HD 39118

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 708 pc 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 “cold” and “hot” refer to the effective temperature of the components. They are separated by 4.7e8 km, and complete an orbit around each other every 2570 day. 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 708 pc in the constellation Orion. 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 10 pc, is –2.53. It is moving away from Earth at a velocity of 4.24km/s.

It was double cataloged in the Henry Draper Catalogue, receiving the designations HD 39118 and HD 39119.

Primary star
The primary has an spectral classification of K0II, meaning that it is a K-type star that has left the main sequence, being now a bright giant star. Currently, it is in the red giant branch of evolution. It is 3.28 times more massive has expanded to 25 times the Sun's size. It emitts a luminosity 535 times the solar luminosity from its photosphere at an effective temperature of 4,550 K, which is around 1,200K cooler than the Sun. 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 under its own axis at a velocity of 4.19 km/s. 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). It has an absolute magnitude about 1.55 magnitudes fainter than the primary companion. The B-type star has an effective temperature of 11,300 K. The B-V index of the hot companion is of –0.09, meaning that it has a typical hue of a B-type star.