SX Phoenicis variable

An SX Phoenicis variable is a type of variable star. These stars exhibit a short period pulsation behavior that varies on time scales of 0.03–0.08 days (0.7–1.9 hours). They have spectral classifications in the range A2-F5 and vary in magnitude by up to 0.7.[1] Compared to the Sun, these stars have a lower metallicity, which means they have a reduced abundance of elements other than hydrogen and helium. They also have relatively high space velocity and low luminosities for stars of their stellar classification.[2][3] These properties distinguish the SX Phoenicis variables from their cousins, the Delta Scuti variables. The latter have longer periods, strong metallicity and large amplitudes.[4]

SX Phoenicis variables are found primarily in globular clusters and galactic halos. The variability cycle has a periodicity-luminosity relation.[5] All known SX Phoenicis variables in globular clusters are blue straggler stars.[6] These are stars that appear more blue (having a higher temperature) than the main sequence stars in the same cluster that have similar luminosities.[7]


The following list contains selected SX Phoenicis variable that are of interest to amateur or professional astronomy. Unless otherwise noted, the given magnitudes are in the V-band.

(in days)
SX Phoenicis[n 1] 6.76 7.53 0.055 A2V
HD 94033 9.46 10.26 0.060 B9III/IV
DY Pegasi 10.00 10.56 0.073 F5
CY Aquarii 10.42 11.20 0.061 B8
AE Ursae Majoris[n 1] 10.86 11.52 0.086 A9
XX Cygni 11.28 12.13 0.135 A5-F5
BL Camelopardalis 12.92 13.25 0.039
BX Sculptoris 13.42 13.71 0.037 A


  1. ^ a b double-mode


  1. ^ "GCVS Variability Types". General Catalogue of Variable Stars. Moscow, Russia: Russian Foundation for Basic Research, Sternberg Astronomical Institute. Retrieved 2010-03-17.
  2. ^ Burki, G.; Mayor, M. (1986). "Radial velocities with CORAVEL - Results on stellar variability and duplicity". Instrumentation and research programmes for small telescopes; Proceedings of the IAU Symposium. Christchurch, New Zealand: Dordrecht, D. Reidel Publishing Co. p. 392. Bibcode:1986IAUS..118..385B.
  3. ^ Breger, M. (Jan 1980). "The nature of dwarf Cepheids. V - Analysis and conclusions". Astrophysical Journal, Part 1. 235: 156. Bibcode:1980ApJ...235..153B. doi:10.1086/157620.
  4. ^ McNamara, D. H. (April 1995). "Period-luminosity relations of SX Phoenicis stars". Astronomical Journal. 109 (4): 1751–1756. Bibcode:1995AJ....109.1751M. doi:10.1086/117405.
  5. ^ Templeton, Matthew. "Variable Star of the Season: Delta Scuti and the Delta Scuti variables". American Association of Variable Star Observers.
  6. ^ Jeon, Young-Beom; Lee, Myung Gyoon; Kim, Seung-Lee; Lee, Ho (July 2004). "SX Phoenicis Stars in the Globular Cluster NGC 5466". The Astronomical Journal. 128 (1): 11. arXiv:astro-ph/0404069. Bibcode:2004AJ....128..287J. doi:10.1086/421735.
  7. ^ Santolamazza, P.; Marconi, M.; Bono, G.; Caputo, F.; Cassisi, S.; Gilliland, R. L. (June 2001). "Linear Nonadiabatic Properties of SX Phoenicis Variables". The Astrophysical Journal. 554 (2): 1124–1140. arXiv:astro-ph/0102029. Bibcode:2001ApJ...554.1124S. doi:10.1086/321408.
Blue straggler

A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3. Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main-sequence cluster stars, blue stragglers seem to be exceptions to this rule. The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found.

Known Space

Known Space is the fictional setting of about a dozen science fiction novels and several collections of short stories written by Larry Niven. It has also become a shared universe in the spin-off Man-Kzin Wars anthologies. ISFDB catalogs all works set in the fictional universe that includes Known Space under the series name Tales of Known Space, which was the title of a 1975 collection of Niven's short stories. The first-published work in the series, which was Niven's first published piece was "The Coldest Place", in the December 1964 issue of If magazine, edited by Frederik Pohl. This was the first-published work in the 1975 collection.

The stories span approximately one thousand years of future history, from the first human explorations of the Solar System to the colonization of dozens of nearby systems. Late in the series, Known Space is an irregularly shaped "bubble" about 60 light-years across.

Within the Tales of Known Space, the epithet "Known Space" refers to a relatively small region in the Milky Way galaxy, one centered on Earth. In the future that the series depicts, spanning roughly the third millennium, humans have explored this region and colonized many of its worlds. Contact has been made with other species, such as the two-headed Pierson's Puppeteers and the aggressive felinoid Kzinti. Stories in the Known Space series include events and places outside of the region called "Known Space" such as the Ringworld, the Pierson's Puppeteers' Fleet of Worlds and the Pak homeworld.

The Tales were originally conceived as two separate series, the Belter stories set roughly from 2000 to 2350 CE and the Neutron Star / Ringworld stories set in 2651 CE and later. The earlier, Belter period features solar-system colonization and slower-than-light travel with fusion-powered and Bussard ramjet ships. The later, Neutron Star period features faster-than-light ships using "hyperdrive". Niven implicitly joined the two settings as a single fictional universe in the short story "A Relic of the Empire" (If, December 1966), by using background elements of the Slaver civilization from the Belter series as a plot element in the faster-than-light setting. In the late 1980s—having written almost no Tales of Known Space in more than a decade—Niven opened the 300-year gap in the Known Space timeline as a shared universe, and the stories of the Man-Kzin Wars volumes fill in that history, bridging the two settings.

Messier 107

Messier 107 or M107, also known as NGC 6171, is a very loose globular cluster in the constellation Ophiuchus, and is the last globular cluster in the Messier Catalogue. It was discovered by Pierre Méchain in April 1782, then independently by William Herschel in 1793. Herschel described it as a "globular cluster of stars, large, very rich, very much compressed, round, well resolved, clearly consisting of stars". It was not until 1947 that Helen Sawyer Hogg added it and three other objects discovered by Méchain to the list of Messier objects. The cluster is located 2.5° south and slightly west of the star Zeta Ophiuchi.M107 is close to the galactic plane at a distance of about 20,900 light-years from Earth and 9.8 kly (3.0 kpc) from the Galactic Center. The orbit of this cluster carries it through the galaxy between 9.2–12.4 kly (2.82–3.79 kpc) from the Galactic Center, with the perigalactic distance laying within the galactic bar region. It is an Oosterhoff type I cluster with a metallicity of –0.95 and is considered part of the halo population. There are 22 known RR Lyrae variable stars in this cluster and a probable SX Phoenicis variable.

NGC 5053

NGC 5053 is the New General Catalogue designation for a globular cluster in the northern constellation of Coma Berenices. It was discovered by German-British astronomer William Herschel on March 14, 1784 and cataloged as VI-7. In his abbreviated notation, he described it as, "an extremely faint cluster of extremely small stars with resolvable nebula 8 or 10′ diameter, verified by a power of 240, beyond doubt". Danish-Irish astronomer John Louis Emil Dreyer reported in 1888 that the cluster appeared, "very faint, pretty large, irregular round shape, growing very gradually brighter at the middle".This is a metal-poor cluster, meaning the stars have a low abundance of elements other than hydrogen and helium—what astronomers term metallicity. As recently as 1995, it was considered the most metal-poor globular cluster in the Milky Way. The chemical abundances of the stars in NGC 5053 are more similar to those in the dwarf galaxy Sagittarius Dwarf Spheroidal Galaxy than to the Milky Way halo. Along with the kinematics of the globular cluster, this suggests that NGC 5053 may have been stripped from the dwarf galaxy.There are ten known RR Lyrae variable stars in this cluster with masses ranging from 68% to 78% of the solar mass. Nine of these variables were reported by German astronomer Walter Baade in 1928, and the tenth by American astronomer Helen Sawyer in 1946. The cluster hosts 27 known blue stragglers, of which five are short period SX Phoenicis variable stars.NGC 5053 is a relatively low mass cluster with a low core concentration factor of 1.32. It sports a stream of tidal debris to the west with a projected length of 1.7 kpc. This stream may have been created through shock-induced processes. The cluster is located less than 1° from Messier 53 and the two have nearly the same distance modulus, which corresponds to a spatial separation of around 2 kpc. There is a tidal bridge joining M53 to NGC 5053, suggesting the pair may have interacted in the past. The cluster is following an orbit through the Milky Way that has a perigalacticon distance of 9 kpc and an orbital eccentricity of 0.84. At present, it is 18.4 kpc from the Galactic Core, with a radial velocity of 42.0±1.4 km/s.

Outline of astronomy

The following outline is provided as an overview of and topical guide to astronomy:

Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).

SX Phoenicis

SX Phoenicis is a variable star in the southern constellation Phoenix. With an apparent visual magnitude ranging around 7.33, it is too faint to be seen with the naked eye. It is located 272 light years from the Sun, as determined from an annual parallax shift of 12 mas.This is the prototype SX Phoenicis variable, which means it is a population II, high amplitude, pulsating variable. SX Phoenicis has a very low metallicity, with only 4% the proportion of iron of the Sun. A member of the galactic halo, it has a retrograde orbit around the center of the Milky Way, and has an extremely high peculiar velocity of 323.2+12.7−13.3 km/s. From its movement through space, it can be a member of the Kapteyn group, a moving group of stars with a similar movement as Kapteyn's Star.Discovered to be variable by Olin J. Eggen in 1952, SX Phoenicis has been targeted in many studies of its light curve and spectrum. These observations revealed that SX Phoenicis has two pulsations periods of 0.055 and 0.043 days, which correspond to radial pulsations in the fundamental mode and in the first overtone, respectively. Other pulsation frequencies, combinations of these two, are also observed. The pulsations also cause the radial velocity of the star to vary by 38 km/s, with the same periods as the light variation. There is evidence that the pulsation periods change in a timescale of decades, with a possible cyclic variation period of 43 ± 10 years. Overall, the visual apparent magnitude of the star varies between 6.76 and 7.53.SX Phoenicis is a A-type main-sequence star with a stellar classification of A2 V. During the 1940s it was classed as a "probable subdwarf" because of its low luminosity for the spectral class. On average, it has a luminosity of 5.89 times the Sun's luminosity and an effective temperature of 7,700 K. During the primary pulsation cycle, the temperature varies between 7,230 K at minimum brightness up to 8,170 K at maximum brightness; when the two pulsation cycles are appropriately phased, the temperature can reach 8,400 K. Similarly, the pulsations cause the radius of the star to change, which is evidenced by changes in the surface gravity. Stellar evolution models by Petersen and Christensen-Dalsgaard (1996), taking into account the pulsating behavior of the star, indicate that the properties of the star are consistent with a mass of 1.0 solar mass and an age of 4 billion years.The origin of SX Phoenicis, and of SX Phoenicis variables in general, remains unclear. While its properties are well explained by standard stellar evolution, the observation of SX Phoenicis variables in old globular clusters indicates that these stars are blue stragglers, presumably formed by the merger of two stars or by interactions in a binary system. This explains why SX Phoenicis seems to be a young star, despite belonging to the halo population. In this scenario, SX Phoenicis was formed as a close binary star, whose components merged and originated a rejuvenated star, which started evolving as a single star.


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