Q9.    Why does a star leave the main sequence? What source of energy doe sit use after it has left the main sequence?  Answer

Q1.    Why can observations of a star cluster be used to test models of stellar evolution? How are these observations used to test the models?  Answer

Q2.    How is a star cluster different from a constellation?  Answer

Q3.    How are the results of stellar evolution calculations tested?   Answer

Q4.    How can we test the models of an evolving star with observations of actual stars?  Answer

Q5.    How are models of an evolving star tested against observations?   Answer

A1.    If the two stars in a binary star system are close enough to each other, they may exchange matter at certain stages of their evolution. In this case, the originally more massive star evolved first to the red giant stage. When it became big enough, it began to shed matter to its companion. This continued until it died, and became a white dwarf. The companion, which originally was a low mass main sequence star, gained extra matter during this process and became a more massive main sequence star. It has not yet evolved off the main sequence.

A2.    When hydrogen is exhausted at the center of a star, that region begins to slowly collapse. This collapse heats the material around the core slightly, which raises the temperature in hydrogen around the center to the temperatures required for nuclear reactions there. This creates a shell of nuclear reactions around the old core of the star. These changes inside the star cause it to leave the main sequence in the HR diagram and move toward the red giant portion of the diagram.

A3.    A star forms as a fragment of a large, cold interstellar cloud collapse under the influence of the force of gravity of the cloud fragment. Because the atoms are falling, they gain speed. Collisions turn this motion into increased random motion, which is by definition an increase in the temperature of the gas.

A4.    The matter in a star does not collapse because there is a balance between the outward force of gas pressure and the inward force of gravity at every point within the star.

A5.    Massive stars are at the top of the main sequence, with brightness that are hundreds of thousands of times greater than the sun’s brightness. While a massive star has more fuel than the sun (10-50 times as much), it is using its available fuel at a far greater rate than the sun does, and will use it up sooner than the sun will.

A6.    Hydrostatic equilibrium requires that the force of gravity at every point within the star is balanced by the force of the gas pressure pushing outward. The sum of the masses of the shells of the star must equal the total mass of the star. The sum of the energy produced (or consumed) in each shell must equal the total brightness of the star. Energy moves from the center toward the surface through either radiation, convection, or conduction.

A7.    The fact that stars shine requires that they evolve. The light that leaks away from the star is the result of collisions between atoms exciting electrons. Those collisions are required to support the structure of the star against the force of gravity. As energy (light) leaks away from the star, it must be replaced in order to maintain the balance between gravity and the pressure exerted by colliding atoms. This new energy requires a source or fuel, which ultimately will be completely consumed. This event will then require the star to change or evolve.

A8.    If we plot random stars on an HR diagram (color versus true brightness), we find that the vast majority lie on the Main Sequence, with less massive stars near the bottom and progressively more massive stars at higher positions. Since we observe these stars at random stages of their lives, the concentration of stars on the Main Sequence tells us that most stars spend most of their lives on the Main Sequence. The fact that the stars are arranged in mass sequence on the Main Sequence tells us that a given star remains relatively fixed at one position on the Main Sequence, and does not move along the Main Sequence as it ages.

A1.    A model of a star consists of a table of numbers that describe the physical conditions, such as temperature and density, at each point inside the star. The four principles used to compute such a model are: hydrostatic equilibrium (at every point within the star the outward force of the gas pressure exactly equals the inward force of gravity), the continuity of mass (the sum of the masses of each layer of the star equals the total mass of the star), the continuity of energy (the sum of the energy created or consumed in each layer equals the energy released at the surface), and energy transport (energy is carried from the centered toward the surface by some combination of conduction, convection, and radiation).

A1.    Observations of stars in a cluster can be used to test models of stellar evolution because all the stars in the cluster formed at about the same time. Their positions in an HR diagram can be compared to models of various masses, each evolved to the same age. If the pattern of observed stars can be matched to that of the models for many different clusters, then it shows the models match what we see in star clusters.

A2.    A constellation is a random grouping of stars in our sky. Some may be close while others are far away. Stars in a cluster are close together in space and form a physical grouping of stars.

A3.    Predictions of the position of stars of many different masses but a certain age are compared in an HR diagram to observations of stars in a single star cluster. If the pattern of models in the HR diagram can be made to match the pattern of observations, then it seems reasonable to assume that all the stars in that cluster formed at the same time, given by the age of the models. If this process is repeated many times for many different clusters, and a match is always found, then we can have some confidence that the models correctly predict the pattern of aging in stars.

A4.    Stellar evolution models are tested by comparing the pattern of stars observed in the HR diagram of a star cluster, in which all the stars have the same age, with models evolved to a fixed age. If we can match the pattern of models evolved to a fixed age with a number of different stars clusters, it shows that the models are correctly predicting the changing characteristics of the stars in the clusters.