Minimum aperture for globulars and galaxies

(Tony Flanders) writes:

Even a 60mm scope is big enough to classify most of the Messier galaxies
as spiral versus elliptical with a reasonably high degree of confidence,
and M31 is obviously a spiral to the naked eye. Not that you can see
the spiral arms, but no elliptical galaxy could possibly have that
shape and that kind of central condensation.

You are exaggerating _grossly_ on both acounts, IMO. First, a small
scope (most scopes, really) typically shows just the central hub of
spiral galaxies, and these are not much differen from ellipticals.
Second, one only has to glance over at neighboring M110 to see an
elliptical galaxy that looks very similar to the naked eye view of M31.

pej
--
Per Erik Jorde

Per Erik Jorde posted:

Even a 60mm scope is big enough to classify most of the Messier galaxies
as spiral versus elliptical with a reasonably high degree of confidence,
and M31 is obviously a spiral to the naked eye. Not that you can see
the spiral arms, but no elliptical galaxy could possibly have that
shape and that kind of central condensation.


You are exaggerating _grossly_ on both acounts, IMO. First, a small
scope (most scopes, really) typically shows just the central hub of
spiral galaxies, and these are not much differen from ellipticals.
Second, one only has to glance over at neighboring M110 to see an
elliptical galaxy that looks very similar to the naked eye view of M31.

Well, he might be exagerating a little, but a good 60mm refractor when used at
a *low* enough power under reasonably dark skies will show M31 and its much
fainter extended outer haze. Its core region is usually the only thing which
shows up well at the moderate powers that many of the common "department
store" refractors use, but drop that power to 20x and the faint vaguely patchy
outer extensions become quite easy to see, along with the marked dropoff of
light intensity along the northwest side marking the location of the first
major dark lane. This kind of brightness profile (small brighter nuclear
region and extended faint outer haze) is enough to show that M31 is a spiral
galaxy, although it does not show what kind it is nor does it show much of the
arm structure. M110 on the other hand does not show the kind of smaller and
markedly brighter central core which would hint at it being a spiral. Its
smooth edges, slowly increasing brightness level with decreasing distance from
the center, and the lack of a smaller fairly well-defined brighter (but
non-stellar) core indicate that it is probably an elliptical galaxy, an
impression which is confirmed with larger apertures.
It is quite easy to use the overall brightness profile of galaxies which
show little other structure to get at least an idea of whether they are
spirals or ellipticals. In addition, with an 8 or 10 inch telescope, I can
usually tell what kind of galaxy I am looking at for a *large* number of
galaxies as faint as 13th magnitude (or sometimes fainter). With that
aperture, I can usually see at least some indications of structure (mottling
or irregular edges, small bright nucleii, dark lanes) that can point to the
object being a spiral without necessarily seeing the spiral arms. Of course,
with a ten inch, there are a number of galaxies which will visibly show the
spiral structure, so for those at least, there is little doubt of the
classification. Even without those, the brightness profile is sometimes
enough on its own to tell whether a galaxy is an elliptical or a spiral.
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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Per Erik Jorde wrote in message ...

First, a small
scope (most scopes, really) typically shows just the central hub of
spiral galaxies, and these are not much differen from ellipticals.

Well, central hub means different things to different people, but
it's true that when a galaxy is at the edge of visibility, you rarely
see the full extent as listed in the catalogs. But the fact remains
that whenever I see a new galaxy -- and most of those are pretty faint
for whatever scope I'm using -- I try to classify it as a spiral,
ellipse, or irregular. I'm certainly wrong sometimes, but I'm right
much more often than not.

I've even seen a fair number of galaxies right at the edge of
averted vision, where I wasn't 100% sure if I was seeing them
at all, but I was 100% sure that if I was, it must be an edge-
on spiral because no elliptical ever has such a thin aspect ratio.

It would be fun to try this particular experiment with the Messier
galaxies and a 60mm scope, but alas, I probably have the classification
of most Messier galaxies buried somewhere in my mind, so it wouldn't
be a fair trial. Maybe if somebody else found them for me, and didn't
tell me which one I was looking at ...

Second, one only has to glance over at neighboring M110 to see an
elliptical galaxy that looks very similar to the naked eye view of M31.

I'm sceptical, but I'll give it a whirl next time I'm under dark skies.

- Tony Flanders