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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Step 7: How bright is that star?


We’ve been rating stars by their brightness for at least 2,000 years and the operation is really quite simple, though the whole system may seem backwards to you. In any event, it’s worth learning because the different brightness of stars is one of the main ways we can tell one from the other and make some sense out of the night sky.

Stars are ranked in brightness by a numbering system called “magnitude” – the lower the magnitude number, the brighter the star. Generally, the brightest star we see is magnitude 0 and the dimmest we see is magnitude 6. If that feels backwards to you, think of it this way. Stars of magnitude 1 – very bright – are “first class” stars. Dimmer stars are magnitude 2 – “second class” stars, etc.

Simple – yes, but of course there are exceptions and several ifs, ands, and buts that make the whole business of brightness quite fascinating!

First, 10 stars ( if you include our Sun) are brighter than magnitude 1 – so they are magnitude zero and if brighter than that,  we go into a minus system. Magnitude -1 is brighter than 0, which is brighter than one.

The brightest star we see is our Sun. It is magnitude -26.7! The next brightest object is the full Moon, at magnitude -12. Then comes the planet Venus at -4.7 (sometimes) and Jupiter, usually about -2.3 or -2.4.  Other than the Sun, the  brightest star in our sky is Sirius, at magnitude -1.46. We don’t get to an actual  magnitude 1 star until Betelgeuse, which is magnitude .5.  Confused? Don’t be. We call Betelgeuse first magnitude because it falls in the range of .5 to 1.5. No star is exactly a “1” in brightness, though Spica comes very, very close at .96.

Of approximately 6,000 stars bright enough to be seen by the average person from some spot on Earth, only 22 are of first magnitude or brighter. Quite an exclusive club!  These include all the ones we learn as signpost stars and, of course, some of them are visible only from the Southern hemisphere.

While most books will tell you that the faintest star we see with the unaided eye is magnitude 6, this too is only an approximation and doesn’t really represent today’s  world that well.  Generally speaking, in our light-polluted regions, the faintest star we are likely to see is magnitude 4. If you live well away from city lights, you probably see to magnitude 5.   On mountain tops, well above the thicker parts of our atmosphere and free of light pollution, some observers with excellent eyes and skills can see stars as faint as magnitude 8.5 with the unaided eye. In my own area, which is semi-rural, I’m very happy when I can see a magnitude 5 star with my naked eye.

With binoculars or telescopes we see stars that are much fainter, but that’s another story. Here we’re focused on what we can see with no optical aid.

Many familiar star patterns are made up primarily of second magnitude stars, such as Orion’s Belt and most of the stars of the Big Dipper.  The North Star (Polaris) is second magnitude as well as one other star in this asterism. But the other five stars in the Little Dipper  are magnitudes 3, 4 and 5. This makes the Little Dipper a good object to use to get a sense of magnitude.

On a star chart it is common to represent magnitude by making circles of difference size to represent the star and its brightness. That really isn’t the same as brightness, but it does give us an idea of what to expect. So, for example, here’s how the Starry Nights Software represents the stars of the Little Dipper.

Study the four stars that make up the cup of the Little Dipper for agood idea of the magnitude scale. It includes magnitude 2,3,4, and 5 stars.

Study the four stars that make up the cup of the Little Dipper for agood idea of the magnitude scale. It includes magnitude 2,3,4, and 5 stars.

You’ll find that when you get to know stars of different brightness, the magnitude system makes general sense to our eyes. That is, there seems to be about the same gap in  brightness between a magnitude 1 and a magnitude 2 star as there is between a magnitude 2 and magnitude 3 star.

While the magnitude system has been around since the early Greeks, it was given a mathematical foundation about 150 years ago when it was decided that there would be a gap of 2.512 times between each level of brightness. That means a first magnitude star is 2.5 times as bright as a second one – and exactly 100 times as bright as a star at magnitude six – five magnitudes fainter.

At that time it was thought this logarithmic progression matched the way the human eye responded to brightness. It does so, but only approximately.  Still, the system is convenient, stars are routinely listed this way, and the system easy enough to become acquainted with while observing.

One last note – don’t expect to simply walk out of your nicely lit house and see faint stars. At first only the brightest will be visible to you. Your eyes need time to adjust to the dark. After about 15 minutes you’ll see much fainter stars and after about half an hour you can assume that the faintest star you see is the faintest you will see that night.

But also keep in mind that what you can see at any given moment varies in a number of ways, including:

  1. How good your eyes are at detecting faint objects.
  2. How clear your skies really are – astronomers quickly learn that all “clear” nights are not created equal!
  3. Your physical condition at the moment, including how tired you are and whether or not you have been recently drinking alcohol or smoking, both of which can impair night vision.
  4. The height of the star above the horizon – when you look at stars near the horizon, you are looking through a lot more air then when you are looking at stars directly overhead.
  5. Your experience in looking at faint objects   – yes, you can actually learn to see dim objects by looking slightly to one side of them, a technique called “averted vision” that takes practice to do well.

All of this helps make star-gazing more fun. Variety is, indeed, a tasty spice, and no two nights of observing are exactly the same.  Nor is it all as cut and dry as it seems.  An experienced, older observer can actually see more, in some instances, than a younger, inexperienced observer whose eyesight is technically better.

3 Responses

  1. […] Step 6 – How bright is that star? […]

  2. […] Step 6 – How bright is that star? […]

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