9. Stellar Evolution and Stellar Death

Question 1

Explain the zero-age main-sequence

Answer 1

Zero Age Main Sequence (ZAMS) is the time when a star first joins the main sequence on the Hertzsprung-Russell diagram (HR diagram) by burning hydrogen in its core through fusion reactions. After this time the star enters a phase of stellar evolution that is quite stable, and steadily processes Hydrogen into heavier elements.

Question 2

Appreciate the difference in timescales for stellar formation and evolution, as a function of mass

Answer 2

The most massive stars spend only a few million years on the main sequence. A star of 1 solar mass remains there for roughly 10 billion years, while a star of about 0.4 solar mass has a main-sequence lifetime of some 200 billion years, which is longer than the current age of the universe. (Bear in mind, however, that every star spends most of its total lifetime on the main sequence. Stars devote an average of 90% of their lives to peacefully fusing hydrogen into helium.)

Question 3

Describe evolution on the main-sequence

Answer 3

When a main-sequence star has consumed the hydrogen at its core, the loss of energy generation causes its gravitational collapse to resume and the star evolves off the main sequence. The path which the star follows across the HR diagram is called an evolutionary track.

Question 4

Explain how star clusters can be used to determine ages

Answer 4

Because all of a clusters’s stars were born at the same time, we can measure a cluster’s age by finding the main-sequence turnoff point on an H-R diagram of its stars. The cluster’s age is equal to the hydrogen-fusion lifetime of the hottest, most luminous stars that remain on the main sequence

Note: Star Clusters are groups of gravitationally bound stars. They range in size from a few 10’s of stars up to more than 1 million stars. The nice thing about a cluster of stars is that all the stars are at the same distance and it seems that clusters have only a single burst of star formation, so all the stars in one cluster are the same age.

Question 5

List the different types of clusters and their main properties

Answer 5

There are three main types of star clusters: globular clusters, open clusters, and stellar associations.

Globular clusters: nearly symmetrical round systems of, typically, hundreds of thousands of stars. The diameters of globular star clusters range from 50 light-years to more than 450 light-years. Most of them are in a spherical halo (or cloud) surrounding the flat disk formed by the majority of our Galaxy’s stars.

Open clusters: are found in the disk of the Galaxy. They have a range of ages, some as old as, or even older than, our Sun. Open clusters are smaller than globular clusters, usually having diameters of less than 30 light-years, and they typically contain only several dozen to several hundreds of stars

Stellar associations: An association is a group of extremely young stars, typically containing 5 to 50 hot, bright O and B stars scattered over a region of space some 100–500 light-years in diameter. Associations also contain hundreds to thousands of low-mass stars, but these are much fainter and less conspicuous.