Download distance to the centre of the Milky Way.

The Size of the Milky Way;
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 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?
A First
Problem:
Obscuration!
Working at Other Wavelengths Helps
(but that‘s a fairly recent development)
Historical Problems
1.
2.
Modern 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 detectors
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...
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.
Herschel (1790) Star Counts
He 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.
He merely counted how many stars of
different brightness he could see in
various directions.
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)
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.
The ‘Kapteyn Universe’
He deduced that we are very near the centre of a
small lens-shaped system, some thousands of
light years across.
This:
Not
this:
That is, flattened, not elongated like a cigar.
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.
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.
Herschel’s Model and the Kapteyn Universe:
These 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?
Meet Harlow Shapley
The man
…and his famous Harvard desk
One of His Research Interests
- here, M13 in Hercules
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.)
Look Towards Sagittarius…
Why Are There So Many
Globular Clusters in the South?
Two possibilities:
1.
We are at the centre of the M.W., but the
majority of the globulars are offset to one side.
2.
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).
An Analogy:
New York City
Where’s the centre? What landmarks tell us? The
prominent skyscrapers! – not the small shops and
houses.
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.
Shapley’s Interpretation
A Less Cluttered Drawing
Distances Derived
From 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.
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.
Compare Shapley to Herschel
Over the
centuries:
-
Wright (top)
Herschel
Kapteyn
Shapley
the modern view