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How is it known that the Milky Way is a spiral galaxy ?



 
 
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  #1  
Old May 29th 06, 05:08 PM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?

Hi,

The question is as the title : given that we see it edge on, then how is it
known that the Milky Way is a spiral galaxy with arms and all ?

Thanks,

--
Boo
  #2  
Old May 29th 06, 05:58 PM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?


"Boo" wrote in message
...
Hi,

The question is as the title : given that we see it edge on, then how is
it known that the Milky Way is a spiral galaxy with arms and all ?

Thanks,


The first evidence was provided by Radio astronomy, where radio observations
of atomic Hydrogen wavelengths showed the existence of large scale
structures that could be interpreted as arms. (a rotating disk model allows
us to determine where a body of gas is placed by measuring its relative
speed to us).

Later observations for radio emissions for other atoms provided stronger
evidence for 'arms', but there were continuing problems in determining the
distances to these large scale features within the galaxy.

Correlating optical observations of 'young' features and radio observations
gave stronger evidence of spiral structure.

http://www.cfa.harvard.edu/~reid/background.html

As is usual, the accepted model of the Milky Way is based on a number of
strands of evidence that contribute towards a consistent model.


  #3  
Old May 30th 06, 01:23 AM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?

Boo wrote in
:

Hi,

The question is as the title : given that we see it edge on, then how
is it known that the Milky Way is a spiral galaxy with arms and all ?

Thanks,


It's not only a spiral. It appears to be a barred spiral.

http://www.seds.org/Messier/more/mw.html

Klazmon.
  #4  
Old May 30th 06, 03:08 AM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?


OG wrote:
"Boo" wrote in message
...
Hi,

The question is as the title : given that we see it edge on, then how is
it known that the Milky Way is a spiral galaxy with arms and all ?

Thanks,


The first evidence was provided by Radio astronomy, where radio observations
of atomic Hydrogen wavelengths showed the existence of large scale
structures that could be interpreted as arms. (a rotating disk model allows
us to determine where a body of gas is placed by measuring its relative
speed to us).


I thought that it was determied a spiral by Herschel and his
sister..er..er ..Herschel.

  #5  
Old May 30th 06, 09:57 AM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?


Boo wrote:

The question is as the title : given that we see it edge on, then how is it
known that the Milky Way is a spiral galaxy with arms and all ?


The most convincing direct observations are from radio astronomy using
spectroscopy of the neutral hydrogen 21cm spectral line. The amount of
material and its velocity can be deduced along a line of sight. A
short reasonably accessible intro is online at:

http://homepage.smc.edu/balm_simon/f...milky_way.html

Regards,
Martin Brown

  #6  
Old May 30th 06, 10:03 AM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?

"Weatherlawyer" wrote in message
oups.com...

OG wrote:
"Boo" wrote in message
...
Hi,

The question is as the title : given that we see it edge on, then

how is
it known that the Milky Way is a spiral galaxy with arms and all ?

Thanks,


The first evidence was provided by Radio astronomy, where radio

observations
of atomic Hydrogen wavelengths showed the existence of large scale
structures that could be interpreted as arms. (a rotating disk model

allows
us to determine where a body of gas is placed by measuring its relative
speed to us).


I thought that it was determied a spiral by Herschel and his
sister..er..er ..Herschel.


Caroline. No, William Herschel determined by star counts that our star
system was somewhat flattened, and a few thousand light years* across, but
he was not able to determine anything about spiral structure. Lord Rosse
was the first to observe spiral structure visually in an external galaxy,
M51, at least half a century after Wm. Herschel. But at that time the true
nature of nebulae was unknown.

The Milky Way was widely believed to be a spiral by analogy with external
galaxies before the radio astronomy result, but only after Hubble's
discovery of the true distance to nearby galaxies.

*The underestimate was due in part to not realising the importance of
interstellar dust in obscuring the view of distant stars.


--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

  #7  
Old May 30th 06, 12:21 PM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?


Mike Dworetsky wrote:

I thought that it was determied a spiral by Herschel and his
sister..er..er ..Herschel.


Caroline. No, William Herschel determined by star counts that our star
system was somewhat flattened, and a few thousand light years* across, but
he was not able to determine anything about spiral structure. Lord Rosse
was the first to observe spiral structure visually in an external galaxy,
M51, at least half a century after Wm. Herschel. But at that time the true
nature of nebulae was unknown.

The Milky Way was widely believed to be a spiral by analogy with external
galaxies before the radio astronomy result, but only after Hubble's
discovery of the true distance to nearby galaxies.

*The underestimate was due in part to not realising the importance of
interstellar dust in obscuring the view of distant stars.


It strikes me that we still don't know all we think we do about our
home despite all the theories. We don't know for a fact what is at the
heart of the sun never mind nebulae.

When god parted the waters, was this space dust a layer of the watery
deep? God knows. No man alive can say at any rate. The only thing we
can say for certain apparently, is that we still don't have a parallax
on any of the stars yet.

  #8  
Old May 30th 06, 03:27 PM posted to uk.sci.astronomy
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Default OT: Herschel (was: How is it known that the Milky Way is a spiral galaxy ?)

Weatherlawyer wrote:
Herschel and his sister..er..er ..Herschel.


If he was Herschel, surely she was Frauschel? Or may, being anglicised,
she was merely Hischel?

I'll get me coat....

Best,
Stephen

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  #9  
Old May 30th 06, 03:46 PM posted to uk.sci.astronomy
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Default How is it known that the Milky Way is a spiral galaxy ?

Thanks too all for the responses and links.

--
Boo
  #10  
Old May 30th 06, 06:48 PM posted to uk.sci.astronomy
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Default OT: Herschel (was: How is it known that the Milky Way is a spiral galaxy ?)


Stephen Tonkin wrote:
Weatherlawyer wrote:
Herschel and his sister..er..er ..Herschel.


If he was Herschel, surely she was Frauschel? Or may, being anglicised,
she was merely Hischel?


No, it was him and herschel.

 




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