How do we know there are other galaxies?

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Goals for Today

1) Identify the role of distance indicators in proving

i. the size of the Galaxyii. the Sun’s position in the Galaxyiii. the existence of other galaxies

2) Describe how dust can affect distance measurements for different types of objects (stars, globular clusters, external galaxies).

3) Apply the P-L relation and inverse square law to find distances.

Everyone takes the limits of his own vision for

the limits of the world.

– Arthur Schopenhauer

Milky Way = huge band of gas and stars:

star distribution as deduced from star counts by

William Herschel in 1785 puts the solar system

near the center

Where is a real edge? Where does dust create an “edge”?

Center of Milky Way?

he finds

distances to

globular

clusters

using

variable

stars

In 1920, Shapley maps the 3D locations of globular

clusters and realizes the Milky Way is larger than

previously thought, and we’re not at the center!

Why doesn’t extinction put us at the center?

Think-Pair-Share 1

Why did Harlow Shapley believe the Milky Way was

much larger than previously thought?

A) he saw that globular clusters make a distribution

centered far away from the Sun

B) he realized dust was affecting star counts

C) he saw that globular clusters exist far above and

below the main Milky Way

D) he realized that the diffuse light of the Milky Way

is all distant, unresolved stars

Think-Pair-Share 2

What key difference between globular cluster positions and

star counts allowed Shapley to conclude the Sun wasn’t

in the middle of the known universe?

A) a 3D distribution is more informative than a 2D one

B) there isn’t much dust in front of globular clusters

C) the gravity needed to hold together the globular clusters

would obviously be more than the Sun could provide

With the MW being huge, Shapley

thinks the “spiral nebulae” must be

inside it just like the other nebulae

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But others think they’re “island universes”

Lord Rosse builds world’s

largest telescope in 1845…

In Ireland??!

…and draws pictures of the

Whirlpool Nebula

Immanuel Kant in 1755, Rosse in mid-1800’s,

Curtis in the Great Debate of 1920

Using variable stars, Hubble

shows Shapley is wrong,

because the distance to

Andromeda is huge.

Ironically, Shapley had used

variable stars to get distances

to his globular clusters and thus

the shape of the Milky Way.

Shapley himself coins the term

“galaxy”: another Milky Way

Edwin Hubble “wins” the Shapley-Curtis debate

• Henrietta Leavitt figured out that Cepheid variables were periodic, with period P related to L… that meant she could find d! (How?) A similar relation works for RR Lyraes.

• Leavitt was one of the Harvard College Observatory “computers” – Shapley’s army of cheap smart labor, barred from using telescopes

How did they get distances from variable stars?

the original “computers”

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Think-Pair-Share 3

Edwin Hubble figured out that the Andromeda “nebula” lay

outside our own “island universe” using:

a) (d/dsun)2 = (L/Lsun)/(b/(Lsun /pc2))

b) d=sqrt(L/b)

c) d2=L/(4πb)

d) the P-L relation for RR Lyraes

With no correction for dust, which

will be higher than it should be?

a) L b) b c) P d) d

Peer Leading: computing distances

Compute the distance in pc to an RR Lyraestar with period 1 day and apparent brightness 8x10-4 Lsun/pc2. You may read off the vertical axis at a convenient number.