I have relooked and relooked at this unit, and it just seems to me that I am being difficult to comprehend. The basic point of this rather lengthy page is to teach you that astronomers have been able to determine more than just how bright a star appears to be, but can actually determine how bright a star is, how far away the star is, and what that star is doing at that particular point in its life.

Measuring Star Light

Stars hold the keys for astronomers who wish to learn about the secrets of the Universe. Due to thermonuclear fusion reactions in the core of all stars, electromagnetic energy is created when heavier elements are forged from lighter ones. In this manner, stars are the creators of light in the Universe, and when that light eventually arrives at the Earth, we see a point of light in the sky. To a human eye, most stars look pretty much the same. The eye has two kinds of photoreceptor cells; cones which detect colors, and rods which detect black and white. The cone cells do not operate very well in the darkness, so the rod cells take over. These are the cells which are excited by the starlight, but the brain only detects white stars. If you take a camera with a shutter that can remain open for a longer period of time, and point the camera at the stars overhead, you would find a photograph with many differently colored stars ... reds, yellows, oranges, whites, and even bluish ones are all up there, but it is hard for the human cone cells in the eye to detect those colors. Astronomers have developed sensitive instruments to study starlight in greater detail. Additionally, you learned on the previous page that starlight is actually a broad range of electromagnetic radiation from gamma rays to radio waves. Since to many of the portions of the electromagnetic spectrum are blocked by Earth's atmosphere, astronomers have been forced to wait until the mid-1960's to really study stars in all their glory. When scientists launched telescopes above the atmosphere and tuned their instrumentation to shorter or longer wavelengths, a great deal more of stars was available for study than ever known before. The purpose of this page is to share with you some of the basic information about starlight, teach how astronomers measure the distance to stars, and lead you into a discovery of the many varieties of tools astronomers utilize to study stars.

Basic Starlight

Stars come in all different colors and sizes. Some stars are just big beyond belief. Our sun has a diameter of 1,392,000 km, and with this huge size, we could fit 1.3 million earths inside. Yet our sun is just an average-sized star. Betelguese, the left shoulder star in the constellation Orion, is well over 1,000,000,000 km in diameter, and Mu Cephi, the 14th brightest star in the constellation Cepheus is even larger than that. On the opposite end of the diameter chart and tiny white dwarf stars which are barely the size of the Earth. Even smaller are the neutron stars whose diameters are less than the size of Minneapolis, Minnesota. And Black Holes do not have any size at all ... since their entire mass is compacted into what is zero volume. In the days of ancient Greek Astronomy, stars were thought to occupy a sphere just beyond the orbital sphere of Saturn. Stars are actually much, MUCH father away than that. The closest neighbor star to our sun is roughly 4.3 light years, or 40,000,000,000,000 km distant from the Sun. Stars at the galactic center of the Milky Way are 30,000 light years away. Stars in really distant galaxies are up to 15 billion light years from here. It is amazing to think of these distances, but just as incredible to realize that we can see the starlight from these distance places. This leads us to the first conclusion:

1) Looking at starlight is like looking back in time. For instance, the light from our sun requires 8 minutes to reach Earth. When you are looking at the Sun, you do not see the Sun as it is, but as it appeared to be 8 minutes ago. If it were to suddenly explode, you would have 8 minutes of ignorant bliss before the collective death of the entire solar system. Comforting thoughts before retiring to bed. When look at a star like Betelgeuse, you are looking at a star whose light left there

When you look at the Andromeda Galaxy, you are seeing light which left 2.9 million years ago and is just arriving here today.

The farther out into space you look, the farther back into time you see. Astronomers actually believe they can see all the way back to when the Universe was about 300,000 to 700,000 years old. This was a time before stars or galaxies even formed. It is a glow of light from the original Big Bang expansion, and though we have instruments sensitive enough to see farther out, nothing apparently is there. Astronomers use starlight to try and determine the age of the Universe and learn about the early conditions when stars first formed.

A second conclusion is drawn from the appearance of the starlight in the night sky.

2) All stars are not equally bright. The Greek astronomer Hipparchus (120 B.C.) assigned levels of brightness to distinguish one star from another categorically. Hipparchus drew upon an aged compilation of star catalogues and developed a system to describe the brightness of 1080 stars relative to each other. He listed six classes of stars, with Class 1 stars being the brightest and Class 6 stars being the most faint which a human eye can detect. The larger the number the more dim the star is. Since the time of Hipparchus, optical devices for looking at stars have advanced greatly and many stars were found to be present in the sky, but more dim than the sixth class. Careful recording of stars and more precise methods of measuring brightness have resulted in a modification of the Hipparchus system. Stars more bright than class one stars, and some of the Solar System's planets that reflect sunlight, have been found to actually be brighter than the Class 1 stars of Hipparchus' day. Those such bright objects have been assigned magnitude values of 0 or even negative numbers. The Hubble Telescope has detected stars of the 28th class. Astronomers have organized the improved system to yield a difference in brightness between 5 classes of stars to be 100 times. A first class star is exactly 100 times more bright than a sixth class star. We call a stars brightness its MAGNITUDE, and its brightness class MAGNITUDE CLASS. For each change in magnitude class, a star is 2.512 times more dim. A second magnitude star is 2.512 times dimmer than a first magnitude star. A third magnitude star is (2.512)X(2.512) times dimmer. A fourth magnitude star is (2.512)e3 times dimmer. The exponent is an indication of the difference in brightness. A difference in magnitude scale of (2.512)5 = 100. With the system better organized, it has been possible to assign a correct magnitude value to every star in the sky. The brightest star is Sirus (-1.42). The Sun has a magnitude of (-26.8), and the Moon (-12.5). This means that the Sun is (2.512)e26.8 times more bright than a 0 magnitude star.

Magnitude Levels of Six Classes of Stars. Notice that the larger the number, the dimmer the star.

Magnitude Class
Difference in Brightness
Logarithmic scale of 2.512 x between magnitude levels starting at sixth magnitude



2.51 x 2.51 x 2.51 x 2.51 x 2.51



2.51 x 2.51 x 2.51 x 2.51



2.51 x 2.51 x 2.51



2.51 x 2.51






Astronomers use the magnitude class of stars as a tool to measure the distance to them, using a mathematical formulae developed by Isaac Newton. It is most important that astronomers utilize the relationship between how bright a star appears to be and how bright a star actually is to determine the distance to that star. In this manner, astronomers can begin to map the location of stars near our own Sun in a three-dimensional map. To learn about this relationship between a star's two magnitudes, please move forward to stellar magnitudes.

At any time during these three pages, you should try to actually go outside and see some of the things I am talking about in this unit. The Star Observation is your second outdoor observation and the focus of this exercise will be to notice color and special sites of interest. You can return at any time to the Introduction to Light and Telescope Page, the Syllabus, or the Home page.

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