1.3 Hubble's Tuning Fork

Question 1

What are the two primary observables used to classify galaxies?

Answer 1

Their shape and their size

Question 2

How can we very broadly classify galaxies based on their size?

Answer 2

Classify as giants (e.g. MW) and dwarfs (Magellanic clouds)

Question 3

Describe Hubble’s tuning fork

Answer 3

Early galaxies: Ellipticals and lenticulars
Evolve into at the bifurcation
Late galaxies: Spirals and barred spirals
Also have irregular and peculiar galaxies

Question 4

Give the notation for elliptical galaxies, and a rough gauage on shape

Answer 4

E0 E3 E7

Go from perfect circles, to ellipses

Question 5

Give notation for Lenticulars and a rough gauage on shape

Answer 5

S0

Nucleus bulge, and a disk

Question 6

Give notation for spirals and barred spirals

Answer 6

Spirals: Sa Sb Sc/Sd
Barred spirals: SBa, SBb, SBc, SBd
Start with very curved arms, then straighten up

Question 7

State the physical properites of elliptical galaxies

Answer 7

Smooth light distribution, and no evident structure 
Red colours (absence of young stars)
Very little gas and very common in galaxy clusters

Question 8

State the equation used to determine the number for an elliptical galaxy e.g. E0 E7

Answer 8

n = 10(1-b/a) where b is the minor axis, and a is the semi axis
e.g. for a circular galaxy, b = a -> E0

Question 9

State the physical properites of lenticular galaxies

Answer 9

Have more structure than ellipticals

- Show a nucleus (bulge) and a disk but no spiral arms

Question 10

State the physical properites of irregular/peculiar galaxies

Answer 10

Common at the low mass end and show no coherent structure

Peculiars have odd shapes and thought to be due to galactical interactions

Question 11

State the physical properites of spiral galaxies

Answer 11

Sn form from tight to open and the nucleus decreases in size down the sequence

• Red disk, blue arms meaning arm formation
• Common in low density environments and have a similar luminosity to MW

Question 12

What happens to the spiral arms of low-luminosity spirals and high-luminosity spirals

Answer 12

Low: Arms can be broken into irregular patches (flocculant)
High: Can have arms > 270 deg (grad design)

Question 13

State the difference between leading and trailing spiral arms, and which is more common?

Answer 13

Leading: Curve in opposite direction to galaxy rotation
Trailing: Curve in the same direction as galaxy rotation
Nearly all observed galaxies have trailing arms

Question 14

What is the pitch angle?

Answer 14

The angle between the tangent to the spiral arm and the tangent to the local circle