Astronomy - Third Exam

Where does evidence of a supermassive black hole at the center of the Milky Way come from?

1) direct observations of stars that orbit it 2) X-rays from material that is falling in 3) strong radio emission from the region of the accretion disk

Spiral structure is created by

1) star formation 2) bar-shaped bulge 3) gravitational interaction with other galaxies

Why are globular clusters important?

1) they provide information about how the Milky Way formed and evolved 2) Provide information about how stars evolve 3) they reveal the size of the Milky Way, and Earth's location in it

How do you rank globular clusters with main-sequence turnoffs at the following temperatures

A cooler star has a lower main-sequence mass. The hotter the star, the younger it is

Eta Carinae is an extreme example of ___

A massive star. Eta Carinae is very high mass, and as a result is also driving off a huge amount of its material into space, yet another reason it is an extreme example

The end result of the CNO cycle is that four hydrogen nuclei become one helium nucleus?

Although the CNO cycle uses other nuclei as catalysts, the end result is hydrogen fusion into helium.

In the Hubble scheme for classifying galaxies, what kind of galaxy is the Milky Way?

Barred spiral. Clearly a spiral galaxy and appears to have a small bar

In spiral galaxies, why do stars form primarily in the spiral arms?

Because of the sudden increase in density. Stars form where there is star-forming material, which is largely piled up in denser spiral arms

why is dark matter different than normal matter?

Dark matter does not emit, absorb, or scatter light. Dark matter does not interact at all with light

Rank, in increasing order the following in terms of the fraction of the Milky Way's mass that they represent

Dark matter, stars in the disk, stars in the bulge, stars in the halo. 90 percent of the mass of a galaxy comprises dark matter. In therms of normal matter, the disk dominates. The stellar halo, although large, is very low density and the lowest typical mass.

The dominant factor in the formation of galaxies is the distribution of ___ in the early universe

Dark matter. Microscopic fluctuations in the distribution of dark matter were the seeds of structure today

If all the stars in an elliptical galaxy travel in random directions in their orbits, what type of elliptical galaxy would it be?

E0. E0 is spherical and thus results from perfectly randomized orbits

Which galaxy type describes a galaxy shaped like a rugby ball?

E5. A rugby ball is ellipsoidal and thus would have a large E number

Does a pulsar change in brightness because its size pulsates?

False. A pulsar spins and pulses because the "lighthouse" is rotating

Is most of the mass of a galaxy is in stars?

False. About 90 percent of a galaxy's mass is dark matter

Does the disk of the Milky Way contain most of its mass?

False. Dark matter halo contains most of its mass

When iron fuses into heavier elements, does it produce energy?

False. Iron can't fuse and give off energy.

Is most of the Milky Way visible because it is so close?

False. Most of the Milky Way is obscured by dust in the disk.

Were most of the stars in the Milky Way formed at the same time?

False. Star formation happens in episodes and is a continual process.

Does the milky way rotate like a solid disk?

False. The disk follow Kepler's laws.

Is an E7 more spherical than an E0 galaxy?

False. The larger the E number, the more elliptical the galaxy

Does uranium form in the core of a star?

False. Uranium is made during energetic explosion of a supernova.

Why does the interior of an evolved high-mass star have layers like an onion?

Heavier atoms fuse closer to the center, because the temperature and pressure are higher there. High-mass stars burn successively heavier elements in their cores while continuing to burn the lighter elements in ahsal

What are cosmic rays?

High-energy particles

Where are the youngest stars in the Milky Way galaxy?

In the disk. The disk, and specifically the spiral arms, is where the youngest stars are located, since this is where star formation is most active.

Where did elements heavier than iron originate?

In the explosions of high-mass stars. Elements heavier than iron can't be fused inside a star since fusion of these atoms requires energy. Thus they are created only in explosive events like supernovae.

Why can't iron fusion support a star?

Iron absorbs energy when it fuses. To fuse two iron nuclei, energy must be absorbed since elements heavier than iron are also more massive than their consituents

How does the mass of the supermassive black hole at the center of the Milky Way compare with that found in most other spiral galaxies?

It is much less massive. Our supermassive black hole is on the smaller side of most found in other galaxies

If it were possible to watch a high-mass star move to the right, along the topmost of these post-main-sequence lines, what would you observe happening to the star in terms of its size?

It would become larger. The star moves to the right as roughly constant luminosity, so it becomes red and giant

If it were possible to watch a high-mass star move to the right, along one of these post-main-sequence lines, what would you observe happening to the star in terms of its color?

It would become redder. As the star moves to the right in the H-R diagram, it cools off and becomes redder

If it were possible to watch a high-mass star move along the topmost of these post-main-sequence lines, what would you observe happening to the star in terms of its luminosity?

It would remain about the same. A post-main-sequence massive star remains at roughly constant luminosity as it loops around in the H-R diagram

In the context of spiral galaxies, Kepler's laws could be used to estimate

M, the mass of the galaxy. Kepler's laws use orbital characteristics of starts to determine the mass interior to that star's orbit

A rotation curve plots ___ versus distance from the galaxy center

Orbital speed. Radial speed is motion toward or away from us; rotation implies orbital speed around the galactic center

In order for a variable star to be useful as a standard candle, its luminosity must be related to its

Period of variation. A variable star that has a relationship between its period and luminosity allows the former to yield the latter, from which distance can be determined by comparing the luminosity to the brightness

Larger galaxies form from the merging of small protogalaxies. This process is similar to the formation of what?

Planets. Sometimes called the "bottom up" building scenario, small things combine to form bigger ones, whether planets from dust or galaxies from star clusters

Which galaxy types have a spherical bulge and a well-defined disk?

Spiral and Barred spiral. All spiral have a bulge and disk

What does the abundance of a star's heavy elements indicate about the age of the star?

Stars with higher abundance are younger. The elements in a star's atmosphere were placed there when the star formed, and early in the universe there were very few massive elements, so a star with many of them must have formed recently

The start of photo disintegration of iron in a star sets off a process that always results in a ___

Supernova. All core collapses produce supernovae. Most produce neutron stars, whereas the most massive progenitors produce black holes

How has the magnetic field of the Milky Way been detected?

Synchrotron radiation from cosmic rays. The glow of synchrotron radiation from cosmic rays spiraling in the galactic magnetic field is a direct confirmation of the presence of large-scale fields

What are the differences between cold and hot dark matter?

Temperature, composition, and the way they clump under the influence of gravity. Hot and cold dark matter particles move at different speeds are believed to be dominantly made of different species, and the fast-moving particles are not easily influenced by gravitational fields.

Why is detailed observation of the structure of the Milky Way difficult?

The Solar System is embedded in the dust and gas of the disk. Dust obscures much of what we see

What does the concept of galactic habitable zone not consider?

The distance of a planet from its central star. The galactic habitable zone considers where stars could form that may host life-bearing planets, but does not discuss the particulars of the solar system themselves.

Why are older stars found farther from the midplane of a galactic disk?

The stars have lived long enough to move there. Older stars have been "kicked out" of the mid-plane by gravitational interactions with other stars and massive molecular clouds

If the sun were a member of the globular cluster, that cluster's H-R diagram would fall between

The sun is 5 billion

What causes the differences in various types of AGNs?

The viewing angle. The main difference between different types depends on the viewing angle, with a smaller contribution from the amount of accretion feeding the black hole

Halo stars are found in the vicinity of the Sun. What observational evidence distinguishes them from disk stars?

Their speed. Halo stars have very large velocities in/out of the disk, as opposed to disk stars, which are moving around the center of the galaxy in the direction of the disk

Why are globular clusters so useful in determining the size of the galaxy?

They are large, bright, and evenly distributed. They can be used to study overall properties of the galaxy

Why are most of the Milky Way's satellite galaxies so difficult to detect?

They are very faint. It is very hard to detect many galaxies because they have "low surface brightness." which means that their light is spread out (diffuse).

What is the fate of the Large and Small Magellanic clouds?

They will become part of the Milk Way. We already see tidal streams of stars form the Magellanic Clouds being stripped off by the Milky Way's gravity

Are AGNs small but extremely luminous?

True. An AGN may be as small as our Solar System, but can outshine all the stars in a galaxy

Is a spiral galaxy's dark matter distributed spherically?

True. Dark matter halo is roughly spheroidal

Electrons and protons can combine to become neutrons?

True. Electrons and protons can be driven together to form a neutron and a neutrino

Can a supernova be as bright as its entire host galaxy?

True. For a short time, a supernova can produce more light than all the other stars in its host galaxy.

Stars in the instability strip are pulsating variable stars?

True. Stars on the instability strip pulsate. Such pulsations change a star's brightness making it a variable.

Can supernova remnants be viewed at all wavelengths?

True. Supernova remnants emit across the electromagnetic spectrum, from the radio to X-ray

Does a globular cluster spend most of its time in the galaxy's halo?

True. The disk is think and the halo is very large, so halo clusters spend most of their time in the latter

X-ray binaries are similar to another type of system you have studied:

a system consisting of progenitors of Type Ia supernovae.

The existence of supermassive black holes at the center of massive galaxies is

a theory

The layers in a high-mass star occur roughly in order of ___

atomic number. The layers of high-mass stars correspond to which elements are being fused, and heavier elements are fused at higher temperature, meaning closer to the core

What contributes to the largest percentage of total mass of spiral galaxy?

dark matter. Up to 90 percent of a galaxy's mass is dark matter

True or false, dust does not shine in visible light, so it is considered dark matter?

false. Dark matter does not interact at all with any light whereas dust absorbs and emits light (dust absorbs UV and visible light, and emits mostly in the infrared), making it normal matter

What is the dominant force in the formation of galaxies?

gravity. Gravity is the dominant force in all processes of formation of macroscopic astronomical objects (planets, stars, galaxies, etc.).

Large groups of galaxies are formed and held together by what?

gravity. Gravity is what holds most all astronomical objects together.

Astronomers determine the radius of an AGN by measuring

how quickly its light varies. An object can't vary in time any faster than the time it takes for light to cross it. This gives an upper limit to the size of any variable object

In astronomy, isotropy means that the universe is the same ___

in all directions

The best evidence for dark matter in the Milky Way comes from the observation that the rotation curve

is quite flat at great distances from the center. Flat rotation curve can only be explained by the presence of a significant amount of matter that we can't see

Why does a pulsar pulse?

its magnetic axis crosses Earth's line of sight. A pulsar is spinning neutron star whose magnetic axis is different from its spin axis, so that magnetic axis acts like a lighthouse

For a galaxy, what does the term morphology refer to?

its shape. Morphology is a description of the appearance, shape, and size

galaxies are classified according to what?

shape

Arrange the following elements in the order they burn inside the nucleus of a high-mass star during the star's evolution: helium, oxygen, hydrogen, neon, silicon, carbon

silicon, helium, carbon, oxygen, hydrogen, neon

Luminous matter includes

stars, dust, and gas. Luminous matter is any kind of matter than interacts with light

In a high-mass star, hydrogen fusion occurs via what?

the CNO cycle. High-mass stars fuse hydrogen via the CNO cycle, low-mass stars by the proton-proton chain

Pulsations in a Cepheid variable are controlled by ___

the ionization state of helium. In a Cepheid variable, a partial ionization zone of helium can absorb or release energy, which trows off the usual condition of hydrostatic equilibrium and drives the star to pulsate.

How do astronomers know that dark matter is present in galactic halos far from the center of the galaxy?

the speeds of orbiting stars remain constant

Why are all quasars very distant?

the universe has evolved. Quasars are an early stage in galaxy evolution, so only very distant galaxies are being observed while still young

The Big Bang is a what?

theory

Spiral arms have ___ stars than the rest of the disk?

younger. Spiral arms are the locations of large-scale star formation, which means they contain a younger star population