Astronomy Exam 4

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UCF Cooney

Our sun is considered to be a...

low-mass star

Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?

a main-sequence star is hotter and dimmer than it was as a protostar

Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?

a few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage

We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?

brown dwarfs would outnumber all ordinary stars

Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?

fusion reactions are producing more energy more rapidly in a red giant star

Why can the fusion of carbon occur in intermediate and high-mass stars but not in low-mass stars?

it is because the cores of low-mass stars never get hot enough for carbon fusion

Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen---16, neon---20, and magnesium---24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?

at the end of a high-mass star's life, it produces new elements through a series of helium capture reactions

The following statements about various stages of core nuclear burning (hydrogen, helium, carbon, and so on) in a high mass star are true...

- as each stage ends, the core shrinks and heats further
- each successive stage is shorter and shorter
- each successive stage creates an element with a higher atomic number and atomic mass number
- as each stage ends, the reactions that occured in previous stages continue in shells around the core

Which event marks the beginning of a supernova?

the sudden collapse of an iron core into a compact ball of neutrons

Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?

Betelgeuse would remain a dot of light, but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime

A spinning neutron star has been observed at the center of a...

supernova remnant

Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?

planetary nebula formation is more common

Characteristics of low-mass stars:

-have longer lifetimes
-final corpse is a white dwarf
-end life as a planetary nebula
-the Sun is an example

Characteristics of high-mass stars:

-have higher fusion rate during main sequence life
-late in life fuse carbon into heavier elements
-end life as a supernova

According to current understanding, the two most abundant elements in the universe were made...

in the Big Bang (Hydrogen and Helium)

Elements with even atomic numbers tend to be more abundant than neighboring elements with odd atomic numbers. What nuclear process explains why this is the case?

Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional helium nucleus.

The observational data for the element abundances agree quite well with what we expect based on our current understanding of nuclear fusion and stellar evolution. But imagine the data had turned out to be different. Which of the following differences, if it had actually been observed, would have forced us to rethink our entire picture of stellar evolution?

the abundance of elements heavier than uranium turned out to be greater than the abundance of carbon

The following statements are true of degeneracy pressure...

-degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold
-degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives
-electron degeneracy pressure can only be created by interactions among the electrons in an object, but not of neutron degeneracy pressure
-black holes form when gravity overcomes degeneracy pressure

The more massive a white dwarf, the....

smaller its radius

What prevents a white dwarf from having a mass greater than the white dwarf limit (or Chandrasekhar limit)?

electron degeneracy pressure depends on the speeds of electrons, and as a white dwarf's mass approaches the white dwarf limit, its electron speeds are already approaching the speed of light

The white dwarf that remains when our Sun dies will be mostly made of...


The following statements are true about accretion disks...

-the gas in the inner parts of the disk is hotter htan the gas in the outer parts of the disk
-the gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk
-the primary factor is NOT mass, but rather it is whether the white dwarf is in a close binary system where gas from its companion can spill over
-accretion disks are made primarily of hydrogen and helium gas

According to present understanding, a nova is caused by...

a fusion of hydrogen on the surface of a white dwarf

The following statements are true about differences between novae and supernovae...

-novae are much less luminous than supernovae
-supernovae and novae eject gas into space, often creating a nova remnant that remains visible for many years
-novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems
-the same star can undergo novae explosions more than once, but can undergo only a single supernova

Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?

no, because it is not orbited by another star

Which of the following best describes what would happen if a 1.5 solar mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your home town?

the entire mass of the Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star

The Voyager spacecraft has a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why do you think the scientists chose pulsars rather than some other type of star?

pulsars are easy to identify by their almost perfectly steady periods of pulsation

The following statements about pulsars are true...

-all pulsars are neutron stars, but not all neurtron stars are pulsars
-any neutron star may appear to us as a pulsar only if it has beams of radiation sweeping by us with each rotation, regardless of whether it is in a close binary system
-a pulsar must have a very strong magentic field and rotate quite rapidly
-pulsars are kept from collapsing by neutron degeneracy pressure

How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?

the accretion disk around a neutron star is much hotter and emits higher-energy radiation then the accretion disk around a white dwarf

The following statements concerning black hole masses and Schwarzschild radii are true...

-for black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers
-the more massive the black hole, the larger the Schwarzschild radius
-even an object as small as you could become a black hold if there were some way to compress you to a size smaller than your Schwarzschild radius
-there is no theoretical limit to the size of a black hole

Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?

time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted, as predicted by general relativity

The following statements about black holes are true...

-although we are not 100% certain that black holes exist, we have strong observational evidence in favor of their existance
-if you watch someone else fall into a black hole, you will never see them cross the event horizon. however, they will fade from view as the light they emit (or reflect) becomes more and more redshifted
-if you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon
-a spaceship might pass quite safely by the black hole at a distance of 10,000 kilometers, but at 10,000 kilometers from the center of the main-sequence star the spaceship would be deep inside the star. (Of course, the spaceship would be destroyed long before it got that close to the star's center.)

When we see X rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would force us to immediately rule out the possibility of a black hole?

sudden, intense, X-ray bursts (x-ray bursts take place on the surface of a neutron star. a black hole has no surface, and hence cannot have X-ray bursts)

If a neutron star or black hole is accreting material from its companion in a close binary, which of the following observatories would offer us the best chance for discovering this fact?

Chandra X-Ray Observatory

The following statements about gamma ray bursts are true...

-based on their distribution in the sky, we can rule out a connection between gamma ray bursts and X-ray binaries in the Milky Way galaxy
-gamma ray bursts are apparently among the most luminous events that ever occur in the universe
-gamma ray bursts were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth
-in at least a few cases, we have detected the sources of gamma ray bursts in other wavelengths. indeed, it was such observations that convinced astronomers that the bursts occur in distant galaxies

Imagine that an advanced civilization lives on a planet orbiting at a distance of 10 AU (1,500 million kilometers) from a close binary star system that consists of a 15 Msun red giant star and a 10 Msun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed. Why is this?

the red giant will probably supernova within the next million years

Consider again the civilization described in the previous question. (They live on a planet orbiting at a distance of 10 AU from a close binary star system that consists of a 15 Msun red giant star and a 10 Msun black hole surrounded by an accretion disk.) One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least cause significant damage)?

the X-rays from the accretion disk (the accretion disk radiates prodigious amounts of X rays, as evidenced from the fact that we see these types of systems as X-ray binaries. they will kill him long before he begins to feel effects from tidal forces)

Consider again the civilization described in the previous question. (They live on a planet orbiting at a distance of 10 AU from a close binary star system that consists of a 15 Msun red giant star and a 10 Msun black hole surrounded by an accretion disk.) Through a bizarre (and scientifically unexplainable) fluctuation in the space-time continuum, a copy of a book entitled Iguoonos: How We Evolved appears on your desk. As you begin to read, you learn that the book describes the evolution of the people living in the star system described above. In the first chapter, you learn that these people evolved from organisms that lived 5 billion years ago. Which of the following statements should you expect to find as you continue to read this book?

they evolved on a different planet in a different star system, and moved to their current location (the total lifetime of the massive stars in their system is only a few million years. if they evolved 5 billion years ago, then they must have moved to their current location)

Although from afar we'll never see an in-falling rocket cross the event horizon, it will still eventually disappear from view. Why?

its light will become so redshifted that it will be undetectable

If you were inside a rocket that falls toward the event horizon, from your own viewpoint you would...

accelerate as you fall and cross the event horizon completely unhindered

Characteristics of a white dwarf supernova...

-star explodes completely, leaving no compact object behind
-can only occur in a binary system
-can occur in a very old star cluster
-has a brighter peak luminosity
-spectra always lack strong hydrogen lines

Characteristics of a massive star supernova...

-can only occur in a galaxy with ongoing star formation
-black hole or neutron star left behind

The following observations fit in with our current understanding of supernovae...

-a massive star supernova leaves behind no detectable compact object
-a massive star in a binary system explodes
-a white dwarf supernova in a galaxy of only old stars
-two massive star supernovae occur in the same young star clusters

The following observations do not fit in with our current understanding of supernovae...

-an isolated star like our Sun explodes as a white dwarf supernova
-a young (5 million years) star explodes as a white dwarf supernova

Why do we call dark matter "dark"?

it emits no radiation that we have been able to detect

Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?

there could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did NOT contain dark matter. How would their rotation curves be different?

the orbital speeds would fall off sharply with increasing distance from the galactic center (this is what happens to the rotation curve of objects orbiting a centralized mass, such as the planets in our solar system)

How does gravitational lensing tell us about the mass of a galaxy cluster?

using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which is distorts the light of galaxies behind it

If WIMPs really exist and make up more of the dark matter in galaxies, the following are all their characteristics...

-they are subatomic particles
-they can neither emit nor absorb light
-they tend to orbit at large distances from the galactic center

Why isn't space expanding within clusters of galaxies?

their gravity is strong enough to hold them together even while the universe as a whole expands

The following statements about galaxies and large-scale structures such as voids, clusters, superclusters, sheets, and filaments are...

-galaxies and clusters and the structures in which they are embedded have grown around tiny density enhancements that were present in the early universe
-voids began their existance as regions in the universe with a slightly lower density than the rest of the universe
-many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members
-clusters and superclusters seem to lie on great chains and sheets, suggesting they fall along a "framework" that was determined in the very early universe

Based on current evidence, a supercluster is most likely to have formed in regions of space where...

the density of dark matter was slightly higher than average when the universe was very young

According to current evidence, how does the actual average density of matter in the universe compare to the critical density?

the actual density, even with dark matter included, is less than about a third of the critical density

Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?

dark energy does not exist and there is much more dark matter than we are aware of to date

What are the important properties of stars?

luminosity, temperature, mass

What powers stars?

-the sun generates energy via nuclear fusion reactions
-Hydrogen is converted into Helium in the Sun's core
-the mass lost in this conversion is transformed into energy
-the amount of energy is given by Einstein's equation: E=mc^2
-given the Sun's mass, this will provide enough energy for the sun to shine for 10 billion years

Describe the life cycle of stars.

1. main sequence stars convert H -> He in their cores
-the star is table, in balance (gravity vs. pressure from H fusion reactions)
2. the core begins to collapse
-H fuel in core runs out
-H shell heats up and H fusion begins there
-there is less gravity from above to balance this presure so the outer layers of the star expand
3. as the core is collapsing/heating it is called a Red Giant
-the He core collapses until it heats to 10^8 K
--He fusion begins (He->C)
--sometimes called the "triple-a process"
--it is now a helium core-fusion star
4. when the star exhausts its He fuel
-the C core collapses
-low & intermediate-mass stars don't have enough gravitational energy to heat to 6x10^8 K (temperature where Carbon fuses)
5. the He & H burning shells overcome gravity
-the outer envelope of the star is gently blown away
-this forms a planetary nebula
-the core becomes a white dwarf, eventually cooling into something like a planet-sized diamond

How do stars of different masses live differently?

-high-mass stars live much shorter lives than low-mass stars
-high-mass stars can fuse elements heavier than Carbon
-high-mass stars are far less numerous than low-mass stars

What are the different results of the deaths of stars of different masses?

-high-mass stars die as a supernova; low-mass stars die as a planetary nebula
-high-mass stars may leave either a neutron star or a black hole behind; low-mass stars leave a white dwarf behind

Describe the battle between gravity and pressure at the end of the lives of stars.

-the life of any star can be described as a battle between gravity and pressure
-gravity always wants to collapse the star
-pressure holds up the star (the type of star is defined by what provides the pressure)

What are the various types of galaxies and how do they differ?

-spiral: contain large amounts of gas and dust (thus produce many young stars), barred, unbarred, similar structure to our own Milky Way
-elliptical: contain little gas and dust (consist mostly of older stars), great ranges in sizes (dward ellipticals- 10 million stars, giant ellipticals- trillions of stars)

Describe the battle between the two main influences on the evolution of the universe as a whole. (gravity, expansion)

-gravity usually tries to collapse things
-expansion in which everything was given an initial push

What might be an eventual fate of the universe?

-too much stuff (closed universe, eventual recollapse: Big Crunch!)
-just right (critical universe, Big Freeze)
-not enough stuff (open universe, Big Freeze)

Neutron Degeneracy Pressure

supports neutron stars (created when high-mass stars die)

Electron Degeneracy Pressure

supports white dwarfs (created when low-mass stars die)

Where do gamma-ray bursts occur?

outside of our solar system

What can happen to a white dwarf in a close binary system?

-can acquire hydrogen from its companion through an accretion disk that swirls toward the white dwarf's surface
-as hydrogen builds it may begin nuclear fusion and cause a nova
-in extreme cases, accretion may continue until the white dwarf limit is exceeded and it will explode as a white dwarf supernova

What can happen to a neutron star in a close binary system?

-can accrete hydrogen from their companions, forming dense, hot accretion disks
-the hot gas emits strongly in X rays, so we see these systems as X-ray binaries
-in some systems, frequent bursts of helium fusion occur on the neutron star's surface, causing x-ray bursts

How does the balance between pressure and gravity act as a thermostat to regulate the core temperature?

-a drop in core temperature decreases fusion rate, which lowers core pressure, causing the core to contract and heat up
-a rise in core temperature increases fusion rate, which raises core pressure, causing the core to expand and cool down

How does the balancing point between pressure and gravity depend on a star's mass?

-in high-mass stars it results in a higher core temperature, a higher fusion rate, greater luminosity, and a shorter lifetime
-in low-mass stars it results in a cooler core temperature, a slower fusion rate, less luminosity, and a longer lifetime

What kind of gas cloud is most likely to give birth to stars?

a cold, dense gas cloud

Approximately what core temperature is required before hydrogen fusion can begin in a star?

10 million K

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