HR Diagrams and Star Life Cycle

The Life Cycle of Stars

Stars and the HR Diagram

• The HR Diagram is a graph that compares the surface temperature of a star with its magnitude or how bright the would be if they were all the same distance from Earth.

• Luminosity is a term that measures how much energy leaves a star in a certain period of time.

• General Rule: As luminosity increases, brightness increases.

What do the axes on this chart show us about stars?

Temperature

Luminosity

Spectral Class

Magnitude

What affects luminosity?

• Temperature: As temperature increases, luminosity increases.

• Size: As size increases, luminosity increases.

• Most Luminous? Large and Hot

• Least Luminous? Small and Cool

Multiple Choice

What is the approximate surface temeprature of the sun according to the HR diagram?

• 5,500 K

• 6,000K

• 7,200 K

• 15,000K

Multiple Choice

Would the surface temperature of a red supergiant be higher or lower than a white dwarf?

• Higher

• Lower

Poll

What is the color of the stars with the highest surface temperature?

• Red

• Yellow

• White

• Blue

Multiple Choice

Most of the stars on the HR diagram are classified as which type of star?

• White Dwarfs

• Main Sequence

• Giants

• Supergiants

Multiple Choice

How is it possible for white dwarf stars to have a lower luminosity than the sun, even though the sun has a cooler surface temperature.

• The sun is smaller than a white dwarf.

• The sun is larger than a white dwarf.

Multiple Choice

Star A has a surface temperature of 10,000K and medium brightness. What color is it?

• White

• Red

• Yellow

• Blue

Multiple Choice

What kind of star is Sirius, the Dog Star?

• Main Sequence

• White Dwarf

• Blue Giant

• Red Supergiant

The Life Cycle of a Star

• All stars begin life as dust in a nebula.

• Gravity begins to form masses of hydrogen in the nebula.

• When the heat in the protostar reaches 15,000,000 C, nuclear fusion starts and a star is born!

This is the Pillars of Creation within the Eagle Nebula, part of M16, viewed by the Hubble Telescope

2 Pathways:

The mass of the star that forms in the nebula determines how the star will burn fuel:

--small stars burn their fuel more slowly and reach lower temperatures.

--large stars burn through their fuel rapidly and reach higher temperatures.

Common Ground:

All Stars begin life as a H -> He fusing Main Sequence star.

No matter the mass of a star, as its hydrogen supply begins to run low, the core cools and the star takes on a red color, forming either a Red Supergiant or a Red Giant.

Different Endings:

Large stars can end up as black holes or pulsars.

Small stars can end up as a Nova, Supernova or a Black Dwarf.

Multiple Choice

If all Main Sequence stars are "young" stars, what can the color tell you about the star?

• Its size

• how long it will stay part of the Main Sequence

• Both options are correct

• Neither option is correct