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Vega
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Vega (a Lyr / a Lyrae / Alpha Lyrae) ( or ) is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus. It is a relatively nearby star at only from Earth, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood.
Vega has been extensively studied by astronomers, leading it to be termed, "arguably the next most important star in the sky after the Sun".
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Encyclopedia
Vega (a Lyr / a Lyrae / Alpha Lyrae) ( or ) is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus. It is a relatively nearby star at only from Earth, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood.
Vega has been extensively studied by astronomers, leading it to be termed, "arguably the next most important star in the sky after the Sun". Historically, Vega served as the northern pole star at about 12,000 BCE and will do so again at around 14,000 CE. Vega was the first star, other than the Sun, to have its photograph taken and the first to have its spectrum photographed. It was also one of the first stars to have its distance estimated through parallax measurements. Vega has served as the baseline for calibrating the photometric brightness scale, and was one of the stars used to define the mean values for the UBV photometric system.
This star is relatively young when compared to the Sun. It has an unusually low abundance of the elements with a higher atomic number than that of helium. Vega is also a suspected variable star that may vary slightly in magnitude in a periodic manner. It is rotating rapidly with a velocity of 274 km/s at the equator. This is causing the equator to bulge outward because of 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 the Earth, Vega is being observed from the direction of one of these poles.
Based upon an excess emission of infrared radiation, Vega has a circumstellar disk of dust. This dust is likely the result of collisions between objects in an orbiting debris disk, which is analogous to the Kuiper belt in the Solar System. Stars that display an infrared excess because of dust emission are termed Vega-like stars. Irregularities in Vega's disk also suggest the presence of at least one planet, likely to be about the size of Jupiter, in orbit around Vega.
Visibility
Vega can often be seen near the zenith in the mid-northern latitudes during the evening in the Northern Hemisphere summer. 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. At latitudes to the north of +51° N Vega remains continually above the horizon as a circumpolar star. On about July 1, Vega reaches midnight culmination when it crosses the meridian at that time.
This star lies at a vertex of a widely-spaced asterism called the Summer Triangle, which consists of the zero-magnitude stars Vega in the constellation Lyra and Altair in Aquila, plus the first magnitude star Deneb in Cygnus. 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.
The Lyrids are a strong meteor shower that peak each year during April 21–22. When a small meteor enters the Earth's atmosphere at a high velocity it produces a streak of light as the object is vaporized. During a shower, a multitude of meteors arrive from the same direction, and, from the perspective of an observer, their glowing trails appear to radiate from a single point in space. In the case of the Lyrids, the meteor trails radiate from the direction of Lyra, and hence are sometimes called the Alpha Lyrids. However, they actually originated from debris emitted by the comet C/1861 G1 Thatcher and have nothing to do with the star.
Physical properties
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 only one billion years, a tenth of our Sun's. The current age of this star is between 386 and 511 million years, or up to about half its expected total main sequence life span. 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 of the Sun and its full luminosity is about 37 times the Sun's value. If Vega is variable, then it may be a Delta Scuti type with a period of about 0.107 days.
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 requires a temperature of 16 million K, which is higher than the core temperature of the Sun, but is more 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 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.
The energy flux from Vega has been precisely measured against standard light sources. At 5480 Å, the flux is 3,650 Jy with an error margin of 2%. 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. The lines of other elements are relatively weak, with the strongest being ionized magnesium, iron and chromium. The X-ray emission from Vega is very low, demonstrating that the corona for this star must be very weak or non-existent.
Rotation
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.
The pole of Vega—its axis of rotation—is inclined no more than five degrees from the line-of-sight to the Earth. The equator of Vega has a rotation velocity of 274 km/s (for a rotation period of about 12.5 hours), which is 93% of the speed that would cause the star to start breaking up from centrifugal effects. This rapid rotation of Vega produces a pronounced equatorial bulge, so the radius of the equator is 23% larger than the polar radius. (The estimated polar radius of this star is 2.26 ± 0.02 solar radii, while the equatorial radius is 2.78 ± 0.02 solar radii.) From the Earth, this bulge is being viewed from the direction of its pole, producing the overly large radius estimate.
The local gravitational acceleration at the poles is greater than at the equator, so, by the Von Zeipel theorem, the local luminosity is also higher at the poles. This is seen as a variation in effective temperature over the star: the polar temperature is near 10,000 K, while the equatorial temperature is 7,600 K. As a result, if Vega were viewed along the plane of its equator, then the luminosity would be about half the apparent luminosity as viewed from the pole.[From the poles, the star presents a circular profile, while from the equator the star appears as an ellipse. The cross-sectional area of the star's elliptical profile is only about 81% of the cross-sectional area of the star's polar profile, so less energy is received along the plane of the equator. Any additional difference in luminosity is accounted for by the temperature distribution. From the Stefan–Boltzmann law, the energy flux from Vega's equator will be about:
or 33% of the flux from the pole.] 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 Vega has a convection zone around the equator, while the remainder of the atmosphere is likely to be in almost pure radiative equilibrium.
If Vega was actually a slowly rotating, spherically-symmetric star and it was radiating the same energy as viewed from the Earth, then the luminosity of Vega would be 57 times the luminosity of the Sun. This value is much larger than the luminosity of a typical slowly rotating star with the same mass as Vega. Thus the discovery of fast rotation of Vega resolved this discrepancy. The true full luminosity of Vega is about 37 times the luminosity of the Sun.
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 for improved instrument calibrations.
Element abundance
Astronomers term "metals" those elements with higher atomic numbers than helium. 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:
.] (Compare this, for example, to a three-fold 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. 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-type star. 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.
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.
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 redshift (or blueshift) of the star's spectrum. Precise measurements of this redshift give a value of −13.9 ± 0.9 km/s. 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 milli-arcseconds (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, 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 = −13.9 ± 0.9, V = −6.3 ± 0.8 and W = −7.7 ± 0.3, for a net space velocity of 17 km/s. 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 sky, the star is slowly brightening as proper motion causes it to approach the Sun. Vega will eventually become the brightest star in the sky in around 210,000 years, will attain a peak brightness of magnitude –0.81 in about 290,000 years and will be the brightest star in the sky for about 270,000 years.
Based on this star's kinematic properties, it appears to belong to a stellar association called the Castor Moving Group. This group contains about 16 stars, including Alpha Librae, Alpha Cephei, Castor, Fomalhaut and Vega. All members of the group are moving in near parallel with similar space velocities. Membership in a moving group implies a common origin for these stars in a open cluster that has since become gravitationally unbound. The estimated age of this moving group is 200 ± 100 million years, and they have an
average space velocity of 16.5 km/s.[U = −10.7 ± 3.5, V = −8.0 ± 2.4, W = −9.7 ± 3.0 km/s. The net velocity is:
]
Planetary system
Infrared excess
One of the early results from the Infrared Astronomy Satellite (IRAS) was the discovery of excess infrared flux coming from Vega; beyond what would be expected from the star alone. This excess was measured at wavelengths of 25, 60 and 100 µm, and came from within an angular radius of 10 arcseconds (10″) centered on the star. At the measured distance of Vega, this corresponded to an actual radius of 80 astronomical units (AU), where an AU is the average radius of the Earth's orbit around the Sun. It was proposed that this radiation came from a field of orbiting particles with a dimension on the order of a millimeter, as anything smaller would eventually be removed from the system by radiation pressure or drawn into the star by means of Poynting-Robertson drag. The latter is the result of radiation pressure creating an effective force that opposes the orbital motion of a dust particle, causing it to spiral inward. This effect is most pronounced for tiny particles that are closer to the star.
Subsequent measurements of Vega at 193 µm showed a lower than expected flux for the hypothesized particles, suggesting that they must instead be on the order of 100 µm or less. To maintain this amount of dust in orbit around Vega, a continual source of replenishment would be required. A proposed mechanism for maintaining the dust was a disk of coalesced bodies that were in the process of collapsing to form a planet. Models fitted to the dust distribution around Vega indicate
that it is a 120 AU-radius circular disk viewed from nearly pole-on. In addition, there is a hole in the center of the disk with a radius of no less than 80 AU.
Following the discovery of an infrared excess around Vega, other stars have been found that display a similar anomaly that is attributable to dust emission. As of 2002, about 400 of these stars have been found, and they have come to be termed "Vega-like" or "Vega-excess" stars. It is believed that these may provide clues to the origin of the Solar System.
Debris disk
By 2005, the Spitzer Space Telescope had produced high resolution infrared images of the dust around Vega. It was shown to extend out to 43″ (330 AU) at a wavelength of 24 µm, 70″ (543 AU) at 70 µm and 105″ (815 AU) at 160 µm. These much wider disks were found to be circular and free of clumps, with dust particles ranging from 1–50 µm in size. The estimated total mass of this dust is 3 times the mass of the Earth. Production of the dust would require collisions between asteroids in a population corresponding to the Kuiper Belt around the Sun. Thus the dust is more likely created by a debris disk around Vega, rather than from a protoplanetary disk as was earlier thought.
The inner boundary of the debris disk was estimated at 11″ ± 2″, or 70–102 AU. The disk of dust is produced as radiation pressure from Vega pushes debris from collisions of larger objects outward. However, continuous production of the amount of dust observed over the course of Vega's lifetime would require an enormous starting mass—estimated as hundreds of times the mass of Jupiter. Hence it is more likely to have been produced as the result of a relatively recent breakup of a moderate-sized (or larger) comet or asteroid, which then further fragmented as the result of collisions between the smaller components and other bodies. This dusty disk would be relatively young on the time scale of the star's age, and it will eventually be removed unless other collision events supply more dust.
Observations with the CHARA array at Mt. Wilson in 2006 revealed evidence for an inner dust band around Vega. Originating within 8 AU of the star, this dust may be evidence of dynamical perturbations within the system. This may be caused by an intense bombardment of comets or meteors, and may be evidence for the existence of a planetary system.
Possible planets
Observations from the James Clerk Maxwell Telescope in 1997 revealed an "elongated bright central region" that peaked at 9″ (70 AU) to the northeast of Vega. This was hypothesized as either a perturbation of the dust disk by a planet or else an orbiting object that was surrounded by dust. However, images by the Keck telescope had ruled out a companion down to magnitude 16, which would correspond to a body with more than 12 times the mass of Jupiter. Astronomers at the Joint Astronomy Centre in Hawaii and at UCLA suggested that the image may indicate a planetary system still undergoing formation.
Determining the nature of the planet has not been straightforward; a 2002 paper hypothesizes that the lumps are caused by a roughly Jupiter-mass planet on an eccentric orbit. Dust would collect in orbits that have mean-motion resonances with this planet—where their orbital periods form integer fractions with the period of the planet—producing the resulting clumpiness.
In 2003 it was hypothesized that these lumps could be caused by a roughly Neptune-mass planet having migrated from 40 to 65 AU over 56 million years, an orbit large enough to allow the formation of smaller rocky planets closer to Vega. The migration of this planet would likely require gravitational interaction with a second, higher mass planet in a smaller orbit.
Using a coronagraph on the Subaru telescope in Hawaii in 2005, astronomers were able to further constrain the size of a planet orbiting Vega to no more than 5–10 times the mass of Jupiter. Although a planet has yet to be directly observed around Vega, the presence of a planetary system can not yet be precluded. Thus there could be smaller, terrestrial planets orbiting closer to the star. The inclination of planetary orbits around Vega is likely to be closely aligned to the equatorial plane of this star. From the perspective of an observer on a hypothetical planet around Vega, the Sun would appear as a faint 4.3 magnitude star in the Columba constellation.[The Sun would appear at the diametrically opposite coordinates
from Vega at α=, δ=, which is in the western part of Columba. The visual magnitude is given by ]
Etymology and cultural significance
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, 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. It is the brightest of the successive northern pole stars.
Among the northern Polynesian people, Vega was known as whetu o te tau, the year star. For a period of history it marked the start of their new year when the ground would be prepared for planting. Eventually this function became denoted by the Pleiades.
The Assyrians named this pole star Dayan-same, the "Judge of Heaven", while in Akkadian it was Tir-anna, "Life of Heaven". In Babylonian astronomy, Vega may have been one of the stars named Dilgan, "the Messenger of Light". To the ancient Greeks, the constellation Lyra was formed from the harp of Orpheus, with Vega as its handle. For the Roman Empire, the start of autumn was based upon the hour at which Vega set below the horizon.
In Chinese mythology, there is a love story of Qi Xi in which Niu Lang (Altair) and his two children (ß and ? Aquilae) are separated from their mother Zhi Nü (Vega) who is on the far side of the river, the Milky Way ??. However, one day per year on the seventh day of the seventh month of the Chinese lunisolar calendar, magpies make a bridge so that Niu Lang and Zhi Nü can be together again for a brief encounter. The Japanese Tanabata festival is also based on this legend. In Zoroastrianism, Vega was sometimes associated with, Vanant, a minor divinity whose name means "conqueror".
The name Wega (later Vega) comes from a loose transliteration of the Arabic word meaning "falling", via the phrase ‘, which sources translate as "the falling eagle" or "the swooping vulture", as this constellation was represented as a vulture in ancient Egypt,
and as an eagle or vulture in ancient India. The Arabic name then appeared in the western world in the Alfonsine Tables, which were drawn up between 1215–70 by order of Alfonso X.
Medieval astrologers counted Vega as one of the Behenian stars and related it to chrysolite and winter savory. Cornelius Agrippa listed its kabbalistic sign under Vultur cadens, a literal Latin translation of the Arabic name. Medieval star charts also listed the alternate names Waghi, Vagieh and Veka for this star.
Vega became the first star to have a car named after it when Chevrolet launched the Vega in 1971. Other vehicles named after Vega include the ESA's Vega launch system and the Lockheed Vega aircraft.
See also
External links
- Freeview videos provided by the Vega Science Trust.
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