Exam 3

Which feature of a galaxy seems to be related to the mass of the galaxy's central supermassive black hole?

the size of the galaxy's central bulge

What main conclusion did the Herschels draw from their star counts?

The Milky Way is a disk of stars with the sun near the center.

This graph shows the period-luminosity relation for Cepheid variables. If the period of a type I Cepheid is 10 days, what is its absolute magnitude?

-4.3

What color is the central bulge? Why?

The central bulge is red, due to the presence of old objects, such as red dwarfs and red supergiants.

If all elliptical galaxies had 3 different diameters (a different length, width, and height) would you ever see an elliptical galaxy that looks circular in a photograph?

No, not if all three diameters are different.

What is the period of the pulsar whose pulsations are shown here as a graph of intensity emitted versus time?

1.3 sec

What is the distance to a galaxy that has a recessional speed of 3,500 km/s if the Hubble constant is 70 km/s/Mpc?

50 Mpc

What does the observed heavy element abundance tell us about a star?

A low percentage of metals indicates that a star formed long ago.

Which of the following describes the gravitational redshift?

A reduction in the energy of photons as they move away from objects.

All white dwarfs have masses in a small range—from about 0.5 to almost 1.4 M. Why?

A white dwarf cannot support its own weight by its degenerate pressure if its mass is greater than 1.4 solar masses. Stars with main sequence masses less than about 0.5 solar masses cannot ignite the helium fusion required to become white dwarfs.

What did the Herschels find when they counted stars in 683 regions around the Milky Way?

About the same number of stars in each direction.

Why do the millisecond pulsars spin so fast?

Accretion of matter from a nearby binary companion spins them up.

If neutron stars have hot surface temperatures, why aren't they very luminous?

Although neutron stars are hot, they are very small and have little surface area from which to radiate, so their luminosity is low.

Why does the short length of pulsar pulses eliminate normal stars as possible pulsars?

An object cannot emit pulses that are shorter than the time it takes light to cross its diameter.

Rank the four quasars in the figure below in order of their likely distance from us (from closest to farthest). Rank the quasars in order of their amount of redshift (from smallest to largest).

B2 1128+31 PKS 1217+02 4C 73.18 B2 1206+32A B2 1128+31 PKS 1217+02 4C 73.18 B2 1206+32A

How are neutron stars and white dwarfs similar? How do they differ?

Both are faint and difficult to detect.Both are produced by dying stars. Both eventually cease to shine.Both are compact objects. White dwarfs are produced by the final gravitational contraction of the cores of low- to medium-mass stars. Neutron stars are produced from the collapsing core of a massive star as it undergoes a supernova explosion.

How would you find the distance to a very far galaxy?

By using the Hubble law.

Could the black hole in the nucleus of the Milky Way Galaxy be the remnant of a single dead star? Why or why not?

No. This black hole has a mass of several million M, far too massive to result from a single star.

How did astronomers conclude that pulsars actually could not be pulsating stars?

Normal stars are much too large to pulse with a period as small as the period of a pulsar.

Which size object best represents the size of a white dwarf—red giant, Earth, Jupiter, red dwarf, or Sun-like star?

Earth

What gives elliptical galaxies a redder color than spiral galaxies?

Elliptical galaxies have less gas and dust and thus less on-going star formation.

Why is look-back time not important for nearby galaxies?

Galaxies change very slowly.

What does the degree of gravitational lensing seen in this image indicate?

Galaxy clusters contain much more mass than is visible.

Who discovered that when viewed through a telescope, the Milky Way is resolved into thousands of individual stars?

Galileo Galilei

Why are pulsars so hot?

Gravitational energy was converted into thermal energy during formation.

How was William Parsons (Lord Rosse) in 1845 able to see spiral structure in some nebulae, whereas others had not noticed this spiral structure before?

His 72 inch diameter telescope was the largest in the world.

Why do astronomers propose that the Milky Way Galaxy contains a lot of dark matter?

The Galaxy's rotation curve flattens out at great distances.

Why can't massive stars generate energy from iron fusion?

Iron fusion consumes energy.

In what sense is a black hole actually black?

It absorbs all light within the Schwarzschild radius and emits no light itself.

What does the flatness of the rotation curve shown here reveal about this galaxy?

It has a lot of nonluminous (dark) matter in its outer regions.

How can a neutron star not be a pulsar?

Its magnetic field may be too weak to generate beams of radiation. A pulsar may be too old and rotate too slowly to pulse.A pulsar's magnetic field may not sweep past Earth.Its magnetic field may be too weak to generate beams of radiation and a pulsar may be too old and rotate too slowly to pulse.

This is an image of the starburst galaxy NGC 1569. What is believed to be the cause of the recent rapid rate of star formation in this galaxy?

NGC 1569 has recently collided with another galaxy.

Why do you expect neutron stars to spin rapidly?

Neutron stars are formed by the collapse of massive stars. Since all stars rotate, the principle of conservation of angular momentum predicts that as a massive star collapses it must rotate faster to conserve angular momentum.

If the Sun has a Schwarzschild radius, why isn't it a black hole?

Not all of the Sun's mass is inside its Schwarzschild radius.

Why are metals less abundant in older stars than in younger stars?

Older stars were formed during the early history of the Galaxy, when the interstellar medium was largely hydrogen and helium. Since then, supernovae have enriched the interstellar medium with metals created in the cores of older stars. Some of these metals are later incorporated into new, younger stars.

Why have no black dwarfs yet been observed in our galaxy?

Our galaxy is too young for any to have formed.

How are population II stars different than the sun, a population I star?

Population II stars are lower in metals than population I stars.

What does the short length of pulsar pulses tell you?

Pulsars must have much smaller diameters than normal stars.

What evidence is there that quasars occur in distant galaxies?

Quasars are often surrounded by hazy features whose spectra resemble those of normal galaxies

Quasars have which of the following characteristics?

Quasars have bright emission-line spectra.

Galaxies in close pairs are three times more likely to be Seyfert galaxies than are isolated galaxies. What general conclusion can be drawn from this statistical fact?

Seyfert galaxies are very likely the result of galaxy interactions.

Why is a supernova type Ia standard candle better to use in measuring very long distances than either the brightest globular cluster or Cepheid Variable star standard candles?

Supernovae type Ia are more luminous.

This globular cluster map (black border) shows the sun far from the center of the galaxy using the distances determined by Shapley in 1918 (units are kiloparsecs). The rotation curve graph on the right shows the sun's location with corrections made to Shapley's original measurement. According to these two diagrams, how big was Shapley's error?

The corrected distance is about half of that of Shapley's original measurement.

Why are the three very distant galaxies pointed out in this image (see white arrows) red in color?

The extreme recessional speed causes a large Doppler shift.

What observational evidence leads us to believe that AGNs contain supermassive black holes?

The gas in the disk is orbiting at high speeds and short-duration fluctuations in brightness limit the size of the object at the center of an AGN.

How does the lighthouse model explain pulsars?

The lighthouse model explains pulsars as spinning neutron stars that emit beams of radiation from their magnetic poles . As they spin, they sweep the beams around the sky like lighthouses.

What color are the spiral arms? Why?

The spiral arms are blue, due to the presence of hot objects, such as O and B stars.

What main conclusion did Shapley draw from his measurements of the distances to the globular clusters?

The sun is far from the center of the Milky Way.

How does the unified model explain the two kinds of Seyfert galaxies?

The unified model explains that type 1 and type 2 Seyfert galaxies are really the same kind of galaxy being seen from different viewing angles.

What can happen to the white dwarf in a close binary system when it accretes matter from the companion giant star?

The white dwarf can ignite the new matter and flare up as a nova and accrete too much matter and detonate as a supernova type Ia.

Why do neutron stars have an upper mass limit?

There is a maximum mass that can be supported against gravity by the outward pressure of degenerate neutrons.

Why can't the lowest-mass stars have the same fate as the sun-like stars?

They are not massive enough to collapse to such a high density.

How can collisions affect the shapes of galaxies?

They are twisted and distorted by tidal forces.

How can white dwarfs be both very hot and have very low luminosity?

They have small surface area.

Seyfert galaxies have a spectrum with broad emission lines of ionized elements. What other unusual features do Seyfert galaxies have?

They have small, highly luminous nuclei that fluctuate rapidly.

Suppose that you monitor the apparent brightness of a supernova and notice that 50 days after it reached maximum brightness, it has dimmed by 2 magnitudes. Examine the light curves in this figure. Which type of supernova are you most likely observing?

Type II

Why would astronomers naturally assume that the first discovered millisecond pulsar was relatively young?

When a pulsar first forms, it is spinning fast, and its rotation begins to slow as it radiates energy into space.

Why do you expect neutron stars to have a powerful magnetic field?

When a star collapses into a neutron star, its magnetic field is squeezed into a small volume, which can make the field as much as a billion times stronger.

How can a black hole emit X-rays?

X-rays emitted just before material crosses the event horizon can escape the gravitational pull.

What is a starburst galaxy? How is a starburst galaxy likely to be formed? What is the likely future of a starburst galaxy?

a galaxy going through a burst of star formation from a galactic merger It will eventually run out of gas and dust for star formation.

What must actually be measured to use the Hubble law to determine the distance to a galaxy?

a galaxy's redshift

What is the lower limit for the mass of neutron stars?

about 1.4 solar masses

The density of the sun is about 1.4 grams per cubic centimeter. Density is mass divided by volume. The volume of a sphere is proportional to the radius cubed. According to this graph, what is the density of a one-solar-mass spherical white dwarf?

about 3,000,000 grams per cubic centimeter

In a few billion years, our own Milky Way may merge with the Andromeda Galaxy. Which type of galaxy is most likely to result from such a merger?

an elliptical galaxy

Rank these objects from oldest to youngest: a) the Solar System b) the Universe c) the Milky Way Galaxy

b > c > a

What is the classification of the Milky Way Galaxy?

barred spiral

Why is it difficult to give exact dimensions for the Galaxy's visible halo?

because we are on the inside looking out

Why is it difficult to give exact dimensions for the Galaxy's disk?

because we are on the inside looking out because of interstellar dust

What is the probable next step in a star's evolution after the white dwarf stage? How many of the objects from the previous question have been observed? What is the reason for the answer to the above question?

black dwarf none There has not been enough time for these objects to form.

What prevents gravity from shrinking a white dwarf to a smaller size?

degenerate electrons

What prevents neutron stars from contracting to a smaller size?

degenerate neutrons

When viewing a distant galaxy, the amount of look-back time in years is equal to which of the following?

distance to the galaxy in light years

What is the classification of a galaxy that is round, with no spiral arms and no disk component?

elliptical

Which type of galaxy has the largest range in both size and mass?

elliptical

Which type of star eventually develops several concentric zones of active shell fusion?

high-mass stars

What is "look-back time?"

how long the light from an object takes to reach us the same as the distance in light-years to an object

Consider the following elements or groups of elements: hydrogen; helium; carbon, nitrogen and oxygen; iron. Rank the elements or groups in order of abundance in the universe (most abundant first). Rank the elements or groups in order of atomic mass (most massive first).

hydrogen, helium, carbon, nitrogen and oxygen, iron iron, carbon, nitrogen and oxygen, helium, helium

Where are the youngest stars in the Milky Way Galaxy located?

in the flattened disk

What did William Parsons (Lord Rosse) think the spiral nebulae were?

island universes

Consider the following possible stages in the evolution of a star like our sun: black dwarf, giant, main-sequence, planetary nebula, supernova, white dwarf. Rank the stages in the order they occur. Leave out any stages that will not occur in the evolution of a star of similar mass to the sun.

main-sequence giant planetary nebula white dwarf black dwarf

What property does matter inside the event horizon of a black hole retain?

mass, angular momentum, and electric charge

What is the origin of the X-ray bursters?

matter accreting onto a neutron star in a binary system

What may be responsible for the observed gamma-ray bursters?

neutron star merger and hypernovae

Can the Sun ever become a black hole? Why or why not?

no The Sun's mass is too small.

Galaxies with active star formation also have which of the following?

plenty of gas and dust O and B associations emission nebulaa bluish tint

What is at the center of Seyfert galaxies?

supermassive black holes

Which type of supernova leaves NO core remnant?

supernova type Ia

Edwin Hubble resolved the Shapley-Curtis debate in 1924 by measuring the distance to large, bright spiral nebulae. What distance method did Hubble employ?

the Cepheid variable star method

What must be known to estimate the distance to a galaxy with the Hubble law?

the Hubble constant the recessional velocity

What must be measured to determine distance by the Cepheid variable star method?

the apparent magnitude and the period of pulsation of the variable star

What is the Hubble law a relationship between?

the distance and recessional velocity of a galaxy

What parameter must we know to determine the linear diameter and luminosity of a particular galaxy?

the distance to the galaxy

Which of the following is the least important factor in galactic evolution?

the elemental composition of the material from which galaxies form

What event marks the end of every star's main-sequence life?

the end of hydrogen fusion in the core

Why do we expect that neutron stars spin rapidly?

the law of conservation of angular momentum

What was the topic of the Shapley-Curtis Debate of 1920?

the location of the spiral nebulae

Ultimately a sun-like star will become which of the following?

white dwarf

Consider the following objects: black hole, neutron star, white dwarf. Rank the objects in order of increasing mass. Rank the objects in order of increasing size. Rank the objects in order of decreasing density.

white dwarf (least)neutron star black hole black hole (smallest)neutron star white dwarf black hole (greatest)neutron star white dwarf

What evidence can you give that the nucleus of our galaxy contains a supermassive black hole?

x-ray emissions high orbital speeds of stars very large mass