy-classification [2026/02/11 11:23] – asadcourses:ast403:galaxy-classification [2026/02/11 11:34] (current) – shuvo
Line 12: Line 12:
 Hubble categorized galaxies into three primary morphological groups: Ellipticals (E), Spirals (S and SB), and **Irregulars (Irr)**. The Hubble sequence, visually codified in the tuning-fork diagram, serves as the foundational morphological taxonomy for galaxies, categorizing them into **ellipticals (E), lenticulars (S0/SB0), normal spirals (S), barred spirals (SB), and irregulars (Ir)**. The "handle" of the fork represents elliptical galaxies, which are classified from E0 to E7 based on their apparent ellipticity (ϵ=1−β/α), though this classification is complicated by projection effects where the orientation of the spheroid to our line of sight masks its potentially triaxial intrinsic geometry. Lenticulars occupy the junction of the fork as a transitional class, possessing a disk but lacking defined spiral arms. The two primary branches distinguish normal spirals from barred spirals, with both tracks subdivided into types Sa through Sc (and later Sd/Sm) based on a diminishing bulge-to-disk luminosity ratio, more loosely wound arms with pitch angles increasing from roughly 6° to 18°, and an increasing resolution of the arms into discrete H II regions and stellar clumps. Physically, moving from "early-type" galaxies on the left to "late-type" galaxies on the right correlates with a decrease in the average mass-to-light ratio ($M/L_B$), an increase in the mass fraction of gas and dust, and bluer color indices (B−V) that indicate more active, recent star formation [25.2, Table 25.1]. Although Hubble originally conjectured that this diagram represented an evolutionary path, modern astrophysics treats the "early" and "late" designations as purely historical terminology rather than a temporal sequence. Refinements to this system include luminosity classes (I–V) based on arm definition and specific notations for inner rings to account for the structural complexity revealed by modern imaging Galaxies on the left of the diagram (ellipticals) are called early types, while those on the right (spirals/irregulars) are late types, though this is not an evolutionary sequence. Hubble categorized galaxies into three primary morphological groups: Ellipticals (E), Spirals (S and SB), and **Irregulars (Irr)**. The Hubble sequence, visually codified in the tuning-fork diagram, serves as the foundational morphological taxonomy for galaxies, categorizing them into **ellipticals (E), lenticulars (S0/SB0), normal spirals (S), barred spirals (SB), and irregulars (Ir)**. The "handle" of the fork represents elliptical galaxies, which are classified from E0 to E7 based on their apparent ellipticity (ϵ=1−β/α), though this classification is complicated by projection effects where the orientation of the spheroid to our line of sight masks its potentially triaxial intrinsic geometry. Lenticulars occupy the junction of the fork as a transitional class, possessing a disk but lacking defined spiral arms. The two primary branches distinguish normal spirals from barred spirals, with both tracks subdivided into types Sa through Sc (and later Sd/Sm) based on a diminishing bulge-to-disk luminosity ratio, more loosely wound arms with pitch angles increasing from roughly 6° to 18°, and an increasing resolution of the arms into discrete H II regions and stellar clumps. Physically, moving from "early-type" galaxies on the left to "late-type" galaxies on the right correlates with a decrease in the average mass-to-light ratio ($M/L_B$), an increase in the mass fraction of gas and dust, and bluer color indices (B−V) that indicate more active, recent star formation [25.2, Table 25.1]. Although Hubble originally conjectured that this diagram represented an evolutionary path, modern astrophysics treats the "early" and "late" designations as purely historical terminology rather than a temporal sequence. Refinements to this system include luminosity classes (I–V) based on arm definition and specific notations for inner rings to account for the structural complexity revealed by modern imaging Galaxies on the left of the diagram (ellipticals) are called early types, while those on the right (spirals/irregulars) are late types, though this is not an evolutionary sequence.
  
-[{{:un:tuningfork.jpg?800|caption}}]+[{{:un:tuningfork.jpg?800|Hubble's tuning-fork diagram.}}]
  
courses/ast403/galaxy-classification.1770834230.txt.gz · Last modified: by asad
Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki