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Subject: Why isn't the ecliptic in the same plane as the Milky Way galactic disk?

Date: Thu Apr 13 13:36:33 2000
Posted by Doug
Grade level: grad (science) School: none
City: Omaha State/Province: NE Country: USA
Area of science: Astronomy
ID: 955650993.As
Or..why don't the planets orbit the sun in the same plane as the sun orbits the galactic center? I have been searching for the answer to this question ever since my then 10 year old son asked it about 2 years ago while stargazing from a dark site. Imagine my surprise when I found your site and a previous question which was very similar to my son's! The answer to question left me a little confused though! The writer suggested that the angular momentum of the stars was related to the original cloud that collapsed to form them and not the galaxy angular momentum. Reviewing the information found at on angulat momentum did not help me understand the answer. As I understand the answer to, if the stars orbit is not related to the galaxy spin, then I would expect the stars to orbit the galaxy at a multitude of random orbits and no disk to form. I guess an easier way to say it is the original answer does not answer the question of what causes the original angular momentum of the original cloud? And more likley the original answer answers my question but i don't understand it!

Part of the problem here is a confusion between rotation and revolution. Think of rotation as spinning, and revolution as orbiting.

The answer to question 902372110.As says that the spin of stars is not necessarily related to the Galactic rotation. The rotation of the Galaxy as a whole is of course defined by the orbits (revolution) of the stars, and the vast majority of stars we see orbit the Galaxy in the same direction. That's why it has a plane at all!

While the stars revolve around the center of the Galaxy, they spin on their own axis like a top. The direction of that axis has nothing necessarily to do with the Galaxy; they can point any which way.

The idea is, a star forms from a gas cloud. This cloud is orbiting the center of the Galaxy, so the stars that form from it will also orbit the Galaxy. However, that cloud may suffer a random collision and collapse. When it does, it itself starts to spin, and the direction of that spin depends far more on internal dynamics than the outside Galaxy. So while the stars that form from it will orbit the Galaxy, they will spin on their own axes randomly.

That is why the ecliptic, the plane of Earth's orbit, is not aligned with the plane of the Galaxy. As a matter of fact, they are currently tilted with respect to one another by about 60 degrees!

©2008 Phil Plait. All Rights Reserved.

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