Editing Vega (section)

==Physical characteristics==
Vega's [[spectral class]] is A0V, making it a blue-tinged white [[main-sequence star]] that is [[nuclear fusion|fusing]] [[hydrogen]] to [[helium]] in its core. Since more massive stars use their fusion fuel more quickly than smaller ones, Vega's main-sequence lifetime is roughly one billion years, a tenth of the Sun's.<ref name=ajss40_733/> The current age of this star is about 455&nbsp;million years,<ref name=apj708_1_71/> or up to about half its expected total main-sequence lifespan. After leaving the main sequence, Vega will become a class-M [[red giant]] and shed much of its mass, finally becoming a [[white dwarf]]. At present, Vega has more than twice the mass<ref name=nature440_7086_896/> of the Sun and its [[bolometric luminosity]] is about 40 times the Sun's. Because it is rotating rapidly, approximately once every 16.5 hours,<ref name=petit2022/> and seen nearly pole-on, its apparent luminosity, calculated assuming it was the same brightness all over, is about 57 times the Sun's.<ref name=apj645_1_664/> If Vega is variable, then it may be a [[Delta Scuti variable|Delta Scuti type]] with a period of about 0.107&nbsp;day.<ref name=asp93_2_333/>

Most of the energy produced at Vega's core is generated by the carbon–nitrogen–oxygen cycle ([[CNO cycle]]), a [[Stellar nucleosynthesis|nuclear fusion]] process that combines [[proton]]s to form helium nuclei through intermediary nuclei of carbon, nitrogen and oxygen. This process becomes dominant at a temperature of about 17&nbsp;million&nbsp;K,<ref name=salaris_cassisi2005/> which is slightly higher than the core temperature of the Sun, but is less efficient than the Sun's [[proton–proton chain]] fusion reaction. The CNO cycle is highly temperature sensitive, which results in a [[convection zone]] about the core<ref name=apj601_1_512/> that evenly distributes the 'ash' from the fusion reaction within the core region. The overlying atmosphere is in [[Radiative transfer|radiative equilibrium]]. This is in contrast to the Sun, which has a [[radiation zone]] centered on the core with an overlying convection zone.<ref name=padmanabhan2002/>

The energy flux from Vega has been precisely measured against standard light sources. At {{val|5480|fmt=commas|u=Å}}, the flux density is {{val|3650|fmt=commas|u=Jy}} with an error margin of 2%.<ref name=apj161_1015/> The visual spectrum of Vega is dominated by [[absorption line]]s of hydrogen; specifically by the hydrogen [[Balmer series]] with the [[electron]] at the n=2 [[principal quantum number]].<ref name=richmond/><ref name=clayton1983/> The lines of other elements are relatively weak, with the strongest being ionized [[magnesium]], [[iron]] and [[chromium]].<ref name=mnras197_57/> The [[X-ray]] emission from Vega is very low, demonstrating that the [[stellar corona|corona]] for this star must be very weak or non-existent.<ref name=aaa318_215/> However, as the pole of Vega is facing Earth and a polar [[coronal hole]] may be present,<ref name=apj229_661/><ref name=sao389/> confirmation of a corona as the likely source of the X-rays detected from Vega (or the region very close to Vega) may be difficult as most of any coronal X-rays would not be emitted along the line of sight.<ref name=sao389/><ref name=apj213_5_874/>

Using [[spectropolarimetry]], a [[magnetic field]] has been detected on the surface of Vega by a team of astronomers at the [[Observatoire du Pic du Midi]]. This is the first such detection of a magnetic field on a spectral class A star that is not an [[Ap and Bp stars|Ap]] [[chemically peculiar star]]. The average line of sight component of this field has a strength of {{val|−0.6|0.3}} [[Gauss (unit)|gauss (G)]].<ref name=aaa500_3_L41/> This is comparable to the mean magnetic field on the Sun.<ref name=sd20090726/> Magnetic fields of roughly 30 G have been reported for Vega, compared to about 1 G for the Sun.<ref name=apj229_661/> In 2015, bright [[starspot]]s were detected on the star's surface—the first such detection for a normal [[A-type star]], and these features show evidence of [[rotational modulation]] with a period of 0.68&nbsp;day.<ref name=aa577_A64/>

===Rotation===
Vega has a rotation period of 16.3 hours,<ref name=petit2022/> much faster than the Sun's rotational period but similar to, and slightly slower than, those of [[Jupiter]] and [[Saturn]]. Because of that, Vega is significantly [[oblate spheroid|oblate]] like those two planets.

When the radius of Vega was measured to high accuracy with an [[Astronomical interferometer|interferometer]], it resulted in an unexpectedly large estimated value of {{val|2.73|0.01}} times the [[radius of the Sun]]. This is 60% larger than the radius of the star Sirius, while stellar models indicated it should only be about 12% larger. However, this discrepancy can be explained if Vega is a rapidly rotating star that is being viewed from the direction of its pole of rotation. Observations by the [[CHARA array]] in 2005–06 confirmed this deduction.<ref name=apj645_1_664/>

[[File:Vega compared with the Sun.jpg|thumb|Size comparison of Vega (left) to the Sun (right)]]

The pole of Vega—its axis of rotation—is inclined no more than five degrees from the line-of-sight to the Earth. At the high end of estimates for the [[stellar rotation|rotation]] velocity for Vega is {{val|236.2|3.7|u=km/s}}<ref name=apj708_1_71/> along the equator, much higher than the observed (i.e. [[projected rotational velocity|projected]]) rotational velocity because Vega is seen almost pole-on. This is 88% of the speed that would cause the star to start breaking up from [[Centrifugal force|centrifugal]] effects.<ref name=apj708_1_71/> This rapid rotation of Vega produces a pronounced equatorial bulge, so the radius of the equator is 19% larger than the polar radius, compared to just under 11% for Saturn, the most oblate of the Solar System's planets. (The estimated polar radius of this star is {{val|2.362|0.012}} [[solar radius|solar radii]], while the equatorial radius is {{val|2.818|0.013}} solar radii.<ref name=apj708_1_71/>) From the Earth, this bulge is being viewed from the direction of its pole, producing the overly large radius estimate.

The local [[surface gravity]] at the poles is greater than at the equator, which produces a variation in [[effective temperature]] over the star: the polar temperature is near {{val|10000|fmt=commas|ul=K}}, while the equatorial temperature is about {{val|8152|fmt=commas|u=K}}.<ref name=apj708_1_71/> This large temperature difference between the poles and the equator produces a strong [[gravity darkening]] effect. As viewed from the poles, this results in a darker (lower-intensity) limb than would normally be expected for a spherically symmetric star. The temperature gradient may also mean that Vega has a convection zone around the equator,<ref name=apj645_1_664/><ref name=noao2006/> while the remainder of the atmosphere is likely to be in almost pure [[Radiation zone|radiative equilibrium]].<ref name=adelman2004/> By the [[Von Zeipel theorem]], the local luminosity is higher at the poles. As a result, if Vega were viewed along the plane of its [[equator]] instead of almost pole-on, then its overall brightness would be lower.

As Vega had long been used as a [[calibrator star|standard star for calibrating telescopes]], the discovery that it is rapidly rotating may challenge some of the underlying assumptions that were based on it being spherically symmetric. With the viewing angle and rotation rate of Vega now better known, this will allow improved instrument calibrations.<ref name=science317_5836_325/>

===Element abundance===
In astronomy, those elements with higher [[atomic number]]s than helium are termed "metals". The [[metallicity]] of Vega's [[photosphere]] is only about 32% of the abundance of heavy elements in the Sun's atmosphere.<ref group=note name=metal/> (Compare this, for example, to a threefold metallicity abundance in the similar star Sirius as compared to the Sun.) For comparison, the Sun has an abundance of elements heavier than helium of about Z<sub>Sol</sub>&nbsp;=&nbsp;{{val|0.0172|0.002}}.<ref name=apj644_2_1291/> Thus, in terms of abundances, only about 0.54% of Vega consists of elements heavier than helium. [[Nitrogen]] is slightly ''more'' abundant, [[oxygen]] is only marginally less abundant and [[sulfur]] abundance is about 50% of solar. On the other hand, Vega has only 10% to 30% of the solar abundance for most other major elements with [[barium]] and [[scandium]] below 10%.<ref name=apj708_1_71/>

The unusually low metallicity of Vega makes it a weak [[Lambda Boötis star]].<ref name=bicds38_137/><ref name=apj548_2_77/> However, the reason for the existence of such chemically peculiar, [[spectral class]] A0–F0 stars remains unclear. One possibility is that the chemical peculiarity may be the result of [[diffusion]] or mass loss, although stellar models show that this would normally only occur near the end of a star's hydrogen-burning lifespan. Another possibility is that the star formed from an [[interstellar medium]] of gas and dust that was unusually metal-poor.<ref name=mnras301_4_1099/>

The observed helium to hydrogen ratio in Vega is {{val|0.030|0.005}}, which is about 40% lower than the Sun. This may be caused by the disappearance of a helium [[convection zone]] near the surface. Energy transfer is instead performed by the [[radiative process]], which may be causing an abundance anomaly through diffusion.<ref name=apj348_712/>

===Kinematics===
The [[radial velocity]] of Vega is the component of this star's motion along the line-of-sight to the Earth. Movement away from the Earth will cause the light from Vega to shift to a lower [[frequency]] (toward the red), or to a higher frequency (toward the blue) if the motion is toward the Earth. Thus the velocity can be measured from the amount of shift of the star's spectrum. Precise measurements of this [[blueshift]] give a value of {{val|−13.9|0.9|u=km/s}}.<ref name=rgcrv1966/> The minus sign indicates a relative motion toward the Earth.

Motion transverse to the line of sight causes the position of Vega to shift with respect to the more distant background stars. Careful measurement of the star's position allows this angular movement, known as [[proper motion]], to be calculated. Vega's proper motion is {{val|202.03|0.63|ul=milliarcseconds}} (mas) per year in [[right ascension]]—the celestial equivalent of [[longitude]]—and {{val|287.47|0.54|u=mas/y}} in [[declination]], which is equivalent to a change in [[latitude]]. The net proper motion of Vega is {{val|327.78|u=mas/y}},<ref name=majewski2006/> which results in angular movement of a degree every {{val|11000|fmt=commas|u=years}}.

In the [[galactic coordinate system]], the [[Space velocity (astronomy)|space velocity]] components of Vega are (U, V, W) = {{nowrap|({{val|−16.1|0.3}}, {{val|−6.3|0.8}}, {{val|−7.7|0.3}}) km/s}}, for a net space velocity of {{val|19|u=km/s}}.<ref name=aaa339/> The radial component of this velocity—in the direction of the Sun—is {{val|−13.9|u=km/s}}, while the transverse velocity is {{val|12.9|u=km/s}}.{{cn|date=October 2023}} Although Vega is at present only the fifth-brightest star in the night sky, the star is slowly brightening as proper motion causes it to approach the Sun.<ref name=moulton1906/> Vega will make its closest approach in an estimated 264,000 years at a [[perihelion]] distance of {{cvt|4.04|pc|ly|lk=off|order=flip}}.<ref name=aa575_A35/>

Based on this star's kinematic properties, it appears to belong to a stellar association called the [[Castor Moving Group]]. However, Vega may be much older than this group, so the membership remains uncertain.<ref name=apj708_1_71/> This group contains about 16 stars, including [[Alpha Librae]], [[Alpha Cephei]], [[Castor (star)|Castor]], [[Fomalhaut]] and Vega. All members of the group are moving in nearly the same direction with similar [[Space velocity (astronomy)|space velocities]]. Membership in a moving group implies a common origin for these stars in an [[open cluster]] that has since become gravitationally unbound.<ref name=inglis2003/> The estimated age of this moving group is {{val|200|100|u=million years}}, and they have an average space velocity of {{val|16.5|u=km/s}}.<ref group=note name=space_velocity/><ref name=aaa339/>