# Stars/Vega

Vega is the fifth brightest star in the sky, and the brightest in the constellation Lyra, overhead in summer to observers in the northern hemisphere.
Vega has been extensively studied, leading it to be termed "arguably the next most important star in the sky after the Sun".

## Astronomy

"Vega rotates in less than a day, while the Sun's rotation period is 27 days."
From Earth, Vega is observed from the direction of one of its poles.
Photometric measurements of Vega during the 1930s appeared to show that the star had a low-magnitude variability on the order of ±0.03 magnitudes (around ±2.8%).
This range of variability was near the limits of observational capability for that time, and so the magnitude of Vega was measured again in 1981 at the David Dunlap Observatory and showed some 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.
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 on its abstract that "The long-term (year-to-year) variability of Vega was confirmed".

## Astrophysics

The first person to publish a star's parallax was Friedrich Georg Wilhelm von Struve, when he announced a value of 0.125 arcseconds (0.125″) for Vega.
Struve's initial result was actually close to the currently accepted value of 0.129″, as determined by the Hipparcos astrometry satellite.

## Stars

At one time (1932), Vega was listed as a double star.
In 1963, Vega is listed as a visual double star.
In 1983, it was listed as a double star. This was repeated in 1994. It is still listed as a double star in 1996. The update in 2001 also lists it. The star with Vega is 56.41 arcsec away and is designated as BD+38 3238D of unknown spectral class. That these two stars are undifferentiated between double star or binary star for some 70 years at only 25 lyrs away is remarkable.
Vega was the northern pole star around 12,000 BC and will be so again around the year 13,727, when the declination will be +86°14'.
Vega is the brightest of the successive northern pole stars.

## Electromagnetic interactions

An extensive convection zone is not required, and any star with magnetic field strengths and geometry similar to the Sun's will possess a corona.
Magnetic fields on the order of ~30 gauss have been reported for Vega (~1 gauss for the Sun), so perhaps these substantially higher average field strengths compensate for the expected reduced convective activity, resulting in surface X-ray luminosities comparable to the quiet Sun.
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. They "report the detection of a magnetic field on Vega and argue that Vega is probably the first member of a new class of yet undetected magnetic A-type stars."
The "polarization [is] a Zeeman signature [that] leads to a value of [B$_l$ =] -0.6 ± 0.3 G for the disk-averaged line-of-sight component of the surface magnetic field."
"The strength of Vega magnetic field is about 50 micro-tesla, which is close to that of the mean field on Earth and on the Sun."

## Spectral distributions

Vega is a nearby optical spectral type A0V star at about 25 lyrs. Its polar effective temperature is near 10,000 K, while its equatorial effective temperature is 7,600 K.
Vega was the first star other than the Sun to be photographed and the first to have its spectrum recorded.
Each of the annotated spectra at right display portions of the spectrum at the top of this resource:
* the wavelength domain from 380.0 nm to 500.0 nm,
* the 500.0-600.0 nm domain,
* the 600.0-700.0 nm domain,
* the 700.0-800.0 nm domain,
* the 800.0-900.0 nm domain, and
* the 900.0-1020.0 nm domain.
Each of these images at right has various element lines labeled: calcium, hydrogen, iron, magnesium, nitrogen, oxygen, silicon, sodium, and titanium.
Molecular lines occur for H$_2$O and O$_2$ which are likely from Earth's atmosphere or the accretion disk around Vega.
The applicable astronomies are
* violet astronomy, blue astronomy, cyan astronomy, and green astronomy,
* green astronomy, yellow astronomy, and red astronomy,
* red astronomy and near infrared astronomy,
* near infrared astronomy,
* near infrared astronomy, and
* near infrared astronomy.
The principal low atomic number elements of interest, whose lines need to be identified are
* hydrogen,
* helium,
* lithium,
* beryllium,
* boron,
* carbon,
* nitrogen,
* oxygen,
* fluorine, and
* neon.
Element lines and wavelengths can be verified using these resources:
* Alexander Kramida; Yuri Ralchenko; Joseph Reader (September 24, 2012). NIST Atomic Spectra Database, Version 5. Gaithersburg, Maryland, USA: National Institutes of Standards and Technology (NIST). http://www.nist.gov/pml/data/asd.cfm. Retrieved 2013-01-20.
* Jean E. Sansonetti; W. C. Martin; S. L. Young (December 9, 2011). Handbook of Basic Atomic Spectroscopic Data. Gaithersburg, Maryland, USA: Physical Measurement Laboratory, NIST. http://www.nist.gov/pml/data/handbook/index.cfm. Retrieved 2013-01-24.
The element lines to look for and verify whether present or not are
* hydrogen, all high intensity lines detected and state properly labeled.
* helium, the two major lines at 388.86 and 706.519 nm are possible but may overlap the hydrogen line at 388.90 nm.
* lithium - no lines detected.
* beryllium - no lines detected.
* boron - two strong lines are possible, the 412.1933 nm as a shoulder on H I 410.17 nm line, while the other 448.705 nm overlaps the labeled 448.11 Mg II line.
* carbon, the line at 723.642 nm is possible but unlabeled.
* nitrogen, high intensity lines labeled, but the 868.028 N I is possible as an unlabeled shoulder.
* oxygen - the 777.42 nm O I may be mislabeled and should be 777.194 nm O I which is the stronger line.
* fluorine - the 685.603 nm F I line is possible.
* neon - not detected.
For the heavier elements:
* sodium - the two Na I lines are properly labeled.
* magnesium - all of the lines may be wrong and mislabeled.
* calcium - all high intensity lines present and labeled, the Ca K is a Ca II line.
* titanium - the Ti II line at 439.50 nm is properly labeled as are the others.
* silicon - lines are properly labeled.
* iron - all are labeled properly, because the 501.84 nm line is only Fe II, which differentiates Fe I lines from Fe II lines.

## X-rays

According to SIMBAD, Vega (alpha Lyrae) (delta Sct type variable) is an X-ray source in the first Einstein catalog (1E).
The X-ray "counts observed from [...] Vega in the HRI are very likely to be due entirely to UV contamination from the photospheric emission, and hence the X-ray luminosities for [...] Vega [...] should be replaced by upper limits at least one order of magnitude lower, i.e., log L$_{X,Vega}$ < 26.6 [...] Thus the HRI observation of Vega is completely consistent with the upper limit obtained in the IPC, and the only remaining inconsistency concerning the X-ray emission from Vega is the rocket experiment by Topka et al. (1979), who report a detection of Vega in a 5 s pointing yielding 7 counts; however, in our opinion these authors do not convincingly rule out the possibility of UV contamination. Note in this context that the IPC's used for Topka et al.s' (1979) rocket flight and for the Einstein Observatory were not identical."
"Many types of main sequence stars emit in the X-ray portion of the spectra. In massive stars, strong stellar winds ripping through the extended atmosphere of the star create X-ray photons. On lower mass stars, magnetic fields twisting through the photosphere heat it sufficiently to produce X-rays. But between these two mechanisms, in the late B to mid A classes of stars, neither of these mechanisms should be sufficient to produce X-rays. Yet when X-ray telescopes examined these stars, many were found to produce X-rays just the same."
"The first exploration into the X-ray emission of this class of stars was the Einstein Observatory, launched in 1978 and deorbited in 1982. While the telescoped confirmed that these B and A stars had significantly less X-ray emission overall, seven of the 35 A type stars still had some emission. Four of these were confirmed as being in binary systems in which the secondary stars could be the source of the emission, leaving three of seven with unaccounted for X-rays."
"The German ROSAT satellite found similar results, detecting 232 X-ray stars in this range. Studies explored connections with irregularities in the spectra of these stars and rotational velocities, but found no correlation with either. The suspicion was that these stars simply hid undetected, lower mass companions."
Either "the main star truly is the source, or there are even more elusive, sub-arcsecond binaries skewing the data."
On July 27, 1977, at 05:41:48.1 UTC, an Aerobee 350 or boosted Black Brant launched from White Sands Missile Range using Vega as a reference by its star tracker to update its position while maneuvering between X-ray targets automatically observed Vega with its X-ray telescope for 4.8 s.
The quantity of detected photons (7) in the band 0.2-0.80 keV corresponds to an X-ray luminosity L$_X$ ≈ 3 x 10$^{28}$ erg s$^{-1}$.
"The ANS 3 σ upper limit for Vega (2.5 x 10$^{28}$ ergs s$^{-1}$) is only slightly lower than our flux measurement."
"Because the X-ray [luminosity] of Vega [is] much closer to that of the Sun than to the typical galactic X-ray sources which have been detected to date, it is natural to consider processes analogous to solar coronal activity as the explanation for the X-ray activity."
"Vega is thought to be a solitary star, and therefore noncoronal X-ray-producing mechanisms seem to be excluded".
"Vega is the first solitary main-sequence star beyond the Sun known to be an X-ray emitter".
Vega's "computed X-ray surface luminosity [...] is comparable to that of the quiet Sun [...] Note, however, that because of our very short exposure, the average level of coronal emission may vary significantly from our single measurement."
"Using estimates of the stellar [radius] derived from stelar structure calculations, we obtain [a] surface X-ray [luminosity] of ~6.4 x 10$^4$ ergs cm$^{-2}$ s$^{-1}$ for Vega [that falls] within the range of solar coronal X-ray emission, which can vary between ~8 x 10$^3$ ergs cm$^{-2}$ s$^{-1}$ in coronal holes and ~3 x 10$^6$ ergs cm$^{-2}$ s$^{-1}$ in active regions".
Magnetic "field activity, leading to coronal heating, may account for Vega's X-ray emission because of inhomogeneous distribution of surface magnetic flux and associated coronal activity."
That Vega is regarded as an X-ray source rests on one 4.8 s star-tracking observation by one sounding rocket flight carrying an X-ray detector flown on many flights that yields trustworthy results.
"Vega is a pole-on, highly oblate, rapid rotator [...] the star exhibits extreme limb darkening and a large decrement in effective temperature from pole to equator. [...] the best fittingmodel (T$_{eff}$ pole=10150 K, T$_{eff}$ eq = 7900 K, θ = 3.329 mas) has the pole inclined 5° to line of sight and rotates at 91% of the angular speed of break-up, resulting in a temperature drop of 2250 K from center to limb. [...] the total luminosity [...] is emitted in a highly non-homogeneous manner with five times more UV flux being emitted from the pole as is emitted in the equatorial plane, while the visible through near-IR flux is some 70% greater at the pole than that of the equatorial plane and 54% greater than that expected from a slow or non-rotating A0 V star."
A 'polar coronal hole' is a coronal hole that occurs above one or both rotational poles of a star that has a coronal cloud around it.
"The radiant emission from coronal holes is greatly diminished relative to other coronal regions".
The "emission is proportional to the integral of the square of the electron density along the line of sight [...] Data of this type are therefore heavily influenced by regions of high density along the line of sight--the low corona for disk observations, and denser structures surrounding coronal holes for limb observations."
An "analysis of the northern polar region during the period 1973 June 29 to July 13 [...] can be summarized as follows. The boundary of the hole is essentially axisymmetrc about the polar axis and is nearly radial from 3 to 6 R$_⊙$. The boundary at these heights is located at 25° ± 5° latitude, although it is of much smaller extent (boundary ~65° latitude) as observed near the solar surface with the American Science and Engineering (AS&E) X-ray experiment on Skylab [...] the increase of the polar hole's cross sectional area from the surface to 3 R$_⊙$ is approximately 7 times greater than for a purely radial boundary."
For α Lyr, log F$_X$/F$_V$ = -6.79 (variable X-ray source), log L$_X$ 27.6 erg s$^{-1}$ (variable X-ray source). Upper limits were log F$_X$/F$_V$ = -7.4 and log L$_X$ 27.0 erg s$^{-1}$.
A coronal cloud is not a diffuse, homogeneous hot atmosphere, but one or more strongly structured topologically closed features dominated by magnetic confinement.

## Ultraviolets

According to SIMBAD, Vega (alpha Lyrae) (delta Sct type variable) is an ultraviolet source from the CEL, EUVE, and TD1 catalogs.

## Infrareds

The images at the right has a 24-micron image in blue on the left and a 70-micron infrared image on the right. "The [debris] disc extends to at least 815 astronomical units." "The images are 3 arcminutes on each side."
BD+38 3238D is unspecified or unlocated within the infrared images but should be within the debris disc.
According to SIMBAD there are ten objects within one arcminute of Vega including BD+38 3238D, a submillimeter source (JCMTSE J183656.4+384709), and eight infrared sources: [ MHW2003] 1-8.
"The infrared excesses are well modeled by two components, a warm belt close to the star, and a cooler belt farther out. The clear separation of the belts could be explained by the presence of planets clearing the gap."
The graph at left shows the clear separation of infrared belts for Vega. This separation may "be explained by the presence of planets clearing the gap."

## Metallicities

Vega has an unusually low abundance of the elements with a higher atomic number than that of helium.
Its metallicity [Fe/H] = −0.5 dex.
The metallicity of Vega's photosphere is only about 32% of the abundance of heavy elements in the Sun's atmosphere. For a metallicity of −0.5, the proportion of metals relative to the Sun is given by
    $10^{-0.5}=0.316$.