HD 122563
Encyclopedia
HD
122563 is an extremely metal poor
red giant
star, and the brightest metal poor star in the sky. Its low heavy element content was first recognized by spectroscopic analysis
in 1963. For more than twenty years it was the most metal-poor star known, being more metal-poor than any known globular cluster
, and it is the most accessible example of an extreme Population II
or Halo
star.
As the most extreme metal-poor star known, HD 122563's composition was crucial in constraining theories for galactic chemical evolution
; in particular, its composition peculiarities provided signposts for understanding the accumulation of heavy elements by stellar nucleosynthesis
in the Galaxy
.
For example, it has an excess of oxygen
,[O/Fe] = +0.6, while the proportions of strontium
, yttrium
, zirconium
, barium
and the lanthanide
elements suggest that the s-process
has made no contribution to the material present in the star: in HD 122563, all these elements are products of the r-process
instead. The implication is that the star formed at a time and place where there had not been enough time for any previous generation of stars to have produced s-process elements, though there was r-process material present.
its spectral type is given as F8IV, but its color index
indicates a surface temperature much cooler than an F8 star should be. Because the spectral type of a star in the A to K star regime is judged by the relative strengths of the absorption lines
of the metals relative to the hydrogen Balmer lines, the extreme metal deficiency results in weak metal lines and yields a spuriously early spectral type. If the spectral classification is performed including the metal deficiency, the result is a rather later type.
Henry Draper Catalogue
The Henry Draper Catalogue is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the Henry Draper Extension , published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the...
122563 is an extremely metal poor
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...
red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
star, and the brightest metal poor star in the sky. Its low heavy element content was first recognized by spectroscopic analysis
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
in 1963. For more than twenty years it was the most metal-poor star known, being more metal-poor than any known 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...
, and it is the most accessible example of an extreme Population II
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...
or 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...
star.
As the most extreme metal-poor star known, HD 122563's composition was crucial in constraining theories for galactic chemical evolution
Chemical evolution
Chemical evolution may refer to:*Nucleosynthesis, the creation of chemical elements in the universe either through the Big Bang, or supernovae*Abiogenesis, the transition from nonliving elements to living systems...
; in particular, its composition peculiarities provided signposts for understanding the accumulation of heavy elements by stellar nucleosynthesis
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
in the Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
.
For example, it has an excess of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
,
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
, yttrium
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
, zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
and the lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
elements suggest that the s-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
has made no contribution to the material present in the star: in HD 122563, all these elements are products of the r-process
R-process
The r-process is a nucleosynthesis process, likely occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich atomic nuclei that are heavier than iron. The process entails a succession of rapid neutron captures on seed nuclei, typically Ni-56,...
instead. The implication is that the star formed at a time and place where there had not been enough time for any previous generation of stars to have produced s-process elements, though there was r-process material present.
Spectral type
The spectral type of HD 122563 is one of characteristics which initially indicated its peculiarity. In the Bright Star CatalogueBright Star Catalogue
The Bright Star Catalogue, also known as the Yale Catalogue of Bright Stars or Yale Bright Star Catalogue, is a star catalogue that lists all stars of stellar magnitude 6.5 or brighter, which is roughly every star visible to the naked eye from Earth. It is currently available online in its 5th...
its spectral type is given as F8IV, but its color index
Color index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...
indicates a surface temperature much cooler than an F8 star should be. Because the spectral type of a star in the A to K star regime is judged by the relative strengths of the absorption lines
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
of the metals relative to the hydrogen Balmer lines, the extreme metal deficiency results in weak metal lines and yields a spuriously early spectral type. If the spectral classification is performed including the metal deficiency, the result is a rather later type.