Go outside some evening this month and locate the familiar constellation pattern of Orion the Hunter.
Surrounding Orion are the northern hemisphere constellations, and scattered throughout this area of our night sky are many of
the brightest stars we see during the year. In fact, eight of the twenty-five brightest stars in our sky are located within
the constellations surrounding Orion. Also within this group are two of the closest stars, Sirius at 8.6 light
years and Procyon at 11.4 light years.
Not only are these particular stars among the brightest in our skies, but they also demonstrate a
diversity of colors. Interestingly, there is a relationship between the color of a star, its temperature, and its brightness.
The temperature of a star determines its surface color and luminosity (the star's actual output of energy, which we see as how
bright it appears). In general, cool stars such as Betelgeuse are reddish; medium temperature stars such as our Sun are
yellow-orange; and hotter stars, such as Sirius and Rigel, are blue or bluewhite. The hottest stars, logically,
are also be the brightest stars. However, the size of the star and its distance from the Earth play important roles in how bright
the star appears.
Click on the graphic to see it full size. The brightness of a star can be described in two ways-apparent magnitude and absolute magnitude. Apparent
magnitude is based on how bright the star appears to be when compared to other stars in the sky. This is why the Sun appears to
be the brightest star during the day, and Sirius appears to be the brightest star at night. On the other hand, absolute magnitude
is a measure of how bright the star is when compared to other stars, and this comparison is done as if all the stars were the same
specific distance from the Earth. Using a standard magnitude scale, a star differs in brightness by a factor of approximately 2.5
for each whole number difference. For example a first magnitude star is about 2.5 times dimmer than a 0 magnitude star, a second
magnitude star is about 2.5 x 2.5 times dimmer, and so on down the line. The dimmest star we can see with the unaided eye is a sixth
magnitude star, but this is only possible under excellent viewing conditions. Negative values are used for the very brightest objects.
Look at the bright stars surrounding Orion. Some, such as Sirius at 8.6 light years (l.y.) away and Aldebaran
(60 l.y.), are relatively close as star distances go, while others such as Betelgeuse (1400 l.y.) and Rigel (1400 l.y.) are quite distant.
Yet they all appear to be about the same brightness, albeit different colors and temperatures.
It turns out that cool stars can appear to be bright if they are very large, similar to the red super-giant,
Betelgeuse. With an estimated radius of 241,401,600 kilometers, the relatively dim light from this red star is spread out over a very
large surface area, which increases its apparent brightness considerably. By comparing the distance and apparent brightness of
Betelgeuse to that of Aldebaran, another cool red star, it can be concluded that Betelgeuse must be larger than Aldebaran because
it is about 35 times further away from the Earth.
Stars may also appear to be bright, regardless of their size, if their distance from Earth is not too great.
Our Sun, for example, is less than 1,609,344 kilometers in diameter, but it appears to be the brightest because it is the closest to
Earth. Sirius is approximately twice the diameter of the Sun and is the seventh nearest star at a distance of about 8.6 light years,
yet it is the second brightest star. This is due to its temperature rather than its proximity to the Earth. Put the Sun at the same
distance from Earth as Sirius, however, and the it would not be visible to us at all.
Distance not only plays an important role in apparent brightness, it is also a measure of how long it has
taken the light from a star to travel to the Earth. The light year as a distance unit is based on how far light will travel in one
year at the speed of light. The speed of light is 299,338 kilometers per second and, in one year, light can travel
nearly 9,656,064,000,000 kilometers. In other words, the light you see from the Sun took eight minutes to travel the
approximately 149,668,992 kilometers to Earth, while the light from Sirius takes nearly nine years to arrive. Light
from Betelgeuse, as a comparison, takes nearly 1,400 years to make the trip to Earth.
Extending this idea of time and light travel quite a bit further is a video that illustrates this relationship
with regard to galaxies.
Watch this video and another video, 'Two Small Pieces of Glass' at my Qué tal Theater
