The Life Cycle of Stars

Nebulae

Protostar “childhood” lasts 1 million yrs for a sun-like star The molecular cloud condenses because of gravitational forces

Protostar ignition Protostars have central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract. When contraction is complete, the fusion process begins. The Main Sequence stage (90% of stars) can last 3 million to hundreds of billions of years.

“Main Sequence” Stars

Rules for thinking about stars Large stars (blue and white) are hot and short lived (millions of years) Small stars (yellow, orange and red) are cooler and long lived (billions of years) Stars change over the course of their births, lives and deaths The characteristics of stars are clues to the stage of their lives that they are in.

The Hertzsprung-Russel Diagram

Luminosity: how much energy a star emits Larger stars emit more energy Surface temperature is related to color

Lab 3 Temperature and Luminosity of Stars

Problems How can the size of a star be determined by its temperature and luminosity? How can an H-R diagram be used to chart the life cycle of a star?

Procedure You have been given a table containing information about 15 stars and a chart containing an H-R diagram. Draw in dividing lines between each of the temperature categories shown above the H-R diagram. Color in each of these categories with the appropriate shade. You should notice three distinct groups of stars on the H-R diagram, one running diagonally across the chart and two in separate corners of the chart. The stars in the group that runs diagonally across the chart are part of the Main Sequence – label this on the chart. Label the following: Red giants, White dwarves, Blue giants, and Red dwarves. Plot each star onto the H-R diagram. Fill in the spectral color and star type for each star on the data table.

Questions List the colors of stars from the color of the hottest star to the color of the coldest star. What color are the stars in the top right corner of the H-R diagram? _______________ What color are the stars in the bottom left corner of the H-R diagram? _______________ What color(s) are the stars in the group seen diagonally across the center of the diagram?

How do the red giant stars differ from the white dwarf stars in terms of absolute magnitude and temperature? What is the range in absolute magnitude of the main sequence stars? The sun is part of which group on the H-R diagram? ______________ What is the surface temperature of the sun? _________________

For the remaining questions, circle the most appropriate term in the parenthesis. Antares has a lower absolute magnitude than that of (Arcturus, Mintaka) Supergiants are stars with relatively (high, low) temperatures and high absolute magnitudes. Red giants (are, are not) main sequence stars.  The absolute magnitude of Hadar is (higher, lower) than that of the sun. The temperature of Sirius B is (higher, lower) than the sun. Red dwarfs (are, are not) main sequence stars.  Altair and Van Maanen’s star have very similar surface temperatures, but very different absolute magnitudes (brightness, or luminosity). Explain.