HD 39118 (HD 39119, HR 2024) is a spectroscopic binary star in the constellation Orion, close to the celestial equator. It is made up from three stars: a cool primary (a K-type giant star) and a hot secondary, which is a binary star formed from a B-type main-sequence star and an A-type main-sequence star. A 2021 estimate derive a distance of to HD 39118, and it is moving away from Earth at a speed of 4.24 km/s. The apparent magnitude is 5.976, making it visible to the naked eye only from dark skies.
Characteristics
HR 2024 is a spectroscopic binary (more precisely a single-lined spectroscopic binary) made up of a cool primary and a hot secondary, which is also a binary star. The designations “cool” and “hot” refer to the relative effective temperature of the components. They are separated by , and complete an orbit around each other every . The orbital eccentricity is equivalent to 0.3.
HD 39118 can be seen in the northern celestial hemisphere, close to the celestial equator, at a distance of in the constellation Orion. Data about this star can be seen here. It has an apparent magnitude of 5.976. At this magnitude, it is visible to the naked eye only in dark skies, being close to the limiting magnitude to naked-eye vision of 6.5. The absolute magnitude, i.e. its brightness if it was seen at a distance or , is –2.53. It is moving away from Earth at a velocity of 4.24km/s.
The Henry Draper Catalogue recognises that the spectrum is composite: the designation HD 39118 is applied to component A with spectral class G0; HD 39119 is applied to component B with spectral class A0, although both entries have the same position and magnitude.
Primary star The primary has a spectral classification of K0II, meaning that it is a K-type star that has evolved away from the main sequence and is now a bright giant star. Currently, it is on the horizontal branch, fusing helium in its core. HD 39118's database entry at VizieR. It is 3.28 times more massive than the Sun and has expanded to 25 times the Sun's size. It emits a luminosity 535 times the solar luminosity HD 39118's database entry at VizieR. from its photosphere at an effective temperature of , which is around cooler than the Sun. HD 39118's database entry at VizieR. It has a subsolar metallicity, having an abundance of iron on its surface equivalent to 46% that of the Sun. The age of the primary is estimated at 263 million years, much younger than the Sun (4.6 billion years) despite its advanced evolutionary stage. It rotates on its own axis at a projected velocity of 4.19 km/s. HD 39118's database entry at VizieR. The B-V index is of 1.12, giving it the typical orange hue of a K-type star.
Hot companion The hot companion is made up of two other stars, one is a late B-type main-sequence star (spectral type B7V/B8V) and the other is an early A-type main-sequence star (spectral type A0V). They have a combined brightness about 1.55 magnitudes fainter than the cooler primary star. The B-type star has an effective temperature of 11,300 K. HD 39118's database entry at VizieR. Although stars cannot be resolved, it is calculated that the combined B-V index of the hot companions is –0.09, meaning that it has a typical hue of a B-type star.
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