Editing Vega (section)

==Observational history==
[[File:The first image of Vega.jpg|left|thumb|upright=1.2|"On the night of July 16–17, 1850, [[John Adams Whipple|Whipple]] and [[William Cranch Bond|Bond]] made the first [[daguerreotype]] of a star (Vega)"]]

[[Astrophotography]], the [[photography]] of celestial objects, began in 1840 when [[John William Draper]] took an image of the [[Moon]] using the [[daguerreotype]] process. On 17 July 1850, Vega became the first star (other than the Sun) to be photographed, when it was imaged by [[William Cranch Bond|William Bond]] and [[John Adams Whipple]] at the [[Harvard College Observatory]], also with a daguerreotype.<ref name=allen1963/><ref name=barger_white2000/><ref name=pasp2_10_249/> In August 1872, [[Henry Draper]] took a photograph of Vega's [[Astronomical spectroscopy|spectrum]], the first photograph of a star's spectrum showing absorption lines.<ref name=paps24_166/> Similar lines had already been identified in the spectrum of the Sun.<ref name=aip/> In 1879, [[William Huggins]] used photographs of the spectra of Vega and similar stars to identify a set of twelve "very strong lines" that were common to this stellar category. These were later identified as lines from the Hydrogen [[Balmer series]].<ref name=klaus2002/> Since 1943, the [[stellar spectrum|spectrum]] of this star has served as one of the stable anchor points by which other stars are classified.<ref name=baas25_1319/>

The distance to Vega can be determined by measuring its parallax shift against the background stars as the [[Earth]] orbits the Sun. [[Giuseppe Calandrelli]] noted stellar parallax in 1805-6 and came up with a 4-second value for the star which was a gross overestimate.<ref name=gore1904>{{cite book | author=Gore, J.E. | url=https://archive.org/details/studiesinastrono00gorerich/page/42/mode/1up | title=Studies in astronomy |publisher=Chatto & Windus |year=1904 |place=London |page=42}}</ref>  The first person to publish a star's parallax was [[Friedrich Georg Wilhelm von Struve|Friedrich G. W. von Struve]], when he announced a value of 0.125 arcsecond ({{val|0.125|u="}}) for Vega.<ref name=berry1899/> [[Friedrich Bessel]] was skeptical about Struve's data, and, when Bessel published a parallax of 0.314″ for the star system [[61 Cygni]], Struve revised his value for Vega's parallax to nearly double the original estimate. This change cast further doubt on Struve's data. Thus most astronomers at the time, including Struve, credited Bessel with the first published parallax result. However, Struve's initial result was actually close to the currently accepted value of 0.129″,<ref name=debarbat1988/><ref name=astroprof/> as determined by the ''[[Hipparcos]]'' [[astrometry]] satellite.<ref name=aaa474_2_653/><ref name=aaa323_L49/><ref name=GSM/>

The brightness of a star, as seen from Earth, is measured with a standardized, [[logarithmic scale]]. This [[apparent magnitude]] is a numerical value that decreases in value with increasing brightness of the star. The faintest stars visible to the unaided eye are sixth magnitude, while the brightest in the night sky, [[Sirius]], is of magnitude −1.46. To standardize the magnitude scale, astronomers chose Vega and several similar stars and averaged their brightness to represent magnitude zero at all wavelengths. Thus, for many years, Vega was used as a baseline for the calibration of absolute [[photometry (astronomy)|photometric]] brightness scales.<ref name=garfinkle1997/> 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.<ref name=ajss45_83/>

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 [[A-type main-sequence star|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: {{nowrap|''U'' − ''B''}} = {{nowrap|''B'' − ''V''}} =&nbsp;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]].<ref name=apj117_313/> Thus, Vega has a relatively flat electromagnetic spectrum in the visual region—wavelength range 350–850 [[nanometer]]s, most of which can be seen with the human eye—so the flux densities are roughly equal; 2,000–{{val|4000|fmt=commas|u=[[Jansky|Jy]]}}.<ref name=eso20020306/> However, the flux density of Vega drops rapidly in the [[infrared]], and is near {{val|100|u=Jy}} at {{val|5|ul=micrometers}}.<ref name=mcmahon2005/>

[[File:Star-Vega.png|thumb|upright|Photograph of Vega captured in March 2025]]
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%<ref group=note name=deltaLum/> 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]].<ref name=asp93_2_333/> This is a category of stars that oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.<ref name=araa33_1_75/> 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.<ref name=merezhin/><ref name=hayes1984/> 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".<ref name=gray2007/> Also, a 2011 article affirms that "The long-term (year-to-year) variability of Vega was confirmed".<ref name=butkovskaya2011/>

Vega became the first solitary [[main-sequence star]] beyond the Sun known to be an X-ray emitter when in 1979 it was observed from an imaging X-ray telescope launched on an [[Aerobee]] 350 from the [[White Sands Missile Range]].<ref name=apj229_661/> In 1983, Vega became the first star found to have a disk of dust. The [[Infrared Astronomical Satellite]] (IRAS) discovered an excess of infrared radiation coming from the star, and this was attributed to energy emitted by the orbiting dust as it was heated by the star.<ref name=nature307_5950_441/>