Re: a question about milky way and constellations
- From: canopus56 <canopus56@xxxxxxxxx>
- Date: Thu, 09 Aug 2007 11:40:42 -0700
On Aug 8, 2:32 pm, "beow...@xxxxxxxxxxx" <r.oeler...@xxxxxxxxx> wrote:
<snip>
i mean, when i see
casseopia, or cygnus, *where* am i looking in terms of earth and the
galaxy? Am i looking towards the center of the galaxy? away from the
galaxy center but through our arm of the galaxy, etc.?
Mellinger's Virtual Reality MilkyWay Galaxy will help you pair up
those constellations with the band of the Milky Way's disk and the
core of the Milky Way.
http://canopus.physik.uni-potsdam.de/~axm/mwpan_vr.html
Most planisphere wheels have the outline of the Milky Way printed on
them.
In addition to the many good recommendations listed above in this
thread, I'd add Tirion's _Binocular Astronomy_. It has sections
describing each arm of the Milky Way as it can be seen during each
season.
http://www.willbell.com/handbook/hand2.htm
Another excellent out-of-print book that can sometimes be found in
libraries is _The Guide to the Galaxy_ Cambridge University Press 1994
by Heather Couper and Nigel Henbest. This runs through objects that
are visible in each galactic arm.
Presently, we are entering August - the "Grand Display" month, in
which the spinward disk of the Milky Way from the Galactic Core are
seen _through_ the nearby stars in Sagittarius, Cygnus and ending at
Perseus Double Cluster between Cass and Perseus.
Taking the galactic core at the 0 deg point and tracing a 120 deg arc
from Sag, to Aql, to Cyg (90 degs from Sag), thence to Cass and
Perseus, you are looking at the spinward direction of arms of the
Milky Way spiral galaxy. The problem interpreting this are a 3-D
object is that everything is so distant you have no depth perception.
A northern hemisphere view of the horizon looks something like this -
http://members.csolutions.net/fisherka/astronote/Clarkxref/img/OrbitAE_9PM.jpg
- with the Milky Way's disk about 30-40 degrees above the eastern
horizon at 9 pm.
The anti-galactic center (180 degs from Sag) in the direction of the
constellation Orion is under your feet, now behind the Earth.
You are embedded inside and are surrounded by the Orion-Cygnus Arm.
Most of the bright stars that you see are a halo of stars within 400
parsecs or 1,200 light-years of the Earth.
Looking directly at Sag you are looking through the next arm in from
our Orion-Cygnus Arm - the Sagittarius Arm and towards the galactic
core.
Along our 120 deg arc from Sag to Aql (at 20 to 30 degs from the
core), you are also looking down the spinward direction of the
Sagittarius-Carina Arm. The next arm in towards the galactic core in
this same direction is the Scutum-Crux Arm. Almost no objects that
are visible in amateur class telescopes can be seen in the Scutum-Crux
Arm.
The outer edge of the core of the Milky Way can be seen in M24, the
Small Sagittarius Star Cloud. This is really just a hole in the
clouds in the Sagittarius and Scutum Arms that are letting part of the
bright galactic core peak through.
http://www.seds.org/messier/m/m024.html
http://www.library.gatech.edu/Barnard_Project_W/plate/Bar-pt1-pl027_sm.jpg
Sag), you are looking down the spinward side of our Orion CygnusFrom Aql through Cygnus (40 degs to 90 degs from the galactic core in
Arm.
The solar system's apparent direction of travel is at an oblique angle
relative to the galactic disk. You are moving "up" of out the galactic
plane towards a point in the constellation Hercules - that sits above
the galactic plane. This is called the solar apex.
http://members.csolutions.net/fisherka/astronote/Clarkxref/img/SolarApexPM.jpg
The true motion of the solar system (as opposed to its apparent
motion) is somewhat different. The solar system moves in a wave
pattern in a direction that is out of the galactic plane, but over
time, gravity pulls it back in towards the galactic plane. Because
the solar system is orbiting in a spinning disk, the true motion of
the solar system is towards Aql, not Hercules.
When you look towards Cygnus and down the spinward direction of the
Orion Arm, your view is blocked by large gas clouds between 250 and
750 parsecs distant (about 750 and 2,000 light-years). Supernovae
have evacuated cavities or "cells" inside these clouds. Deneb (alf
Cyg) above the Great Cygnus Drift and Northern Coalsack is an
example.
Along the arc from the galactic core lies Cass and Perseus at about
120 to 130 degrees from the core. Here, you are looking at an oblique
angle out the spinward side of the Perseus Arm. The Perseus Arm is
the next galactic arm outside of the Cygnus-Orion Arm - away from the
galactic center. The showcase Perseus Double cluster are open
clusters that are exiting the "inside" of the gas wall of the Perseus
Arm.
http://www.seds.org/messier/xtra/ngc/n0869.html
Since you are looking "out" of the galactic disk, note that the bright
star clouds found in Cygnus, Scutum and Sagittarius are no longer
seen. The "Milky Way" appears thinner and dimmer. There is less star
formation going in the outer arms.
When Orion swings around into view next fall, you will be looking
through Orion and towards the inside edge of the outer Perseus Arm.
Orion is located 180 degrees from the galactic core in Sag.
Also note that the two great star forming clouds in our local part of
the galaxy - M42 in Orion and M8 in Sagittarius are located about 180
degrees apart. M8, the Lagoon nebula is embedded in the "back" or
"outside" of the next arm in - the Sagittarius-Carinia Arm. M42 - the
Great Orion Nebula - is in a cloud between the Orion and Perseus
Arms.
http://www.seds.org/messier/m/m042.html
http://www.seds.org/messier/m/m008.html
Hope that helps. - Canopus56
.
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