Classifying Stars – the Hertzsprung-Russell Diagram

Classifying Stars All stars are huge spheres of glowing gas They are made mostly of hydrogen, and they make energy by nuclear fusion Stars are classified by their physical characteristics: Size Temperature Brightness

Size of Stars Size of Stars Many stars are the same size of the sun, which is medium sized Very large stars are called giant or supergiant stars If Betelgeuse were located where our sun is, it would be large enough to fill the solar system as far out as Jupiter White dwarf stars are about the size of the Earth Neutron stars are even smaller, only about 20km in diameter

Size Comparison

Color and Temperature of Stars A star’s color reveals its temperature The coolest stars appear reddish in color The hottest stars in the sky appear slightly bluer than the sun The Color of a Star Song - kidsknowit

Brightness of a Star Stars differ in brightness – the amount of light they give off The brightness of a star depends upon its size and temperature How bright a star looks from Earth depends on how far the star is from Earth and how bright the star actually is A star’s brightness is described by its Apparent magnitude Absolute magnitude

Brightness of a Star

Apparent Magnitude A star’s apparent magnitude is its brightness as seen from Earth Just like a flashlight looks brighter the closer it is to you, a star looks brighter the closer it is to Earth

Absolute Magnitude A star’s absolute magnitude is the brightness the star would have if it were at a standard distance from Earth Stars in a globular cluster are all at about the same distance from Earth, so scientists can study clusters to determine absolute magnitude

The Hertzsprung-Russell Diagram Two of the most important characteristics of stars are temperature and absolute magnitude Ejnar Hertzsprung and Henry Norris Russell created graphs to show a relationship between the temperature of stars and brightness Most stars form a diagonal band called the main sequence In the main sequence, surface temperature increases as brightness increases

H-R Diagram The sun is in the middle of this diagram. It is used for comparison The vertical axis shows the star’s luminosity or absolute magnitude The horizontal axis shows the star’s surface temperature, the axis is reversed from the way you normally see it, it decreases from left to right

Animated HR Diagram - sunshine The Life and Death of Stars Animated HR Diagram - sunshine

Hidden Violent Universe (5:20)