8.11P-8.12P - Absolute magnitude and HR diagram

Absolute magnitude

The apparent brightness of a star is how bright it looks from Earth. Apparent brightness depends on two things: how much energy the star emits and how far away it is. A nearby dim star can appear brighter than a distant powerful star.

Absolute magnitude is a fairer comparison because it asks: how bright would the star appear if it were at a standard distance? For stars, this standard distance is 10 parsecs, which is about 32.6 light-years.

The word magnitude has an unusual scale. Lower magnitude numbers mean brighter stars. Very bright stars can have negative absolute magnitudes, while dim stars have larger positive absolute magnitudes.

For example, a star with absolute magnitude -5 is brighter than a star with absolute magnitude +5. The minus sign is not an error; it means the star is very bright on the magnitude scale.

Absolute magnitude does not tell you how bright the star looks from Earth tonight. It tells you how bright it would look if distance were made the same for comparison.

HR diagram axes

An HR diagram is a graph used to show patterns in star properties. In this lesson, the vertical axis is absolute magnitude and the horizontal axis is surface temperature or spectral class.

The vertical scale is usually drawn with the brightest stars at the top. Because the magnitude scale runs backwards, that means more negative absolute magnitudes are near the top and larger positive absolute magnitudes are near the bottom.

The horizontal scale is also easy to reverse by mistake. Hotter stars are placed on the left and cooler stars on the right. A diagram may label the horizontal axis with temperature values, or with spectral classes from hot to cool. In a monochrome diagram, colour words such as blue, white, yellow, orange and red should be written as labels rather than shown with coloured marks.

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The axis direction matters because it controls where each star group belongs. If a question asks you to draw the main components, full credit depends on the labelled regions being in the right parts of the diagram, not just on naming them somewhere on the page.

Main sequence

The largest feature on an HR diagram is the main sequence. It is drawn as a broad diagonal band from the upper left to the lower right.

At the upper-left end, main sequence stars are hot and very bright. At the lower-right end, main sequence stars are cool and dim. The Sun is a main sequence star, but it is not at either extreme; it sits around the middle of the main sequence.

The main sequence is not a single track that every star moves along during one lifetime. For this lesson, treat it as a region where many stars are found when their surface temperature and absolute magnitude are plotted.

In an exam drawing, the main sequence should be the easiest feature to recognise: a diagonal strip sloping downwards from left to right. If it is drawn from lower left to upper right, the temperature direction or brightness direction has probably been reversed.