Zosma is a mag. +2.5 star marking the point on the top of the constellation Leo’s body that connects to the Lion’s tail.
This is the IAU-approved name for Delta (δ) Leonis, derived from the Greek word for ‘girdle’.
However, its Arabic name, Dhur, meaning ‘the lion’s back’, might have been more appropriate.
Zosma is a main sequence star with a spectral classification A4 V: a white (A4) main sequence dwarf (V). It is 2.1 times larger than the Sun and 15 times more luminous.
Unlike the Sun’s leisurely rotation velocity of 2km/s, Zosma rotates with a projected rotational velocity of 180 km/s.
This is uncorrected for the inclination of the rotational axis; correct the angle so the axis would be at right angles to our line of sight and the speed would be around 280km/s.
As a consequence, up close Zosma would show a noticeable equatorial bulge.
Parallax measurements put the star at a distance of 58.4 lightyears from the Sun.
Direct measurements of Zosma’s proper motion – direction and velocity through space – show it is probably part of a collection of stars known as the Ursa Major Moving Group.
This group would have formed in the same location together around 500 million years ago.
Zosma’s hydrogen fusion period is around 1 billion years and consequently we know the star is about halfway through its ‘normal’ life cycle.
At the end of this cycle its helium core will shrink and its outer layers expand, causing it to become an orange giant.
This guide originally appeared in the March 2023 issue of BBC Sky at Night Magazine.
