Our Place Within It The Visible Milky Way The Milky Way as Seen From Texas from Canada we can t see the Southern Cross The Obvious Questions How big is the Milky Way and where are we located At the ID: 593835
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Slide1
The Size of the Milky Way; Our Place Within ItSlide2
The Visible Milky WaySlide3
The Milky Way as Seen From Texas(from Canada, we can’t see the Southern Cross)Slide4
The Obvious QuestionsHow big is the Milky Way, and where are we located? At the
centre
? Off to one side?
Why is it flattened?
Does it have any structure (like spiral arms)
?
Are
there other systems like it?
How massive is it?
How did it form and evolve?Slide5
A First Problem: Obscuration!Slide6
Working at Other Wavelengths Helps
(but that
‘
s a
fairly recent
development)Slide7
Historical ProblemsModern astronomy
‘
grew up
’
in the Northern Hemisphere, so the southern skies were not well known.
The early work was necessarily done
‘
by eye
’
since there were no photographic techniques or electronic detectorsSlide8
The First Speculations
Wright
’
s
Grindstone model (~1750).
Very qualitative:
we are near the middle of a flattened slab of stars.
But he did not attempt to ascribe a ‘size’ to it...Slide9
For More Precise Answers:
Carry Out ‘Star Counts’
Analogy: look at the people around you, to see if you are in the
middle
or near the
edge
of a crowd.Slide10
Herschel (1790) Star CountsHe used telescopes to
see more
stars (since this makes
fainter
ones visible), but was still just working
‘
by eye
’
. He could not take photographs, for instance.
H
e
merely
counted
how many stars
of different
brightness
he
could see in various
directions.Slide11
Herschel’s
‘
Map
’
We Were Near the Centre
He believed the
‘
bays and indentations
’
were real (he did not know about obscuring dust that can block our view)Slide12
A Century Later:
The
Photographic
Era
[late 1800s]
Long
-exposure photographs show
many more stars
(fainter than just the eye + telescope can see)
. This
yields better statistical results.
Kapteyn
(early 1900s) did such star counts in various directions (the
“
Selected Areas
”
) using photographic plates.Slide13
The ‘Kapteyn Universe
’
He deduced that we
are very near the
centre
of a small lens-shaped system, some thousands of light years across.
Slide14
This:
Not
this
:
That is, flattened, not elongated like a cigar.Slide15
A Brief Digression:
Matters
of
Definition
Note the changing use of the word
“universe”
In ~1900,
“
Universe
”
= the distribution of stars within
which
we find
ourselves (i.e. the Milky Way
). It was believed to lie in a vast (perhaps infinite)
void.
In ~1920s,
other
such
systems were first
recognized
(as we will see) and called
“
Island
universes
.”
This term was soon replaced by the word
galaxies
.Slide16
Modern Terminology
“
Universe
”
=
everything
:
the entire ensemble of stars, planets, galaxies, gas, dust, radiation, dark matter, empty space,
as far as we can see and beyond, perhaps to infinite distance.
The word “cosmos” is equivalent, and is the origin of terms like “cosmology” and “cosmological.”
The universe
contains
the
galaxies.Slide17
Herschel’s Model and the Kapteyn
Universe:
T
hese Were Disturbing
Findings
Remember
Copernicus: he had removed the Earth from
the
‘
center of everything.
’
We
were thereafter just
one planet among many
.
Nothing special about us!
Why then should our
sun
be in so uniquely privileged a location in the Milky Way?
What makes us so very special?Slide18
Meet Harlow Shapley
The
man …and
his
famous Harvard deskSlide19
One of His Research Interests- here, M13 in HerculesSlide20
The Significance of Globular Clusters
There
are ~150 of them in our own Galaxy.
But
you
can see more of them from Chile and Australia than from the
north
!
(From the southern hemisphere, there are literally dozens of them overhead at midnight in
June, although not generally visible to the unaided eye.
) Slide21
Look Towards Sagittarius…Slide22
Why Are There So
Many
Globular Clusters in
the South?
Two possibilities:
We
are at the
centre
of the M.W., but the majority of the
globulars
are offset to one side.
The system of globular clusters tells us where the center of the M.W. is, and shows us that
we are off to one side of it
(and the Earth is tipped at an angle). Slide23
An Analogy:New
York City
Where
’
s
the
centre
? What landmarks tell us
? The prominent skyscrapers!
–
not the small shops and houses.Slide24
The Galaxy Seen
‘Sideways On’
The
yellow object
represents the solar system. The North Pole of the Earth is tipped in the direction of the red arrow. This explains why people living in North America don’t see many globular clusters, but those in Chile or Australia see lots of them!
The
red X
is the
centre
of the MW, according to Shapley.Slide25
Shapley’
s
Interpretation
A Less Cluttered Drawing
Slide26
Distances DerivedFrom measured brightnesses of some of the stars in the globular clusters (in particular, by studying some
variable stars
of a characteristic luminosity), Shapley was able to derive the distances to many of the clusters.
The
average
distance, of course, represented the
distance to the
centre
of the Milky Way. Slide27
So, in 1918:
The
‘
Universe
’
was Resized
After Shapley: we knew we were about
2/3 of the way out from the
very
centre
of a huge
stellar system
,
now known to be about
100,000 light years in diameter.
Note
that Shapley actually
overestimated
the
distances somewhat,
because he
didn
’
t fully understand the effects of the obscuring dust.
But
this changed
understanding
was still
absolutely
correct in principle!
We are in no special place
!
The Sun is just one very average star among the billions in the Galaxy.Slide28
Compare Shapley to HerschelSlide29
Over the centuries:
-
Wright (top)
- Herschel
-
Kapteyn
- Shapley
- the modern view