The collision of two or more galaxies of comparable size to form a single, larger galaxy. For example, the peculiar elliptical galaxy and strong radio source Centaurus A is believed to result from the merger of an elliptical galaxy with a somewhat smaller, gas-rich spiral galaxy. A major merger is usually defined as one where the smaller galaxy has more than a quarter the mass of the larger one, while the term minor merger refers to a merger between galaxies with a greater mass difference than this.