Upgrade to remove ads
Chapter 18 Astronomy
Terms in this set (30)
When mass transfer from an evolved companion onto a white dwarf occurs, why is an accretion disk formed?
It is formed due to the laws of conservation of angular momentum. The white dwarf is such a small target. The donor star's mass has some initial angular momentum as it falls toward the white dwarf, and unless it's "aimed" perfectly, it will "miss" the white dwarf and orbit it, forming a disk.
What are the key characteristics of a white dwarf star?
It's the leftover core of a dead low-mass star (after the star explodes in a planetary nebula). It's hot when it first forms (100,000 K), cools over time. Made of electron degenerate matter. About the size of Earth, but gets smaller as more mass is added to it
When the Sun becomes a white dwarf, of what will it primarily be made? Can a white dwarf be made of iron?
Carbon. No, iron is only created inside high-mass stars, and those explode in a supernova and form neutron stars.
Which is larger in radius: A 1MSun white dwarf or a 1.3MSun white dwarf?
A 1 solar mass white dwarf. Degenerate matter shrinks as more mass is added.
What is the Chandrasekhar Limit?
1.4 solar masses. Above that, and the white dwarf cannot support itself (via electron degeneracy pressure) against the crushing force of gravity and can explode in a supernova.
What are the similarities and differences between a protostellar disk and a white dwarf accretion disk?
Protostellar disks and white dwarf accretion disks are similar in that they are formed due to the conservation of angular momentum, and also particles in them obey Kepler's Laws. They are different due to the fact that a white dwarf is more dense and compact, which causes the disk to be hotter.
How do novae occur? How do they compare to supernovae? Can they recur?
Novae occur when an evolving star puffs up, becomes a red giant and donates some of its loosely bound, hydrogen-rich atmosphere to a compact binary companion white dwarf. The hydrogen builds up on the surface of the white dwarf, where gravity is quite intense and compresses the gas to a high density. The temperature increases to 10 million K, which is the triggering temperature for hydrogen fusion. When hydrogen fusion erupts on the surface, it causes a runaway reaction and subsequent explosion that we call a nova. They are much less luminous than supernovae. They can recur on timescales of every 1,000 to 100,000 years. The more massive the white dwarf, the shorter the time between recurrence.
How is a white dwarf supernova different from a massive star supernova?
White dwarf supernovae occur when either 1) two white dwarfs collide and push each other over the Chandrasekhar limit of 1.4 solar masses, or 2) an evolving star donates mass onto its binary white dwarf companion. A massive star supernova occurs when a high-mass star collapses and explodes.
The two types of supernovae also brighten and fade over time slightly differently, which is important to know if using them as a standard candle.
Why are white dwarf supernovae useful as fudge? Do they have any disadvantages?
They are standard candles and they are luminous and can be seen across the Universe. The disadvantage is that they occur randomly and you have to know which type they are.
About how large is a neutron star? What range of masses can it have?
They are about 15 km in radius (about the size of a small city). The masses range from between 1.4 to 3 solar masses.
How were the first exoplanets discovered in 1992?
Exoplanets were found orbiting a pulsar located in the constellation Virgo. They were found by studying the delay in timing of the pulses due to the gravitational effects of the planets on the pulsar's rotational speed.
How fast can neutron stars spin? How can a neutron star's spin rate change?
They can spin 625 times per second and change their rotation speed by adding mass from a companion.
Define 'inertial reference frame' and 'spacetime'?
A frame of reference in which a body with zero net force acting up on it is not accelerating. Such a body is at rest or moving at a constant speed in a straight line.
Under what circumstances is it necessary to take into account relativistic effects?
When the speed of the object is approaching a significant fraction of the speed of light.
What is the speed of light? What can go faster than it? What always goes the speed of it (in a vacuum)?
186,282 miles/second, or 3x10^8 m/s. Nothing. Light
How does the boxcar experiment illustrate that the time between two events depends on the observer's frame of reference?
The boxcar experiment illustrates the relativity of the timing of events by showing an experiment whereby the time between events changes depending on what reference frame you're in. Event 1 is the lightbulb turning on, and event 2 is the light being detected back at the detector. Depending on the frame of reference, the observer sees the path taken by the light to be longer or shorter, and therefore sees the time between events as longer or shorter. This is due to the fact that light has a finite travel speed.
Do moving clocks (clocks that appear to be moving with respect to your frame of reference) count slower or faster? What about moving rulers?
Moving clocks count slower. Moving rules are shorter.
1What is the Lorentz Factor? If given a table of values, could you use it to quantify length contraction and time dilation like in the examples with dog and cat in the slides
The Lorentz factor is the number by which one multiplies the time/length of an object to quantify how much that time is dilated/length is contracted.
Why is our observation of muons on the surface of the Earth only explainable using the concepts of time dilation and length contraction?
Muons are subatomic particles that are created by collisions in our upper atmosphere. They travel at speeds close to the speed of light downward toward Earth's surface. Their lifespan, according to classical physics, should cause them to decay before reaching the ground, and we should not be able to detect them from the surface of Earth. However, we do detect muons at the surface. Their detection is due to the effects of relativity. In Earth's reference frame, the muons's clocks are counting slower. In the frame of reference of the muon, the Earth's surface appears to be approaching it at nearly the speed of light. Because moving rulers are shorter, the distance between the upper atmosphere (where they're created), and the surface, is length contracted.
Briefly summarize the twin paradox and its resolution
Two twins start out on Earth. One gets into a spaceship and moves away at relativistic speeds, turns around, and then returns some time later. From the frame of reference of the twin on Earth, the twin in the spaceship looks like she's moving. From the frame of reference of the twin in the spaceship, the twin on Earth looks like she's moving. Because both perceive the other as being the one in motion, and because moving clocks count more slowly, it is unclear which twin will have had less time pass.
The resolution is that special relativity only applies when the reference frames are inertial (not accelerating). Because the twin in the spaceship was accelerating (and therefore, changing reference frames many times), she is the one who is younger when she returns.
What is the difference between special and general relativity?
Special relativity only applies to situations with inertial reference frames. General relativity applies even in accelerating reference frames (changing speeds, changing directions, or near a massive object).
What is the Equivalence Principle?
The Equivalence Principle is a basic postulate of general relativity, stating that at any point of space-time the effects of a gravitational field cannot be experimentally distinguished from those due to an accelerated frame of reference.
What is the rubber sheet analogy? What are its limitations?
The rubber sheet analogy is a way to visualize how mass distorts spacetime. It is limited by the fact that it is a 2D representation of a 4D phenomenon, and can cause one to gain an over-simplified understanding of spacetime distortion.
What happens to a person who is falling into a black hole? What does an outside observer see, and what does the person who is falling see/experience? Why might you rather fall into a supermassive massive black hole than a stellar mass black hole?
They get torn apart (spaghettified). An outside observer would see person appearing redder and their clock slowing down. A supermassive black hole has a more gently sloped spacetime distortion (the tidal force would not be as strong). This gentler slope would allow you to get much closer to the black hole (perhaps even within the event horizon) before being torn apart.
Does time count faster or slower on Everest compared to sea level?
Clocks that are deeper within gravitational wells are slower. Therefore, clocks at the height of Mount Everest would have gained 39 hours over the 4.6 billion years that Earth has existed.
What are observational pieces of evidence of relativity?
Gravitational lensing, GPS, precession of Mercury's orbit.
What experiment first detected gravitational waves? What produces gravitational waves?
LIGO (Laser Interferometry Gravitational-wave Observatory). Gravitational waves are produced when massive objects are accelerated, such as two neutron stars merging.
What is a black hole?
It's a hole in spacetime, a singularity, an object of infinite density, an object so dense that its escape velocity exceeds the speed of light.
Define event horizon and Schwarzschild radius. If the Sun were turned into a black hole, how large would its Schwarzschild radius be? Would Earth be sucked in?
The event horizon is an imaginary sphere around the black hole that represents the surface where the escape velocity reaches the speed of light. The Schwarzschild radius is the radius of the event horizon. If the Sun were compressed down into a black hole, its Schwarzschild radius would be 3 km. Earth would continue orbiting normally because the Sun's mass didn't change, just its density. Mass is what governs the force of gravity between them.
Why is Cygnus X-1 considered some of the best observational pieces of evidence that black holes truly do exist in nature?
Cygnux X-1 is a binary system in which one of the members is a high-mass evolved star, and the other is a compact, unseen massive companion. The flickering of the light from the accretion disk around the unseen companion, as well as the emission of X-rays from the inner part of the hot accretion disk, proves that it is very compact. The fact that it is compact and massive constrains its density such that it must be a black hole.
YOU MIGHT ALSO LIKE...
Chapter 18: The Bizarre Stellar Graveyard
Astronomy Midterm II
Astro Test 3
OTHER SETS BY THIS CREATOR
busa midterm 3
BUSA 2106 Midterm 1
astro bullshiz final