Astro Ch. 11 - Deaths of Stars
Describe the two types of supernovae and explain how each is produced.
A Type I supernova is the explosion and complete destruction of a dead star called a white dwarf. (The Sun, after it dies, will become a white dwarf.) If the white dwarf is made of carbon (the end product of the thermonuclear reactions that took place during the star's life), and if it is a member of a binary system, a Type I supernova can potentially occur.
Core collapse
A Type II supernova (plural: supernovae or supernovas) results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40-50 times, the mass of the Sun (M☉) for this type of explosion.
Electron degenerate matter
A collection of free, non-interacting particles with a pressure. In a mixture of particles, such as ions and electrons in white dwarfs or metals, the electrons may be degenerate, while the ions are not.
Supernova
A star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.
Variable star
A star whose brightness as seen from Earth (its apparent magnitude) fluctuates.
Nova
A white dwarf--a normally very faint star--undergoing an explosion on its surface that results in a rapid, temporary increase in luminosity. Novae eventually fade back to normal, usually after a few weeks or months.
Explain how white dwarfs in binary-star systems can become explosively active.
A white dwarf--a normally very faint star--undergoing an explosion on its surface that results in a rapid, temporary increase in luminosity. Novae eventually fade back to normal, usually after a few weeks or months. The white-dwarf stage represents the end point of a star's evolution. If the distance between the two stars is small enough, the dwarf's tidal gravitational field can pull matter--primarily hydrogen and helium--away from the surface of its main-sequence or giant companion. The system becomes a mass-transferring binary. A stream of gas leaves the companion through the Lagrange point and flows onto the dwarf.
Planetary nebulae
An expanding cloud of gas left behind by a star's collapse.
Accretion disk
Disk around a black hole that emits light and x-rays.
Special theory of relativity
E=MC^2 1. The laws of physics are the same for all observers in space regardless of the motion of the observer. 2. The speed of light is the same for all observers in space regardless of the motion of the observer.
Summarize the evolutionary stages followed by a Sun-like star once it leaves the main sequence and describe the resulting remnant.
Eventually, hydrogen becomes completely depleted at the center, the nuclear fires there cease, and the location of principal burning moves to higher layers in the core. An inner core of nonburning pure helium starts to grow. Without nuclear burning to maintain it, the outward-pushing gas pressure weakens in the helium inner core. They run out of hydrogen in their core. About 10 billion years after the star arrived on the main sequence, the helium core begins to contract. The star first evolves to the right on the diagram, its surface temperature dropping while its luminosity increases only slightly. The star's roughly horizontal track from its main-sequence location (stage 7) to stage 8 on the figure is called the subgiant branch. By stage 8, the star's radius has increased to about three times the radius of the Sun. The nearly vertical (constant temperature) path followed by the star between stages 8 and 9 is known as the red giant branch of the H-R diagram
Relate the phenomena that occur near black holes to the warping of space around them.
General theory of relativity - gravity is not a real force, it's the curvature of spacetime. Time dilation - the stronger the gravitational field, the slower your clock will tick (gravitational red-shift). Equivalence principle - Acceleration looks like gravity.
General theory of relativity
Gravity is not a real force, it's the curvature of spacetime. Time dilation - the stronger the gravitational field, the slower your clock will tick (gravitational red-shift). Equivalence principle - Acceleration looks like gravity.
Contrast the evolutionary histories of high-mass and low-mass stars.
High-mass stars evolve much faster than their low-mass counterparts. The more massive a star, the more ravenous is its fuel consumption and the shorter its main-sequence lifetime. All evolutionary changes happen much more rapidly for high-mass stars because their larger mass and stronger gravity generate more heat, which speeds up all phases of stellar evolution. The Sun will spend a total of some 10 billion years on the main sequence, but a 5-solar-mass B-type star will remain there for only a few hundred million years. A 10-solar-mass O-type star will depart in only 20 million years or so. This trend toward much faster evolution for more massive stars continues even after the main sequence.
Explain the origin of elements heavier than helium and discuss the significance of these elements for the study of stellar evolution.
How and where did all these elements form? Were they always present in the universe, or were they created after the universe formed? Since the 1950s, astronomers have come to realize that the hydrogen and most of the helium in the universe are primordial--that is, these elements date back to the very earliest times. All other elements in our universe result from stellar nucleosynthesis--that is, they were formed by nuclear fusion in the hearts of stars. A key point in understanding the creation of heavy elements is that large nuclei can be built from smaller ones by nuclear fusion. We might naturally theorize that all the heavy elements have been created in this way. In this scenario, the ultimate source of the heavy elements is the lightest and simplest of all--hydrogen. To test this idea, we must consider not just the list of different kinds of elements and isotopes but also their observed abundances. This curve is derived largely from spectroscopic studies of stars, including the Sun. This model combines all the known elements into eight distinct groups based on the number of nuclear particles (protons and neutrons) that they contain. Any theory proposed for the creation of the elements must reproduce these observed abundances. The most obvious feature is that the heavy elements are much less abundant than most light elements.
Discuss some of the ways in which the presence of a black hole might be detected.
If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.
Black hole
If the guest star formed from a type II supernova is greater than 3 solar masses, a black hole is formed. A black hole is a region of space having a gravitational field so intense that no matter or radiation can escape.
Describe how black holes are formed and discuss their effects on matter and radiation in their vicinity.
In fact, we know of no force that can counteract gravity beyond the point at which neutron degeneracy pressure is overwhelmed. If enough material is left behind after a supernova, as may happen in the case of an extremely massive progenitor star, gravity finally wins once and for all, and the central core collapses forever. As the core shrinks, the gravitational pull in its vicinity eventually becomes so great that even light itself is unable to escape. The resultant object therefore emits no light, no radiation, no information whatsoever. Astronomers call this bizarre end point of stellar evolution, in which a massive core remnant collapses in on itself and vanishes forever, a black hole. The event horizon is the region within which no event can ever be seen, heard, or known by anyone outside. Even though there is no matter of any sort associated with it, we can think of the event horizon as the "surface" of a black hole.
Cepheid variable
Intrinsic variables which pulsate in a predictable way. In addition, a Cepheid star's period (how often it pulsates) is directly related to its luminosity or brightness. Cepheid variables are extremely luminous and very distant.
Explain the nature and origin of pulsars, and account for their characteristic radiation.
Most pulsars emit their pulses in the form of radio radiation. These electromagnetic flashes at different frequencies are all synchronized--that is, occurring at regular, repeated time intervals--as we would expect if they arose from the same object. The period of most pulsars is usually short--ranging from about 0.03 to 0.3 s, corresponding to a flashing rate of between 3 and 30 times per second. A few pulsars are clearly associated with supernova remnants, although not all such remnants have a detectable pulsar within them. There is a Crab pulsar at the center of the Crab supernova remnant. They show that the Crab also pulses in X rays. By observing the velocity and direction that the Crab's ejected matter is traveling, astronomers can work backward to pinpoint the location in space at which the explosion must have occurred and where the supernova core remnant should be located. That is precisely the region of the Crab Nebula from which the pulsating signals arise. The Crab pulsar is evidently all that remains of the once-massive star (The Guest Star) whose supernova was observed in 1054.
Neutron degenerate matter
Neutron degeneracy is analogous to electron degeneracy and is demonstrated in neutron stars, which are partially supported by the pressure from a degenerate neutron gas. This may happen when the core of a white dwarf star above the vicinity of 1.4 solar masses, the Chandrasekhar limit, collapses and is not halted by the degenerate electrons, or more typically when the core of a massive star collapses.
Neutron star
Neutron degenerate matter. Gravity in the neutron star is balanced by an outward force due to neutron degeneracy. Believed to be created by type II supernovae, i.e., explosions of high-mass stars. They contain strong magnetic fields created by the collapse of the star, which significantly intensifies the original weak magnetic field of the star.
Describe the properties of neutron stars and explain how these strange objects are formed.
Neutron stars are extremely small and very massive. Composed purely of neutrons packed together in a tight ball about 20 km across, a typical neutron star is not much bigger than a small asteroid or a terrestrial city, yet its mass is greater than that of the Sun. With so much mass squeezed into such a small volume, neutron stars are incredibly dense. Their average density can reach 1017 or even 1018 kg/m3, nearly a billion times denser than a white dwarf. A single thimbleful of neutron-star material would weigh 100 million tons--about as much as a good-sized terrestrial mountain. Even the density of a normal atomic nucleus is "only" 1017 kg/m3. First, newly formed neutron stars rotate extremely rapidly, with periods measured in fractions of a second. This is a direct result of the law of conservation of angular momentum (see Chapter 15), which tells us that any rotating body must spin faster as it shrinks. The collapsing iron (and, later, neutron) core of an evolved star is no exception to this law. Even if the core of the progenitor star were initially rotating quite slowly (once every couple of weeks, say, as is observed in many upper main-sequence stars), it would be spinning a few times per second by the time it had reached a diameter of 20 km. The second important property of a young neutron star is its strong magnetic field. The original field of the progenitor star is amplified by the collapse of the core because the contracting material squeezes the magnetic field lines closer together. This squeezing increases the field strength to a value on the order of a trillion times that of Earth's field (millions of times stronger than the fields found even in the hearts of the most violent solar flares). In time, theory indicates, our neutron star will spin more and more slowly as it radiates its energy into space, and its magnetic field will diminish. However, for a few million years after its birth, these two properties combine to provide the primary means by which this strange object can be detected and studied.
Type II supernova
Occurs among very young stars of large mass. Results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40-50 times, the mass of the Sun (M☉) for this type of explosion.
Pulsars
Pulses from a few seconds to over 2000 times per second.
Chandrasekhar Limit
Shows the effect of gravity on atoms. Stars that are greater than 1.4 solar masses cannot become white dwarf stars because the mass is too great.
Discuss the observations that help verify the theory of stellar evolution.
Star clusters. Every star in a given cluster formed at the same time, from the same interstellar cloud, with virtually the same composition. Only the mass varies from one star to another. This allows us to check the accuracy of our theoretical models in a very straightforward way. Having studied in some detail the evolutionary tracks of individual stars, we now consider how their collective appearance changes in time.
Supergiants
The largest stars in the universe. They can be thousands of times bigger than our Sun and have a mass up to 100 times greater. The largest known supergiant star, VY Canis Majoris, is up to 2,100 times the size of the Sun (based on upper estimates).
Event horizon
The lip of a black hole within which nothing can be seen and nothing can escape, because the necessary escape velocity would equal or exceed the speed of light (a physical impossibility).
Supernova remnant
The structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way. Discovered by radio wave detection. Visible wavelengths are produced by supernova remnants but they are blocked by the interstellar medium.
Type I supernova
Type 1 occurs among old stars of small mass, probably white dwarfs. It is not known how a small-mass star can release the very large amounts of energy needed to explain type 1 Supernovas. Scientists generally believe that this must involve binary systems, two stars revolving around each other. In such a system one of the stars is a white dwarf, a small, dense star near the end of its nuclear burning phase. After attracting matter from the companion star for some time, the white dwarf eventually collapses with a great rush. It condenses into a neutron star and ejects matter outward. This rebound of matter is thought to be the supernova.
