La Superba
La Superba is one of only 20 known stars that contains a relatively high abundance of carbon-13 in its outer atmosphere. The carbon-13 — produced mainly from fusing helium near its core — bubbles up to the surface in a process called, “dredge up.” There it interacts with other elements to form carbon compounds that absorb shorter wavelengths of light, leaving only the red spectrum for us to see. The high abundance of carbon in La Superba makes it one of the reddest stars known to exist.
La Superba, also known by its latin name, Y Canum Venaticorum, is giving us a preview of how the sun will look when it ages to the point when all the hydrogen
GALAXY WATCH
in its core has fused into heavier elements. This has raised the core temperature of La Superba considerably, making it expand large enough that, if it replaced our Sun, its diameter would reach beyond the orbit of Mars.
La Superba is also a variable star. Although it is 22,000 times brighter than the sun at its minimum and 87,000 times at its maximum, its distance from Earth (760 light-years) makes it difficult to see with the naked eye. If you use 10x50 binoculars, you’ll be able to easily see it, including it’s deep-red tint.
On the Hertzsprung-Russell stellar evolution diagram (H-R Diagram), La Superba is known as an asymptotic giant branch (AGB) star. When a low to medium mass star (one half to 8 solar masses) has fused all the hydrogen in its core into heavier elements, it
becomes unstable. The heavier elements require more pressure and higher temperatures to continue the nuclear fusion process. This causes the star to evolve into a red giant as it reaches new equilibriums between the expansion pressures in its interior and the gravitational collapsing pressures from its overall mass. All the while, the star expels its outer atmosphere into space at ever-increasing rates. The mass loss causes the star to convulse as it temporarily shrinks and then expands again as fusion temperatures continue to increase. The overall changes cause the star to become cooler on its surface but much more luminous because of its ever-increasing size. That’s when it becomes an AGB star — because it forms a curve (surface temperature versus luminosity) on the H-R diagram that scribes an asymptotic line (comes close to but doesn’t quite match the curve it produced during its initial red giant phase).