Exam 3 Astronomy
What distinguishes a collapsing cloud from a protostar and a protostar from a star? (cc)
(Collapsing cloud/prostar)the existence of a photosphere, meaning that the inner part of the cloud becomes opaque to its own radiation, signaling the slowing of the collapse phase. (prostar/star) Nuclear fusion in the core and equilibrium between pressure and gravity.
The Chandrasekhar mass limit is?
1.4 solar masses
What is the size of a typical interstellar cloud that is dense enough to form stars?
10 parsecs
What temperature is required to initiate nuclear fusion in a stellar core?
10,000,000 Kelvins
What is the typical age for a globular cluster?
12 billion years
What is a carbon denotation supernova?
A Type I supernova that occurs when a carbon-oxygen white dwarf in a binary system gains mass, collapses, and explodes as its carbon ignites
Describe the properties of neutron stars, and explain how these strange objects are formed.
A core-collapse supernova may leave behind a remnant—an ultracompressed ball of material called a neutron star. Neutron stars are extremely dense and, at formation, are predicted to be very hot, strongly magnetized, and rapidly rotating. They cool down, lose much of their magnetism, and slow down as they age.
What results in a Nova?
A nova is the result of a white dwarf in a binary system drawing hydrogen rich material from its companion.
The super nova of 1054 AD produced?
A remnant still visible to the naked eye, crab nebula M-1
What is a Nova?
A star that suddenly increases greatly in brightness then slowly fades back to its normal appearance over a period of months.
A type II supernova is produced when ?
A stars core collapses and shoots off the outer layers
lighter elements fused into elements more massive than iron?
A supernova
Explain the nature and origin of pulsars, and account for their characteristic radiation.
According to the lighthouse model neutron stars, are magnetized/rotating, send bursts of electromagnetic energy into space, produced by charged particles confined by the strong magnetic fields- pulsar. The pulse period is the rotation period of the neutron star. Because the pulse energy is beamed into space and because neutron stars slow down as they radiate energy into space, not all neutron stars are seen as pulsars.
Explain the origin of elements heavier than helium, and discuss the significance of these elements for the study of stellar evolution.
All elements heavier than helium are formed by stellar nucleosynthesis—the production of new elements by nuclear reactions in the cores of evolved stars. Elements heavier than carbon tend to form by helium capture rather than by the fusion of more massive nuclei. At high enough core temperatures,photodisintegration breaks apart some heavy nuclei, providing helium-4 nuclei for the synthesis of even more massive elements, up to iron. Elements beyond iron form by neutron capture in the cores of evolved stars.
Why don't we see pulsars at the centers of all supernova remnants?
Because (1) not all supernovae form neutron stars, (2) the pulses are beamed, so not all pulsing neutron stars are visible from Earth (3) pulsars spin down and become too faint to observe after a few tens of millions of years.
Why are observations of star clusters so important to the theory of stellar evolution? (cc)
Because a star cluster gives us a "snapshot" of stars of many different masses, but of the same age and initial composition, allowing us to directly test the predictions of the theory.
Why does the iron core of a high-mass star collapse?
Because iron cannot fuse to produce energy. As a result, no further nuclear reactions are possible, and the core's equilibrium cannot be restored.
Why is stellar evolution important to life on Earth?
Because it is responsible for creating and dispersing all the heavy elements out of which we are made. In addition, it may also have played an important role in triggering the collapse of the interstellar cloud from which our solar system formed.
Why is it important to understand the evolution of binary stars?(cc)
Because many, if not most, stars are found in binaries, and stars in binaries can follow evolutionary paths quite different from those they would follow if single.
If stars in a cluster all form at the same time, how can some influence the formation of others? (cc)
Because stars form at different rates—high-mass stars reach the main sequence and start disrupting the parent cloud long before lower-mass stars have
Why does fusion cease in the core of a low-mass star?(cc)
Because the core's contraction is halted by the pressure of degenerate (tightly packed) electrons before it reaches a temperature high enough for the next stage of fusion to begin.
Why does a star get brighter as it runs out of fuel in its core?(cc)
Because the nonburning inner core, unsupported by fusion, begins to shrink, releasing gravitational energy,heating the overlying layers and causing them to burn more vigorously, thus increasing the luminosity.
Why would you never actually witness an infalling object crossing the event horizon of a black hole?
Because the object would appear to take infinitely long to reach the event horizon, and its light would be infinitely red shifted by the time it got there.
How did astronomers know, even before the mechanisms were understood, that there were at least two distinct physical processes at work in creating supernovae?
Because the two types of supernova differ in their spectra and their light curves, making it impossible to explain them in terms of a single phenomenon.
Why are the elements carbon, oxygen, neon, and magnesium, whose masses are multiples of four, as well as the element iron, so common on Earth?
Because they are readily formed by helium capture, a process common in evolved stars. Other elements (with masses not multiples of four) had to form via less common reactions involving proton and neutron capture.
Describe how black holes are formed, and discuss their effects on matter and radiation in their vicinity.
Beyond solar masses the star can no longer support itself against its own gravity, and it collapses to form a black hole. A regionof space from which nothing can escape.Very massive stars, after exploding as supernovae, form blackholes rather than neutron stars. The radius at which the escape speed from a collapsing star equals the speedof light is called the Schwarzschild radius. The surface of an imaginary sphere, of radius equal to the Schwarzschild radius, surrounding a black hole is called the event horizon
How do astronomers "see" black holes?
By observing their gravitational effects on other objects, and from the X-rays emitted as matter plunges toward the event horizon.
Rather than solar systems or multiple star systems, some stars belong to larger families called?
Clusters
What is the essential evolutionary difference between high-mass and low-mass stars?(cc)
Fusion in high-mass stars is not halted by electron degeneracy pressure. Temperatures are always high enough that each new burning stage can start before degeneracy becomes important. Such stars continue to fuse more and more massive nuclei, faster and faster, eventually exploding in a supernova.
Outline the basic characteristics of gamma-ray bursts and some theoretical attempts to explain them.
Gamma-ray bursts are energetic flashes of gamma rays observed about once per day, distributed over the entire sky. In some cases, their distances have been measured, placing them far away from us and implying that they are extremely luminous.
What inevitably forces a star like the sun to evolve away from being a main sequence star?
Helium builds up in the core, while the hydrogen burning shell expands.
Contrast the evolutionary histories of high-mass and low-mass stars.
High mass stars evolve more rapidly because larger mass results in higher temperature. High mass never initate a helium flash and fuse carbon. They become supergiants and die explosively.
What is a Type I supernovae?
Hydrogen poor and have a light curve similar to that of a nova.
What is a type II supernovae?
Hydrogen rich and have have a characteristic plateau in the light curve a few months after maximum. A Type II supernova is a core-collapse supernova.
A proposed explanation of gamma ray burst is?
Hyper nova black holes and bipolar jets
What will happen to an isolated neutron star that accumulates more than about 3 solar masses of material?
It will overcome the neutron degeneracy in its interior, forming a black hole.
Summarize the stages in the death of a typical low-mass star, and describe the resulting remnant
Low mass stars never become hot enough to fuse carbon. Such a star continues to ascend the asymptotic-giant branch until its envelope is ejected into space as a planetary nebula the core becomes visible as a hot, faint, and extremely dense white dwarf, whereas the planetary nebula diffuses into space, carrying helium and some carbon into the interstellar medium. The white dwarf cools and fades,eventually becoming a cold black dwarf
what type of main sequence stars live longest?
M type
what type of main sequence stars live the longest?
M-type
At which stage in a Sun-like star's life is its core the least dense?
Main Sequence
Explain how the formation of a star depends on its mass.
Massive fragments within interstellar clouds tend to produce the most massive protostars, the most massive stars. Low fragments = low mass stars. Evolutionary track is always the main sequence.
Stars with core remnants between 1.4 and 2.5 solar masses will end their lives as?
Neutron stars
How do Newton's and Einstein's theories differ in their descriptions of gravity?
Newton's theory describes gravity as a force produced by a massive object that influences all other massive objects. Einstein's relativity describes gravity as a curvature of space-time produced by a massive object; that curvature then determines the trajectories of all particles—matter or radiation—in the universe.
Will the Sun ever become a nova?
No, because it is of low mass and not a member of a binary-star system.
What can we detect from matter that has crossed an event horizon?
Nothing
Do stars evolve along the main sequence? (cc)
No—different parts of the main sequence correspond to stars of different masses. A typical star stays at roughly the same location on the main sequence of most of its lifetime.
Are all supernovae expected to lead to neutron stars?
No—only Type II supernovae. According to theory, the rebounding central core of the original star in a Type II supernova becomes a neutron star.
Which statement about star birth is false?
Nuclear reactions begin in the core by stage 4
The elements heavier than H and He were created by?
Nucleosynthesis in massive stars.
What spectral type that is still around formed most recently?
O-type
Explain the difficulties in observing black holes, and describe some ways in which a black hole might be detected.
Once matter falls into a black hole, it can no longer communicate with the outside. However, on its way in, it can form an accretion disk and emit X-rays, just as in the case of a neutron star.
What compelling evidence links pulsars to neutron stars?
Only a small, very dense source could rotate that rapidly without flying apart.
A ______ Is a relatively gentle mass-loss event which allows a star to peacefully readjust its structure into a white dwarf configuration.
Planetary nebula
What is the force that keeps a main sequence star from collapsing on itself?
Radiation pressure
Describe the nature of interstellar shock waves, and discuss their possible role in the formation of stars.
Shock wave: wave of matter, may be generated by a new born star or super nova, that pushes material outward into the surrounding molecular cloud.the material tends to pile up, forming a moving shell of dense gas. Shock waves may trigger the mechanism needed to initiate star formation.
What is the connection between X-ray sources and millisecond pulsars?
Some X-ray sources are binaries containing accreting neutron stars, which may be in the process of being spun up to form millisecond pulsars.
Summarize the sequence of events leading to the formation of a star like our Sun
Stars form when an interstellar cloud collapses under its own gravity and breaks up into pieces. A cold interstellar cloud fragments into many smaller clumps of matter, from which stars eventually form. As a collapsing prestellar fragment heats up and becomes denser, it eventually becomes a protostar- a warm, very luminous object that emits mainly infrared radiation. Eventually, a protostar's central temperature becomes high enough for hydrogen fusion to begin, and the protostar becomes a star
Explain why stars form in clusters, and distinguish between open and globular star clusters.
Stars from in clusters because it is the end result of a collapse of cloud. Open cluster: loosely bound collection of tens to hundreds of stars, a few parsecs across generally found in the plane of the Milky Way. Globular star cluster: tightly bound, spherical collection of hundred and sometimes millions of stars spanning about 50 parsecs across. Distributed in the halos around the Milky Way and other galaxies.
Explain how the evolution of stars in binary systems may differ from that of isolated stars.
Stars in binary systems can evolve quite differently from isolated stars because of interactions with their companions.
Explain why stars evolve off the main sequence.
Stars leave the main sequence when the hydrogen in their cores is exhausted.Low-mass stars evolve much more slowly; high-mass stars much faster
What makes the subject of star formation so difficult and complex?
Stars live too long to be observed from birth to death.
Summarize the sequence of events leading to the violent death of a massive star.
Stars more massive than 8 solar masses form heavier elements in their cores, at a more rapid pace.As they do so, their cores form a layered structure consisting of burning shells of successively heavier elements. The process stops at iron, whose nuclei can neither be fused together nor split to produce energy. As a star's iron core grows in mass, it eventually becomes unable to support itself against gravity and begins to collapse. At the high temperatures produced during the collapse, iron nuclei are broken down into protons and neutrons. The protons combine with electrons to form more neutrons. Eventually, when the core becomes so dense the collapse stops and the core rebounds, sending a violent shock wave out through the rest of the star. The star explodes in a core-collapse supernova
Stars who collapse into a black hole proceed demise with a?
Supernova
What kind of variable stars are pre-Main Sequence stars undergoing gravitational contraction and exhibiting erratic changes in their luminosities?
T Tauri
Stage 4-6 of star formation is when the object can exhibit violent surface activity producing extremely strong prostellar winds. The phase is called?
T Tauri Phase
What basic competitive process controls star formation? (cc)
The competing effects of gravity, which tends to make an interstellar cloud collapse, and heat (pressure), which opposes that collapse
Describe some of the observational evidence supporting the modern theory of star formation.
The dark interstellar regions near emission nebulae often provide evidence of cloud fragmentation and protostars. Radio telescopes are used in studying the early phases of cloud contraction and fragmentation; infrared observations allow us to see later stages of the process. Many wellknown emission nebulae, lit by several O- and B-type stars, are partially engulfed by molecular clouds, portions of which are fragmenting and contracting, with smaller sites forming protostars.
The region around a black hole from which no radiation can escape is?
The event horizon (Schwarzschild radius)
Outline how the universe continually recycles matter through stars and the interstellar medium.
The processes of star formation, evolution, and explosion form a cycle that constantly enriches the interstellar medium with heavy elements and sows the seeds of new generations of stars. Without the elements produced in supernovae, life on Earth would be impossible.
What is the Helium Flash?
The rapid fusion of helium in the electron- degenerate helium core of a red giant.
At what stage of its life will our sun become a black hole?
The sun will never become a black hole.
Present the observations that help verify the theory of stellar evolution.
Theory of stellar evolution can be tested by studying star clusters. Massive stars leave MS first and so on.
What are gamma-ray bursts, and why have they posed such a challenge to current theory?
They are energetic bursts of gamma rays, They pose a challenge because they are very distant, and hence extremely luminous, but their energy originates in a region less than a few hundred kilometers across. There also appear to be two distinct types, with different energy generation mechanisms.
What are X-ray bursters?
They are neutron stars on which accreted matter builds up, then explodes in a violent nuclear explosion.
Relate the phenomena that occur near black holes to the warping of space around them.
To a distant observer, light leaving a spaceship that is falling into a black hole would be subject to gravitational redshift as the light climbed out of the hole's intense gravitational field. At the same time, a clock on the spaceship would show time dilation—the clock would appear to slow down as the ship approached the event horizon. The observer would never see the ship reach the surface of the hole.
What are the two classifications of supernovae?
Type I and type II
How can a "snapshot" of the universe today test our theories of the evolution of individual objects?(cc)
We assume that we observe objects at many different evolutionary stages, and that the snapshot therefore provides a representative sample of the evolutionary stages that stars go through.
Present the observational evidence for the occurrence of supernovae in our Galaxy.
We can see evidence of a past super nova in the form of a supernova remnant a shell of exploded debris surrounding the site of the explosion and expanding into space at a speed of thousands of kilometers per second.
How can astronomers "see" stars evolve in time?(cc)
We can't observe a single star evolve, but we can observe large numbers of stars at different stages of their lives, and hence build up an accurate statistical picture of stellar evolutionary tracks.
Outline the events that occur as a Sun-like star evolves from the main sequence to the giant branch.
When the central nuclear fires in the interior of a solar-mass star cease, the helium in the star's core is still too cool to fuse into anything heavier. With no internal energy source, the helium core is unable to support itself against its own gravity and begins to shrink. At this stage, the star is in the hydrogen-shell-burning phase, with a nonburning helium core surrounded by a layer of burning hydrogen. The energy released by the contracting helium core heats the hydrogen-burning shell, greatly increasing the nuclear reaction rates there. The star becomes much brighter, while the envelope expands and cools. A low-mass star like the Sun moves off the main sequence on the H-R diagram first along the subgiant branch and then almost vertically up the red-giant branch
Explain how white dwarfs in binary-star systems can become explosively active.
White dwarf draws hydrogen rich material from its companion. The gas spirals inward in an accretion disk and builds up on the white dwarfs surface eventually becoming hot and dense enough for hydrogen to burn
What is the physical size of a typical white dwarf star?
about the size of earth
Where are prostars located on the HR diagram ?
above and to the right of the main sequence
A star located on the asymptotic giant branch of the HR diagram?
after the horizontal branch
Stars in clusters have approximately the same?
age
Isolated main-sequence stars as massive as 10 to 12 times the mass of the Sun may still manage to avoid going supernova. Why?
because they can also have strong stellar winds
Our Sun, along with most of the stars in our neighborhood, probably formed?
billions of years ago.
Prestellar objects in which nuclear fusion never starts are referred to as ?
brown dwarfs
All stellars in a stellar cluster have roughly the same ?
distance
Two important properties of young neutron stars are?
extremely rapid rotation and a strong magnetic field.
In contrasting the distribution of the X−ray and gamma ray burst we find?
gamma ray bursts are far beyond our Galaxy, at cosmological distances, and spread all over the sky, not in the plane of our Galaxy.
What basic competitive process controls star formation?
gravity, makes interstellar cloud collapse and heat(pressure) collapse.
Most stars, probably all stars, formed?
in a cluster of stars.
When a star's inward gravity and outward pressure are balanced, the star is said to be?
in hydrostatic equilibrium.
Where was supernova 1987a located?
in our companion galaxy, the Large Magellanic Cloud
An iron core cannot support a star because?
iron cannot fuse with other nuclei to produce energy.
A solar mass star will evolve off the main sequence when?
it builds up a core of inert helium.
Virtually all the carbon-rich dust in the plane of the galaxy originated in?
low-mass stars.
For gravity to contract a spinning interstellar cloud, there must be sufficient?
mass
Most of the energy of the supernova is carried outward via a flood of?
neutrinos
An object more massive than the Sun, but roughly the size of a city, is a?
neutron star
List and explain some of the observable properties of neutronstar binary systems.
neutron star in a close binary system can draw matter from its companion,forming an accretion disk. The material in the disk heats up before it reaches the neutron star, making the disk a strong source of X-rays. As gas builds up on the star's surface, the star eventually becomes hot enough to fuse hydrogen. When hydrogen burning starts, it does so explosively, and an X-ray burster results. The rapid rotation of the inner part of the accretion disk causes the neutron star to spin faster as new gas arrives on its surface. The eventual result is a very rapidly rotating neutron star—a millisecond pulsar.
What can escape a black hole event horizon?
nothing
A surface explosion on a white dwarf, caused by falling matter from the atmosphere of its binary companion, creates what kind of object?
nova
what property of a type 1 supernovae makes them useful in determining distances?
peak brightness is consistent and very brilliant
The event horizon has a ?
radius equal to the schwarzchild radius
The "Helium Flash" occurs at what stage in stellar evolution?
red giant
The average density of neutron stars are?
similar to the density of an atomic nuclei
Why might we expect multiple episodes of star formation to occur in some locations?(cc)
star formation may be triggered by some external event, which might cause several interstellar clouds to start contracting at once. Alternatively, the shock wave produced when an emission nebula forms may be sufficient to send another nearby part of the same cloud into collapse.
Neutron stars have very?
strong bi-polar magnetic fields.
Stars whose core collapses into a neutron star or black hole precede their demise with a?
supernova
At temperatures of ________ K, photons can split apart nuclei until only protons and neutrons are left in photodisintegration
ten billion
Explain how the Sun will eventually come to fuse helium in its core, and describe what happens when that occurs.
when the nuclear fires in the interior of a solar mass cease, the helium in the star's core is still too cool to fuse into anything heavier. With no internal energy the core is unable to support itself against its own gravity and begins to shrink. This stage is known as hydrogen shell burning phase, with nonburning helium core surrounded by a layer of burning hydrogen. The energy released by the contracting heliumn core heats the hydrogen-burning shell, greatly increasing the nuclear reaction rates there. The star becomes much brighter, while the envelope expands and cools.
A solar type star will end its life as what kind of object?
white dwarf
The order of revolutionary stages of a star like the sun would be main sequence, giant, planetary neubla, and finally?
white dwarf
Why does a type 1 supernova show little or no hydrogen?
white dwarfs contain no hydrogen