Ch 11 Jane Lucas

Jane Lucas
Astronomy 100-Week 12 – Chapter 11
Astrochemistry
Properties of Stars
Properties of Thermal Radiation
Patterns Among Stars
Star Clusters
Astrochemistry – As the universe cools the complexity of chemical reaction increases and life appears.
Properties of Stars: Include luminosity, surface temperature, and mass.  We learned how to measure each of these properties.  To measure stellar luminosities we calculate its apparent brightness and its distance, which we can measure through stellar parallax.  To measure stellar temperatures we measure the star’s surface temperature from its color or spectrum.  We classify stars according to the sequence of spectral types OBAFGKM, which runs hottest to coolest.  To measure stellar masses we use Newton’s version of Kepler’s third law in binary star systems.  Types of binary systems are visual, eclipsing, and spectroscopic. So we need to know the orbital period and separation of the two stars.
Properties of Thermal Radiation:

1. Hotter objects emit more light per unit area of all frequencies.
2. Hotter objects emit photons with a higher average frequency.

Patterns Among Stars: Graphs revealed unsuspected patterns of stars.  Graphs such as the Hetzsprung-Russell (H-R) diagram became one of the most important tools in astronomical research.  The diagram plots the stars according to their surface temperatures and luminosities.  Stars spend most of their lives fusing hydrogen into helium in their cores, and stars in this stage of life are found in the H-R diagram in a narrow band known as the main sequence.  Giants and supergiants are to the upper right of the main sequence and white dwarfs are to the lower left.  Stars on the main sequence are all fusing hydrogen into helium in their cores, and a star’s position along the main sequence depends on its mass.  High-mass stars are at the upper left end of the main sequence, and the masses of stars become smaller as we move toward the lower right end.  Giants and supergiants are stars that have exhausted their central core of supplies of hydrogen for fusion and are undergoing other forms of fusion at a prodigious rate as they near the ends of their lives.  White dwarfs are the exposed cores of star that have already died, meaning they have no further means of generating energy through fusion.
Star Clusters:  There are two types of star clusters called open clusters, and globular clusters.  Open clusters contains up to several thousand stars and are found in the disk of the galaxy.  Globular clusters contains hundreds of thousands of stars, all packed closely together.  They are found mainly in the halo of the galaxy.  We can measure a star cluster’s age by finding the main-sequence turnoff point of its stars on the H-R diagram.  All the star clusters were born at the same time.  The cluster’s age is equal to the core hydrogen fusion lifetime of the hottest, most luminous stars that remain on the main sequence.  Open clusters are much younger than globular clusters, which can be as old as 13 billion years.
In conclusion we learned in Chapter 11 that all stars are primarily made of hydrogen and helium.  The differences between stars are due to the differences in their mass and stage of life.  Also, stars spend most of their lives as main-sequence stars that fuse hydrogen into helium in their cores.  The most massive stars, which are also the hottest and most luminous, live only a few million years.  The least massive stars, which are coolest and dimmest, will survive until the universe is many times its present age.  The key to recognizing these patterns among stars was the H-R diagram.  Moving on to the end of the chapter with the studying of star clusters, which we can measure its age by using the H-R diagram.