GalaxiesChapter 16

Galaxies

• Star systems like our _______ _____• Contain a few __________to tens of _______of stars.• Large variety of shapes and sizes

Galaxy Diversity

The Hubble Deep Field:

10-day exposure on an apparently empty field in the sky

Even seemingly

__________regions of

the sky contain

__________of very

_____, very distant

__________

:

Galaxy ClassificationSa

Sb

Sc

E0 = _________

Small ________;

loosely wound __________

E1

E6

E0, …, E7 Large _________;

tightly wound ________

E7 = Highly ____________

Gas and Dust in GalaxiesSpirals are rich in

______and _______Ellipticals are almost

devoid of ____and ___

Galaxies with disk and bulge, but no dust are termed ______

Barred Spirals

• Some spirals show a pronounced ______structure in the center• They are termed _________spiral galaxies

• Sequence:

____, …, ______,

analogous to regular spirals

Irregular GalaxiesOften: result of galaxy _________/ ___________

Often: Very active star ___________(“Starburst galaxies”)

Some: Small (“dwarf galaxies”) satellites of larger galaxies

(e.g., Magellanic Clouds)

Large Magellanic

Cloud

NGC 4038/4039

The Cocoon Galaxy

Distance Measurements to Other Galaxies (2): The Hubble Law

E. Hubble (1913):

Distant galaxies are moving _________from our Milky Way, with a recession velocity, vr, proportional to their distance d:

vr = H0*d

H0 ≈ 70 km/s/Mpc is the Hubble constant

• Measure vr through the __________effect infer the _____________

The Extragalactic Distance Scale

• Many galaxies are typically millions or billions of _______________from our galaxy.

• Typical distance units:

Mpc = ______________= 1 million __________

Gpc = ______________= 1 billion __________

• Distances of Mpc or even Gpc The light we see left the galaxy _______or __________of years ago!!

• “Look-back times” of _______or _______of years

Supermassive Black HolesFrom the measurement of stellar _________near the center of a galaxy:

Infer mass in the very center

______black holes!

Several million, up to more than a billion ______ __________!

Supermassive black holes

Our Galaxy Cluster: The Local Group

Milky Way Andromeda galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Galactic Cannibalism• Collisions of large with small galaxies often result in complete _______of the _______galaxy.• ______galaxy is “swallowed” by the ______one.

• This process is called “galactic cannibalism”

NGC 5194