Space Science

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Absolute Magnitude is the star’s actual brightness. Apparent Magnitude is how bright a star appears from Earth. The H-R Diagram is based on the actual brightness of a star.

If we place two stars were the same distance from the Earth, and measured their brightness, this would be their absolute magnitude.

Demo: Compare a flashlight and a pen light.

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Surface Temperature

Astronomers use color as a guide to determine the temperature.

The hottest star is 50,000 °C

The coolest star is around 3,000 °C

Remember heating the metal rod in lab? The colors changed as the metal became hotter.

Remember the Surface Temperature is less here than the core of the star. We will look at this concept in depth when we study the sun and it’s layers.

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So now what?

The stars are plotted on the diagram according to their surface temperature and absolute magnitude.

Once the stars are placed on the diagram, astronomers have noticed clustering of the plotted stars.

These clusters are grouped together into the various stages of a stars life cycle.

Let’s look at those briefly.

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Main Sequence Stars

Make up 90% of the stars

These are medium sized stars

Our sun is a main sequence star

Sizes vary from 1/10 that of our Sun to 10 times that of our Sun

Stars stay in this part of their life cycle for a long time; most of their “lives”

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Giant Stars

These stars are the next size up from the Main Sequence stars

They are 10-100 times as large as our Sun

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Super Giants

The largest stars, larger than the giant stars

Their diameters are 1,000 times that of our Sun

A star this size would extend past Mars from where our Sun is now if compared to our Sun’s current size

Due to their size, they are the shortest lived stars and die off quickly

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White Dwarfs

These are smaller than the sun and even the Earth

The smallest know white dwarf has a diameter that is less then the distance across Asia

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Neutron Stars

These are the smallest stars

A typical neutron star is 16 km in diameter

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