EVOLUTION OF STARS

Click on one of these topics to see questions related to that topic:

saturnbutton1.JPG (21728 bytes)Star Death saturnbutton1.JPG (21728 bytes)Star Death - Neutron Stars
saturnbutton1.JPG (21728 bytes)Star Death - White Dwarfs saturnbutton1.JPG (21728 bytes)Star Death - Super Novae
saturnbutton1.JPG (21728 bytes)Star Death - Planetary Nebulae saturnbutton1.JPG (21728 bytes)Home Page
saturnbutton1.JPG (21728 bytes)Star Death - Black Holes

 

saturnbutton1.JPG (21728 bytes)Star Death Questions

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

Q1.    Explain the concept of degeneracy as applied to either electrons or neutrons. What role does this phenomenon play in dead stars?  Answer

Q2.    What is meant by the term "death" when applied to stars? What causes the death of stars?  Answer

Q3.    What causes the death of a star? What is the initial consequence of death for any star?  Answer

Q4.    The final state of a dead star can be one of three types of objects. What are they? How is each observed? What determines which a given star will become?  Answer

Q5.    Why do low mass stars die a quiet death and massive stars a more violent death?  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Neutron Stars Questions

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

Q1.    What is a pulsar? How are they commonly observed? How are those observations explained?  Answer

Q2.    How are neutron stars observed? Be sure to explain the physical process which generates the radiation we observe.  Answer

Q3.    How were neutron stars first observed? What unusual or unexpected property did these observations have? How are these observations connected to neutron stars?  Answer

Q4.    Describe the physical properties (mass, density, and size) of a neutron star. Why do they not continue to collapse indefinitely?  Answer

Q5.    What are the two ways that neutron stars are commonly observed? Explain what is producing the radiation in each case.  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - White Dwarfs Questions

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

Q1.    What is the ultimate fate for stars like the sun? Describe the properties of such objects, and explain how they can be stable.  Answer

Q2.    When a low mass star dies, why is the collapse gradual and not catastrophic? Be sure to explain the principle behind your answer.  Answer

Q3.    Describe the physical properties of white dwarf stars – what would one look like if we could see it up close?  Answer

Q4.    What is the final state for a dead, low mass star? Describe the physical properties of such an object?  Answer

Q5.    What is the Chandrasekhar limit? What does it tell us about the death of massive stars?  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Super Novae Questions

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

Q1.    Describe briefly the five observable consequences of a supernova explosion.  Answer

Q2.    Why do massive stars die explosively? What are the five consequences of the explosion?  Answer

Q3.    If a supernova occurred in our part of the galaxy, what would we see in our sky both initially and over the course of a few months?   Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Planetary Nebulae Questions

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

Q1.    What event occurs just before a star like the sun dies? How will this event ultimately be visible to us?  Answer

Q2.    The death of a low mass star occurs fairly quickly, compared to normal time scales for stellar evolution. Why are we able to easily find such objects in transition? Answer

Q3.    In what form do we observe medium mass stars as they die (i.e., during the process of dying, not afterwards)? What caused this phenomenon?   Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Black Holes Questions

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

Q1.    What is meant by the term, "Black holes have no hair"? What are the only three properties of a black hole that can be measured by an external observer?  Answer

Q2.    How do the properties of a black hole change as its mass is increased?  Answer

Q3.    What is the specific definition of a black hole? How can we tell that a binary star system contains a black hole, as opposed to a neutron star?  Answer

Q4.    Why does matter falling toward a black hole become stretched?   Answer

Q5.    What is meant by the term escape velocity? What is the definition of a black hole?  Answer

Q6.    What would happen to Earth’s orbit if the sun were replaced by a black hole of equal mass? Explain.  Answer

Q7.    Compare the size and density of a black hole with a mass equal to the sun’s mass and one with a 100 billion times the sun’s mass.  Answer

Q8.    In what way do we think we have observed "star-like" black holes? How do we know they are black holes?  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death Answers

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

A1.    Only a certain number of particles of a given type can be packed into a given volume of space. Once this limit is reached, any additional particles must have a much greater than normal energy in order to occupy the same volume. This increased energy of the particles greatly increases the pressure they exert on each other. Since this increased pressure depends only upon the degree to which the particles are packed together, and not on their temperature, this high pressure will permanently stabilize a dead star against the inward force of gravity. Electrons in white dwarfs and neutrons in neutron stars supply this degenerate pressure.

A2.    A star dies when stable nuclear reaction no longer occur within the star. Death occurs when the star is unable to maintain the temperature required for nuclear reactions in any of the fuel that may remain in the star. For example, the out layers of the star may still contain plentiful hydrogen, but the temperature there is much too low for nuclear reactions. The most massive stars may have tiny cores of iron, which can no longer fuse to release energy at any temperature.

A3.    A star dies when gravity is no longer able to maintain the temperatures required for fusion reactions in the remaining fuels. For example, if hydrogen is still present near the surface of the star, it will not be hot enough for fusion reactions of hydrogen. Deeper in the star, where the temperature is high enough for hydrogen fusion, there remains no hydrogen. Helium in those layers, however, may not be hot enough for those reactions. If all fuels fall in this situation, then no nuclear reactions are possible anywhere within the star. When this occurs, the pressure within the star will gradually diminish and hydrostatic equilibrium will be lost. Gravity will become stronger than pressure, and the star will begin to collapse.

A4.    Dead stars can be either a white dwarf (seen as a very dim but very hot star), a neutron star (seen as a very regular pulsar and as a low mass x-ray emitting companion to a normal star), or a black hole (seen as an x-ray emitting companion to a normal star with a mass above the limit for neutron stars). The final mass of the star determines which of these end points it becomes. Low mass stars die as white dwarfs, while only the most massive stars become black holes.

A5.    As a low mass star collapse, the electrons within it become degenerate. They are able to provide a stable source of pressure to balance the gravity of the star when it reaches the size of a white dwarf. In more massive stars, there is no electron degeneracy to slow the collapse. By the time electrons might become degenerate, they have fused with protons to become neutrons. Without anything to slow the collapse, a massive star implodes on itself and then rebounds in a violent explosion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Neutron Stars Answers

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

A1.    A pulsar is a rapidly spinning, highly magnetic neutron star. As it spins the bright spot created by particles trapped in the magnetic field of the star flashes into view, producing brief flashes of energy. These flashes of light are seen as very regular pulses of (usually) radio light called pulsars.

A2.    Neutron stars are observed as pulsars – radio objects which turn on and off several times a second with extraordinary precision. These radio pulses are produced by the interaction of charged particles around the neutron star with the rapidly spinning, highly magnetic star. As the "hot spot" near the magnetic pole of the star spins into view, we see the pulse of radio waves emitted there.

A3.    Neutron stars were first observed as pulsars – sources of radio waves that turned on and off rapidly and extremely precisely. The pulses were so uniform that they were first thought to be electronic glitches, and then possible signals from another civilization. Further study showed that neutron stars would be rapidly and uniformly rotating and that they would emit radio waves from a hot spot created by a very powerful magnetic field. As the neutron star, this hot spot is repeated brought into our line of sight.

A4.    A neutron star must have a mass less than about 2-3 times the mass of the sun. They are about 10-20 miles in diameter, and have densities a trillion times ordinary matter. At those densities, all the particles have combined to form neutrons, and the neutrons have become degenerate. This neutron degeneracy provides sufficient pressure to establish hydrostatic equilibrium.

A5.    The magnetic field of a rapidly rotating neutron star produces radio waves that become visible whenever the pole of the star comes into view. This produces rapid, highly regular pulses of radio waves. If a neutron star exists in a binary star system in which the other star is shedding matter onto the neutron star, x-ray will be emitted by the matter as it falls toward the neutron star.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - White Dwarfs Answers

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

A1.    Upon its death, a low mass star slowly collapses to become a white dwarf star – hardly bigger than Earth, a hundred to a thousand times dimmer than our sun, and extremely dense. The collapse is halted when the electrons within the star have become completely degenerate. All the available low energy levels for the electrons have been filled, so they cannot be further compressed without added a great deal of energy to them.

A2.    When any star dies it begins to collapse as the pressure weakens against gravity. In a low mass star, the center of the star is already partially degenerate. Degenerate electrons exert a very strong pressure force because they have reached the "maximum" density allowed for electrons. Any further compression of the electrons requires that they be elevated to much higher energies, which leads to the great pressures. Since this degeneracy pressure does not leak away like ordinary pressure, the partially degenerate core helps to support the star and leads to a gradual collapse as the outer layers settle onto the stable core.

A3.    White dwarfs are very small (about the size of Earth, or 10,000 km in diameter), very hot (20,000-50,000K initially), and very dim (more than 1,000 times dimmer than the sun).

A4.    A dead, low mass star ends its life as a white dwarf star. It is about the size of Earth, only 10,000 kilometers in radius, but has a density of about 10-15 tons per teaspoon. It is also extremely dim, about 100 to 1,000 times dimmer than the sun, but initially has a very hot surface because of the collapse that led to this final state.

A5.    The Chandrasekhar limit is the maximum mass a white dwarf can have – about 1.4 times the mass of the sun. Any star with a mass greater than the Chandrasekhar limit at the time of its death cannot become a white dwarf. Such stars die a more violent death in a supernova explosion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Super Novae Answers

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

A1.    When a supernova explosion occurs, we see (1) a bright flash of light (about a billion times brighter than the sun lasting for a few months), (2) an expanding cloud of gas which appears years later and expands rapidly from the site of the explosion, (3) the creation of elements heavier than iron, (4) the stimulation of star formation in nearby molecular clouds as the expanding material moves through them and compresses the cold gas, and (5) a stellar remnant in the form of a neutron star or black hole.

A2.    Unlike the low mass stars, there is nothing left to support a massive star after it dies. Gravity quickly overwhelms the pressure, and the star collapses rapidly. At some point the density becomes so great that the falling matter rebounds and is explosively ejected from the star. The consequences of this explosion include: (1) bright flash of light seen as a supernova, (2) creation of an expanding cloud of gas, (3) creation of a great variety of heavy elements not produced by normal fusion reactions, (4) stimulation of star formation in neighboring clouds of gas as the expanding debris "snow plows" through them, and (5) the production of a dense central core left behind after the explosion.

A3.    If a nearby supernovae occurred, we would see an extremely bright "new star" in the sky. It would be bright enough to cast shadows on the ground, and would rival the Moon in brightness. This new star would gradually fade in brightness, but would remain visible to the naked eye for several months.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Planetary Nebulae Answers

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

A1.    The last gasp of energy production for low mass stars like the sun gently puffs the outer layers of the star away from the star. This slowly expanding cloud of gas eventually becomes visible to us as a planetary nebula, when it has become big enough for us to see and the star has become hot enough to cause the gas to begin to glow.

A2.    Just before a low mass star dies, it gently expels the outer layers of the star. These slowly expanding layers of gas eventually become large enough to see in a telescope. When the central star becomes very hot during its collapse to a white dwarf, its light excites the gas to make it visible to us. These round, glowing clouds of gas are easily recognized in photographs, which draws attention to the dying star in the center.

A3.    A dying medium mass star is seen as a planetary nebula. The outer layers of the star are gently puffed off the star just before it dies. As they expand away from the star, they glow from the light of the star and produce a visible ring around the star.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Star Death - Black Holes Answers

***NOTE: After clicking on the link Answer click the back button on your browser to go back to the question***

A1.    Once matter has fallen into a black hole, outside observers cannot know anything about it (that is, whether or not it has hair) except for its mass, electric charge, and amount of rotation.

A2.    With increasing mass, the size of a black hole increases but the overall density (defined as mass divided by the volume of the event horizon) decreases. Massive black holes are neither small nor dense.

If a black hole is formed as a result of a supernova explosion, what observation can we take to "observe" it? Describe interpretation of the observations and the analysis of those observations which eliminates other possible objects.

"Stellar" black holes can be observed in x-ray binary star systems. If the black hole is orbited by an ordinary star, matter may be pulled from the surface of the ordinary star to form an accretion disk around the black hole. This rapidly swirling matter will be heated to millions of degrees, which leads to the emission of the x-rays. By determining the characteristics of the orbit from observations of both the x-rays and the ordinary star, we can determine the mass of the compact object. If it is greater than the mass limit for neutron stars, we conclude that it is a black hole.

A3.    A black hole is any object whose minimum escape speed is greater than the speed of light. The minimum escape speed is the smallest speed required to permanently leave an object. If a black hole exists within a binary star system, it may pull matter from its companion. This in-falling matter will give off x-rays as it approaches the black hole. Neutron stars may do the same thing, but their mass cannot be greater than about 3 times the mass of the sun. Hence, if you find an x-ray emitting binary star system with a star more massive than about 3 times the sun, you can tell that it contains a black hole.

A4.    As matter falls toward a black hole, one side will be slightly closer to the black hole than the other side. The side that is closer will feel a stronger force of gravity, and will fall more rapidly than the more distant side. As the matter gets closer to the black hole, this effect becomes more and more noticeable as the object is stretched out.

A5.    The escape velocity is the minimum speed required to leave the surface of an object and never return. It is the minimum speed required to overcome the gravity of the object. A black hole is any object whose escape velocity exceeds the speed of light. Since nothing can move faster than the speed of light, nothing can leave the surface of a black hole.

A6.    If the sun were replaced by a black hole of equal mass, Earth’s orbit would not be altered at all. We are so far from the sun that the force of gravity out here would be unaltered.

A7.    A solar mass black hole is very tiny (about 4 miles across) but extremely dense. A galactic black hole is much larger (about 0.03 light years) but has an average density about equal to air.

A8.    Black holes are observed in some x-ray binary star systems. By observing the normal star which orbits the black hole, we can determine the mass of the compact object. If this mass is greater than the maximum mass for a neutron star, then we conclude that a black hole is the only other possibility.