From: Ronald Wong (ronwong@inreach.com)
Date: Wed Sep 01 2004 - 01:13:43 PDT
Message-Id: <l03102802bd5b21e8ea7c@[209.209.18.107]> Date: Wed, 1 Sep 2004 01:13:43 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: Shape of the Milky Way
While browsing through Pinhole's archive, I noticed that some interesting
questions have gone unanswered.
Here's one from Erick Kielich in January of this year:
>I had a student ask me how astronomers figured out the shape of our
>galaxy, and I admit I was stumped. Can anyone help me with this? We have
>studied different galaxy shapes and all that, but how did astronomers
>determine the shape of something they could not see from afar?
Basically, it started with Galileo's discovery that the milky way was not a
luminous, gaseous cloud but a large collection of stars. A later
investigation of the distribution of the stars in the milky way - including
those too faint to be seen by Galileo - lead to the conclusion that they
were more or less in a plane.
Early in the last century, a study of the distribution of the globular star
clusters that are found in our galaxy showed that they were grouped in a
spherical fashion with the center of the sphere in the plane of the stars
of the milky way. This lead to the conclusion that the center of this
sphere was probably the center of our galaxy. That this was indeed the case
became evident when it was later discovered that many of the gaseous nebula
found in the night sky were really spiralling pinwheels of stars - like our
closest neighbor, the Andromeda galaxy (you can still find it referred to
as the Andromeda nebula in older textbooks) and that those which had
globular clusters had them grouped about the center of their galaxy.
Our galaxy is filled with interstellar clouds of dust. These clouds prevent
us from seeing the parts of our galaxy that lie behind them. As a result,
the actual distribution of all the stars and clouds of interstellar gases
in our galaxy had to wait until the latter half of the 20th century when
devices were developed that allowed us to "see" the universe in terms of
radio and infrared radiation. Such forms of radiation can penetrate these
clouds and have allowed us to fill in the missing blanks, As a result, we
now have a more detailed picture of what our galaxy looks like.
The story is not over. The picture is still incomplete. So determining the
exact shape of our galaxy continues to be a work in progress.
Take a peek at:
http://users.erols.com/arendt/Galaxy/mw.html
There is material there that you may find useful should you bring up the
topic of galaxies again next year.
ron
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