Absolute and Relative Magnitude

On a clear night, it looks like the sky is full of stars. Some are bright and some you can barely see. How bright are the stars in the night sky? The question is a little more complex than it sounds. The actual brightness of a star depends on the stage it is in and the rate of fusion taking place. The apparent brightness in the night sky depends on the energy being given off and how far away the star is. Please look at the following image showing the brightness of the stars.

HR Diagram

Notice that stars with the same temperatures can also have different luminosity. Many stars are larger and brighter than the Sun. However, they look dim because they are light years away. Stars may look very similar in a picture, but be very different due to distances and light output.

Astrophysicists use physical laws and theories to interpret the measurements we gather from stars. We can then use this data to determine temperature and composition of those stars. A star's luminosity is consistent and tells us the:

- Rate at which energy is being emitted into space
- Rate at which fusion is occurring in the core

Luminosity of a star is the total amount of energy it radiates into space each second. We can abbreviate this using the letter L. Luminosity can be measured in watts like a light bulb. A typical home light bulb is:

60 to 100 W (watts)

A typical star is:

4x10²⁶ W

We use the Sun as a standard measurement of luminosity. The Sun's luminosity is 4x10²⁶W. This is set to:

L₀ as a set value

So a star with a luminosity of 5L₀ would be:

4x10²⁶W x 5=2x10²⁷W

Learn about the Inverse Square Law in the Interactive below!

Learn about the Inverse Square Law in the Interactive below!

There are two methods for determining the distance of a star.

Standard-Candles Method

Early history used the standard for light and called it standard candle.
This was the rate at which a candle made of a specific wax gave off light.
Stars that have a well-determined luminosity can be used as a reference for other stars and is called standard candles.
These stars must have well determined luminosities.
This is an unusual aspect of their spectra.
They must pulsate.

Magnitude System

Magnitude 1 is the brightest in the night sky.
Magnitude 6 is barely visible.
This system uses apparent magnitude which describes how a star appears.
Lower numbers indicate brighter stars.
Stars can even have a negative magnitude.
A 1^st magnitude star is 100 times brighter than a 6^th magnitude star.

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