Distance of spiral nebulae

From: Jonathan Clarke (jdac@alphalink.com.au)
Date: Wed Jan 23 2002 - 17:28:45 EST

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    Hi Glenn

    There were two issues: 1) whether the faint points of light in the spiral
    nebulae were stars and 2) how far they were.

    From my brief reading on the subject, William Parsons, the 3rd earl of Rosse
    demonstrated the spiral nature of some nebulae. He speculated that the points
    of light were stars first with M51 in 1850, and some of his drawings show
    stars. Before we criticise Whewell for disputing this in 1853 we need to know
    how accepted these observations were. The fact that Shapley could argue in 1920
    that spiral nebulae were gas clouds suggests that they were contentious.
    Remember that the observations were visual, recorded by sketches, under poor
    conditions, Ireland is one of the worst places to put a large telescope (wet
    climate, low altitude). There is an interesting web page on Birr Castle at
    http://www.birrcastle.com/birr/astronomy/astframes.html and on Parsons at

    The issue was resolved by Curtis in 1920 when the light from spiral nebulae was
    shown as spectroscopically identical with starlight and quite different from
    emission nebulae. For the history of the great debate see

    Until the distance to spiral nebulae could be resolved the question became how
    far these spiral nebulae were. Were they galaxies similar in size to ours or
    clusters of faint stars. Their distance could not be resolved using the H-R
    diagram as the stars were too faint and too close together to resolve
    individually using the spectroscope. This is where Henrietta Leavitt's work on
    using Cephid variables proved decisive when applied by Edwin Hubble to M31 in
    1923 (see previous web reference).

    Although Parsons was correct, his observations were controversial and not
    vindicated for 70 years. While celebrating Parson's vision we should avoid Whig
    views of history while. Whewell was wrong, but was he right for good or bad
    reasons. We know he originally was sympathetic to the purality of worlds. Did
    he change his ideas because of his theology changed or because of scientific
    evidence? If you could get his earlier work it would make an interesting
    comparison. But seeing that people like Robert Chambers and Christian Huygens
    were Christians and Thomas Chambers a "woolly" Anglican with deistic tendencies,
    the connection often made between belief in a plurality of inhabited worlds or
    of island universes and atheism, seems weak. But it would be a fruitful area of
    historical research.



    Glenn Morton wrote:

    > Hi George,
    > >-----Original Message-----
    > >From: george murphy [mailto:gmurphy@raex.com]
    > >Sent: Wednesday, January 23, 2002 2:02 AM
    > >To: Glenn Morton
    > >
    > > You've read Whewell & I only summaries of him, & I don't
    > >want to dispute
    > >your basic point about the tendentious character of his argument.
    > > But in the mid-18th century "the nebulae" included a
    > >number of different
    > >kinds of objects. There were some objects that did indeed consist
    > >of individual
    > >stars with or without other "nebulosity"
    > >that are within the Milky Way, like the Orion Nebula. But there
    > >were others
    > >like M31 that we now know to be external galaxies but that
    > >couldn't be resolved
    > >into stars at that time. The latter are the ones that had been
    > >hypothesized to
    > >be "island universes" by some, but this claim was disputed by others. It's
    > >really this "island universe" idea that was being debated &, as I
    > >noted, the
    > >debate wasn't settled till ~70 years after Whewell wrote.
    > > I'll be out of touch for ~1week. (Some will say I have
    > >been for a long
    > >time!)
    > I will answer you and await your response when you get back, if you care to
    > respond. From what I recall of my time as an astronomy major in college
    > (many years ago and quickly and forcibly moved to physics) the controversy
    > about the galaxies which Hubble solved was NOT whether or not they were
    > collections of stars--that was settled prior to Whewell for all except
    > Whewell. What the issue was, concerned the location of these collections of
    > stars. Where the nebulas within the Milky Way or outside of it? That is
    > what Hubble solved. Since all my astronomy books are locked up in a Houston
    > storage vault, I can only offer this as support:
    > " In the early 1920's Hubble played a key role in establishing just what
    > galaxies are. It was known that some spiral nebulae (fuzzy clouds of light
    > on the night sky) contained individual stars, but there was no consensus as
    > to whether these were relatively small collections of stars within our own
    > galaxy, the 'Milky Way' that stretches right across the sky, or whether
    > these could be separate galaxies, or 'island universes', as big as our own
    > galaxy but much further away. "http://www.netlabs.net/hp/tremor/hubble.html
    > And even in the great 1920 debate between Shapley and Curtis, where Shapley
    > was arguing against the nebulae being outside of our galaxy, Shapley
    > acknowledged that there were stars in the nebula and used this as an
    > argument in his favor saying:
    > "c. If our galaxy approaches the larger order of dimensions, a serious
    > difficultly at once arises for the theory that spirals are galaxies of stars
    > comparable in size with our own: it would be necessary to ascribe impossibly
    > great magnitudes to the new stars that have appeared in the spiral nebulae.
    > "http://antwrp.gsfc.nasa.gov/htmltest/gifcity/cs_nrc.html
    > THus I respectfully submit the issue in the 20's was not whether the nebulae
    > were stars but how far the stars were!

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