Accepted Estimates

I try to read as much as I can about the latest discoveries of the
Universe. I find there are considerable differences in the estimates
of size and distances, though.

More times than not, the diameter given for the Milky Way is 100,000
light years. The estimated number of stars in it varies a lot and I've
read where they range from 100 billion to 200 billion stars.

Andromeda is estimated to be from 100,000 to 200,000 light years in
diameter. The number of stars it has is estimated to be from 100
billion to 500 billion stars.

There are varied estimates of from 100 to 250 billion galaxies in the
Universe.

It seems there are two general lines of theory about the overall
Universe. One theory says it's finite. I've read where the estimated
size may range from 55 billion to nearly 160 billion light years in
diameter. The other theory says the universe is infinite. I'm very
interested in both theories.

Even trying to find the correct average diameter of the Solar System
proved to be difficult.

Understandably, none of these estimates and/or theories can be
absolutely precise. Ever increasing technology will also bring
continuous changes. What is considered to be the most accepted
source(s) for current info on each of these many sizes and distances?

Chuck wrote:


It seems there are two general lines of theory about the overall
Universe. One theory says it's finite. I've read where the estimated
size may range from 55 billion to nearly 160 billion light years in
diameter. The other theory says the universe is infinite. I'm very
interested in both theories.



Ref: http://www.aip.org/pnu/2004/split/685-1.html
Number 685 #1, May 12, 2004 by Phil Schewe and Ben Stein

Our Universe Has a Topology Scale of at least 24 GPC

Our universe has a topology scale of at least 24 Gpc, or about 75
billion light years, according to a new analysis of data from the
Wilkinson Microwave Anisotropy Probe (WMAP).

What does this mean? Well, because of conceivable hall-of-mirrors
effects of spacetime, the universe might be finite in size but give us
mortals the illusion that it is infinite. For example, the cosmos might
be tiled with some repeating shape, around which light rays might wrap
themselves over and over ("wrap" in the sense that, as in video games,
something might disappear off the left side of the screen and reappear
on the right side).

A new study by scientists from Princeton, Montana State, and Case
Western looks for signs of such "wrapped " light in the form of pairs
of circles, in opposite directions in the sky, with similar patterns in
the temperature of the cosmic microwave background. If the universe
were finite and actually smaller than the distance to the "surface of
last scattering" (a distance that essentially constitutes the edge of
the "visible universe," and the place in deep space whence comes the
cosmic microwaves), then multiple imaging should show up in the
microwave background.

But no such correspondences appeared in the analysis. The
researchers are able to turn the lack of recurring patterns into the
form of a lower limit on the scale of cosmic topology, equal to 24
billion parsecs, a factor of 10 larger than previous observational
bounds. (Cornish, Spergel, Starkman, Komatsu, Physical Review Letters,
upcoming article; contact Neil Cornish, 406-994-7986,
.)

Chuck wrote:
Understandably, none of these estimates and/or theories can be
absolutely precise. Ever increasing technology will also bring
continuous changes. What is considered to be the most accepted
source(s) for current info on each of these many sizes and distances?

Galactic star counts, believe it or not, are not of much interest to
most professional astronomers. It depends on what you say a star is.
Astronomers are more interested in how the stars are distributed with
respect to mass, metallicity, orbital motion, age, etc. than they are
in how many there are. That's why the estimates vary quite a bit.

The diameter of the Milky Way disc is indeed about 100,000 light-years.
I don't know the diameter of the halo.

It's probably best not to work with the absolute size of the universe,
but with its curvature. At the moment of the Big Bang, the curvature
was great, and its subsequent expansion has made it nearly zero.

There's no one place to get all of the most up-to-date stuff; you
probably would have to read astronomy journals. Maybe someone else
knows a good clearinghouse?

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

I prefer the Greek myth, the story where where Eros and Chaos start the
universe. It's far more lyrical, and makes just about as much sense as
infinite density and curvature magically transforming into perfectly
uniform mass distribution with zero curvature ;-)


Brian Tung wrote:


It's probably best not to work with the absolute size of the universe,
but with its curvature. At the moment of the Big Bang, the curvature
was great, and its subsequent expansion has made it nearly zero.

Tim Killian wrote:
I prefer the Greek myth, the story where where Eros and Chaos start the
universe. It's far more lyrical, and makes just about as much sense as
infinite density and curvature magically transforming into perfectly
uniform mass distribution with zero curvature ;-)

Perhaps! But nature isn't obliged to make sense. The Greek myth does
not, for example, explain the proportions of hydrogen and helium in the
universe. I can't dispute that it's more lyrical for most people,
though.

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

Perhaps the "dark matter" is Eros, and the "dark energy" is Chaos. The
two conspired long ago to create a resonance that balances the ratio of
hydrogen to helium. There, the Greek myth is now consistent with your
big bang! ;-)

Brian Tung wrote:

Tim Killian wrote:

I prefer the Greek myth, the story where where Eros and Chaos start the
universe. It's far more lyrical, and makes just about as much sense as
infinite density and curvature magically transforming into perfectly
uniform mass distribution with zero curvature ;-)


Perhaps! But nature isn't obliged to make sense. The Greek myth does
not, for example, explain the proportions of hydrogen and helium in the
universe. I can't dispute that it's more lyrical for most people,
though.

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

Tim Killian was saying
I prefer the Greek myth, the story where
where Eros and Chaos start the
universe.

I think Chaos spent a lot more time on my part of the universe than
Eros...
Marty

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.
For years I had a very wrong notion of how the BB occurred, thinking it
was much like an explosion. At that time, I sort of thought about it
in this way:

If an explosion was set off in mid air, it would tend throw everything
left of the bomb material outward in all directions. Within a second,
there would nothing left where the explosion occurred. If the bomb was
made in a precise manner, then all remaining material from it would be
thrown out fairly equally. At an exact time, the debris would form
something roughly similar to a very thick skinned balloon. This would
all happen in a second or two of the explosion. The only difference I
thought is that this would all be slowed way down in the BB.
Obviously, all people connected to astronomy have known all along this
not the case at all for the "Big Bang". But, for amateurs like
me..............no.

Then, about fifteen years ago, I was finally put on the right track. I
was fortunate enough to have "The Expansion of Pure Energy" explained
to me. The simple title, "Big Bang", really threw me and it is still
misleading to many people, today.


Sam ---- Thanks for your writing on the finite/infinite universe. It's
truly a mind-boggling thing to try and understand, but nonetheless I'm
bound determined to learn as much as I can. The idea that a beam light
appears to curve over enormous distances in the mirror effect is a real
attention getter.

I bought the book "The Astronomical Companion" you recommended to me
awhile back and it's been great reading! I've learned much from from
this book and it's been a big help towards understanding galactic
planes.


Brian ---- Thanks for putting in your links with your post. I skimmed
over most of your extensive material and will go back and read much of
what you've written. Many good hours of reading to be spent here! In
your FAQ the explanation of the earth axis and it's orbital plane is
very well explained.

Purely as a hobby, I'm setting up a model of the Local Group. I'm
establishing the elliptic plane along with the galactic planes of the
galaxies considered to be in the Local Group, in three dimensions. The
model won't even be close to scale, but when finished I'll have a
better picture of how the Solar System fits in our small part of the
Universe.


Tim ---- One way to make life a lot less complicated is to NOT even try
and learn about all these things. I've got this big problem though
------- I'm curious .... I am REALLY curious how all this stuff goes
together ....... lol. In high school I can recall reading about Eros,
Chaos and the gang, but you have to admit that it's more fun trying to
learn the facts .......... right?

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.

I hesitate to mention is here but... do you believe in a Creator? I do and
from my personal experience, I consider many of the theories proposed by man
to be just plain faulty. Yes, we've figured out a few things, but there are
far more concepts that will never be determined.... although some will
continue to try.

Good luck coming up with a reasonable estimate.

Jason

"Chuck" wrote in message
oups.com...
I try to read as much as I can about the latest discoveries of the
Universe. I find there are considerable differences in the estimates
of size and distances, though.

More times than not, the diameter given for the Milky Way is 100,000
light years. The estimated number of stars in it varies a lot and I've
read where they range from 100 billion to 200 billion stars.

Andromeda is estimated to be from 100,000 to 200,000 light years in
diameter. The number of stars it has is estimated to be from 100
billion to 500 billion stars.

There are varied estimates of from 100 to 250 billion galaxies in the
Universe.

It seems there are two general lines of theory about the overall
Universe. One theory says it's finite. I've read where the estimated
size may range from 55 billion to nearly 160 billion light years in
diameter. The other theory says the universe is infinite. I'm very
interested in both theories.

Even trying to find the correct average diameter of the Solar System
proved to be difficult.

Understandably, none of these estimates and/or theories can be
absolutely precise. Ever increasing technology will also bring
continuous changes. What is considered to be the most accepted
source(s) for current info on each of these many sizes and distances?

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.

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