Ask Us

Space Physics:
Milky Way Galaxy

What Does Our Galaxy Look Like?

How do we know what the Milky Way looks like?

This question has been answered by our sister site, Imagine the Universe!.

Beth Barbier

Shape of Our Galaxy

Why does our galaxy look like a disk? Why are all of the stars rotating at the same "level," and not spread out in a big cluster like a ball?

Galaxies form from condensing gas. If there is enough rotational energy in the material, it will flatten out as it condenses and end up as a spiral galaxy. (As opposed to ellipticals, which have little to no rotational energy.)

You can see a movie of galaxy formation at the Carnegie Observatories website. Choose the spiral galaxy movie.

Dr. Louis Barbier
(November 2001)


Where Does Galaxy End?

How can you tell where our galaxy ends and another begins?

Galaxies are large groups of stars that are held together by their mutual gravitational attraction. Although they are very large (the Milky Way galaxy that the Sun is in is 100,000 light years across) the distance between galaxies is even larger (the Andromeda galaxy is 2 million light years away). There is a lot of nothing between the galaxies. So it is easy to tell where one ends (when there are no more stars) and where the next begins (where the stars start again).

For more information on galaxies, you can check our sister site, StarChild.

Dr. Eric Christian

Does Speed of Galaxy Affect Time?

What is the velocity the Milky Way or Local Cluster moves from the origin of the Big Bang, and does this speed affect our time? For instance, would time move much faster if the galaxies/Universe ceased movement?

The Milky Way appears to be moving at 600 km/sec relative to the primordial background radiation (the remnant of the Big Bang). For more information on this, you can check the Astronomy Picture of the Day archives.

This speed is much less than the speed of light, however, so the effect on time is negligible.

Dr. Eric Christian

Black Hole and the Solar System

Would the solar system, if still remaining after the Sun goes supernova, be eventually pulled spiralling into the black hole that's in the center of the Milky Way?

You've got two common misperceptions here. A black hole doesn't have any more gravitational attraction than the star or whatever that formed it. If the Sun instantaneously turned into a black hole, the Earth and all the other planets would orbit just the same with no change in gravity. Because black holes are much more concentrated, however, you can get closer to them than you could to the Sun and get to a region where the force of gravity is much higher. But at the distance of the Earth there is no difference.

The other common misperception is that the Sun is going to go supernova. Only the largest of stars can go supernova. Our Sun is much too small. It will eventually expand to a red giant, and then contract down to a white dwarf.

Dr. Eric Christian