Download spiral nebulae

What Are the
Spiral Nebulae?
The State of Play
Following Shapley and Oort, we knew that

The galaxy is a big flat rotating system of stars
But that’s all we knew. We did not know, for example, that
it has a spiral stucture – proof of that only came in the
1950s.
For all we knew, the Milky Way might still be a uniquely
large object in the universe, with no counterparts at all.
But Consider the So-Called Spiral Nebulae
By the 1920s, astronomers knew of many spiral nebulae in
the night sky. Their varied appearances convinced us
that they are flat. And so is our Milky Way! Two
obvious questions arise:
1.
2.
Are the spiral nebulae also gigantic star systems like
our own flattened Milky Way? Or might they instead be
small objects (maybe solar systems in formation)?
Is it possible that the Milky Way is itself a spiral?
The‘Spiral Nebulae’
First sketched by Rosse (in 1845)
- no photographic techniques back then!
Eventually, Though
One of the very earliest astronomical photographs
Modern Images Are
Of Course Much Better!
M31, in Andromeda
We See
Them at
Various
Angles
Close Up
note the smaller [more remote?] spirals in the background
The Dominant Belief
in the early 1920s
[just 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.’
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
The Key Question:
Could This Be a Solar System in Formation?
The Spiral Nebulae
Were Thought to be Nearby
[which implies that they must be small]
There were two reasons for this belief.
1.
Their distribution in space
2.
Their apparent states of rotation
1. Their Spatial Distribution
Suppose instead that the nebulae are huge
(like our own Milky Way) and randomly
distributed throughout a vast universe, then:
we should see them all over the sky, in all
directions
(The ones will be relatively big and bright;
many others will be smaller and faint)
That’s Not What Was Seen!
There is a So-Called ’Zone of Avoidance’
No spiral nebulae at all are seen in the
broad coloured-in region of the sky!
The ‘Zone of Avoidance’
Lies in a Special Location
It is aligned with the plane of the Milky Way
itself.
The spirals, whatever they are, seem to
‘avoid’ the Milky Way!
The Correct, Modern Understanding:
Interstellar Obscuration
We see small remote galaxies ‘above and below’ this
nearby galaxy, but can’t see through it!
Our own Milky Way likewise blocks our view.
Dust Gets
in the Way!
We live in a gas-rich
dusty galaxy with
obscured views.
Resolving the \Problem:
Our Galaxy in Infrared Light
(not possible in Shapley’s day!)
Long wavelengths help cut though the interstellar‘fog.’
An Understandable
Misinterpretation
No one fully appreciated the effects of dust and
obscuration, so the belief was that the spiral
nebulae must be satellites of the Milky Way –
objects that ‘avoid’ the densest parts of the
galaxy for some reason.
(Maybe they get disrupted by tidal forces?)
2. The Apparent Rotations of
the Spiral Nebulae
Some of the spiral nebulae seemed clearly to be rotating
measurably. That in itself seemed sensible enough,
as it would explain why the nebulae were flattened.
(Indeed, we now know that they are indeed rotating,
just as the Milky Way is.)
But there is an important implication: for this rotation to
be directly observable in the way reported, the
nebulae must be small [for reasons to be explained]
And the fact that we can see them the way we do, this in
turn implied that they must be nearby
How Was the Rotation Detected?
It’s simple in principle: take several photographs, many
years apart, and see if the dots of light change position
systematically! (Do we see the ‘pinwheel’ turning?)
Image 1 (say, 1910)
Image 2 (1920)
Would You Expect to See Rotation?
If this spiral nebula pictured is indeed something like a
solar system in formation, you would expect to see
systematic rotation, in a consistent direction,
whether clockwise or counter-clockwise. The
brighter ‘dots’ in the nebula might be, say,
something like ‘Jupiters in formation,’ and should
orbit the growing star at the center fairly quickly.
(The spiral arms might, for example, be trailing
behind.)
Adrian van Maanen’s
‘Discovery’
The arrows in this picture
show the sense in which
various features seemed to
have moved, by comparison
to an earlier picture.
The actual amount of the
motion was very small,
nearly immeasurable – the
arrows show the directions.
That Kind of Motion is
Fine on Small Scales…
Pinwheels can spin
very quickly – a
few times a
second!
But Now Think Astronomically
Could this object spin
on a timescale of
decades or centuries,
as van Maanen’s
observations implied?
NO! -- Not If It’s Huge!
If this object is indeed comparable to our Milky Way, it
must be about one hundred thousand light years
across. The path followed by an object travelling
around and around the centre must be tens or
hundreds of thousands of light years in length.`
To give rise to the ‘observed’ rotational motions, over a
period of just a decade or two, the stars within them
would need to be moving very much faster than the
speed of light -- which is simply not possible.
So: The Enforced Conclusion
If van Maanen’s observations of rotation are correct,
the nebulae must be

small objects (like solar systems, perhaps) and

relatively nearby, since we see them in sone detail
This suggests that they were presumably associated
with the Milky Way, in the way Shapley believed.
The Modern Understanding
Quite simply, van Maanen’s
careful measurements
were simply wrong.
Repeated tests have shown
that no such motions can be
detected. He probably ‘saw’
what he subconsciously
expected to see.
Can We Resolve the Question
Once and For All?
Let us determine the true
distance to even one of the
spirals!
We will learn whether it is
(a) a tiny satellite, or instead
(b) a huge object comparable
to the Milky Way.
But how shall we
accomplish this?