Scientists of all types spend a lot of their careers figuring out how to calibrate the when, where, and how of everything we see. In astronomy, we don’t have rocks to dig through, but we do have stars that build up myriad galaxies, and in their latter days, many of these stars will shine in particular ways that allow us to use them as calibration tools. 

You may be familiar with supernovae being used as distance indicators. When two white dwarf stars of the same mass explode, they give off the same amount of light, and we can determine which is closer and which is farther by how bright they appear. While explosions are always going to catch a person’s attention, other stars take a more subtle approach. Stars like our Sun form what are called planetary nebulae when they die. 

In its final millennia, the Sun will exhale its outer layers, and the core that will be left behind will illuminate that expanding shell (or shells) of material. The light that the core (a white dwarf star) gives off is pretty well known, and as the nebula expands over time, it will fade in a way that is directly proportional to its size. If a planetary nebula can be observed, its distance can be determined by measuring how big it appears and how bright it appears. Since only one distance value will fit both the size and distance, this is a useful technique for any galaxy close enough to allow us to use telescopes to see its planetary nebulae as more than just a point on the sky. 

In the early 2000s, folks started studying planetary nebulae in the Andromeda galaxy and now, thanks to the amazing resolution of the MUSE instrument on the Very Large Telescope, astronomers have been able to make out nebulae in the galaxy NGC 474, which is 110 million light-years away, or 55 times further away than Andromeda. Remarkably, this distorted galaxy could easily be mistaken for a planetary nebula if glanced at in an image set. 

Planetary nebulae exist in every large galaxy in our modern universe, including ours. This means we can measure them locally and use them to calibrate more… I was going to say globally, but in this case, the word is universally.

More Information

AIP press release

“Toward Precision Cosmology with Improved PNLF Distances Using VLT-MUSEI. Methodology and Tests,” Martin M. Roth et al., 2021 July 22, The Astrophysical Journal