as a baseline for the calibration of absolute photometric brightness scales. However, this is no longer the case, as the apparent magnitude zero point is now commonly defined in terms of a particular numerically specified flux. This approach is more convenient for astronomers, since Vega is not always available for calibration and varies in brightness. The UBV photometric system measures the magnitude of stars through ultraviolet, blue and yellow filters, producing U, B and V values, respectively. Vega is one of six A0V stars that were used to set the initial mean values for this photometric system when it was introduced in the 1950s. The mean magnitudes for these six stars were defined as: = = 0. In effect, the magnitude scale has been calibrated so that the magnitude of these stars is the same in the yellow, blue and ultraviolet parts of the electromagnetic spectrum. Thus, Vega has a relatively flat electromagnetic spectrum in the visual region—wavelength range 350–850 nanometers, most of which can be seen with the human eye—so the flux densities are roughly equal; 2,000–. However, the flux density of Vega drops rapidly in the infrared, and is near at . Photometric measurements of Vega during the 1930s appeared to show that the star had a low-magnitude variability on the order of ±0.03 magnitude (around ±2.8% luminosity). This range of variability was near the limits of observational capability for that time, and so the subject of Vega's variability has been controversial. The magnitude of Vega was measured again in 1981 at the David Dunlap Observatory and showed some slight variability. Thus it was suggested that Vega showed occasional low-amplitude pulsations associated with a Delta Scuti variable. This is a category of stars that oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity. Although Vega fits the physical profile for this type of variable, other observers have found no such variation. Thus the variability was thought to possibly be the result of systematic errors in measurement. However, a 2007 article surveyed these and other results, and concluded that "A conservative analysis of the foregoing results suggests that Vega is quite likely variable in the 1–2% range, with possible occasional excursions to as much as 4% from the mean". Also, a 2011 article affirms that "The long-term (year-to-year) variability of Vega was confirmed". Vega became the first solitary main-sequence star beyond the Sun known to