The benefits of a dark sky
Some days ago, I read an article entitled “Why Can't I See That Galaxy?”1 and then I understood why in a dark sky I was able to see M33 with my binoculars, but in my hometown I spent several nights with both the telescope and the binoculars unable to find it.
The rule seemed to be simple: the darker sky is the better the view of DSOs is. But the matter I had to settle was why some DSOs can’t be seen even if they have the same apparent magnitude as stars that can easily be seen? Let me travel around the topic though my sketching stories.
On the 24th of June, 2012, my second try at sketching, I drew the Andromeda galaxy as seen from a very light polluted sky (Bortle 9). The core of the galaxy was bright enough to locate easily. I found it on the way from Mirach (the only easily visible star in the LPS) to the west. M31 seemed like a cotton ball hanging from the sky. Besides the enormous amount of light pollution, I was just taking my first steps in DSOs observation. My eyes and mind were also untrained in how to see DSOs and therefore I would never see the whole galaxy. Actually, I realized that when I performed the sketch as I thought I had seen something like the galaxy’s arms but in the wrong way (North-South).
In contrast to the previous observation, I finally got a better view of M31 in a little town located 2 hours south from San Francisco. I got a very dark sky because the giant redwood trees were blocking all the lights coming from the biggest cities nearby San Jose and Santa Cruz. They were also blocking the view below 45 degrees over the horizon but the sky was so dark that without eye adaptation I could easily see the Milky Way.
With a sky like that (Bortle 3) I could not pass up the opportunity to sketch. I mounted the binos on the tripod and looked for M31. These were ideal conditions to observe the sky, but there was also a neighbor’s light just in front of my field of view about 100 meters away. I just could not keep my night vision well adapted, but anyway, I decided to sketch. I could see the Andromeda Galaxy like an enlarged oval with a very bright well-defined circular shape core. The arms were much dimmer than the core, but I still could see them. I compared this sketch with my previous M31 sketch and realized than I could also see M32 this time. What a massive difference a dark background makes.
Andromeda’s apparent magnitude is 3.4, but it does not show up that bright in every spot. In fact, this scale of magnitude corresponds to an object about the same size as a star. But because it is much larger than a star, a better way to know how bright it can be seen, is by its surface brightness which is the brightness of a certain given area (not that simple but that’s the general idea). I found out that for objects like Andromeda (with a very dense and bright core), the surface brightness score (13.56) did not fit at all, since I could see the core even from a Bortle 9 sky with my binos, but I could not see stars of that magnitude. There is more to uncover in this matter but for now, I feel comfortable with what I know about it.
Now let’s talk about M33: Triangulum galaxy; another galaxy in the local group where the Milky Way is. Andromeda and Triangulum are Milky Way neighbors living very far away: more than 2 and 3 million years-light away. Triangulum galaxy has an apparent magnitude of 5.7 and a surface brightness of 14.34 and it seemed like the brightness had equal distribution through all the visible area.
Once again, I would like to give a plus point to a dark sky over a big telescope under bad skies. And for those who want to see M33, it’s time to go and look for a dark sky!
Edited by Jennifer Steinberg (editor in chief)