CW Leonis

Lore & Background

CW Leonis is believed to be in a late stage of its life, blowing off its own sooty atmosphere to form a white dwarf. Based upon isotope ratios of magnesium, the initial mass of this star has been constrained to lie between 3–5 solar masses. The mass of the star's core, and the final mass of the star once it becomes a white dwarf, is about 0.7–0.9 solar masses.

Its bolometric luminosity varies over the course of a 649-day pulsation cycle, ranging from a minimum of about 6,250 times the Sun's luminosity up to a peak of around 15,800 times. The brightness of the star varies by about two magnitudes over its pulsation period, and may have been increasing over a period of years. One study finds an increase in the mean brightness of about a magnitude between 2004 and 2014. Many studies of this star are done at infrared wavelengths because of its very red colour; published visual magnitudes are uncommon and often dramatically different.

The Guide Star Catalog from 2006 gives an apparent visual magnitude of 19.23. The ASAS-SN variable star catalog based on observations from 2014 to 2018 reports a mean magnitude of 17.56 and an amplitude of 0.68 magnitudes. An even later study gives a mean magnitude of 14.5 and an amplitude of 2.0 magnitudes.

The carbon-rich gaseous envelope surrounding this star is at least 69,000 years old and the star is losing about solar masses per year. The extended envelope contains at least 1.4 solar masses of material. Speckle observations from 1999 show a complex structure to this dust envelope, including partial arcs and unfinished shells.

This clumpiness may be caused by a magnetic cycle in the star that is comparable to the solar cycle in the Sun and results in periodic increases in mass loss. Various chemical elements and about 50 molecules have been detected in the outflows from CW Leonis, among others nitrogen, oxygen and water, silicon, and iron. One theory was that the star was once surrounded by comets that melted once the star started expanding, but water is now thought to form naturally in the atmospheres of all carbon stars.

Reader's Guide

CW Leonis is notable as a carbon star in a late evolutionary stage, shedding its atmosphere to eventually become a white dwarf. Its discovery in 1969 via infrared observations highlighted the importance of infrared astronomy for studying objects obscured by dust. The star's extreme infrared flux—the highest of any object outside the Solar System at 5 μm—makes it a key target for studying mass loss and dust formation in evolved stars. The complex structure of its envelope, including partial arcs and unfinished shells, suggests periodic mass loss possibly linked to a magnetic cycle.

The detection of water and many other molecules in its outflows has informed theories about chemical processes in carbon-rich environments. Its variable brightness and uncertain distance (with a lower estimate of 120 pc) add to its interest, and astrometric measurements suggest it may be the closest carbon star to Earth, possibly with a binary companion. These characteristics make CW Leonis a benchmark object for understanding the late stages of stellar evolution and the enrichment of the interstellar medium.