Expert: Tom Whiting - 6/6/2011

Question
QUESTION: Hello..
I have two questiohs
1-   Is all the stars are spheric like our sun? if yes.. why they weren't discoid ? in other words;; since the sun spins around it's centre it must be a disc, not a sphere.. like the galavy!
2-   In winter the earth is between the sun and the centre of the galaxy.. am I right?
3-   Thank you
4-   Dr. Barzaq

ANSWER: Hi Mohamed,
The sun is not perfectly spherical either... but differs by 0.001% or only about 10-20 miles between it's polar and equatorial diameter, see
http://www.nasa.gov/topics/solarsystem/features/oblate_sun.html
Being the most massive object in the Solar System also makes it the smoothest and least oblate.
Many stars spin very much faster, like in a matter of a few hours...  some fast enough to even look like American footballs (as opposed to the English soccer ball which they call "footballs");  or also called oblate spheroids.

2. Almost, but not exactly. The center of the Milky Way Galaxy is at 17Hr 46 minutes of Right Ascension, and -29 degrees Declination.  Whereas the sun on December 19th is at 17hr 46 minutes of RA but at -23.3 degrees Declination. (Near the star 58 Ophiuchus) So that's about 5.7 degrees north of the galactic center on December 19th.  (one fist-width at arm's length).  The first day of winter for the N. Hemisphere is typically December 21st or 22nd... this year winter begins at 0530 Hr GMT or Universal Time on the 22nd).
This is all coincidental, of course. Due to precession of the Earth's axis, that date of winter changes by one day every approximately 71 years  (25,800 years precession divided by 365 days in a year). That will also slowly change the point in our sky where the winter solstice occurs by a small amount every 71 years too, so in a few thousand years the winter solstice will occur well
west of the galactic center. (Axial precession causes all the cardinal points, both equinoxes and both solstices, to change their location in our sky slowly, once around a complete circle in the sky every 25,800 years.)
It's probably also coincidental that the plane of the Solar System has it's tilt with one edge pointing kind of toward the galaxy center, but our Solar System plane is far removed from the central plane of the Milky Way galaxy itself. That's why we can see the Milky Way so well, crossing from NE to SW when it's overhead in our summer night sky.

So if your original intent is to combine the gravity of the galactic center and the sun to cause a catastrophe, you can forget it... we arrive at that point once a year, and we've been doing it for centuries, once a year.  And we're still here.  The effects of the gravity of the center of our galaxy on our Solar System is barely measureable, let alone catastrophic... if that's what you are thinking. (Remember for the force of gravity, you'd put distance squared, or 25,000 light years squared, in the denominator of the equation for gravitational force!) That would be a huge number if you convert it to, say meters, and square it.
Clear Skies,
Tom Whiting
Erie, PA

---------- FOLLOW-UP ----------

QUESTION: hello..
thank you ..you've fully answerd my second question.. but it's stell not obvious in your answer of my first question, it seems you forgot to clarifay why the sun is not a disc? what makes it have poles?

Answer
I'm not sure what you mean by a "disc"... you mean that you think the sun is just 2 - dimensional
flat object, as opposed to a 3 dimensional sphere?  Which would be a bit crazy, considering that the sun contains 99.86% of the mass of the entire Solar System.  (Jupiter having 0.1% and all the other stuff accounts for only the remaining 0.04%, including the Earth.)  So, surely you don't mean that definition as a "disc". (Plus now, we have satellites almost completely surrounding the entire sun, showing the same picture of a ball, at the same time.  In fact, very soon we will be able to deliver sunspot counts over the ENTIRE sun, and not just the Earth-facing side!)  It will probably be called the "enhanced sunspot count"... like tornadoes.

As far as flattening out, all massive objects, even the much smaller planets, pull themselves gravitationally into a basic ball shape because that presents the lowest surface area per unit mass. And since our sun rotates very slowly (once every 28 days or so at the equator), there is not that much angular momentum (centripetal force) to convert it into a flattened disc, as the Milky Way has done, because its a conglomeration of individual stars are separated by light years. So bodies like the sun and the stars are more like the spherical, elliptical galaxies... as opposed to the rarer spiral galaxies with arms and flattened discs. Remember that the sun is composed of mostly plasma (a hot, highly compressed gas that acts more like a liquid than a gas)... and all liquids in a zero-gravity environment like space, automatically revert to the spherical shape.  So that's why stars are basically ball-shaped, and not flattened discs.
Clear Skies,
Tom