Its albedo, or reflectivity, is high (84 percent of the light falling on it is reflected), but it is so far away from the earth its stellar magnitude (a scale used to describe the brightness of an astronomical object; lower numbers correspond to brighter objects) is only 7.8, which means it is never bright enough to be seen with the naked eye from earth. When observed through a telescope, it appears as a greenish-blue disk without any definite surface markings. However, images from the Voyager 2 probe, which flew past Neptune in 1989, and the Hubble Space Telescope, which observed Neptune in 1994, have revealed dynamic bright and dark spots in Neptune's atmosphere that are thought to be huge storms caused by the difference in temperature between the heat-producing core and the frigid cloud tops. The striking blue color of Neptune is caused by the estimated three percent of methane contained in Neptune's atmosphere.