star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
located approximately 410
light year
A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The first constellati ...
of
Ophiuchus
Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constellati ...
, northwest of the center 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 ...
. It is visible to the naked eye as a faint, blue-white point of light with an
apparent visual magnitude
Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...
of 4.81. The star is moving closer to the Earth with a heliocentric
radial velocity
The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...
of –12 km/s.
This object is notable for being "a rare, nearby example of a young planetary system just entering the last phase of planet formation". There is uncertainty about the
stellar classification
In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...
of this star. It has the nominal classification of B9.5IIIe, a B-type giant star with
emission line
A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
s. However, it has also been classified as an A0 II-IIIe star and as a Herbig Ae/Be star. 51 Ophiuchi is about 300,000 years old with 3.3 times the mass of the Sun and a polar radius 5.7 times the
Sun's radius
Solar radius is a unit of distance used to express the size of objects in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3:
1\,R_ = 6.957\ti ...
. It is radiating three times the
Sun's luminosity
The solar luminosity () is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun.
One nominal s ...
from its
photosphere
The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...
at an
effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
of 9,772 K. The star is spinning rapidly with a
projected rotational velocity
Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface.
The rotation of a star produces an equatorial bu ...
of 267 km/s, close to the critical rotation rate.
Dust and gas disk
51 Ophiuchi has a disk of dust and gas that appears to be a young
debris disk
A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...
and is probably a planetary system in the late stages of formation. This system resembles Beta Pictoris, a well known star with a large debris disk, in several ways: spectral type, the presence of an edge-on disk with both gas and dust, and the presence of variable blue-shifted absorption lines suggesting in-falling comets.
The distance to 51 Ophiuchi is much greater than the distance to Beta Pictoris, and its debris disk is relatively compact. As a consequence, the disk around 51 Ophiuchi requires an
interferometer
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...
to resolve, in contrast to that of Beta Pictoris, which has been observed using visual spectrum imaging. Recent observations of 51 Ophiuchi made with the Keck Interferometer Nuller at the W. M. Keck Observatory show that the disk has two components: a central cloud of large particles ( exozodiacal dust) surrounded by a much larger cloud of small
silicate
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
particles extending to about 1,000
astronomical unit
The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
s. The inner disk has a radius approximately four times the distance between the sun and the Earth, with a density of around 100,000 times that of the dust in the Solar System.
The spectra taken by 2020 have indicated the circumstellar disc is similar in composition to interstellar medium. The dominant species in atomic numbers are hydrogen, helium, oxygen, nitrogen, silicon and iron. Surprisingly, the disk was found to be strongly depleted of carbon and carbon monoxide.