Vega
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Lyra |
| Pronunciation | /ˈviːɡə/[1][2][3] or /ˈveɪɡə/[2] |
| Right ascension | 18h 36m 56.33635s[4] |
| Declination | +38° 47′ 01.2802″[4] |
| Apparent magnitude (V) | +0.026[5] (−0.02...+0.07[6]) |
| Characteristics | |
| Evolutionary stage | Main sequence |
| Spectral type | A0Va[7] |
| U−B color index | 0.00[8] |
| B−V color index | 0.00[8] |
| Variable type | Delta Scuti[6] |
| Astrometry | |
| Radial velocity (Rv) | −13.9 ± 0.9[9] km/s |
| Proper motion (μ) | RA: 200.94[4] mas/yr Dec.: 286.23[4] mas/yr |
| Parallax (π) | 130.23 ± 0.36[4] mas |
| Distance | 25.04 ± 0.07 ly (7.68 ± 0.02 pc) |
| Absolute magnitude (MV) | +0.582[10] |
| Details | |
| Mass | 2.135 ± 0.074[11] M☉ |
| Radius | 2.362–2.818[11] R☉ |
| Luminosity | 40.12 ± 0.45[11] L☉ |
| Surface gravity (log g) | 4.1 ± 0.1[12] cgs |
| Temperature | 9,602 ± 180[13] (8,152–10,060 K)[11][note 1] K |
| Metallicity [Fe/H] | −0.5[13] dex |
| Rotational velocity (v sin i) | 20.48 ± 0.11[11] km/s |
| Age | 455 ± 13[11] Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only 25 light-years (7.7 parsecs) from the Sun, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood. It is the fifth-brightest star in the night sky, and the second-brightest star in the northern celestial hemisphere, after Arcturus.
Vega has been extensively studied by astronomers, leading it to be termed "arguably the next most important star in the sky after the Sun".[17] Vega was the northern pole star around 12,000 BCE and will be so again around the year 13,727, when its declination will be +86° 14′.[18] Vega was the first star other than the Sun to be photographed and the first to have its spectrum recorded.[19][20] It was one of the first stars whose distance was estimated through parallax measurements. Vega has functioned as the baseline for calibrating the photometric brightness scale and was one of the stars used to define the zero point for the UBV photometric system.
Vega is only about a tenth of the age of the Sun, but since it is 2.1 times as massive, its expected lifetime is also one tenth of that of the Sun; both stars are at present approaching the midpoint of their life expectancies. Vega has an unusually low abundance of elements with a higher atomic number than that of helium.[13] Vega is also a variable star that varies slightly in brightness. It is rotating rapidly with a velocity of 236 km/s at the equator. This causes the equator to bulge outward due to centrifugal effects, and, as a result, there is a variation of temperature across the star's photosphere that reaches a maximum at the poles. From Earth, Vega is observed from the direction of one of these poles.[21]
Based on an observed excess emission of infrared radiation, Vega appears to have a circumstellar disk of dust. This dust is likely to be the result of collisions between objects in an orbiting debris disk, which is analogous to the Kuiper belt in the Solar System.[22] Stars that display an infrared excess due to dust emission are termed Vega-like stars.[23] In 2021, a candidate ultra-hot Neptune on a 2.43-day orbit around Vega was discovered with the radial velocity method, additionally, another possible Saturn-mass signal with a period of about 200 days.[24]
Nomenclature[]
α Lyrae (Latinised to Alpha Lyrae) is the star's Bayer designation. The traditional name Vega (earlier Wega[14]) comes from a loose transliteration of the Arabic word wāqi' (Arabic: واقع) meaning "falling" or "landing", via the phrase an-nasr al-wāqi' (Arabic: النّسر الْواقع) , "the falling eagle".[25] In 2016, the International Astronomical Union (IAU) organized a Working Group on Star Names (WGSN)[26] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[27] included a table of the first two batches of names approved by the WGSN; which included Vega for this star. It is now so entered in the IAU Catalog of Star Names.[28]
Observation[]
Vega can often be seen near the zenith in the mid-northern latitudes during the evening in the Northern Hemisphere summer.[29] From mid-southern latitudes, it can be seen low above the northern horizon during the Southern Hemisphere winter. With a declination of +38.78°, Vega can only be viewed at latitudes north of 51° S. Therefore, it does not rise at all anywhere in Antarctica or in the southernmost part of South America, including Punta Arenas, Chile (53° S). At latitudes to the north of 51° N, Vega remains continuously above the horizon as a circumpolar star. Around July 1, Vega reaches midnight culmination when it crosses the meridian at that time.[30]
Each night the positions of the stars appear to change as the Earth rotates. However, when a star is located along the Earth's axis of rotation, it will remain in the same position and thus is called a pole star. The direction of the Earth's axis of rotation gradually changes over time in a process known as the precession of the equinoxes. A complete precession cycle requires 25,770 years,[31] during which time the pole of the Earth's rotation follows a circular path across the celestial sphere that passes near several prominent stars. At present the pole star is Polaris, but around 12,000 BCE the pole was pointed only five degrees away from Vega. Through precession, the pole will again pass near Vega around 14,000 CE.[32] Vega is the brightest of the successive northern pole stars.[14]
This star lies at a vertex of a widely spaced asterism called the Summer Triangle, which consists of Vega plus the two first-magnitude stars Altair, in Aquila, and Deneb in Cygnus.[29] This formation is the approximate shape of a right triangle, with Vega located at its right angle. The Summer Triangle is recognizable in the northern skies for there are few other bright stars in its vicinity.[33]
Observational history[]
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 Bond and John Adams Whipple at the Harvard College Observatory, also with a daguerreotype.[14][19][34] Henry Draper took the first photograph of a star's spectrum in August 1872 when he took an image of Vega, and he also became the first person to show absorption lines in the spectrum of a star.[20] Similar lines had already been identified in the spectrum of the Sun.[35] 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.[36] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[37]
The distance to Vega can be determined by measuring its parallax shift against the background stars as the Earth orbits the Sun. The first person to publish a star's parallax was Friedrich G. W. von Struve, when he announced a value of 0.125 arcsecond (0.125″) for Vega.[38] 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″,[39][40] as determined by the Hipparcos astrometry satellite.[4][41][42]
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 to represent magnitude zero at all wavelengths. Thus, for many years, Vega was used as a baseline for the calibration of absolute photometric brightness scales.[43] 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.[44]
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: U − B = B − V = 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.[45] 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; 2,000–4,000 Jy.[46] However, the flux density of Vega drops rapidly in the infrared, and is near 100 Jy at 5 micrometers.[47]
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%[note 2] 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.[48] This is a category of stars that oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.[49] 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.[50][51] 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".[52] Also, a 2011 article affirms that "The long-term (year-to-year) variability of Vega was confirmed".[53]
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.[54] 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.[55]
Physical characteristics[]
Vega's spectral class is A0V, making it a blue-tinged white main-sequence star that is 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.[56] The current age of this star is about 455 million years,[11] 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[21] of the Sun and its bolometric luminosity is about 40 times the Sun's. Because it is rotating rapidly and seen nearly pole-on, its apparent luminosity, calculated assuming it was the same brightness all over, is about 57 times the Sun's.[12] If Vega is variable, then it may be a Delta Scuti type with a period of about 0.107 day.[48]
Most of the energy produced at Vega's core is generated by the carbon–nitrogen–oxygen cycle (CNO cycle), a nuclear fusion process that combines protons to form helium nuclei through intermediary nuclei of carbon, nitrogen and oxygen. This process becomes dominant at a temperature of about 17 million K,[57] which is slightly higher than the core temperature of the Sun, but is less efficient than the Sun's proton–proton chain reaction fusion reaction. The CNO cycle is highly temperature sensitive, which results in a convection zone about the core[58] that evenly distributes the 'ash' from the fusion reaction within the core region. The overlying atmosphere is in radiative equilibrium. This is in contrast to the Sun, which has a radiation zone centered on the core with an overlying convection zone.[59]
The energy flux from Vega has been precisely measured against standard light sources. At 5,480 Å, the flux density is 3,650 Jy with an error margin of 2%.[60] The visual spectrum of Vega is dominated by absorption lines of hydrogen; specifically by the hydrogen Balmer series with the electron at the n=2 principal quantum number.[61][62] The lines of other elements are relatively weak, with the strongest being ionized magnesium, iron and chromium.[63] The X-ray emission from Vega is very low, demonstrating that the corona for this star must be very weak or non-existent.[64] However, as the pole of Vega is facing Earth and a polar coronal hole may be present,[54][65] 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.[65][66]
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 chemically peculiar star. The average line of sight component of this field has a strength of −0.6 ± 0.3 gauss (G).[67] This is comparable to the mean magnetic field on the Sun.[68] Magnetic fields of roughly 30 G have been reported for Vega, compared to about 1 G for the Sun.[54] In 2015, bright starspots 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 day.[69]
Rotation[]
Vega has a rotation period of 12.5 hours.[70]
When the radius of Vega was measured to high accuracy with an interferometer, it resulted in an unexpectedly large estimated value of 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.[12]
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 rotation velocity for Vega is 236.2 ± 3.7 km/s[11] along the equator, much higher than the observed (i.e. 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 effects.[11] This rapid rotation of Vega produces a pronounced equatorial bulge, so the radius of the equator is 19% larger than the polar radius. (The estimated polar radius of this star is 2.362 ± 0.012 solar radii, while the equatorial radius is 2.818 ± 0.013 solar radii.[11]) 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 10,000 K, while the equatorial temperature is about 8,152 K.[11] 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,[12][71] while the remainder of the atmosphere is likely to be in almost pure radiative equilibrium.[72] 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 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.[73]
Element abundance[]
In astronomy, those elements with higher atomic numbers 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.[note 3] (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 ZSol = 0.0172±0.002.[74] Thus, in terms of abundances, only about 0.54% of Vega consists of elements heavier than helium.
The unusually low metallicity of Vega makes it a weak Lambda Boötis star.[75][76] 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.[77]
The observed helium to hydrogen ratio in Vega is 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.[78]
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 −13.9 ± 0.9 km/s.[9] 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 202.03 ± 0.63 milliarcseconds (mas) per year in right ascension—the celestial equivalent of longitude—and 287.47 ± 0.54 mas/y in declination, which is equivalent to a change in latitude. The net proper motion of Vega is 327.78 mas/y,[79] which results in angular movement of a degree every 11,000 years.
In the galactic coordinate system, the space velocity components of Vega are (U, V, W) = (−16.1 ± 0.3, −6.3 ± 0.8, −7.7 ± 0.3) km/s, for a net space velocity of 19 km/s.[80] The radial component of this velocity—in the direction of the Sun—is −13.9 km/s, while the transverse velocity is 9.9 km/s. 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.[81] Vega will make its closest approach in an estimated 264,000 years at a perihelion distance of 13.2 ly (4.04 pc).[82]
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.[11] This group contains about 16 stars, including Alpha Librae, Alpha Cephei, Castor, Fomalhaut and Vega. All members of the group are moving in nearly the same direction with similar space velocities. Membership in a moving group implies a common origin for these stars in an open cluster that has since become gravitationally unbound.[83] The estimated age of this moving group is 200 ± 100 million years, and they have an average space velocity of 16.5 km/s.[note 4][80]
Possible planetary system[]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b (unconfirmed) | ≥21.9±5.1 M
|