Module 9 ASTR 100

Describe the steps that occur when a heavy star explodes in a supernova. Is this a quick or slow process? Explain.

1)The core becomes electron-degenerate when it is about the size of Earth. The weight bearing down on the iron ash core is too great to be held up by electron degeneracy pressure 2) At these temperatures, the nucleus of the star is filled with photons so energetic that they can break iron nuclei apart. 3) This process, called photodisintegration, absorbs thermal energy and begins tearing apart the nuclei built up by nuclear fusion. At the same time, the density of the core is so great that electrons are forced into atomic nuclei, where they combine with protons to produce neutrons and neutrinos. 4) The collapse of the core accelerates, reaching a speed of 70,000 km/s, or almost one-fourth the speed of light, on its inward fall. All of these events together take place in less than a second.

What is a white dwarf made of? How can it produce energy if there is no fusion going on?

A non-burning ball of carbon, oxygen, and small amounts of some other elements.Within a few thousand years the burned-out core shrinks to about the size of Earth, at which point it has become electron-degenerate and can shrink no further. This remnant of a low-mass star is called a white dwarf. The white dwarf continues to radiate energy away into space because the white dwarf's surface is still incredibly hot and producing energy even though no fusion is going on.

What is a planetary nebula? What creates the different colors seen?

A planetary nebula consists of the remaining outer layers of a star, which were ejected into space as a dying gasp at the end of the star's ascent of the asymptotic giant branch. The colors come from emission lines due to particular atoms and ions.

What are Type Ia supernova? How often do they occur in our galaxy? How are they useful for measuring distances to other galaxies?

A white dwarf that is accumulating mass, however, likely does not quite reach the Chandrasekhar limit. As the star reaches about 1.38 MSun, core pressures and temperatures rise enough to ignite carbon and begin a simmering phase that holds off thermonuclear runaway for a while. Once the temperature reaches about 1.0 3 108 K, the runaway carbon burning involves the entire white dwarf. Within about a second, the whole white dwarf is consumed in the resulting explosion.

What happens to a white dwarf if it gets closer to 1.4 Mo - the Chandrasekhar limit?

Above this mass, even the pressure supplied by degenerate electrons is no longer enough to balance gravity, so the white dwarf will collapse.

What causes a nova to occur on the surface of a white dwarf? Will our Sun ever do this? Why or why not?

White dwarves in binary systems create a runaway effect of explosions, which eventually create a nova. The sun will not do this because it is not in a binary system.

What happens to the mass of heavy stars during the time they are main sequence or giant stars?

An evolving high-mass star may inhabit a region of the H-R diagram known as the instability strip more than once during its post-main-sequence evolution; stars in this region grow brighter and fainter and brighter and fainter repeatedly. The time it takes a star to do this once is known as the period, because the star pulsates with a periodic variation in luminosity

What is happening to the size and luminosity of a star when it is a pulsating variable star?

As a star evolves, it may become a pulsating variable star, which does not find a steady balance between the inward-pushing force of gravity and the outward- pushing force due to radiative pressure. As a result, the star grows alternately larger and smaller.

What happens in the core of a heavier star as soon as the core runs out of hydrogen? How does the star's surface temperature, diameter, and luminosity change during this time?

As the high-mass star runs out of hydrogen in its core, the weight of the overlying star compresses the core. Yet long before the core becomes electron- degenerate, the pressure and temperature become high enough for helium burning. Because the core is not made of degenerate matter, the structure of the star responds to the increase in temperature, but its luminosity changes relatively little. The star makes a fairly smooth transition from hydrogen burning to helium burning.

What happens to the outer parts of an AGB star? Why does this occur?

Before the temperature in the carbon core becomes high enough for carbon to burn, the star loses its gravitational grip on itself and expels its outer layers into interstellar space.

Describe what will happen to the luminosity of our Sun while it is a main sequence star. When will most of this change occur?

Between the time the Sun was born (about 4.6 billion years ago) and the time it will leave the main sequence (about 5.4 billion years from now), its luminosity will roughly double. Most of this change will occur during the last billion years of its life on the main sequence.

What is the relationship between the period and luminosity of a Cepheid variable star? Describe how you can use this to measure distance.

Cepheid variables have periods in the range of 1 to 100 days. The luminosity of a Cepheid variable is related to its period: Short- period Cepheid variables are less luminous than long-period Cepheid variables. This period-luminosity relationship allows astronomers to use Cepheid variables to find the distances to galaxies beyond our own. By observing the period, astronomers can determine the average luminosity.

How is energy primarily lost for heavy stars when they have carbon and heavier elements fusing? What happens to the rate of nuclear reactions when this occurs?

Energy is carried primarily by neutrinos rather than radiation and convection. In a high-mass star, once cooling by neutrinos becomes significant, the outer layers of the star fall inward, drive up the star's density and temperature, and force nuclear reactions to run even faster.

What causes the main sequence life of a star to end?

Eventually, a star exhausts all of the hydrogen fuel in its core. At this point, the innermost core of the star is composed entirely of helium ash. As thermal energy leaks out of the helium core into the surrounding layers of the star, no more energy is generated within the core to replace it. The balance between pressure and gravity that has maintained the structure of the star throughout its life up to this time is now broken. The star's life on the main sequence has come to an end, and its further evolution depends on temperature changes in the core, which govern fusion reactions.

What happens to a heavy star when the core runs out of helium?

Eventually, the high-mass star exhausts the helium in its core. As the core col-lapses, it reaches temperatures high enough to burn carbon. Carbon burning produces even more massive elements, including oxygen, sodium, neon, and magnesium. The star now has a carbon-burning core surrounded by a helium-burning shell surrounded by a hydrogen-burning shell. When carbon is exhausted, neon burning begins; and when neon is exhausted, oxygen begins to burn.

What are free neutrons? Describe how they are responsible for making elements heavier than iron.

Free neutrons, however, have no net electric charge, so there is no electric repulsion to prevent them from simply running into an atomic nucleus. Under normal conditions, free neutrons are rare. Under the conditions of a Type II supernova, however, free neutrons are produced in very large numbers. These are easily captured by massive atomic nuclei and later decay to become protons, forming elements more massive than iron.

Do heavier stars have longer or shorter main sequence lifetimes than less heavy stars?

Heavier stars have shorter lifetimes in comparison to less heavy stars because everything, including the life span, happens faster with heavier stars.

Why does iron not fuse in a heavy star?

Iron does not release energy when it fuses (it absorbs energy instead), the chain of nuclear fusion stops with iron.

What elements were present at the beginning of the universe? Where were all the other elements formed?

Only the least massive chemical elements were present at the beginning of the universe: hydrogen, helium, and trace amounts of lithium, beryllium, and possibly boron. All of the rest of the chemical elements, including a large fraction of the atoms we are made of, were formed in stars through nuclear reactions and then returned to the interstellar medium when the stars exploded.

What carries away most of the energy in a Type II supernova?

Over the next second or so, almost 20 percent of the material in the core is converted into neutrinos. Most fly outward through the star, but at these phenomenal densities, not even neutrinos pass through with complete freedom.

What is the most important property of a star in determining its evolutionary course?

The most important property in a star's evolution is its mass.

What is different about the way hydrogen fusion occurs in heavier stars?

The cycle that high mass stars undergo. Have carbon,nitrogen, and oxygen act as catalysts for hydrogen fusion in the core. Making it proceed at a far higher rate than would be possible by the proton proton chain alone. This process is the same as the proton proton chain but it has a much higher rate of fusion which leads to enormous luminosities and short lifetimes of high mass stars

What happens to the diameter, luminosity and surface temperature of a star when it becomes a horizontal branch star/yellow giant?

The diameter decreases, the luminosity stays the same and the surface temperature of the star increases.

What happens to the diameter, luminosity and surface temperature of a star when it becomes an asymptotic giant branch (AGB) star?

The diameter of the AGB star increases by ~50x again, the luminosity increases by ~1000x and the temperature drops again.

What happens to the diameter, luminosity and surface temperature of a star when it is on the red giant branch?

The diameter of the star increases by ~50x, the luminosity increases by ~1000x and the surface temperature drops.

What is the main sequence turnoff? How can it be used to estimate the age of a cluster of stars?

The point at which the main sequence ends is called the main-sequence turnoff. The farther a star is up the main sequence, the more massive it is, and the shorter its lifetime on the main sequence. We can use this information to find the ages of clusters and, therefore, of the stars in them.

What is the triple-alpha process? What element does it create? What temperatures are required for it to occur?

The triple-alpha process is the two stage process in which helium burns. This process can create the element of carbon. It must be 100 million degrees for this process to occur.

Which of these is true about a Type II supernova? a) A Type II supernova begins when the iron core of a heavy star collapses. b) Nothing is left after a Type II supernova. c) In a Type II supernova, neutrons are split into protons and electrons which creates a shock wave. d) All of the above.

a) A Type II supernova begins when the iron core of a heavy star collapses.

The most important property of a star in determining its evolutionary course is its: a) Mass or weight b) Distance from Earth c) Location in the galaxy d) Location in the sky - ie. what constellation it is in.

a) Mass or weight

When our Sun becomes a red giant, which of the following will happen? a) The surface temperature of the Sun will decrease. b) The luminosity of the Sun will decrease. c) The diameter of the Sun will decrease. d) All of the above.

a) The surface temperature of the Sun will decrease.

Which of these is true about stars? a) High mass stars will only fuse hydrogen and helium - never anything heavier. b) High mass stars, right before they explode, are the largest diameter stars that we see in the galaxy. c) High mass stars live for much longer than low mass stars. d) All of the above.

b) High mass stars, right before they explode, are the largest diameter stars that we see in the galaxy.

Why will our Sun never go through a nova? a) Our Sun will never become a white dwarf. b) Our Sun is not part of a binary system. c) Temperatures inside the core of the Sun are too hot. d) Temperatures inside the core of the Sun are too cold.

b) Our Sun is not part of a binary system.

You see two stars in a binary system. Star A is a blue main sequence star and Star B is a white dwarf. When they first formed, which star was heavier? a) Star A b) Star B c) They had the same weight when they formed. d) Impossible to tell.

b) Star B

Which of the following statements is true? a) The core of a red giant star does not collapse because of all the helium fusion that is occurring. b) White dwarfs can be seen in the sky because of the large amount of fusion going on inside. c) A planetary nebula occurs when a red supergiant star slowly blows off its outer layers. d) All of the above.

c) A planetary nebula occurs when a red supergiant star slowly blows off its outer layers.

Which of these is true about stars? a) Helium fusion turns 3 helium atoms back into hydrogen. b) The yellow giant phase of a star's life ends when the core runs out of hydrogen. c) Higher temperatures are needed for helium fusion than hydrogen fusion partly because helium atoms have a larger force trying to push them apart. d) When a star becomes a red giant, the temperature at the surface will increase.

c) Higher temperatures are needed for helium fusion than hydrogen fusion partly because helium atoms have a larger force trying to push them apart.

Which of these phases in the life of a star like our Sun lasts the longest? a) Red giant b) Red Supergiant c) Main Sequence d) Planetary Nebula

c) Main Sequence

Which of the following best describes the end of our Sun's life? a) Our Sun will always be a main sequence star. b) Our Sun will turn into a white dwarf and then explode in a supernova. c) Our Sun will turn into a white dwarf and then slowly fade away. d) Our Sun will explode as a supernova and then turn into a black hole.

c) Our Sun will turn into a white dwarf and then slowly fade away.

Imagine two variable stars are seen in the sky. Star George is very faint but Star Ringo is bright. Both stars have the same period. Which of the following is true? a) Star George has a smaller luminosity than Star Ringo. b) Star George is located closer to us than Star Ringo. c) Star George and Star Ringo have the same luminosity. d) It is impossible to tell which star has the higher luminosity or which star is closer.

c) Star George and Star Ringo have the same luminosity.

Which of these statements is NOT true? a) Different colors in a planetary nebula come from different gasses that are glowing. b) The lower the mass of the main sequence turnoff the older the cluster is. c) Type Ia supernovae are useful for measuring distances to other galaxies because all Type Ia supernova last exactly 15 seconds. d) If a white dwarf gets close to the Chandrasekhar limit fusion may begin and the white dwarf can explode.

c) Type Ia supernovae are useful for measuring distances to other galaxies because all Type Ia supernova last exactly 15 seconds.

Which of these is NOT true about high mass (>8 Mo) stars? a) High mass stars use carbon, nitrogen, and oxygen to help the hydrogen fusion process occur quicker. b) High mass stars begin helium fusion as soon as the core runs out of hydrogen. c) High mass stars completely skip the red giant phase. d) High mass stars are very common - there are many more high mass stars than low mass stars.

d) High mass stars are very common - there are many more high mass stars than low mass stars.

Which of these is true about a main sequence star? a) Our Sun will be much hotter on the photosphere in 3-4 billion years (ie. a billion years before it stops being a main sequence star). b) The luminosity of a star increases about 1 million times while it is a main sequence star. c) Hydrogen fusion occurs everywhere inside a main sequence star. d) Our Sun will be a main sequence star for about 10 billion years in total (it is about halfway through that time now). e) All of the above.

d) Our Sun will be a main sequence star for about 10 billion years in total (it is about halfway through that time now).

The main sequence life of a star will end when: a) The star is exactly 10 billion years old b) A nearby star explodes as a supernova c) The outer layers of the star are blown away forming a planetary nebula d) The core runs out of hydrogen

d) The core runs out of hydrogen

Which of the following is true about the interstellar medium? a) The interstellar medium is the gas and dust found between stars. b) Interstellar medium blocks shorter wavelength radiation more than longer wavelength radiation. c) Interstellar medium can make a star look redder than it actually is. d) Objects with a lot of interstellar medium between them and Earth can sometimes not be seen with visible light but can be seen in radio waves or infrared radiation. e) All of the above.

e) All of the above.