Terminology: Constellations and stars

One of the things I love about astronomy is you don’t actually need to know anything to appreciate the night sky. But if you want to communicate with others about what you saw or learn what’s out there, you have to have names.

Ever since humans have seen the stars, they’ve seen patterns. These patterns are called asterisms and they can be whatever you want. Some are common and have established names, such as the Big Dipper and Orion’s Belt. It’s important to remember that even though stars appear close to each other in the sky, they’re (usually) not actually connected. Since the stars are so distant, they all look equally far away, but they’re not. It would be like standing in New York City and thinking that because Chicago and San Francisco both lie in the same general direction, that they were closely connected.

Ptolemy formalized 48 of these as constellations. The International Astronomical Union (the international authority for astronomy) modified and expanded this list (including adding constellations that can only be seen from the southern hemisphere) into the modern official 88 constellations. Furthermore, constellations are now defined as areas of the sky, not as patterns of stars. This means that every point in the sky is part of some constellation. The bright stars may form patterns that help us to identify it, but they are not the constellation per se. I like to think of the stars as cities, and the constellations as states or countries. In the United States, some states have several large cities, others may have none. But every point in the United States is part of some state.

Star chart of Orion

Star chart of the constellation Orion. Note that the Orion includes the entire unshaded area, not just the bright stars and connecting lines. The Greek letters are Bayer designations (see below). (IAU/Sky&Telescope/Wikipedia)

The names of stars are bit more complex, since there are an estimated 3 sextillion stars in the observable universe (3 × 1023, or almost a trillion trillion stars)! Clearly, we need some naming scheme. I’ll normally use Johann Bayer’s method. To each star in a constellation, he assigned a lower-case letter from the Greek alphabet, usually following brightness and location characteristics. (After he ran out of Greek letters, he switched to lower-case Latin letters — our alphabet — and then upper-case letters). In his designation, the name of the star consists of the letter followed by the constellation name in the “genitive case” (a possessive form). Either the Greek letter can be used or it can be spelled out as an English word, and either the full constellation name or the standard three-letter abbreviation can be used. So for example, take Rigel, in the constellation Orion. This star will be called Beta Orionis, or β Orionis, or β Ori for short. Many stars also have their own names (like Rigel!). For the twenty or so brightest stars, their individual names are commonly used, but for the rest, Bayer designations are most common. There are stars that Bayer didn’t list, and for those I’ll use John Flamsteed’s names: they’re similar to Bayer’s method, but use a number instead of a Greek letter (for example, 40 Eridani, one of the stars in Eridanus).

For a more thorough discussion, see Sky & Telescope’s excellent Names of the Stars.


April 22, 2012: Happy Earth Day, and focus on Perseus (Alpha Persei)

Earth Day flag from Wikipedia

Earth Day flag, from Wikipedia.

Happy Earth Day! I tried to make the most of this day by spending time outdoors: I went for a 4.5–mile run this afternoon, then went out for an extended observing session at night. I located a nearby park and drove over to check it out. It’s pretty convenient: no streetlights and no obstructing trees or houses next to me, though the houses adjacent to the park do have some lights, and occasional cars drove by.

I focused again today on Perseus. I’m really starting to recognize it as well as its position in the sky. Its low in the sky these days, so I want to study it before it disappears for the season (or more precisely, before the sun moves towards Perseus so that daylight obscures it [or even more precisely, before Earth proceeds further in its orbit so that the sun comes between us and Perseus]). It’s quite easy for me to make out δ Per, α Per, and γ Per. I saw Algol (β Per) pretty clearly earlier tonight. I later picked out η Per, ε Per, and even ν Per (3.75m).

View of Alpha Persei Cluster

Simulated view of the Alpha Persei Cluster, created in Stellarium (Wikipedia).

I’ve also been thinking about trying to focus on one star each day. That way I can try to systematically learn the sky, as well as log the stars I’ve seen. And I can read about the star, too. I selected the brightest star in Perseus for today: Alpha Persei (α Per). Also called Mirfak, it’s a second-magnitude star (1.8m) from our distance, though its absolute magnitude of −5.1 shows it to be a rather bright star. It’s 510 light years away. It’s part of an open cluster of stars called the Alpha Persei Cluster. Apparently you can see the cluster with binoculars. I’ll have to try that out some time.

I know I mentioned this before, but I’ve come to really like Corvus. It’s such a nice shape. Monoceros, however, is my new nemesis. The brightest star in it is still only magnitude 3.9. I stared and stared at the locations where its stars should be, but I saw nothing. I few times I could barely imagine I saw a hint of a glimmer, but I couldn’t convince myself that it was real.

April 17, 2012: An eclipsed star, and two artificial satellites!

I discovered that AccuWeather.com offers forecasts for stargazing. It’s a nice quick glance at the week ahead, and it showed me that tomorrow, Thursday, and Friday were rated a “1” (poor). Today was a “9” (excellent) so I decided to go out as soon as it was dark enough.

My goal today was to again continue familiarizing myself with the overall layout of this section of the sky. Like yesterday, I was observing outside my front door. The conditions are certainly not optimal, with lights, buildings, trees, and cars. But not only will this be the easiest place for me to pop out for a few minutes to take in the sky, but also I want to get used to trying to pick out fainter objects under challenging conditions. My other goals were to study Perseus a bit more and to see if I could identify stars in a constellation I didn’t previously know.

Saturn, Mars, and Venus were all in the sky. Saturn and Spica make a nice bright pair, with Saturn looking softer and Spica harsher. Mars almost looks like a part of Leo, but its red color makes it a misfit among Leo’s stars. Venus is incredibly bright, with a magnitude of −4.35. It’s gorgeous, but its brightness was almost a distraction as I tried to study Perseus. Perseus was pretty low (altitude around 26°), so buildings blocked it and the thickness of the atmosphere and light pollution at that low elevation really concealed it. With difficulty, I could make out η Per (magnitude 3.75).

But I really struggled to see Algol (β Per, usual magnitude 2.05). I finally did manage to see it, but only if I kept my gaze about 10° to the side. I thought that perhaps because it was so much closer to the horizon (altitude around 18°), that was why it was so dim. But looking at my charts later, I read that Algol is not a single star, but rather a triple-star system (Beta Persei A, B, and C). From our point of view on Earth, we are in the same plane in which β Per A and β Per B orbit, so periodically the dimmer β Per B eclipses the brighter β Per A. According to Wikipedia, “Thus, Algol’s magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every two days, 20 hours and 49 minutes during the roughly 10-hour long partial eclipses.” I then found a page on Sky & Telescope that showed that there was brightness minimum today at 21:44 CDT (4/18/2012, 0244 UT), which was right around the time I was observing it. Fascinating! — I can’t wait to see it during a period of normal brightness to compare.

High up in the sky, close to the zenith (altitude 80°), I was able to make out 38 Lyn (magnitude 3.90). That’s the dimmest star I identified tonight. I was not familiar with Lynx, nor with the adjacent Leo Minor, nor with the two nearby stars (λ UMa and μ UMa) that are part of Ursa Major but far from the Big Dipper asterism. It took me a long time to figure out what stars I was looking at.

But one of the highlights of the evening was seeing a slowly moving point of light traveling just below Arcturus (α Boo) and ε Boo in Boötes. I recognized it as an artificial satellite, and described the path into the Voice Recorder app on my iPhone so that I could identify it later. With help from the fantastic site Heavens Above, I was able to identify it as Cosmos 2082 Rocket. NASA’s page informs us that it was a surveillance/military satellite launched by the former U.S.S.R in May of 1990.

Then, a few minutes later, I noticed another moving point, this time in Ursa Major. Again, with the help of Heavens Above, I identified it as Tiangong-1, a Chinese space module that’s a prototype for docking capabilities for a future Chinese space station. It was launched last September.

In retrospect, it wasn’t only chance that I happened to see those two artificial satellites. During the day, the sky is too bright to see practically any satellite. At night, the Earth is between the satellite and the sun; in Earth’s shadow, they’re dark. But close to dawn and dusk, the sun may be below the horizon for us, but a satellite is high up enough that it can still get hit by sunlight.

April 16, 2012: Overall sky survey

Diagram of Cassiopeia

Cassiopeia (IAU/Sky & Telescope)

I spent around 15–20 minutes outside today, just studying the overall sky. I was just out in front of my house, which is convenient, but there is a lot of light pollution from street lights and house lights. The trees and other buildings also obstruct much of the sky. But since the view from outside my front door is going to be the most convenient place to stargaze, I want to be familiar with it. Fortunately, the sky was completely clear, and it wasn’t too cold (about 43°F/6°C, with no wind).

My best view is to the south, with good views to the east and west, but obstructions to the north. The nice thing about tonight’s viewing was that the visible constellations represented a part of the sky with which I’m relatively familiar. The first thing I noticed was Mars nice and high in Leo, Sirius visible just above the horizon, and Venus close to setting. Orion, my favorite constellation, was setting. I could see Betelgeuse (α Ori) and the belt (ζ Ori, ε Ori, and δ Ori).

I’ve been trying to start systematically becoming familiar with the constellations, going quadrant by quadrant. I’ve started with “NQ1”, the eighth of the sky between right ascension 0h and 6h and above 0° declination. Most of the stars in this quadrant aren’t really visible at this time of year, since the sun is currently in this quadrant. I did try to focus on Cassiopeia, but it was close to setting. I could clearly see the five bright stars of the “W” asterism: ε Cas, δ Cas, γ Cas, α Cas, and β Cas. I also tried to study Perseus, since it’s a constellation with which I am really not familiar. I could make out four stars that were more or less parallel to the horizon, at about an altitude of 20°; reviewing my charts now, I assume these are δ Per, α Per, γ Per, and η Per. I’ll hope to recognize Perseus better once it’s more visible.

Overall, I think η Per was the dimmest star I saw tonight — it has an apparent magnitude of 3.75, making it a fourth-magnitude star. It’s not bad given the amount of light pollution.


Welcome to my new blog. I’ve loved astronomy since I was young, though more as an armchair astronomer, reading about stars and galaxies. Several years ago, I started developing an interest in actually observing the night sky, trying to match what I saw to what I knew. That interest has waxed and waned over the years, but I never stopped looking up whenever I was outside at night.

I’ve always been a casual observer, a dilettante. When I think of the term “amateur astronomer”, I envision someone with more dedication, knowledge, and sophistication than I. But recently I’ve been wanting to take stargazing a bit more seriously. In particular, I can only recognize a handful of constellations, and I’d really like to become familiar with the entire night sky. Even without looking at a star chart, I would like to be able to look up and find my way around the sky (a task made harder by light pollution and view obstruction where I live). And eventually, I’d like to be able to do more than pick out the brightest stars in the most salient constellations.

I was browsing Sky & Telescope’s How to Start Right in Astronomy, and one of the author’s suggestions was to keep an astronomy diary. The more I thought about it, the more I liked this idea. What better way to learn the sky than by recording what I’ve seen? I decided to start keeping a log in my notebook, but I also liked the idea of keeping a blog as a companion to my log. I hope to share some of my thoughts and perspectives, and perhaps some of my observations as I explore the night sky.

I’m still at a beginner’s level, so I welcome commentary from all levels — it would be nice to have experience from more advanced astronomers and questions from other beginners. Exploring the night is more fun, and more rewarding, if you can explore with others.