HD 134439 and HD 134440 are a pair of
K-type runaway star
In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.
Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as ...
s, nearly
hypervelocity star
In astronomy, stellar kinematics is the Observational astronomy, observational study or measurement of the kinematics or motions of stars through space.
Stellar kinematics encompasses the measurement of stellar Velocity, velocities in the Milky ...
s, in the constellation
Libra
Libra generally refers to:
* Libra (constellation), a constellation
* Libra (astrology), an astrological sign based on the star constellation
Libra may also refer to:
Arts and entertainment
* ''Libra'' (novel), a 1988 novel by Don DeLillo
Musi ...
. Although they are roughly away, they have one of the highest
proper motion
Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
s of any star in the sky, travelling over 3.5
arcsecond
A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...
s every year.
The stars are believed to originate from outside of the
Milky Way
The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
, likely from a destroyed, unusually dusty
satellite galaxy
A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and Luminosity, luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents ...
.
They have an extremely low
metallicity
In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...
, only about 3.5% that of the
Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
,
with unusual chemical abundances very different from those of stars formed anywhere in the galaxy.
Because of this, they are considered
subdwarf
A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same ...
s, and are less luminous than a
main-sequence
In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...
star of the same
spectral type
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
would be.
Binary system
It is unknown if HD 134439 and 134440 are actually orbiting each other, as at their distance, they would have to be a minimum apart from each other, which would make them one of the widest binary systems known. Further adding to this, using Gaia data, HD 134440 appears to be light years further than HD 134439,
resulting in a true separation of light years from each other. If they are orbiting each other, they would have an orbital period on the scale of million years, making them one of the longest period binary systems known as well.
Destroyed planet
In a 2018 study, it was found that HD 134440 had a noticeably higher metallicity than HD 134439, which could be potentially explained by the star having engulfed a planet orbiting it, suggesting that it may be possible for planets to form even around stars with extremely low content of planet-forming material.
References
{{DEFAULTSORT:HD 134439/HD 134440
K-type subdwarfs
0579.2
074234 5
134439 40
Libra (constellation)
Extragalactic stars
Runaway stars