The halo is believed to be the oldest component of the Galaxy. As such, halo stars are generally older than stars in the disk. In the disk stars are being formed continuously, whereas there is no evidence for any star formation in the Galaxy's halo. When one observes a particular star, it is not always possible to determine its age. Therefore, the way astronomers usually distinguish members of the halo from members of the disk is by measuring their velocities through space. Disk stars have a tightly constrained distribution of velocities, while halo stars generally have very high velocities. In addition, if one is studying a large number of stars, halo stars can be distinguished because they do not exhibit the overall rotation about the center of the Galaxy that disk stars possess.