by neufer » Tue Nov 10, 2009 12:15 pm
john_crompton wrote:I have a supplementary question on this topic... If the universe is expanding as we believe, surely objects as large as galaxies are all so far apart and all moving away from each other at such a rate that a collision shouldn't happen???
Help me out here someone!
With the universe expanding as we believe, objects as large as
galactic clusters are all so far apart and all moving away from each other at such a rate that a
galactic cluster collision shouldn't happen. This situation hasn't happened with galaxies quite yet and astronomers even predict a Andromeda-Milky Way collision in about 3 billion years.
http://en.wikipedia.org/wiki/Andromeda-Milky_Way_collision wrote:
<<The Andromeda-Milky Way collision is a predicted galaxy collision that could possibly take place in approximately 3 billion years' time between the two largest galaxies in the Local Group – the Andromeda Galaxy and the Milky Way which contains Earth.
It is often used as an example of the kind of phenomena associated with such collisions in simulations. As with all such collisions, it is unlikely that objects such as stars contained within each galaxy will actually collide, as galaxies are in fact very diffuse—the nearest star to the Sun is in fact almost thirty million solar diameters away from the Earth. (If the sun were scaled to the size of an American quarter, 24.26 mm (0.955 in), the next closest quarter/star would be 700 km [475 miles] away.) If the theory is correct, the stars and gas contained in Andromeda will be visible to a naked-eye viewer in approximately 2 billion years. If the collision occurs, the galaxies will likely merge into one larger galaxy.
There is, as yet, no way to know whether the possible collision is definitely going to happen or not. The radial velocity of the Andromeda galaxy with respect to the Milky Way can be measured by examining the Doppler shift of spectral lines from stars in the galaxy, but the transverse velocity cannot be directly measured. Thus, while it is known that the Andromeda galaxy is getting closer to the Milky Way by about 120 km/s, there is no way to tell whether it is going to collide or miss. The best indirect estimates of the transverse velocity indicate that it is less than 100 km/s. This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide. A future European Space Agency spacecraft, the Gaia mission, expected to launch in the Spring of 2012, is intended to measure the positions of stars in the Andromeda galaxy with sufficient precision to pin down the transverse velocity.
Such collisions are relatively common however. Andromeda, for example, is believed to have collided with at least one other galaxy in the past.>>
[quote="john_crompton"]I have a supplementary question on this topic... If the universe is expanding as we believe, surely objects as large as galaxies are all so far apart and all moving away from each other at such a rate that a collision shouldn't happen???
Help me out here someone![/quote]
With the universe expanding as we believe, objects as large as [b]galactic clusters[/b] are all so far apart and all moving away from each other at such a rate that a [b]galactic cluster collision[/b] shouldn't happen. This situation hasn't happened with galaxies quite yet and astronomers even predict a Andromeda-Milky Way collision in about 3 billion years.
[quote=" http://en.wikipedia.org/wiki/Andromeda-Milky_Way_collision"]
<<The Andromeda-Milky Way collision is a predicted galaxy collision that could possibly take place in approximately 3 billion years' time between the two largest galaxies in the Local Group – the Andromeda Galaxy and the Milky Way which contains Earth.
It is often used as an example of the kind of phenomena associated with such collisions in simulations. As with all such collisions, it is unlikely that objects such as stars contained within each galaxy will actually collide, as galaxies are in fact very diffuse—the nearest star to the Sun is in fact almost thirty million solar diameters away from the Earth. (If the sun were scaled to the size of an American quarter, 24.26 mm (0.955 in), the next closest quarter/star would be 700 km [475 miles] away.) If the theory is correct, the stars and gas contained in Andromeda will be visible to a naked-eye viewer in approximately 2 billion years. If the collision occurs, the galaxies will likely merge into one larger galaxy.
There is, as yet, no way to know whether the possible collision is definitely going to happen or not. The radial velocity of the Andromeda galaxy with respect to the Milky Way can be measured by examining the Doppler shift of spectral lines from stars in the galaxy, but the transverse velocity cannot be directly measured. Thus, while it is known that the Andromeda galaxy is getting closer to the Milky Way by about 120 km/s, there is no way to tell whether it is going to collide or miss. The best indirect estimates of the transverse velocity indicate that it is less than 100 km/s. This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide. A future European Space Agency spacecraft, the Gaia mission, expected to launch in the Spring of 2012, is intended to measure the positions of stars in the Andromeda galaxy with sufficient precision to pin down the transverse velocity.
Such collisions are relatively common however. Andromeda, for example, is believed to have collided with at least one other galaxy in the past.>>[/quote]