Chapter 10 - The Death of Stars

• What is evidence that stars die?*

  There is no infinite supply of nuclear fuel. When stars run out of nuclear fuel, gravity wins over pressure and the star dies.
• How do high-mass and low-mass stars die?*

  The mass of stars can determine their fate. High-mass stars die first in gigantic explosions called supernovas. Low-mass stars die quiet deaths.
• What are the differences between Nova's and supernovas'?

  Nova:- Eruption on the surface of a stellar remnant. Only a few are discovered each year. They die quiet deaths.

  Supernova: A violent, explosive death of an aging massive star. Die first. One or two are discovered every century. Evidence of many supernovas are found yearly.
• How does a main sequence star produce energy and how long is it on the main sequence?

  A main sequence star produces energy by nuclear fusion (Hydrogen to Helium) and is on the main sequence 90% of its lifetime.
• What are the steps of evolution off the main sequence to expand into a red giant?*

  - Hydrogen in the core is completely converted to helium.

  - Hydrogen burning (hydrogen fusion into helium) ceases in the core.

  - Core begins to shrink.

  - Hydrogen starts burning in a shell around the shrinking core.

  - Hydrogen burning produces more energy than was required for pressure support, causing...

  - Expansion and cooling of the outer layers of the star.

  - Resulting in a Red Giant.
• What is the first thing that happens when Hydrogen is exhausted?

  The hydrogen core becomes completely helium and the star starts evolving rapidly.
• What is happening to the core when the core begins to shrink?

  As the core contract, it grows hotter. The gravitational energy is converted to thermal energy. This is because there is no pressure or energy opposing gravity allowing it to squeeze the core tightly.
• What does the heating helium core do to the star?

  The helium core heats the hydrogen just above it causing the now heated hydrogen to burn/fuse nuclear energy in a shell around the shrinking core.
• What changes does the new hydrogen shell cause to the star?

  The hydrogen shell produces more energy than was required for pressure support which causes the expansion and cooling of the outer layers of the star.
• Why can't helium in the core first produce nuclear reaction?

  Helium cannot overcome coulomb's barrier. The stellar core's temperature is still less than 100,000,000K. There is no energy being produced that can oppose gravity.
• When does expansion and surface cooling occur and why?*

  Expansion and surface cooling occurs during the phase of an inactive helium core and a hydrogen-burning shell. The expansion is due to the new hydrogen shell producing more energy than required and the cooling is because expansion requires energy and energy gets absorbed during the process causing the temperature to lower.
• What is the movement of stars on the H-R diagram as they become red giants?*

  Massive stars evolve from the main sequence into the supergiant region and the less (medium) massive stars evolve into the red giant regions. The larger a star, the greater its radius and luminosity.
• What may occur to the Sun if nuclear fusion stops in its core and why?*

  The Sun may expand to beyond Earth's orbit because the heating of the hydrogen-fusion shell causes all layers to heat and expand dramatically.
• What type of star's core cannot be a source of energy?*

  Helium in the core of low-mass stars (M<0.4 solar masses) can never become a source of energy because it will never reach the temperature of 100 million K needed for fusion.
• How much of a star's mass is the core?

  A core is only 10% of a star's mass.
• Why can low-mass stars (M<0.4 solar masses) never become a source of energy?*

  - They will never reach the temperature of 100 million K needed for fusion. - There is not enough thermal pressure to resist and balance gravity. Gravity ends up squeezing the core tighter making it small. - The contracting core compresses into a new state called degenerate matter.
• What is degenerate matter?*

  Degenerate matter is when the gas is compressed by gravity into a high-density gas where electrons cannot change their energies due to the electron ladder containing two electrons each gathered at the bottom making them unable to slow down, but only speed up to leap up to the top levels.
• What are two distinctive features about the degenerate gas?

  - Degenerate gas resists compression- Pressure of a degenerate gas does not depend on temperature.
• Why is pressure created in a degenerate core?*

  Pressure supports the core against further collapse if the star's mass is less than 1.4 solar masses; this is because quantum mechanics does not allow electrons to be packed arbitrarily close together.
• Why are two electrons of the degenerate gas allowed on the same energy level?

  The electron pair spin in opposite directions.
• What are the two law of quantum mechanics related to degenerate gas?

  1. Moving electrons in a star's core can contain certain amounts of energy.2. Two identical electrons can't occupy the same energy level.
• What is the next stage of nuclear burning in a star and how does it happen?*

  The next stage of nuclear burning is helium fusion through the triple-alpha process. This only occurs when the shrinking, dense, hot core reaches 100 million K in temperature allowing Helium to produce/fuse Carbon.
• What is the triple-alpha process and what are their equations?*

  The triple-alpha process is when three helium nuclei collide to make carbon. The equations are:- ⁴He + ⁴He → ⁸Be + γ (Gamma Ray)- ⁸Be + ⁴He → ¹²C + γ (Gamma Ray)
• Does Helium fuse in low-mass stars?*

  Helium does not fuse in low-mass stars instead a violent reaction termed Helium Flash occurs.
• What happens during a Helium Flash?

  During an helium flash in an low-mass stars whose temperatures have increased to result in the explosion; the core generates more energy per second than an entire galaxy for a few seconds and becomes able to fuse helium emitting it degenerate phase.
• What is an ability a star obtains from fusing helium/helium flash?


  Stars are able to emit both helium and hydrogen fusion energy, pausing the core's contractions.
• How does a star move along the H-R diagram from hydrogen being exhausted to the beginning of carbon fusion?*

  A star first becomes a giant/red giant (or can be a super giant if it is a massive star). When the star starts to fuse helium or a helium flash occurs the star moves towards the left and slightly up. When the helium becomes exhausted and the carbon core takes over the star moves towards the right and slightly down (Curves up).
• Why cannot helium fuse nuclear energy compared to hydrogen?

  Helium cores are too cool to fuse because helium nuclei have positive charges twice that of hydrogen nuclei, meaning helium needs to move very fast to overcome Coulomb's barrier.
• What happens when Helium fusion ceases in the core?

  The process of reheating occurs and carbon is able to fuse in the core. This may be the last nuclear fusion a star can produce before their deaths.
• How does helium fusion depend on mass?

  Stars greater than 3 solar masses contract rapidly, and so heating is faster and less massive stars evolve slowly, but often become degenerate because of how much their cores contract.
• What are the two types of star clusters?

  open clusters and globular clusters
• What are the differences between the two star clusters?

  Open Clusters: - 10 to 100 stars within a 25 pc diameter.- Open transparent appearance (stars are not crowded together).Globular Clusters: - 10⁵ to 10⁶ stars with a 10 to 30pc diameter.- Clusters are sphereical.- Stars are much closer together.
• What are the similarities between open and globular clusters?

  They both contain stars that orbit their center of mass of the cluster.
• What do all stars in a cluster have in common?*

  All stars in a star cluster all have approximately the same age and composition, differing only in mass.
• What occurs to the H-R diagram position of star clusters?*

  The more massive stars are seen to evolve more quickly than less massive ones because if you plot all the stars of a star cluster on an H-R diagram, you will find the high-mass stars are no longer on the main sequence, but rather, in the Red Giant or Red Super giant region (upper right) of the H-R diagram. Many low-mass stars are still in the main sequence.
• What happens to the massive stars of a star cluster?

  The massive stars are no longer on the H-R diagram because they have died, resulting in the main sequence of a star cluster to grow shorter as it ages.
• What is a turn-off point, what does it represent?*

  A turn-off point include stars that are about to die. You can tell the age of a star cluster by looking at its turn-off point. this is the point where stars are evolving to become giants. Old clusters have faint turn-off points.
• How can you estimate the age of a cluster?*

  The lower the stars/star clusters are on the main sequence's turn-off point, the older the cluster. (The shorter the main sequence the older the cluster, and many stars evolved or died off).
• Why do low-mass stars not get hot?

  Low-mass stars have very little gravitational energy so contracting does not get them hot enough.
• Why do low-mass stars and medium-mass stars differ?

  Low-mass and medium-mass stars differ due to their interior convection.