Accepted Estimates

Chuck wrote:

Gentlemen ---- Thanks for taking the time to write your posts. It's
well appreciated and it's a privilege to have you share your knowledge.

My interest in cosmology began in 1973 when I read about the Big Bang.

That was the year I first read about black holes. I still haven't
completely recovered from that one. :-)


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"Brian Tung" wrote in message
...
Jason Washborne wrote:
These theories make about as much sense to me as flying through the sun!
Really, a lot of what is proposed is far from proven. I remember a
textbook
from the 1950's saying that the "Andromeda Nebula" was 75,000 ly away.
Of
course this distance was revised to what it is presently. I think the
truth
might never be known as it's beyond our capabilities to measure
something so
grand. One thing that doesn't help is the way light bends by gravity
creating double images of galaxies, etc. This will surely throw off any
distances.

Almost two years ago, Abe Johnston wrote:
Of course, the aforementioned only applies if our calculated "light
year"
distances of stellar objects is correct. Even today, I still question
those
distances, even using red and blue shift technique. One textbook I have
from 1960 mentioned the "Andromeda nebula" lying at a distance of 75,000
light years. I can't help but wonder if our current distances will
again be
changed 50 years down the road. In any case, we are still seeing these
objects as they WERE.

At that time, I wrote:
That is probably a typo for 750,000 light-years. That distance was the
one initially determined by Edwin Hubble in the early years of the 20th
century by measuring the period of Cepheid variables, whose luminosity
is (as Ioannis mentions) directly related to their period of
variability.

Later, it was discovered that Cepheids come in two varieties--one group
with one relationship between luminosity and period, and another group
with a different relationship. Hubble had thought that the Cepheids in
the Andromeda Galaxy were of one type, but they were actually of the
other.
The upshot was that he had underestimated their distance by about a
factor
of 3.

Yes, I've read about the using the Cepheids as you mention. However, do we
really know for certain that even light from Andromeda isn't somehow being
bent/ refracted/ doubled before it reaches here? The uncertainty is the
only factor I still wonder about.

Jason


Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
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My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Jason Washborne wrote:
Yes, I've read about the using the Cepheids as you mention. However, do we
really know for certain that even light from Andromeda isn't somehow being
bent/ refracted/ doubled before it reaches here? The uncertainty is the
only factor I still wonder about.

I'm not sure what being bent or refracted would do, or what out in space
would do such bending or refracting. We might have extinction, causing
M31 to appear farther than it really is, but again, it's hard to imagine
what might be causing that extinction.

(Extinction meaning dimming through, e.g., intergalactic gas.)

The point isn't to ensure that we have the absolute right answer, anyway.
The point is to understand, as well as possible, the observations we have.
This may require developing a new theory, or it might involve making still
further observations. As best we understand things, M31 is approximately
2.5 million light-years away, though there is still a significant margin
of error. Someday, more observations might adjust that distance, but that
won't mean the work we're doing today is pointless--only that additional
observations have enhanced our understanding of the way things are.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt