What's the opposite side of the sun? (APOD 11 Jul 2008)

[indiaaditya]

Hi All!

http://apod.nasa.gov/apod/ap080711.html
mentions the following statement:

"The arm is labeled in this illustration as the Far 3kpc Arm, located at a distance of 3 kpc (kiloparsecs) or about 10,000 light-years from the galactic center, on the opposite side from the Sun."


How can there be an OPPOSITE SIDE to Sun. I mean it's a sphere so everything essentially is opposite to it. Is'nt it?

Let me know!

Cheers!
Aditya

[henk21cm]

"The arm is labeled in this illustration as the Far 3kpc Arm, located at a distance of 3 kpc (kiloparsecs) or about 10,000 light-years from the galactic center, on the opposite side from the Sun."

How can there be an OPPOSITE SIDE to Sun.

What is meant: the arm is at the side of the galactic center, opposite of the sun. In other words: APOD means that the arm is not at the same side as our sun, you will have to pass the galactic center and then encounter the arm. It is partly obscured by the galactic center/bar.

[bystander]

Does this mean we now believe there are four spiral arms to the Milky Way, again? Or 6? Or are we still at 2?

See http://antwrp.gsfc.nasa.gov/apod/ap080606.html
and http://asterisk.apod.com/vie ... hp?t=14074

[rigelan]

I was just wondering that same question, bystander.

It had been four. Then it was two. Now its up to four again?

Its evidently highly speculative. Maybe the ones on the far side are joined together into one massive arm, and the ones on this side are one massive arm. Then there are sub-arms.

I'm willing to bet its just in how you NAME an arm. Maybe the far 3KPC arm connects up to the near arm of Crux-Scutum. Is it considered one arm or two?

[BMAONE23]

I believe the arm refered to is considered a Minor Arm but that the galaxy still considered to have only 2 Major Arms.

[bystander]

It appears to me that the 3kpc arms connect the Galactic Bar with the Long Bar where the major arms (Perseus & Scutum-Centaurus) seem to begin. The minor arms (Norma and Sagittarius) seem to begin at the end of the Galactic Bar. I wonder which arms (and bar) are older.

I know this picture is an artist's illustration, but i assume it's based on observation. It's decidedly different than the illustration 4 years ago when there were two arms with branches.

http://antwrp.gsfc.nasa.gov/apod/ap050825.html

[apodman]

This picture shows four arms:

http://samhainmoon.com/2008/05/lakshmi.html

8)

[rigelan]

Lakshmi. . . Haha.

So it seems our current model contains these as the different parts of our galaxy:

A double bar in the center.
A set of 3kpc arms around the center.
A Major Arm originating from each end of the Bar = Perseus & Scutum
A Minor Arm originating from each end of the Bar = Norma & Sagittarius
Several other Minor Arms throughout that originate elsewhere or are part of an already named arm = Orion, Cygnus, Carina, Others?.

[bystander]

I think the Cygnus, Outer, and Norma Arm are all the same, the Cynus-Norma Arm? The Carina Arm is the same as the Sagittarius Arm? The Orion Arm is a minor spur of the Sagittarius Arm? or is it the Perseus Arm? I'm so confused?

http://en.wikipedia.org/wiki/Milky_Way#Spiral_arms

Or maybe it's like this - 10 arms.

http://en.wikipedia.org/wiki/Mahadevi

[rigelan]

Something like that. . .

[apodman]

Old views of the "mapped" Milky Way showed a big blind wedge on the "other side" of the large and obscuring Galaxy Center.

Allowing that our methods have improved for seeing through the central region, we should still be surer (correlated by more methods and with a better view) of the arms on "our side" of the galaxy.

So the desire to make a balanced picture and the actuality of publishing balanced illustrations belies the truth that, until we get a z-axis view beamed to us, we will continue to know more about "our side" no matter what advancements are made.

Now compare the quest to discover and name the spiral arms to the quest to do the same for the contents of the solar system. In the solar system we are dealing with objects, and it still took a long time to get a handle on the contents. The arms of the galaxy are not objects to discover, rather associations among great numbers of objects and looser materials.

So not only do we have to observe the objects and other materials that make up the arms, but we have to find evidence that they are associated in a mappable arm. That might take a long time, as a discovery is not a discovery, it's an arguable opinion.

Now consider Pluto. After we had a pretty good handle on the contents of the solar system, we went back and recategorized the planets and other large objects. After the good name of Pluto has been spread around the world for decades, nobody in school will learn it as part of the list of the planets any more. In the distant future, the same will be true of some spiral arm or other. A millennium of school children will grow up knowing its name, and then some day it will be demoted and forgotten.

So, rigelan and bystander and anyone else out there who has studied all the charts, your list is good enough for me for now. But in 10,000 years I bet there will be some changes.

[bystander]

The New Tourist's Guide to the Milky Way - 2006 Feb 27 (4 arms)
http://www.space.com/scienceastronomy/0 ... _tour.html

New Images: Milky Way Loses Two Arms - 2008 Jun 03 (2 arms)
http://www.space.com/scienceastronomy/0 ... -arms.html

[iampete]

. . . The arms of the galaxy are not objects to discover, rather associations among great numbers of objects and looser materials.

So not only do we have to observe the objects and other materials that make up the arms, but we have to find evidence that they are associated in a mappable arm. That might take a long time, as a discovery is not a discovery, it's an arguable opinion. . .

And, of course, one must never forget that the more possible arms configurations, the more arguments/opinions about arms, the more different possible maps, etc., etc., the more papers, theses, and dissertations that can be published.

This is how science works - the more trees that are killed, the closer we get to the truth!!!

[Andy Wade]

And, of course, one must never forget that the more possible arms configurations, the more arguments/opinions about arms, the more different possible maps, etc., etc., the more papers, theses, and dissertations that can be published.

This is how science works - the more trees that are killed, the closer we get to the truth!!!

Reminds me of the great computer Deep Thought from The Hitch Hikers Guide to the Galaxy when it challenged the philosophers who were protesting about the great computer putting them out of business.
They were worried that Deep Thought was going to give them God's telephone number but instead it gave them the answer to "Life the Universe and Everything" as 42, but then posed a further question of "what was the actual question in the first place?" and designed the Earth as an even greater computer to answer this over millions of years of computing, thereby ensuring millenia of questions and discussion for the philosophers, keeping them in work for the rest of their lives.

[astrolabe]

Hello iampete and Andy Wade,

My vision of the nightmare future is, after all the trees are gone, the last barrel of crude will be used to process and then extrude the mother of all delete buttons. This is because instead of holding one's finger over a nuke button down in some bunker (all the warheads were dismantled for use as nuclear fuel by the controlled-explosion theory of hydroelectricity) the bunkers will have people with their fingers poised over delete buttons waiting for the one with the BIG code to issue the ultimate command.

When it's over the one with all the pertinent information wins.

Or the terrorists tunnel down and pull "THE BIG PLUG" out of the wall.

[apodman]

Nobody disconnects a dangerous thinking machine like James Tiberius Kirk.

[starnut]

Hi All!

http://apod.nasa.gov/apod/ap080711.html
mentions the following statement:

"The arm is labeled in this illustration as the Far 3kpc Arm, located at a distance of 3 kpc (kiloparsecs) or about 10,000 light-years from the galactic center, on the opposite side from the Sun."


How can there be an OPPOSITE SIDE to Sun. I mean it's a sphere so everything essentially is opposite to it. Is'nt it?

Let me know!

Cheers!
Aditya

A little lesson on English usage: Opposite side OF the sun is not the same as opposite side FROM the sun.

Gary

[apodman]

Yes, this is astronomy-related. It just might not look that way out of context:

Preparing my gourmet breakfast, I realize we are one pulverized apricot trying to count the other pulverized apricots in a jar of preserves.

And the following (according to questionable authorities) is what's ON the opposite side of the sun:

http://www.zetatalk.com/science/s37.htm

[astrolabe]

Hello apodman,

And the knife is......................?

[apodman]

And the knife is ... ?

The knife is the unknown cosmic phenomenon (surely caused in some manner by the distribution of gravity over time and space) that makes galaxies develop spiral arms.

The jar of preserves is initially irregular, and the knife has to stir it a few times to develop the appearance of spiral structure.

Thank you for asking. This may be the end of the line for this analogy, though.

[kovil]

The Far 3kpc Arm is located on the far side of the galactic center, from our Sun. The near 3kpc Arm is located on our side of the galactic center.

Other than that, the Long Bar going thru the galactic center has a sudden brightening of stars where it touches the spiral arms. As the arms pass the Long Bar they quickly diminish in brightness and stellar activity, suggesting that the galactic electric current flowing in the spiral arm goes into the Long Bar and discontinues flowing, for the most part, along the spiral arms as they go further centerward in their spiral. This suggests the the Long Bar is the primary electrical current path in the galaxy, as the two spiral arms it touches are rampant with electrically overstressed stars which are shining brightly. The other spiral arms are much dimmer with activity. The Galactic Bar is perhaps likened to Earth's axis of rotation, whereas the Long Bar is likened to Earth's magnetic pole and where the inter solar system's electrical activity is drawn to happening on Earth from space.

[apodman]

the Long Bar going thru the galactic center has a sudden brightening of stars where it touches the spiral arms.

You do realize that the illustration on which you base your interesting analysis is an "Artist's Concept", not a photograph. The bright areas to which you refer may not exist.

http://www.spitzer.caltech.edu/Media/re ... -10b.shtml

[Qev]

the Long Bar going thru the galactic center has a sudden brightening of stars where it touches the spiral arms.

You do realize that the illustration on which you base your interesting analysis is an "Artist's Concept", not a photograph. The bright areas to which you refer may not exist.

http://www.spitzer.caltech.edu/Media/re ... -10b.shtml

It's not really unreasonable to predict them being there, though. Other galaxies with this structure do tend to have brighter areas there. Randall's post over in this thread is almost a dead ringer for the proposed Milky Way.

[iamlucky13]

The polar grid overlayed on the image is confusing because it confounds following the arms and it isn't centered on the galactic center. I was starting to think it was some ridiculous artistic touch to make the image look more technical.

However, looking at the big image, I finally realized it was centered on us. Duh!

It's great image for showing the complexity of the galaxy. The main arms are evident, but looking closer the spurs, gaps, and dust lanes wind all over the place. Very cool.

[henk21cm]

The polar grid overlayed on the image is confusing because it confounds following the arms and it isn't centered on the galactic center. I was starting to think it was some ridiculous artistic touch to make the image look more technical.

However, looking at the big image, I finally realized it was centered on us. Duh!

The grid is a representation of the galactic coordinates. That is a coordinate system, meant for projection on a sphere. Lets start simple. An observer thinks, when he/she looks out at night, that all stars are located on a giant sphere, with the observer in the center of the sphere. All stars seem to rotate around the "polar star". Perpendicular on the line "polar star"-observer, there is the equatorial plane.

Since the rotation axis of the earth has an angle of about 23.5 degrees with the plane of its orbit around the sun, the ecliptic makes the same angle with the equatorial plane. One can think or draw a plane through the ecliptic: the ecliptical plane. When one wants to describe the position of the planets and the sun, it is much easier to use the ecliptical plane and accompanying ecliptic coordinates. Usually the ecliptical height is small for the planets: a few degrees. In other words: the ecliptical plane is coplanar with the orbits of the planets around the sun.

Similarly there is a system called galactic coordinates. The plane of that coordinate system is coplanar with the "milky way". The galactic inclination of stars in the galactic plane are usually small. The 0 "meridian" of the galactic coordinates coincides with the galactic center.

If you look carefully, you will see that at the top of the big image "galactic longitude 0" is written, at the line from the sun through the galactic center. The origin of the polar grid is located at the sun.