What things must an astronomer measure to calculate a star's absolute brightness?

Answer 1

Telescopes, combined with spectroscopy are used for the colors.

The apparent brightness can be measured using a telescope with a special "CCD camera".

To measure the "real" brightness ("absolute magnitude") you also need to be able to work out the distance to the star.

Answer 2

Scientists use telescopes to measure the brightness of stars. But before telescopes were invented, scientists judged the brightness of the stars with their naked eyes. They called the brightest stars they could see the first-magnitude stars, and the dimmest stars, sixth magnitude stars. When telescopes were developed, this system had flaws. They could see more stars with the telescope than with the naked eyes. They could also see the differences in brightness as well.

Answer 3

Apparent Magnitude and Absolute Magnitude.

Apparent magnitude compares how bright stars APPEAR to be, while absolute magnitude measures the actual brightness. Medium size stars that are pretty close will have a brighter "apparent magnitude" than a really huge star a very long distance away.

The description of "magnitude" was developed a long time ago and so as our knowledge has increased, keeping the same description sounds silly, but that's the way it is.

Bright stars were called "first magnitude", and slightly dimmer stars were called "second magnitude", while still dimmer stars got designations like "third magnitude" and fourth magnitude". A seventh magnitude star is barely visible at all, even with good eyes and in a dark sky.

With the invention of more sensitive instruments, scientists were able to distinguish fractions of magnitudes; so a magnitude 1 star is a little brighter than a magnitude 1.3 star. When we need to describe things even brighter than "first magnitude" stars, we say that a zero is brighter than a 1, and a negative 1 is brighter than a zero. Bigger negative numbers are even brighter. (You can see, I hope, how trying to maintain an archaic system in the modern age can cause zany results!) So the planet Venus, which is much brighter than any star, is sometimes a "magnitude -3", while the planet Jupiter (which is a little dimmer) is "magnitude -2".

"Apparent" magnitudes tell us how bright a star APPEARS to be, at its distance, while "absolute magnitude" tells us how bright the stars would be if they were all measured from the SAME distance. So the Sun, which is RIGHT HERE, has an apparent magnitude of -26, meaning INCREDIBLY bright, but an absolute magnitude of 4.8 or so, meaning that if all the stars were lined up at the same distance, our Sun would only be a 4th or 5th magnitude star.