Re: How did we realise we were part of the Milky Way Galaxy?
- From: "OG" <owen@xxxxxxxxxxxxxxxxxxx>
- Date: Tue, 29 Sep 2009 00:27:06 +0100
"António Marques" <m.ap@xxxxxxx> wrote in message
news:h9qt6d$f8r$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi all.
I apologise if this is the wrong place to ask.
I'm mostly ignorant of the history of Astronomy. Just the other day I
found myself wondering for the umpteenth time how it as that astronomers
realised that the Earth was part of the Milky Way Galaxy. Specifically, I
gather that:
- the MW was hypothesized to be composed of stars so distant that they
couldn't be observed individually
- similar stellar clouds were hypothesized to be, again, composed of stars
so distant that they couldn't be observed individually
- it was a small leap from that to the hypothesis that the MW was like the
other stellar clouds, only much closer to us
- still I don't see how the inference was made that most stars were
grouped into galaxies, and that the ones we usually see individually are
only different in that they belong in the same galaxy as we do
- neither do I understand how is it that we figured out the shape of the
MWG and our position in it, since we have a logically very poor
observational point in what regards determining those two
Can anyone englighten me?
Thanks.
It's relatively recently really, less than a century.
In the 18th & 19th C William Herschel had plotted the distribution of stars
and thought we lived near the centre of a disc of stars, but it wasn't until
the early 20th C that improved telescope quality and the recognition of
'standard candle' variable stars (in particular the cepheid variables)
allowed measurements of distances to stars on the galactic scale and beyond.
Prior to this period, there was no way to measure the distance of spiral
nebulae, so there was no way determining where they were located on a cosmic
scale. However, the existence of the 'standard candles' allowed two sets of
measurements -
One set of measurements plotted the direction and distance of the globular
clusters of our own galaxy - these were found to be centred on what is now
known to be the centre of the Galaxy in the direction of Sagittarius.
The other set of measurements identified the same standard candle stars in
the Andromeda galaxy which gave us the information that it's distance was
far in excess of any measured in our own Galaxy. It was also possible to
observe that Andromeda galaxy (like our own) has globular clusters
distributed around its centre.
Directly detecting the spiral structure of our own Galaxy is not
straightforward at optical wavelengths, but measurements made at radio
frequencies and using Infra Red detectors gives us results that show the
spiral arms similar to those we see in many external galaxies .
.
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