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Question

What is the theorized maximum mass that a star’s core can have right before collapse in order for it to
become a neutron star and not a black hole (aka what is the maximum mass of a neutron star)?
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We like to say that black holes have no hair. What are the two quantities that describe astronomical
black holes?
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Exam 2 Review

ASTR 2030

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What is the theorized maximum mass that a star’s core can have right before collapse in order for it to become a neutron star and not a black hole (aka what is the maximum mass of a neutron star)? The maximum mass of a neutron star is thought to be 3 MSun.
We like to say that black holes have no hair. What are the two quantities that describe astronomical black holes? The two quantities that describe astronomical black holes are mass and angular momentum (in general we’d also care about charge but we think astronomical black holes have no charge).
What does the Schwarzschild radius describe, and what parameter (that can change) does it depend on? The Schwarzschild radius describes the radius of the event horizon for a black hole, and it is only dependent on the black hole’s mass
Describe the methods of finding stellar mass black holes. 1: If we see things orbiting around a compact object that has mass > 3 MSun then it's a black hole 2: We can measure some unique properties of black holes: event horizon, gravitational lensing 3. Measure gravitational waves from merging black holes
et’s say you have a black hole laying around that you can magically put wherever you want. Describe how you could use it to help you see a really faint star. By placing the black hole between you and the star, the star will appear brighter due to gravitational lensing focusing the light
Algol is a binary star system in which the two stars are a massive blue star on the main sequence and a less massive star becoming a red giant. Explain why this is weird and also how to make sense of it The more massive the star, the faster it evolves. In Algol its the opposite because the two stars evolved close to each other and the now more massive star star stole mass from the now smaller star.
Does matter typically fall directly onto a neutron star or black hole? Why or why not? It usually does not as the matter typically has angular momentum, which causes it to “swirl” around the object and form an accretion disk.
What is the environment and mechanism by which stellar mass black holes (or neutron stars) produce X-rays? Stellar mass black holes (or neutron stars) that produce X-rays are typically in binary systems where mass is being transferred onto the black hole (or neutron star) from its companion. This gas is heated up by friction in the accretion disk
Compare the velocity, angular momentum, temperature, and type of light emitted from material at the outer and inner edges of an accretion disk. Near the outer edge of the disk has low velocity, high angular momentum, low temperature, and emits ultraviolet or visible light while material near the inner edge of the disk has high velocity, low angular momentum, high temperature, and X-ray light
Describe the flow of energy as gas falls towards a compact object to form an accretion disk. Gravitational Potential Energy ⇒ Kinetic Energy ⇒ Heat (via friction) ⇒ Radiation
What is a key way to differentiate a neutron star from a black hole when observing how gas interacts with them? A neutron star could have surface phenomena (e.g. thermonuclear bursts from gas hitting the surface), while a black hole would not
About how many stars are there in a large galaxy (e.g. the Milky Way) and about how many galaxies are there in the observable Universe? There are about 200 billion (2 × 10^11) of each
How many stars are there in the observable Universe then? # Stars = (# Galaxies) ∗ (# Stars/Galaxy ) = (2 × 10^11 Galaxies) ∗ (2 × 10^11 Stars/Galaxy ) = 4 × 10^22 Stars
Were all galaxies initially formed at the same size they are today? If not, how did they change over time? Not all galaxies formed at the size they are today. Galaxies merge with each other, which builds up larger galaxies. The largest galaxies have undergone many mergers.
Andromeda and the Milky Way will eventually merge, and what will happen to the Sun and Solar System when this happens? The Sun will probably move to a new location in the new galaxy, but still stay within the new galaxy. The planets in the Solar System will still orbit the Sun as usual.
What are the three main components of the Milky Way? 1) The Bulge: 1 kpc radius, mainly stars 2) The Disk: 25 kpc radius, gas, dust, and stars 3) The Halo: Spherical in shape, contains globular clusters
What is “the best” type of light to view our galaxy in and why? The infrared (and radio) allows us to see through the dust of our galaxy while still being able to see most of what we would see in the optical (minus dust)
What does adaptive optics allow us to do? It helps to reduce the effect of atmospheric turbulence on our ground based telesope images. This means that we can get a sharper image of objects that are close together without having to go to space
What is at the center of our galaxy and how can we see it? There is a supermassive black hole (Sagittarius A*) at the center of our galaxy. We can see it in the radio and X-ray, and we can also infer information about it (such as its mass) by observing the orbits of stars near it.
How can we measure the mass of our closest supermassive black hole? Our closest supermassive black hole is Sagittarius A*. We can look at the orbits of stars near it and measure their velocities and orbital radii, and this information can give us the mass of the central object using M = rv^2/G
How do we know that the object at the galactic center is not a cluster of stars or a cluster of neutron stars? A cluster of stars would produce infrared light, which we don’t see. A cluster of neutron stars would collide quickly, forming a black hole.
How massive are supermassive black holes compared to our Sun? They are about a million to a billion times more massive (10^6 − 10^9 MSun).
Sometimes supermassive black holes have gas nearby which isn’t being accreted onto the black hole. Give two reasons that could explain this. 1) The event horizon is much smaller than the distance to these gas reserves; therefore if the gas has a lot spin it won’t be accreted directly. 2) If the gas cannot cool effectively (due to low accretion rate and low density), it can heat up and escape
What is the difference between a supermassive black hole, active galactic nucleus (AGN), and a quasi- stellar radio source (quasar)? An AGN is a supermassive black hole that is accreting enough material to produce a considerable amount of light; quasars are a special type, very bright, of AGN. So not all AGN are quasars and not all supermassive black holes are AGN
What is redshift and what is the main cause of redshift for extremely distant galaxies? Redshift is the change of the wavelength of light, making it longer (redder). The main cause of this for distant objects the expansion of the Universe.
What is the typical efficiency for mass to energy conversion in the accretion disk of a black hole and how does it compare to fusion? Black hole accretion disk efficiency is about 10% while fusion efficiency is about 0.7%
Imagine that a black hole is accreting gas at a rate of 1000 MSun/year. What is its luminosity? L = E ̇Mc^2 = 0.1 ∗ (1000MSun/year) ∗ (3 × 10^8m/s)^2 = 0.1 ∗ (1000 ∗ 2 × 10^30kg/3.16 × 10^7s ) ∗ (3 × 10^8m/s)2 = 5.7 × 10^41W
How can we measure the masses of supermassive black holes in nearby galaxies? We measure the average orbital velocity of masers or gas or stars (instead of looking at specific stars) as a function of the distance from the center and calculate the mass using M = rv^2/G
What is the mass of a black hole where the orbital velocity v = 880km/s for a radius of r = 4 × 10^15m? M = rv2/G = 4.0 × 10^15 m (8.8 × 105 m/s)^2/6.67 × 10^−11 m^3 kg^−1 s^−2 = 4.6 × 10^37 kg = 2.3 × 10^7 Msun
What type of object gives us some of the closest orbital velocity measurements to nearby supermassive black holes? Give a rough explanation of what this object is We use masers, which are collections of water molecules that emit microwaves
What type of objects do Professor Comerford and Aimee and the rest of their group study? They study AGN, particularly AGN in merging galaxies and AGN feedback.
If AGN aren’t always active, what causes them to switch off (and back on)? A supermassive black hole's activity is dependent on the amount of mass is available for it to accrete. This amount of material varies over time (e.g. galaxy mergers bring together a lot of gas, so the black holes are often active in galaxy mergers)
What are gravitational waves and what creates them? Give an example. They are perturbations to the curvature of spacetime that propagate at the speed of light. They are caused by the non-axisymmetric motions of massive bodies. Examples: merging black holes, spinning bumpy neutron star, etc.
How did binary pulsars first prove the existence of gravitational waves (and win people the Nobel Prize)? Pulsars are very accurate and reliable clocks, so we can see the time it takes the pulsar to complete an orbit with its companion. If we see the orbital time decrease, it means the radius of the orbit shrank + lost energy due to gravitational waves.
How can we directly detect gravitational waves? We can measure how much two right angled long arms are stretched/squished when a wave passes by. This is how LIGO works. LISA is the same but with a triangle of three arms in space. The ’arms’ are the distance between the Earth and a pulsar!
What are gamma ray bursts and what creates them? Bursts of gamma rays (0.01-1000 s) originate from distant galaxies. They are believed to be caused by neutron star mergers or the collapses of stars > 40 MSun. These events create beams of gamma rays; we see gamma ray bursts when the beam is pointed at us
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