References

1. Introductory Astronomy & Astrophysics (4th Edition), by Zeilek & Gregory
2. The Dynamic Universe (An introduction to Astronomy), Theodore P. Snow
3. Fundamental Astronomy (3rd Edition), by H. Kartlunen, P Kroger, H. Oja, M. Poutanen & K. J. Bonner
4. Universe (5th Edition), by Kaufmann & Freedman
5. University of California, San diego - Centre for Astrophysics & Space sciences
6. NASA's Hubble Space Telescope picture gallery