You Light Up My Life. Or Do You? Bright Stars and Dark Energy

For decades astronomers have used the Astronomical Magnitude Scale to measure the brightness of stars. But what if they're measuring with an inaccurate ruler? And how could that screw up the quest to understand dark energy?

The Astronomical Magnitude Scale (see bottom of post) is a logarithmic scale used to measure the brightness of stars as seen from earth. The sun, our nearest star, is set at a value of roughly -27, but humans anywhere on earth can comfortably see all the way down to a value of zero. For values of +1 to +7, an observer would have to be in the country, away from light pollution. Trying to see anything dimmer than +7 would mean breaking out the telescopes.

Scientists measure the brightness of astronomical objects by comparing the object's brightness to the brightness of reference stars. Vega and Sirius are popular references, since they have defined and well-known magnitudes.


Or do they? Despite the many advances in the practice, focus, and breadth of knowledge of astronomy, the apparent magnitude values of stars have been updated haphazardly, or not at all.

This faulty measuring system has bigger consequences than missing a twinkle or two. Astronomers use apparent magnitude to judge the distance of stars. Far-away stars are measured by determining their color spectrum and their apparent magnitude. The color spectrum of a star can tell scientists how bright it is. Why, then, do we need the Astronomical Magnitude Scale? Because it, in combination with the knowledge of the absolute brightness of a star, helps scientists determine how far away that star is.


A light that's shining in a person's face from two feet away is uncomfortable. From twenty feet away, it's enough to read by. From two hundred feet away, it's just a glimmer. By combining the knowledge of how bright the light is (spotlight, 60 watt bulb, candle flame) and how bright it appears, a person can roughly judge the distance from them to a light. Astronomers use the same process to determine how far away a star is. Imagine, however, that all throughout a person's life, they were told that a ten watt bulb was actually 60 watts. Their entire scale of reference would be thrown off, making it impossible for them to judge the distance of lights.

Once distance goes out the window, many measurements of the effects of dark energy go right out with it. Dark energy is the mysterious force that seems to be making the universe expand faster and faster over time. Scientists use the distance of stars to measure this expansion.

Picture yourself sitting on a rubber band as it is being stretched. Objects that are close by will be receding away at a certain rate of acceleration. However, objects that are farther away will be receding at a faster rate than nearby objects, because every part of the rubber band is expanding. The farther away an object is, the more material between it and you will be stretching out and pushing you apart.

If a scientist were to look at a star they mistakenly think is close by, and see it receding at an unexpectedly fast rate, they will come to erroneous conclusions about how fast the universe is expanding, and how great the force of dark energy is.

For want of a nail, the cosmological constant is lost.

Fortunately, there is a mission planned to take more accurate measurements of reference stars. Absolute Color Calibration Experiment for Standard Stars, or ACCESS, will give accurate and up-to-date information on the magnitude of the stars, and set a standard that can be used to give all scientists consistent data, and let all stargazers know exactly what they're seeing, and where they are seeing it.


New Scientist

Astronomical Magnitude Scale via Harvard

How Stuff Works
