What Are the Spiral Nebulae?

Two Questions (~1920) We see spiral nebulae in the night sky. It is apparent that they are flat. 1. Are they small objects (maybe solar systems in formation)? Or might they instead be gigantic star systems like our own Milky Way? 2. Is the Milky Way a spiral?

We See ‘Spiral Nebulae’ Sketched by Rosse (in 1845)

Another Example [a very early photograph]

Modern Images Are Much Better!

M31, in Andromeda

We See Them at Various Angles Here, Face-On

At an Intermediate Angle

Nearly Edge-On

Completely Edge-On!

Close Up note the smaller [more remote?] spirals in the background

Our Biggest Problem We are within a dusty, gassy galaxy with obscured views. Analogy: arrive in Trafalagar Square during a ‘London fog’, and try to sketch the layout of streets in that city.

Our Galaxy in Visible Light Obscuring gas and dust confuses our view

Our Galaxy in Infrared Light (not possible in Shapley’s day!) The long wavelengths cut though the interstellar ‘fog.’

The Dominant Belief in 1920s - that is, about a century ago The general feeling was that there was only one galaxy, the Milky Way – a huge stellar system sometimes called our ‘universe.’ (That’s not the modern usage.) The spiral nebulae were thought to be something else, perhaps even solar systems in formation. They were believed to be small satellites of our own galaxy, subordinate to it. The spiral nebulae were thought to be something else, perhaps even solar systems in formation. They were believed to be small satellites of our own galaxy, subordinate to it.

Shapley’s Belief

Is This a Solar System in Formation? How could you test that? (More later.)

In 1915: Spiral Nebulae are Nearby There were two reasons for this firm belief: 1. Their distribution in space 2. Their apparent motions

1. Their Spatial Distribution If the nebulae are huge, like our own Milky Way, and found randomly distributed throughout a vast universe, then: we should see them all over the sky, in all directions (nearby ones big and bright, many more little and faint) we should see them all over the sky, in all directions (nearby ones big and bright, many more little and faint)

But No! (in the 1920s) There are parts of the sky where one sees very few spiral nebulae In particular, there seem to be none at all in the plane of the Milky Way.

The ‘Zone of Avoidance’ No spiral nebulae seen here!

Conclusion: They Must Be Associated! The argument was the nebulae must ‘avoid’ this part of the galaxy for some reason. (Maybe they get disrupted by tidal forces?)

Modern Understanding: You Just Can’t See Through!

Remote Nebulae are Easily Hidden!

2. Their Apparent Motions Some of the spiral nebulae seemed to be rotating (spinning) measurably This implies that they must be small and relatively nearby! (we will see why in just a bit)

How Were These Motions Detected? Simple: take several images, decades apart, and see if the dots of light change position systematically! Image 1 (say, 1910) Image 2 (1920) Image 2 (1920)

Expectations If these are like solar systems, you expect to see systematic rotation, in a particular and consistent direction (i.e. clockwise or counterclockwise). This would explain the flattening too.

Adrian van Mannen’s ‘Discovery’

That’s Okay on Small Scales… Pinwheels can spin very quickly! Pinwheels can spin very quickly!

Now Think Astronomically Now Think Astronomically Could this object spin on a timescale of decades or centuries? Yes -- if it’s the size of the Solar System.

But Not If It’s Huge! If these objects are indeed comparable to our Milky Way, they must be about one hundred thousand light years across (and also very far away, to explain why they look so small). To explain the ‘observed’ rotation, the stars within them would need to be moving faster than the speed of light.

The Enforced Conclusion If van Mannen’s observations of rotation are correct, the nebulae must be small objects (like solar systems, perhaps) and small objects (like solar systems, perhaps) and relatively nearby (associated with the Milky Way). relatively nearby (associated with the Milky Way).

The Modern Understanding Quite simply, Van Maanen’s careful measurements were wrong. No such motions can be detected. He probably saw what he subconsciously expected to see.

How Do We Resolve the Question? Let us determine the true distance to even one of the spirals! Let us determine the true distance to even one of the spirals! We will learn whether it is a tiny satellite or instead a huge object comparable to the Milky Way. But how shall we accomplish this?