science & techESA/Hubble2019 · Hercules constellation

Dim, but still distinct

the gist

Hubble's picture of the week shows UGC 11105, a spiral galaxy about 110 million light-years away in Hercules, which was home to a type II supernova in 2019. The image is a lesson in astronomical brightness: apparent magnitude measures how bright objects look from Earth, not how luminous they are, and the scale is reverse logarithmic, so lower numbers are brighter. The galaxy has an apparent magnitude of about 13.6, while the Sun sits at about -26.8, making the Sun roughly fourteen thousand trillion times brighter in our sky. The naked eye can see stars to about magnitude 6, and Hubble has detected objects as faint as 31.

so what

The magnitude scale explains why distant galaxies are invisible without powerful telescopes and why brightness and distance are so easily confused.

fun fact

Betelgeuse is about 21,000 times more luminous than the Sun, yet looks far dimmer because it is much farther away.

ESA/Hubble · CC BY 4.0

Image: ESA/Hubble & NASA, R. J. Foley (UC Santa Cruz)