Astronomy Final

How can we relate the redshift of an object and the density of the universe when that light was emitted?

(1+z)^3 is the factor by which the mass density of the universe has increased in the time it's taken the light to travel to us.

What is the expression for redshift?

* (observed wavelength - rest wavelength) / rest wavelength = v/c for small values

How can we relate the redshift of an object and the size of the universe when that light was emitted?

1 + z is the factor by which the universe has expanded in the time it's taken the light to travel to us. In the time it has taken for certain light to reach us, the universe has already expanded considerably. z = redshift

What are the pieces of evidence that we have for what is at the center of the active galactic nucleus?

1) Large mass: object at the center of the AGN is 10^6 to 10^9 times the mass of the sun 2) Small size 3) Incredibly bright luminosity

What percentage of mass/energy in the universe is dark matter?

23% of mass/energy in the universe is dark matter.

What percentage of mass/energy in the universe regular matter?

4% of the mass/energy in the universe is regular matter.

What percentage of mass/energy in the universe is dark energy?

73% of mass/energy in the universe is dark energy.

How much dark matter exists in the universe?

85% of all matter in the universe is dark matter. Dark matter represents about 25% of the critical density of the universe.

What is a MACHO? Why do we think these aren't the leading cause of dark matter?

A MACHO is a "massive compact halo object." Theoretically, these objects, which include low-mass M dwarfs, brown dwarfs, and jovian planets, should be "dark," ordinary matter in galaxies. We believe these aren't the leading cause of dark matter, however, because the number of MACHOS does not account for all the dark matter in the Milky Way.

What is a Seyfert galaxy?

A Seyfert galaxy is an active galaxy containing hydrogen and oxygen emission lines.

What is a WIMP?

A WIMP is a "Weakly Interacting Massive Particle." These particles are theoretical; they are yet to be discovered. While they would be massive enough to exert gravitational influence, they wouldn't emit light or be bound to any charged matter which could emit light. Because WIMPS are weakly interacting, they wouldn't collapse with a galaxy's disc. They would remain gravitationally bound in the galaxy's halo.

What is a black hole?

A black hole is a region of space in which the matter is so dense that nothing can escape from it - not even light.

What is a core collapse supernova?

A core collapse supernova is an explosion produced by the sudden collapse of neutrons to the center of a high mass star. The supernovae occur once nuclear fusion produces an iron stellar core.

What is a giant star? A subgiant?

A giant star is a star with a larger radius and cooler temperature that is 10-100 times the size of the present star. A subgiant is a star whose envelope is beginning to expand and cool as it drifts away from the main sequence.

What is a neutron star? How do they form?

A neutron star is a compact ball of neutrons supported by degeneracy pressure. Neutron stars are formed during supernovas of high mass stars.

What is a planetary nebula?

A planetary nebula is the collection of a star's outer layers that are pushed away from the star following the second red giant phase.

What is a pulsar, and how do they form?

A pulsar is a rapidly rotating neutron star characterized by strong magnetic fields and beams of radiation streaming from the poles of the star. Pulsars are formed from the collapse of neutrons in high mass stars once the star's core is iron.

What is a quasar, and how were they first discovered?

A quasar is a point-like, unresolved nucleus similar to the image of a star. Quasars have strong emission lines. They were first discovered when a few radio sources matched with optical counterparts that appeared stellar in the late 1950s.

What is a radio galaxy?

A radio galaxy is an active galaxy in which spiral galactic components merge with elliptical galactic components, and emit a powerful radio jet

What is a singularity?

A singularity results from gravity crushing all the matter into a single point.

What is a standard candle?

A standard candle is an object whose luminosity is known and can be used as a means to measure distance.

What is a star doing when it is on the main sequence?

A star is fusing hydrogen into helium in its core when it is on the main sequence.

What is a supernova?

A supernova is an explosion of either a white dwarf or high mass star.

What is a white dwarf supernova, and how do they occur?

A white dwarf supernova is an explosion catalyzed by sudden carbon fusion. These supernovae occur when accretion in a binary star system takes the mass of a white dwarf above the 1.4 M Sun Chandrasekhar limit.

What is an active galaxy?

Active galaxies are galaxies whose nuclei are incredibly luminous and contain rapidly rotating gas clouds at their galactic centers.

What makes active galaxies interesting?

Active galaxies are interesting because they are rare: only 1% of all galaxies are considered active galaxies.

How were active galaxies first discovered?

Active galaxies were first discovered by Carl Seyfert when he found that a small fraction of galaxies have nuclei whose spectra show high ionization emission lines.

What causes the different types of active galactic nuclei we've seen? How does this model explain the huge active galactic nuclei luminosities observed?

Actively accreting (feeding) black holes cause the different types of active galactic nuclei we've seen. This model explains the huge galactic nuclei luminosities we've observed because only matter falling into a black hole releases that much energy from such a small volume.

What is adaptive optics, and why do we need adaptive optics to see the center of the galaxy?

Adaptive optics improves images by an order of magnitude, which is critical for measuring the positions of distinct stars in the tiny region at the Galactic center.

What is an X-Ray binary?

An X-Ray binary is a binary system that consists of a main sequence star and either a neutron star or black hole. The two are close enough so that mass pulled from the main sequence star into the neutron star/black hole.

What happens as a high mass star uses up the available fuel?

As a high mass star uses up the available fuel the star's core goes through successive stages of burning heavier elements until an inert iron core is reached.

What makes up the density of the universe?

Baryonic (normal) matter, like stars and gas, as well as dark matter make up the density of the universe.

What does the study of galaxy rotation say about dark matter?

Because stars in the outer parts of galaxies rotate just as fast as stars in the inner parts of galaxies, then based on our understanding of gravity, there must be dark matter that is causing the outer parts to move at the same speed as the inner parts.

How do we study galaxies back in time in the early universe?

By looking at galaxies that are farther away, we are able to look at galaxies back in time because light travels at a constant, finite speed.

What are Cepheid variables? How can they be used to measure distances?

Cepheid are incredibly bright, pulsating stars with pulsation ranges varying from one to 100 days. Because the pulsation period of a cepheid is closely related to its luminosity, we can find the flux. By comparing luminosity and flux, we can find the relative distance of the Cepheid. d = square root (L/(4piF)) - d is distance - L is luminosity - F is flux

What is Cosmic Microwave Background (CMB) radiation?

Cosmic Microwave Background radiation is leftover radiation from the Big Bang. CMB helps to explain the geometry of the universe (flat) and the small non-uniformities of the early universe.

What is cosmological redshift?

Cosmological redshift is redshift due to the expansion of the Universe, which stretches the wavelength of light passing between galaxies. Galaxies are not moving through space - it's space itself that is actually expanding!

What is Cygnus X-1 and what does it provide evidence for?

Cygnus X-1 is an X-Ray binary source, providing evidence for the existence of black holes.

What is dark energy?

Dark energy is a form of repulsive energy equivalent to negative gravity.

How does dark energy affect the expansion of the universe?

Dark energy, acting as negative gravity, facilitates the accelerated expansion of the universe. The universe's expansion is getting faster!

What is dark matter?

Dark matter is matter content that neither emits nor absorbs light. It can be observed through its gravitational effect on stars and galaxies.

What does it mean for matter to be degenerate?

Degenerate matter is not affected by or dependent upon temperature. Degenerate matter also responds differently to pressure. The radius of degenerate matter decreases as the pressure increases.

What is differential rotation?

Differential rotation is rotation in which material rotates with constant velocity, so angular velocity decreases at larger radii and therefore have a longer period of rotation.

What do white dwarfs do during their lifetime?

During their lifetime, white dwarfs just gradually cool off and get dimmer until they become black.

What were some early theories as to where we were in the Milky Way?

Early theories as to where we were in the Milky Way placed us near the center of the galaxy.

What type of pressure supports a white dwarf?

Electron degeneracy pressure supports a white dwarf.

What are the properties of elliptical galaxies?

Elliptical galaxies -have no disc component, only a spheroidal component -contain very little interstellar medium -appear red (contain mostly red stars) -motions of stars are not disc-like; they are more random

How are galaxies classified?

Galaxies are classified as spiral, elliptical or irregular.

What type of hydrogen fusion occurs in high mass stars?

Hydrogen fusion via the CNO (Carbon, Nitrogen, Oxygen) cycle occurs in high mass stars due to hotter temperatures.

What does it mean to say that the universe's expansion is bound or unbound?

If the universe's expansion is bound, then at some point the universe will stop expanding and then contract. Density > critical density If the universe's expansion is unbound, then the universe will continue to expand forever. Density < critical density.

How do objects move in the disk in comparison to the bulge or halo of the Milky Way?

In the disk of the Milky Way, objects move in nearly circular rotation around the galactic center. In the bulge and halo, objects move in random, elliptical orbits.

What are the properties of irregular galaxies?

Irregular galaxies -are neither spiral nor elliptical -appear blue/white -contain an interstellar medium -were more common when the universe was young -are found generally at farther distances

What is the equation for the luminosity of an AGN based on its mass accretion rate?

L = n * (dm/dt) * c^2 -L is luminosity -N is a constant (generally 0.1) -dm is change in mass -dt is change in time -c is speed of light

How long do stars generally spend on the main sequence? Why do they leave?

Low mass stars generally spend longer than 10^9 years on the main sequence. High mass stars spend a lot less time on the main sequence than low mass stars. Stars leave the main sequence because they can no longer fuse hydrogen into helium in their cores.

How do low-mass stars end their life?

Low-mass stars end their lives as white dwarfs surrounded by planetary nebulae because they never get hot enough to fuse carbon in their cores.

What is main-sequence fitting?

Main-sequence fitting is comparing the brightness of different star clusters' entire main sequences. By measuring how much fainter one star cluster is than another, we can calculate the distance of the cluster.

How large is a neutron star? How massive?

Neutron stars are about as large as a small city (10 km). Neutrons stars are more massive than the sun.

Has Hubble's constant stayed the same throughout history?

No, Hubble's constant has NOT stayed the same throughout history.

Are all neutron stars pulsars?

No, not all neutron stars are pulsars.

Why does nuclear fusion in high mass stars stop at iron?

Nuclear fusion stops at iron because iron requires additional energy to be inputted into the system for reactions to take place. Therefore, iron is a "dead end" in terms of nuclear fusion.

What happens in the evolution of a high mass star?

On the main sequence, a high mass stars fuses hydrogen to helium in its core via the CNO cycle. Once hydrogen is depleted in the core of the star, hydrogen begins fusing in a shell around the inert helium core, creating a red supergiant. The star burns successively heavier elements in its core until an inert iron core is left over. Iron is a dead end for nuclear fusion, so the neutrons in the high mass star collapse to the center, forming a neutron star. The rest of the star blows up in an an enormous explosion, or supernova.

How do astronomers infer the presence of a black hole at the center of the Milky Way? Is this black hole actively accreting?

Orbits of the stars at the galactic center of the Milky way suggest the presence of a black hole. This black hole is NOT actively accreting.

How did we discover pulsars?

Pulsars were discovered when graduate student Jocelyn Bell and advisor Anthony Hewish came across a radio source which "pulsed" regularly every 1.3 seconds.

What made quasars so initially interesting?

Quasars were so initially interesting because their optical spectra looked weird - they contained broad emission lines that didn't match any known atom or molecule.

Which constellation has the Milky Way's galactic center in it?

Sagittarius has the Milky Way's galactic center in it.

What are some of the different lines of evidence that we have for the existence of dark matter?

Some of the different lines evidence we have for the existence of dark matter come from the observations of galaxies and galaxy clusters.

What are the properties of spiral galaxies?

Spiral galaxies -have a disc component either with or without spiral arms -have a spheroidal component (bulge & halo) -contain an interstellar medium of gas and dust -appear white (contain blue and red stars)

How much of the universe's mass is in stars? How much of the universe's mass is in gas?

Stars make up less than 1% of the critical density of the universe. Gas makes up 3-4% of the critical density of the universe.

What do stars do on the horizontal branch, and how long do they stay there?

Stars on the horizontal branch fuse helium into carbon in their cores, leading up to second red giant phase. Stars do not spend very much time on the horizontal branch (in the grand scheme of things).

What is the Chandreskhar limit for a white dwarf? What happens when you add more mass than this limit to a white dwarf?

The Chandreskhar limit for a white dwarf is 1.4 times the mass of the sun. As the white dwarf approaches this mass via accretion, electron speeds approach the speed of light and electron degeneracy pressure is no longer supported. The white dwarf blows up in a supernova.

What does the Hubble constant say about the expansion of the Universe throughout time?

The Hubble constant indicates that the expansion rate of the Universe has changed throughout time.

What is the Hubble diagram?

The Hubble diagram is a diagram that plots recession velocity as a function of distance away from us. Hubble found that the recession velocity of galaxies were proportional to the distance away from us. Therefore, the slope of the line in Hubble's diagram is Hubble's constant.

What is the Milky Way?

The Milky Way is the galaxy containing our Sun; a gravitationally-bound conglomeration of billions of stars.

What is the Schwarzschild radius, and how do we derive this?

The Schwarzschild radius is the radius of the event horizon. This is the radius at which the escape velocity equals the speed of light. Derivation: Rs = 2GM/c^2

What is the asymptotic giant branch?

The asymptotic giant branch signifies the rapid expansion and cooling of stars during the red giant phases.

What is the best method for measuring close distances?

The best method for measuring close distances is radar ranging.

What is the critical density of the universe, and what does this density mean for the eventual fate of the universe?

The critical density of the universe is the mass density required for gravitational pull to equal the kinetic energy of the universe. The value of the Hubble constant gives us the current KE of the universe. Critical density ~ 10^-26 kg/m^3 (several atoms per cubic meter). This means that the universe will eventually become a dark, cold and lonely place, due to the incredible distances that will separate galaxies and stars.

What is the event horizon?

The event horizon is the imaginary spherical surface of a black hole.

What is the helium flash?

The helium flash is the explosive fusion of helium into carbon that occurs when the helium core of a red giant becomes extremely dense and hot. This occurs because degeneracy pressure supports the core instead of normal heat pressure.

What is the inferred mass of the Milky Way's black hole?

The inferred mass is 4 million times the mass of the sun.

How does the mass of a star determine its lifetime? Why?

The mass of a star determines its lifetime because the greater the mass, the greater the luminosity of the star and the faster a star uses up its available fuel. High mass stars on the main sequence, therefore will "die" before low mass stars.

How massive are white dwarfs typically?

The mass of white dwarfs is comparable to the mass of the sun.

What is the maximum mass for a neutron star?

The maximum mass for a neutron star is about 3 times the mass of the Sun.

What is a white dwarf?

The revealed core of a low-mass star at the end of its life, resembling small, hot, dense star. It gradually cools off over time.

What is the rotation curve for our galaxy? Why is this weird at large radii?

The rotation curve for our galaxy is flat (constant), which is weird at large radii because material is rotating more quickly than expected based on the distribution of ordinary matter (stars + gas).

What does the rotation curve of our galaxy mean for dark matter?

The rotation curve of our galaxy suggests that dark matter makes up most of the matter in the Milky Way.

As you increase the mass of a white dwarf, what happens to its size?

The size of a white dwarf becomes smaller if the mass is increased.

What are the structural components of the Milky Way, and what are the properties of these components?

The structural components of the Milky Way are -the Disk: contains most of the galaxy's stars; contains gas, dust; stellar formation; 30 kpc across; several 10^10 times the mass of the sun; spiral appearance -the Bulge: 10^10 times the mass of the sun -stellar Halo: contains globular clusters; 100 kpc across; no gas or dust; 10^9 times the mass of the sun -dark matter Halo: 10^12 times the mass of the sun -Gas: exists in a range of phases (temperature, density, pressure)

What does the study of galaxy clusters (galactic speeds and positions, hot gas, gravitational lensing) say about dark matter?

The study of galaxy clusters via three independent methods (measuring galactic speeds and positions, measuring the temperature and distribution of the hot gas between galaxies, and observing how clusters bend light as gravitational lenses) indicates that clusters NEED to contain far more mass in dark matter than in stars and gas

What is the typical radius of a white dwarf?

The typical radius is comparable to the radius of planet Earth, around 7000 km.

How do the spectra of spiral, elliptical and irregular galaxies compare? How do their typical environments compare?

There are more absorption lines in spiral galaxies than there are in elliptical and irregular galaxies. Spiral galaxies are found in groups of up to a few dozen galaxies, while elliptical galaxies are much more common in huge clusters (hundreds to thousands).

What do we think is at the center of an active galactic nucleus?

There has to be a supermassive black hole at the center of AGNs

Where are we actually in the Milky Way?

We are actually 8 kiloparsecs away from the galactic center of the Milky Way. We are not close to the center at all.

How can we discover white dwarfs? What are they composed of?

We can discover white dwarfs by observing multiple/binary star systems and finding isolated white dwarfs through telescopes. White dwarfs are composed of the remaining cores of dead stars.

How can we interpret the expansion of the universe with kinetic and potential energies?

We can interpret the expansion of the universe by taking and analyzing a spherical piece of the expanding universe. The kinetic energy of this expanding sphere is related to Hubble expansion, and the gravitational potential energy is related to the density inside the sphere. From these pieces of information, we can interpret the universe's expansion.

How can we measure the distances to Type 1A supernovae?

We can measure the distances to Type 1A supernovae by inferring that every progenitor (white dwarf) is roughly the same size. These progenitors, therefore, are standard candles. They can give us the luminosity, and from there we can calculate distance.

How can we see the center of the galaxy?

We can see the center of the galaxy using adaptive optics.

How do we know that the Universe is expanding?

We know that the universe is expanding from the motions (redshifts) of galaxies, and the distances of galaxies.

How do we measure the distance to far-away galaxies?

We measure the distance to far-away galaxies using the Tully-Fisher relation and observing Type 1A (white dwarf) supernovae

How do we measure the mass of the AGN center?

We measure the mass of the AGN center by observing the motions of stars near the galactic center.

How do we measure the redshift of a distant galaxy?

We measure the redshift of a distant galaxy by measuring the galaxy's spectra. Because these galaxies contain features from stars and gas with known spectral wavelengths, it is easy to measure the redshift.

How do we measure the size of the AGN center?

We measure the size of the AGN center by observing the spectral emissions of the center over time.

Why do we observe a pulsation?

We observe a pulsation because the beams of intense radiation coming from the poles of rapidly rotating neutron stars look like pulses as they sweep past Earth.

What do we think that dark matter is composed of?

We think that dark matter may either be composed of known, baryonic (normal) particles or extraordinary, undiscovered particles.

Which wavelengths do we use to measure the properties of the Galactic center?

We use infrared, radio, and X-ray imaging to measure the properties of the galactic center.

What happens when a high mass star explodes?

When a high mass star explodes, the neutrons in the star collapse to the center, forming a dense, small neutron star.

What happens when a high mass star leaves the main sequence?

When a high mass star leaves the main sequence, hydrogen fuses around the inert helium core, expanding and cooling the star. This is similar to the process of low-mass stars.

What happens when a star leaves the main sequence?

When a star leaves the main sequence it can no longer fuse hydrogen into helium in its core. The core of the star shrinks and heats up, while hydrogen fusion begins in the shell surrounding the helium core. The envelope of the star then expands and cools off.

What is the relationship between redshift and the expansion of the Universe?

a) (wavelength observed / wavelength emitted) = (size of the universe now / size of the universe when light was emitted

What is Hubble's law? What is the Hubble constant?

a) v = H0 * d b) H0 is 70 km/s/Mpc

What is relationship between radius and velocity for the planets in our solar system?

v = (2piR)/P -v is speed -R is radius -P is period