How do we know there are other galaxies?user.physics.unc.edu/~sheila/102-301class16.pdf · 2010....
Transcript of How do we know there are other galaxies?user.physics.unc.edu/~sheila/102-301class16.pdf · 2010....
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.