Which galaxy is not a companion of the Milky Way?

A)Large Magellanic CloudB)Small Magellanic CloudC)Triangulum

Quiz Question 1

Which galaxy is not a companion of the Milky Way?

A)Large Magellanic CloudB)Small Magellanic CloudC)Triangulum

Quiz Question 1

v

r

v

r

v

r

v

r

A. B.

C. D.

Which rotation curve best describes the disk of the Milky Way?

Quiz Question 2

v

r

v

r

v

r

v

r

A. B.

C. D.

Which rotation curve best describes the disk of the Milky Way?

Quiz Question 2

•How are galaxies classified?•How are the distances to galaxies measured?•Can we estimate the age of the Universe?!!

Questions About Galaxies

What is the significance of this classification? It’s not clear.

Hubble’s Classification Scheme

Elliptical Spiral

Bulge-to-disk ratio decreases

Fig.

20-

10

Galaxy Classification

Not all galaxies fit neatly into the classification scheme.

Tidal Tail

Polar Ring

Dwarf Elliptical

The SMC—Irregular

Galaxy Classification

Distances to Nearby Galaxies

Fig.

20-

12

•Cepheids are Standard Candles•Cepheid variables are bright and observable out to several Mpc•Cepheids are periodic variable stars•Characteristic timescales are days to months•More luminous Cepheidshave longer periods

The Cepheid P-L Relation

How do the masses of Cepheid variable stars compare to the Sun?

A.Cepheids are less massive than theSunB.Cepheids are about as massive as the SunC.Cepheids are more massive than the Sun

Concept Question: Cepheids

How do the masses of Cepheid variable stars compare to the Sun?

A.Cepheids are less massive than theSunB.Cepheids are about as massive as the SunC.Cepheids are more massive than the Sun

Remember: L ∝ M3

Concept Question: Cepheids

Hubble’s Law

v (km s-1) = H0 (km s-1 Mpc-1) × d (Mpc)

H0 = 70±5 km s-1 Mpc-1

(The uncertainty in H0 is due to statistical and random measurement error.)

So, if we can measure the recessional velocity, we can solve for the distance to a galaxy:

d = v/H0

The Expansion of the Universe

Redshift

Fig. 20-15

Redshift is denoted by z

z = (λobserved-λrest)/λrest

And for nearby galaxies:

v (km s-1) = c (km s-1) × z

So, with c = 3×108 m s-1,

the distance can be derived:

d = cz/H0

Rest frame

Observed frame

The Expansion of the Universe

Which is the observed quantity for a galaxy?

d = cz/H0

A)d, the distanceB)c, the speed of lightC)z, the redshiftD)H0, the Hubble constant

Concept Question

Which is the observed quantity for a galaxy?

d = cz/H0

A)d, the distanceB)c, the speed of lightC)z, the redshiftD)H0, the Hubble constant

Concept Question

Fig.

20-

21

Study these two in your book

These techniques are not perfect, and they build on each other to some extent, but they are some of the tools (there are more) available to us to measure distances to galaxies.

The Cosmic Distance Ladder

v = H0 × d

Notice that the units of the Hubble constant are inverse time:

H0 = 70 km s-1 Mpc-1.

Notice the similarity of these two equations:

d = v × t

d = v × H0-1

⇒ t = H0-1 = 14 billion years!

(Note that this assumes a constant expansion rate…)

A First Estimate of the Age of the Universe