How to Read a Star's Brightness and Color — the Story of Magnitude and Starlight
Why is one star brighter than another, and why do some look red or blue? Here we unravel the meaning of "brightness (magnitude)" and "color" — fun things to know when you look up at the stars.
The unit of brightness — magnitude
A star's brightness is expressed in "magnitudes." Curiously, the smaller the number, the brighter the star. This tradition goes back about 2,000 years to the Greek astronomer Hipparchus, who ranked the brightest stars as first magnitude and the faintest as sixth magnitude.
A difference of one magnitude means a brightness difference of about 2.5 times, and modern astronomers redefined the scale so that a five-magnitude gap (for example, between a first-magnitude and a sixth-magnitude star) corresponds to exactly 100 times. Very bright stars drop to zero or even negative magnitudes — Sirius, the brightest star in the night sky, shines at magnitude -1.46.
Apparent magnitude and absolute magnitude
The brightness we see with our eyes is the "apparent magnitude," and it depends not only on how bright a star truly is but also on how close it is. Even a genuinely brilliant star looks dim if it is far away. So when comparing the true brightness of stars, astronomers use "absolute magnitude," which imagines every star placed at the same distance (32.6 light-years). The "m" value listed on each constellation page of Byulgil is this apparent magnitude.
A star's color is a thermometer
Look closely and you will see that stars come in different colors. That color tells you the temperature of the star's surface. Just as iron heated in a fire glows from red to white to blue, a cooler star is red and a hotter one is blue.
Red stars (such as Betelgeuse in Orion and Antares in Scorpius) have surface temperatures of about 3,000 °C, making them relatively "cool," while blue-white stars (such as Rigel in Orion and Spica in Virgo) are scorching, topping 10,000 to 20,000 °C. Our own Sun is a yellow star somewhere in between.
Try this when you look at the stars
Next time you look up, don't just note how bright the stars are — pay attention to their "color" too. In winter, compare red Betelgeuse and blue Rigel side by side in Orion, and you can clearly sense the color difference between these two stars of different temperatures with your naked eye.