A large number of variable stars in galaxies other than the Milky Way can usefully be observed by amateur astronomers. These are the objects I have observed.
Historically, some star-like objects of varying brightness were catalogued as variable stars. BL Lac is a famous example; others include AP Lib and W Com. They were subsequently found not to be stars at all but the nuclei of galaxies. The optically varying component of the light from the nucleus comes from a relativistic jet of particles being expelled from the vicinity of a supermassive black hole. BL Lac is now catalogued as a blazar, as is Markarian 421 which appears below. Quasars form another species in the AGN (active galactic nuclei) zoo, of which 3C 271 is the brightest and best known.
Stars
| Star | Type | Range | Notes |
|---|---|---|---|
| AE And | SDOR | 15.1 – 17.6 | In M31 |
| AF And | SDOR | 15.4 – 17.6 | In M31 |
| M31N 2008-12a | NR | 18.0 – <23 | In M31 |
| Y Tri | SDOR | 15.4 – 19.1 | In M33 |
Supernovae
| Star | Type | Maximum | Galaxy | Redshift | Notes |
|---|---|---|---|---|---|
| SN 2022 qmx | Ia | 18.8 | Anonymous | 0.35 | Gravitationally lensed. |
| SN 2023ixf | II-L | 11.0 | Messier 101 | 0.000804 | Precursor was a red supergiant, magnitude 25.0 |
Active galactic nuclei
| AGN | Type | Range | Notes |
|---|---|---|---|
| 4C 31.03 | AGN | 12.4 – 19.0 | 4C 31.03 |
| Mrk 421 | BLLAC | 12.0 – 13.9 | Markarian 421 |