An SX Phoenicis variable is a type of variable star

Variable star

A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

. 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

Magnitude (astronomy)

Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

 by up to 0.7. Compared to the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, these stars have a lower metallicity

Metallicity

In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

, which means they have a reduced abundance of elements other than hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. They also have relatively high space velocity and low luminosities for stars of their stellar classification. These properties distinguish the SX Phoenicis variables from their cousins, the Delta Scuti variable

Delta Scuti variable

A Delta Scuti variable is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the...

s. The latter have longer periods, strong metallicity and large amplitudes.

SX Phoenicis variables are found primarily in globular cluster

Globular cluster

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

s and galactic halo

Galactic halo

The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...

s. The variability cycle has a periodicity-luminosity relation. All known SX Phoenicis variables in globular clusters are blue straggler

Blue straggler

Blue stragglers are main sequence stars in open or globular clusters that are more luminous and bluer than stars at the main sequence turn-off 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...

 stars. These are stars that appear more blue (having a higher temperature) than the main sequence

Main sequence

The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

stars in the same cluster that have similar luminosities.