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 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: 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).

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