https://www.legacysurvey.org//viewer/?ra=45.0709&dec=-54.8944&layer=ls-dr8-south&zoom=13

What a pretty galaxy! I wonder how far away it is. The viewer says it’s 3.93 units away… sure wish I knew what that meant!

If you click on the galaxy in the viewer and select “Look up in Simbad”, in there you will find the galaxy’s estimated velocity. If the velocity isn’t available, you could also just multiply the galaxy’s redshift with the speed of light to find it out, z*c = 0.046 * 299792 km/s = 13 790 km/s). To estimate the galaxy’s distance, divide its velocity with Hubble’s constant (69.8km/s/MPc) and you’ll have an estimate of the galaxy’s distance in megaparsecs.

v/h0 = 13 790 km/s / 69.8 km/s/MPc = 197.5 MPc. Converting that to light years gives us about 644 million light years.

(Above I used a redshift of 0.046 and Hubble’s constant of 69.8 so the velocity differs a little from the ones listed on Simbad.)

Distances in cosmology are complicated… if you want to get the gory details, you can use the tool that many pros use for quick calculations, the venerable

https://www.astro.ucla.edu/~wright/CosmoCalc.html

(change the “z” setting to the redshift in question – we have DR9 photo-z for this one of z~0.038 – and click “Flat” to update the calculations)