The Hubble Classification of Galaxies Notes

How did Edwin Hubble classify elliptical galaxies?

Edwin Hubble classified elliptical galaxies from E0 to E7, with E0 representing nearly round galaxies and the numbers increasing as the galaxies became more elongated, with E1 being slightly elongated and E7 being the most elongated

What are the two subcategories of spiral galaxies in Hubble's classification?

barred spirals, which have a bar-shaped structure through the central bulge, and unbarred spirals, which lack the bar

What determines the shape of a galaxy, and how does it change over time?

The orbits of its stars and can change over millions of years; features like spiral arms and central bars in spiral-disk galaxies are not permanent and can form or dissipate over time

What structural features did Hubble use to classify spiral galaxies?

Hubble classified spiral galaxies based on the size of their central bulge, the tightness of their spiral arms, and the smoothness or clumpiness of the arms, with clumpy regions often consisting of star clusters and diffuse nebulae

How are unbarred spiral-disk galaxies classified in the Hubble system?

Unbarred spirals are classified from Sa to Sc, with Sa having a large central bulge and tightly-wrapped, smooth arms, and Sc having a small bulge and loosely-wrapped, clumpy arms

How are barred spiral-disk galaxies classified, and how do they compare to unbarred spirals?

Barred spirals are classified from SBa to SBc, with similar traits to unbarred spirals but with a central bar; SBa has a large bulge and tightly-wrapped arms, while SBc has a small bulge and loosely-wrapped, clumpy arms

What is the Hubble tuning fork diagram, and what types of galaxies does it include?

The Hubble tuning fork diagram arranges elliptical galaxies (E0-E7) as the handle, unbarred spirals (Sa-Sc) and barred spirals (SBa-SBc) as the two prongs, with irregular galaxies (Irr) placed outside the diagram; lenticular galaxies are a transitional type between ellipticals and spirals

What defines a lenticular galaxy, and how does it compare to ellipticals and spirals?

A lenticular galaxy has little interstellar gas and low star formation like an elliptical, but has a flat disk-like shape and high angular momentum like a spiral

How are lenticular galaxies classified in Hubble's system, and how do they relate to the tuning fork diagram?

Unbarred lenticulars are designated S0, and barred lenticulars as SB0; they sit between ellipticals and spirals in the Hubble tuning fork diagram.

What is the full Hubble sequence for unbarred and barred galaxies in the tuning fork diagram?

Unbarred: S0, Sa, Sab, Sb, Sbc, Sc; Barred: SB0, SBa, SBab, SBb, SBbc, SBc

What is the Hubble classification of the Milky Way Galaxy and why?

The Milky Way is classified as type SBb because it has a central bar, a moderate-sized bulge, and moderately-wrapped, moderately clumpy spiral arms

Where is the Solar System located within the Milky Way Galaxy?

About halfway between the galactic center and the outer edge

What did Edwin Hubble originally believe about his tuning-fork diagram?

He believed it represented a galactic evolutionary sequence, with galaxies evolving from ellipticals to lenticulars, to spirals, and finally ending as irregulars

In Hubble's original theory, what type of galaxy did all galaxies start as?

Elliptical galaxies (beginning with E0)

According to Hubble's early theory, what was the final evolutionary stage of all galaxies?

Irregular galaxies

What do astronomers today understand about Hubble's proposed galactic evolutionary sequence?

They understand it is incorrect and that the Hubble sequence does not represent a true evolutionary path

Why do terms like "early-type" and "late-type" galaxies persist in astronomy?

Because they originated from Hubble's outdated evolutionary theory, and the terminology remains in widespread use despite modern understanding

What historical parallel exists between the Hubble sequence for galaxies and the main sequence for stars?

Both were once believed to represent evolutionary sequences, though this is no longer considered accurate in the case of the Hubble sequence

According to traditional usage, which galaxy is considered "later" in the Hubble sequence: E2 or E5?

E5 is considered "later" than E2 in the Hubble sequence

Which comes earlier in the Hubble sequence: E3 or E6?

E3 comes earlier than E6

Which elliptical galaxy is more elongated: E1 or E5?

E5 is more elongated

Which galaxy comes later in the sequence: E0 or E4?

E4 comes later than E0

Which comes earlier in the Hubble sequence: Sa or Sc?

Sa comes earlier than Sc

Which has a larger central bulge: Sb or Sc?

Sb has a larger central bulge

Which comes later in the sequence: Sab or Sbc?

Sbc comes later than Sab

Which galaxy type is earlier: SBb or SBc?

SBb is earlier than SBc

Which has more loosely wrapped, clumpy arms: SBa or SBc?

SBc has more loosely wrapped, clumpy arms

Which comes later in the Hubble sequence: SBab or SBbc?

SBbc comes later than SBab

Which comes earlier: S0 or Sa?

S0 comes earlier than Sa

Which is the earlier barred galaxy: SB0 or SBa?

Sb0 is earlier than SBa