Caption: The apparent relative orbit (right) and true relative orbit (left) of Sirius B around Sirius A.

Collectively Sirius A and Sirius B constitute the binary system Sirius AB.

Features:

1. Image author's caption: "Trajectory of Sirius B relative to Sirius A as seen from Earth and as seen from directly above the orbital plane (i.e., as seen from orbital inclination 0°). The axis is scaled in arcseconds (''). The orbital parameters this plot is based on are taken from van den Bos (1960). The plot has been created with gnuplot based on data generated by solving Kepler's equation. Notes: 1. The timestamps refer to the motion as seen from Earth, i.e. delayed with respect to the true motion by the travel of light by about 8.7 years. North is down, as usual in many sky maps (AKA star charts). 2. The points refer to steps of 1/50 of an orbit rather than to exact 1-year steps which would be 1/50.09 of an orbit." (Somewhat edited.)

2. A relative orbit is the astro-body relative to another astro-body.

   Usually, the astro-body that acts as the origin is the more massive.

3. An absolute orbit is relative to a local inertial frame usually the one in which the center of mass of the two-body system is at rest.

4. A true relative orbit is just the relative orbit viewed at inclination 0°: i.e., viewed with the orbital plane perpendicular to the viewing direction.

5. An apparent relative orbit is the relative orbit as seen projected on the celestial sphere as seen from Earth.
   Recall "apparent" in astronomy does NOT mean false or seeming. It means as seen from Earth.

6. In the image, Sirius A is chosen as the relative orbit origin.

   Sirius A is A1 V star (i.e., a main-sequence A1 star). It has apparent V magnitude -1.47, stellar mass 2.02 M_☉, and luminosity 25.4 L_☉.

   Sirius A is, in fact, the brightest star on the sky (i.e., the star of highest apparent brightness).

7. Sirius A and the much dimmer Sirius B are collectively called Sirius or, less often, Sirius AB.

   However, one can also use Sirius to mean just Sirius A.

   Context tells you what is meant

8. Sirius B is too faint to be seen by naked eye and too close to Sirius A to be resolved by the naked eye even if it could be seen.

   As the image illustrates, the angular separation of Sirius A and Sirius B is order of magnitude 10''

   The human eye angular resolution has the typical and fiducial value 1 arcminute (') = 60'' (see Wikipedia: Naked-eye astronomy). Some sharp-eyed people may be able to do better.

9. Sirius B is a white dwarf star. It has apparent V magnitude 8.44, stellar mass 0.978 M_☉, and luminosity 0.026 L_☉.

   A White dwarf is a post-main-sequence star that has ended its nuclear-burning lifetime, lost much of its original mass, and is now cooling off forever.

10. Currently, Sirius A is the primary star (i.e., the brighter star and hence designated A) and Sirius B is the secondary star (i.e., the dimmer star and hence designated B).

    However, originally Sirius B was the larger, more luminous star.

    The two stars of a binary system form at the same time.

    The more massive one, and therefore brighter one, runs through all phases of its nuclear-burning lifetime faster than the less massive one.

    Ergo, Sirius B was once the primary star.

    It lost a large fraction of its mass due to strong stellar winds in post-main-sequence evolution.