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BASICS .. COSMIC RAYS .. SUN .. SPACE WEATHER

Ask Us

Space Physics:
Galaxies in General

How Are Galaxies Born?
Distance to Galaxies
Number of Galaxies in the Universe
Types of Galaxies
What's Between the Galaxies?
Motion of Galaxies
Spiral Galaxies and Magnetic Fields
Black Hole at the Center of the Galaxy?

  1. How Are Galaxies Born?

    How are galaxies born?

    This isn't our specialty, but you might want to check out the Imagine the Universe! page on this subject.

    Beth Barbier

  2. Distance to Galaxies

    How many galaxies are close to our galaxy? What is the farthest galaxy seen from Earth?

    Our galaxy (the Milky Way) is one of at least 17 galaxies that are called the "Local Group". There are probably other galaxies in the Local Group that we haven't seen yet (blocked by gas or the rest of the Milky Way). The Andromeda galaxy (M31) and the Milky Way are the two largest galaxies in this group. They are all within about two million light years of us.

    The farthest galaxy seen is about 13 billion light years away. You can see the press release at this Hubble web site.

    Dr. Eric Christian

  3. Number of Galaxies in the Universe

    What is an estimate of the number of galaxies in the observable Universe?

    Current estimates are that there are billions to a few tens of billions of galaxies in the observable Universe.

    Dr. Eric Christian

  4. Types of Galaxies

    Do all galaxies appear in a cartwheel '2D'-like format or can they have many arms which extend from the center in a 3D fashion?

    There are roughly three types of galaxies: elliptical, spiral, and irregular.

    • Elliptical galaxies are just relatively uniform balls (although not necessarily spherical) of stars.

    • Spiral galaxies, like our Milky Way, are typically flat disks (plates) with a bulge at the center. The cartwheel arms form in the disk, and so are '2D' not '3D'. The thickness of the arms in the Milky Way is a few hundred light years, but the disk is 100,000 light years in diameter.

    • Irregular galaxies are ones that aren't elliptical or spiral, like the Greater and Lesser Magellanic Clouds.

    More information is available from the Students for the Exploration and Development of Space (SEDS) website.

    Dr. Eric Christian
    (May 2002)

  5. What's Between the Galaxies?

    What exists between galaxies? Is it just a vacuum of space, or do they all butt up to one another?

    There is lots of space between galaxies. The nearest galaxy close in size to the Milky Way is Andromeda, and that is 1,600,000 light years away, although there are smaller ones closer (like the Magellanic Clouds). They do not butt up against each other, although they can collide.

    Dr. Eric Christian

  6. Motion of Galaxies

    Are there any galaxies that aren't moving outward, but inward or in any other direction? If there are, doesn't this prove the Big Bang theory wrong?

    The motions of galaxies have been changed by gravity over the billions of years since the formation of the universe. These are called peculiar motions and don't change the fact that "on the average" galaxies are moving away from us, and the farther they are, the faster they are moving. That is all that is implied by the Big Bang. Not every galaxy has to be exactly moving away from us at an exact speed.

    Dr. Eric Christian
    (May 2002)

    If this is true, then how is it that the Andromeda Galaxy is predicted to collide with the Milky Way Galaxy in roughly 3 billion years? Is one galaxy somehow going to "overtake" the other in the future?

    Galaxies collide all the time (you can check out some pictures on Astronomy Picture of the Day).

    For every galaxy, within a certain distance, gravity will be the dominant factor (over the expansion of the Universe). The larger the galaxy -- and both the Milky Way and Andromeda are larger than average galaxies -- the larger the volume in which their gravity will be the main effect.

    You can estimate the distance by setting the gravitational potential energy equal to the kinetic energy from Hubble expansion (it won't be a sharp boundary because of random velocity scatter of the galaxies).

    GMm/r = mv2/2

    M, here, is the mass of the prime galaxy, m is the mass of the secondary (which cancels out of the equation), v is the Hubble velocity = Hubble constant * r. So you get a distance of

    r = (2GM/H2)1/3

    When I plug in the numbers, I get about a million light years for the Milky Way, which is the right order of magnitude for Andromeda (which is a bit over 2 million light years distant).

    Dr. Eric Christian
    (June 2008)

  7. Spiral Galaxies and Magnetic Fields

    Does the Parker spiral (the spiral shape of the Sun's magnetic field) apply to other astronomical objects, like spiral or barred spiral galaxies?

    The simple answer is just that the magnetic field in the Galaxy is too weak to do much of anything on the level of galactic structure. Gravity is far stronger. We know this because the magnetic field will lead to atomic line splitting (this is the Zeeman Effect: magnetic fields split atomic energy levels into a larger number of levels so the spectral lines are also split, with the pattern and amount of splitting related to the strength of the field). We can measure the spectrum of light passing to us through the Galaxy and look for this signature. It is so weak that we have only estimates for the local magnetic field strength and it is about 1000 times weaker than the magnetic field on the surface of the Earth.

    In addition, there would need to be significant electrical charge and current associated with massive bodies for the magnetic field to have an important dynamic, and we don't see this.

    Lastly, we know that there are some energetic charged particles called cosmic rays that have too much energy to be confined by the magnetic field of the Galaxy. If the magnetic field were strong enough to affect the location of massive stars, these cosmic rays would be confined to the Galaxy and not escape.

    What we do know, in part from line splitting, is that the magnetic field in interstellar space is organized in patches that do not communicate with each other. We are in a local patch, or cloud, within the Milky Way and expect to be there for a few million years. Within that cloud we think the magnetic field is fairly well ordered, but it is different from the field in the next patch over, with both still within out Galaxy. So the Galaxy fails to provide the large-scale ordering of the magnetic field that the spiral arms would seem to suggest.

    As far as I understand, I think the spirals are formed by gravitational instabilities that cause massive objects to clump and I think the gravitational field of the clumps reaches out to other stars and pulls them along to create a more-or-less smooth spiral. At least, that is the mechanism as I understand it.

    Dr. Charles Smith
    (January 2006)

  8. Black Hole at the Center of the Galaxy?

    I have heard that black holes have been found in the centers of other spiral galaxies besides our own. Does the Milky Way have one?

    Probably. You can read more about this at our sister site Imagine the Universe!

    Beth Barbier

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BASICS .. COSMIC RAYS .. SUN .. SPACE WEATHER

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A service of the Heliophysics Science Division at NASA's GSFC

Questions and comments to: cosmicopia@cosmicra.gsfc.nasa.gov
Curator: Dr Eric R. Christian, NASA
Responsible NASA Official: Dr Eric R. Christian

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This file was last modified: October 30, 2008