A "luminous shell of gas" ejected from an "old low-mass star" is called a 'planetary nebula'.
<u>Explanation:</u>
When the red giants stars are in the last stage of their life, they eject a colored ionized gas which forms a shell-like structure around the star.
This structure appears to have a ring shape. It exhibits colors like red, green and blue.
The term 'planetary' in 'Planetary Nebula' is believed to be debatable by a number of astronomers because the planetary nebulas are related to dying stars, not planets.
