Lecture 2

Stars Hubble sequenceGalaxy classificationPhotometry

The role of star formation

About 6 new stars per year are born in our Galaxy

Oph V380 Ori + NGC1999

GMCs contain: dark clouds, cores, Bok globulesGMC mass / solar mass ~ 105

Dark clouds

L57 Barnard 68

Oph Giant Molecular Cloud, 160 pc awaycontains numerous dark clouds

H II regions

Bright ionized hydrogen clouds,illuminated by type O,B stars

IRAS all sky survey in the infrared

8.5 kpc

Can you findthe sun?

Earlier ideasMore recent findings

V = GM(r)/r2

From BT

From Ch.1 BT

Other galaxies M 31 Andromeda galaxy (Sb)companions: M 32 (E2) (foreground object) & M 110 (E6p)

(below M31)

M-object namesare from a catalogueby Charles Messier1740s)

Hubble sequence (tuning fork diagram)

Similar classification applies to other bandpasses(near-IR, in this case)but there’s no guarantee thatthe morphological type willbe the same in the visible (here Hubble sequence is defined) and the otherwavelengths. On the contrary, there are sometimes bars and rings which are revealed only in theUV, IR, or radio wavelengths.

Hubble sequence (tuning fork diagram)

De Vaucouleurs galaxy classification extends the Hubblescheme in orthogonal directions, adding more criteria

De Vaucouleurs galaxy classification

(cross-section at Hubble type Sb)

Bulk Properties of Galaxies

Galaxy Type

Spiral /Barred Spiral M=109 to 1011 Msun, L=108 to 1010 Lsun, Diameter=5-250 kpc

disk: Population I halo:Population II f=77%(f=Percentage of Observed Galaxies)

Elliptical 105 to 1013 105 to 1011 1-205 kpc Population II f=20%

Irregular 108 to 1010 107 to 109 1-10 kpc Population I f=3%

a bright supernova

Some objects in/around galaxies are tinybut almost as bright: supernovae, Active Galactic Nuclei, Quasars (extreme AGNs)

M 104(Sa)

M81 (Sb)

M 51 (Sc) + NGC 5195 (peculiar SBb?)

Are these colliding or is it just projection effect?Can you say what type they are?

Criteria for distinguishing spirals along the sequence of ‘early-type’ to ‘late-type’: Sa--Sb--Sc

1. Bulge-to-disk light (mass) ratio, size of the bulge (decreases)

2. Prominence of spiral arms, arm-interarm contrast (increases)

3. Visibility and number of population I objects such as: H II regions, OB associations, dark lanes of dust

(increases)

4. Pitch angle (openness) of spiral arms (increases)