Structure and Inclination Angle of the Spiral Galaxy M31

The mathematical form, the symmetry of the spiral structure, and the projection of the galactic disc on the image of the spiral galaxy M31 have been directly studied. It is found that M31 has two symmetric arms, i.e., the pitch angles of the two arms are nearly equal. They are 7.7°and 8.0°, respectively. Using the method proposed in this letter, the inclination angle of the galactic disc of M31 is also obtains , which is 77.5° and is in good agreement with previously published results.