IK Pegasi

context: en: At an estimated 1.15 M☉, IK Pegasi B is considered to be a high-mass white dwarf. Although its radius has not been observed directly, it can be estimated from known theoretical relationships between the mass and radius of white dwarfs, giving a value of about 0.60% of the Sun's radius. (A different source gives a value of 0.72%, so there remains some uncertainty in this result.) Thus this star packs a mass greater than the Sun into a volume roughly the size of the Earth, giving an indication of this object's extreme density.The massive, compact nature of a white dwarf produces a strong surface gravity. Astronomers denote this value by the decimal logarithm of the gravitational force in cgs units, or log g. For IK Pegasi B, log g is 8.95. By comparison, log g for the Earth is 2.99. Thus the surface gravity on IK Pegasi is over 900,000 times the gravitational force on the Earth.The effective surface temperature of IK Pegasi B is estimated to be about 35,500 ± 1,500 K, making it a strong source of ultraviolet radiation. Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain essentially unchanged.
qas: question: zh_tw: 如何能測量一顆白矮星的半徑？
answers: en: text: from known theoretical relationships between the mass and radius of white dwarfs

question: zh_tw: 另一文獻有何價值？
answers: en: text: 0.72%

context: en: IK Pegasi (or HR 8210) is a binary star system in the constellation Pegasus. It is just luminous enough to be seen with the unaided eye, at a distance of about 150 light years from the Solar System.
qas: question: zh_tw: 飛馬座 IK 是什麼？
answers: en: text: a binary star system

context: en: Nearly all small and intermediate-mass stars (below about 9 M☉) will end up as white dwarfs once they have exhausted their supply of thermonuclear fuel. Such stars spend most of their energy-producing life span as a main-sequence star. The time that a star spends on the main sequence depends primarily on its mass, with the lifespan decreasing with increasing mass. Thus, for IK Pegasi B to have become a white dwarf before component A, it must once have been more massive than component A. In fact, the progenitor of IK Pegasi B is thought to have had a mass between 5 and 8 M☉.As the hydrogen fuel at the core of the progenitor of IK Pegasi B was consumed, it evolved into a red giant. The inner core contracted until hydrogen burning commenced in a shell surrounding the helium core. To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star. When the core reached a temperature and density where helium could start to undergo fusion this star contracted and became what is termed a horizontal branch star. That is, it belonged to a group of stars that fall upon a roughly horizontal line on the H-R diagram. The fusion of helium formed an inert core of carbon and oxygen. When helium was exhausted in the core a helium-burning shell formed in addition to the hydrogen-burning one and the star moved to what astronomers term the asymptotic giant branch, or AGB. (This is a track leading to the upper-right corner of the H-R diagram.) If the star had sufficient mass, in time carbon fusion could begin in the core, producing oxygen, neon and magnesium.The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about 5 × 108 km (3 AU) in the case of the pulsating AGB star Mira. This is well beyond the current average separation between the two stars in IK Pegasi, so during this time period the two stars shared a common envelope. As a result, the outer atmosphere of IK Pegasi A may have received an isotope enhancement.
qas: question: zh_tw: 它如何彌補溫度的提升？
answers: en: text: the outer envelope expanded to many times the radius

context: en: Stars within the portion of the instability strip that crosses the main sequence are called Delta Scuti variables. These are named after the prototypical star for such variables: Delta Scuti. Delta Scuti variables typically range from spectral class A2 to F8, and a stellar luminosity class of III (subgiants) to V (main sequence stars). They are short-period variables that have a regular pulsation rate between 0.025 and 0.25 days. Delta Scuti stars have an abundance of elements similar to the Sun's (see Population I stars) and between 1.5 and 2.5 M☉. The pulsation rate of IK Pegasi A has been measured at 22.9 cycles per day, or once every 0.044 days.Astronomers define the metallicity of a star as the abundance of chemical elements that have a higher atomic number than helium. This is measured by a spectroscopic analysis of the atmosphere, followed by a comparison with the results expected from computed stellar models. In the case of IK Pegasus A, the estimated metal abundance is [M/H] = +0.07 ± 0.20. This notation gives the logarithm of the ratio of metal elements (M) to hydrogen (H), minus the logarithm of the Sun's metal ratio. (Thus if the star matches the metal abundance of the Sun, this value will be zero.) A logarithmic value of 0.07 is equivalent to an actual metallicity ratio of 1.17, so the star is about 17% richer in metallic elements than the Sun. However the margin of error for this result is relatively large.
qas: question: zh_tw: 被稱作為矮造父變星的星球存在於星際中的哪部分？
answers: en: text: instability strip

question: zh_tw: 光譜分析結果與其他什麼結果相比較？
answers: en: text: computed stellar models

context: en: The spectrum of A-class stars such as IK Pegasi A show strong Balmer lines of hydrogen along with absorption lines of ionized metals, including the K line of ionized calcium (Ca II) at a wavelength of 393.3 nm. The spectrum of IK Pegasi A is classified as marginal Am (or "Am:"), which means it displays the characteristics of a spectral class A but is marginally metallic-lined. That is, this star's atmosphere displays slightly (but anomalously) higher than normal absorption line strengths for metallic isotopes. Stars of spectral type Am are often members of close binaries with a companion of about the same mass, as is the case for IK Pegasi.Spectral class-A stars are hotter and more massive than the Sun. But, in consequence, their life span on the main sequence is correspondingly shorter. For a star with a mass similar to IK Pegasi A (1.65 M☉), the expected lifetime on the main sequence is 2–3 × 109 years, which is about half the current age of the Sun.In terms of mass, the relatively young Altair is the nearest star to the Sun that is a stellar analogue of component A—it has an estimated 1.7 M☉. The binary system as a whole has some similarities to the nearby system of Sirius, which has a class-A primary and a white dwarf companion. However, Sirius A is more massive than IK Pegasi A and the orbit of its companion is much larger, with a semimajor axis of 20 AU.
qas: question: zh_tw: 飛馬座 IK 被歸為哪類發光體？
answers: en: text: A-class stars