Does Brightness Depend On Distance?

OCT 30, 2010

File this posting under common misconceptions:

I just watched a trailer for a show about the Milky Way. It reminded me of a misconception that everyone seems to have, even the producers of the show. If you bring a distant star closer, it gets brighter. We all get that. BUT, if you bring an extended object closer, it gets bigger but it doesn't get brighter. No one seems to get that even though we experience it every day. I'm looking out my window at a white fence about a 100 yards away. It's a sunny day. I walk over to the fence for a closer look. It does not get any brighter. It just appears bigger, that's all. Armchair astronomers always seem to think things would be brighter if they were closer. It's true for stars (except the Sun) because they are unresolved point sources. But, as soon as an object is big enough to be "extended", then it is no longer true.

In the above-mentioned trailer, the voice over says if we could move our solar system to be positioned near the Orion Nebula our night skies would be bright with the light of the Orion Nebula and the video shows a bright Orion Nebula hanging over a familiar Earthly landscape. The truth is that the brightest part of the Orion Nebula is faintly visible to the naked eye and if we were close to it it would still be a faint, albeit much larger, glowing object. In fairness, I should point out an assumption I am making, namely that there is essentially no dust between us and the Orion Nebula. Dust, of course, would tend to make objects appear dimmer as they become more remote. But, I think that's usually not the case with the various nearby nebulae in our Milky Way.

Many thanks to fellow Questar owner Larry Geary for first setting me straight on this misconception a number of years ago.