PS COLUMN

August 26, 2009

Teaching is learning

One day during Obon season (religious holidays of Buddhism) in August, I was compelled to talk about the summer stars under the starry sky for a small circle of people in a small country town where I live in summer time. But, although I am a planetary scientist, I’m not an astronomer; so I don’t have knowledge about which star has what name and what kind of heavenly story they have behind. And so I decided to talk within my knowledge in a chatting like manner about the summer stars such as great triangle in summer.

In this town the air is so clean that we can clearly see The Milky Way and other numberless stars all over the night sky.Expressing plenty is often described as “as many as stars”; truly in this area is observed countless stars in a true sense of the word.

Then how many is “as many as stars”? It is said that the total number of the stars brighter than sixth-magnitude visible with bare eyes is 8,600, and considering the fact that this town cannot see the stars which are only visible in the Southern Hemisphere and those constellations in winter season, the number of the stars shining spread before us must be about 3,000 which is circa one-third of 8,600.

This is how I decided to talk about the stars by beginning from its number. In preparing to answer anticipated questions such as how many stars binoculars can see and how many is observable of the dark stars by large telescope like The Subaru Telescope of National Astronomical Observatory of Japan, I checked it up beforehand in the Science Almanac about the number of the stars in accordance with their brightness. The result is the following chart.

Visual        -1   0   1    2      3      4        5       6        7          8             9
Magnitude

Total           2   7  12  67  190   710   2000 5600  1.6E+04  4.3E+05  1.2E+05
Number
of
Stars

From this chart we can see that the larger and the darker the stars are, the more there are in its number. We can also find out there exists regularity that by each upgrade of the magnitude, the number of the stars increases by triple.

Then how many stars can we see by binoculars of 3cm lens aperture? As the human pupil is said to be as large as 6mm, 3cm binoculars can collect the light of (30/6) square i.e. as more light as 25 times than human eyes, which means the binoculars will be able to see the stars as dark as 25 times than human eyes. Since the brightness of the stars decreases by one 2.5th for each upgrade of the magnitude, the star as dark as 25 times will be 3.5th magnitude by binoculars rather than 6th magnitude visible for human eyes. That is to say, a man using binoculars of 3cm aperture can see the stars of 9.5th magnitude. From the number of the stars of the above chart, he can see over 120,000 stars. A little tool of binoculars can really expand the observable night sky.

Telescope of 8.2m lens aperture of The National Observatory can observe, from the calculation in the same way as above, the stars as dark as 16th magnitude than human eyes; observable as far as 22nd magnitude. In accordance with the rule that the number of the stars increases by triple for each increase of magnitude, The Subaru Telescope can see as many stars of 6th grade as 43 million times (3 raised to the16th power) i.e. observable number of the stars approximately 240 billions. Astronomers are definitely unable to see all the stars of the night sky in their life span. All the above tells us how powerful the large telescope is.

I’ve never done such calculation as above. In preparing for talk on the stars of night sky, I was led to study something new. I used to have similar experience in teaching at university. I often times recognized how superficial knowledge I had in my giving lectures to the students. I had truly learned the golden lesson for teacher many times, “learning by teaching”.

By the way, you want to know what happened to “star watching event” of Obon? On August 15, our town was covered by thick clouds all over the sky unable to see a single star; previous and following nights were so clear with starry sky though. Bad deeds of lecture’s daily life must be to blame for.

                                (Translated by The Planetary Society of Japan)