About the Image
Also prominent in the local group is the Triangulum Galaxy (M33), Leo I, and NGC 6822. There are over 30 galaxies that are considered to be in the local group, and they are spread over a diameter of nearly 10 million light years, with the center of them being somewhere between the Milky Way and M31. M31 and the Milky Way are the most massive members of the Local Group, with M33 being the 3rd largest. Both M31 and the Milky Way have dwarf galaxies associated with them.
The dynamics of the Local Group are changing, and some astronomers speculated that one day the two large spirals in it (M31 and the Milky Way) may collide and merge to form a giant elliptical galaxy. It is also possible that the Local Group may one day merge with the next nearest big galaxy cluster, the Virgo Cluster.
How Do We Calculate Distances of This Magnitude?
For more information about Cepheids, please read the section on calculating distances in the Milky Way.
Other methods that are used to find the distances to galaxies can be found on the ABCs of Distance.
Why Are These Distances Important To Astronomers?
We can actually study individual objects (ie a specific star) in Local Group galaxies, even at distances as great as 2,000,000 parsecs. But even with the sharp vision of the Hubble Space Telescope, this can't be done much beyond the Local Group. It's not just the apparent faintness of stars at such large distances; the crowding of stars in a small patch of the sky is a more important problem.
How can astronomers figure out how severe this crowding problem is? Each side of a picture element (pixel) defines an angle on the sky, determined by the design of the instrument. Let's use something very small, like 0.1 arcsecond as our angle*.
If the galaxy is 5,000,000 parsecs away, you can use trigonometry to figure out that 0.1 arcsecond corresponds to ~2.5 parsecs. So each pixel covers 2.5 by 2.5 arcseconds (times the 'thickness' of the galaxy) - which is likely to contain several stars at least. This makes it more difficult to study individual objects!
*1 arcsecond is a unit of angle that's 1/3600 of a degree. So, 0.1 arcsecond is 1/12,960,000 of a full circle. Knowing this, we can calculate that, at the distance of 5,000,000 parsecs (which is the radius of this circle), 0.1 arcsecond is about 2.5 parsecs.