Chapter 24 Book Questions

What is the most likely range of values for Hubble's constant? What are the uncertainties in its value?

70 km/s/Mpc (kilometers per megaparsec) The uncertainty in Hubble's constant translates directly into similar uncertainty in all distances determined from Hubble's law.

What are some of the problems astronomers encounter in measuring the distances to faraway galaxies?

Because distance-measurement techniques ultimately rely upon the existance of very bright objects whose luminosities can be inferred by other means. Such objects become increasingly hard to find and calibrate the farther we look into intergalactic space

What distinguishes one type of spiral galaxy from another?

Both are composed of a flattened, star-forming disk with a large central galactic bulge and spiral arms. However, one type also has an elongated bar of stellar and interstellar medium passing through the centre and extending beyond the bulge, into the disk. Spiral galaxies of both types are also characterized by their size and the corresponding tight or looseness of their spiral arms.

How is the Tully-Fisher relation used to measure distances to galaxies?

By measuring the amount of broadening in emission lines, we can determine a galaxy's rotation. Once we know the rotation, we can determine luminosity. Comparing a galaxy's luminosity with its apparent brightness gives us its distance.

What is the evidence that the radio lobes of some active galaxies consist of material ejected from the galaxy's center?

Centarus A is a member of a small cluster of galaxies. Its radio lobes are roughly symmetrical, jutting out from the center of the galaxy and perpendicular to the dust lane, suggesting that they consist of material ejected in opposite directions from the galactic nucleus.

active galaxies are very luminous because they

Contain black holes in their cores

How do we know that the energy-emitting regions of many active galaxies must be very small?

If an object's luminosity fluctuates rapidly, then it must be compact; an object cannot 'flicker' in less time than radiation takes to cross it.

How does the use of Hubble's Law differ from the other extragalactic distance-measurement techniques we have seen in the text?

It does not use the inverse-square law. The other methods all provide a way of determining luminosity of a distant object, which then is converted to a distance using the inverse-square Law. Hubble's law gives a direct connection between redshift and distance

The energy emission from an active galactic nucleus does not resemble a black body curve. Why is this important?

It means that the energy source can't simply be the summed energy of a huge number of stars- some other mechanism must be at work

What is Hubble's Law?

Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is directly proportional to its distance away.

In what ways are large spirals like the Milky Way and Andromeda not representative of galaxies as a whole?

Most galaxies are not large spirals- the most common galaxy types are dwarf ellipticals and dwarf irregulars

How do we know that quasars are extremely luminous?

Quasaras are very far away, therefore they appear small and unimpressive to us. However, they are extremely luminous. At their distances from our galaxy (240 Mpc) it's amazing that we can see them at all.

What are standard candles and why are they important in astronomy?

Standard candles are easily recognizable astronomical objects whose luminosities are confidently known. They are important because they extend the distance-measurement ladder by allowing us to use its estimated luminosity in comparison to apparent brightness to figure out an object's distance.

What was it about the spectra of quasars that was so unexpected and surprising?

The amount of redshifting is very large, they have very high recession velocities, meaning they are receding away from us remarkably fast.

How does the accretion onto a supermassive blackhole power the energy emission from the extended radio lobes of a radio galaxy?

The energy is generated in an accretion disk in the central nucleus of the visible galaxy, then transported by jets out of the galaxy and into the lobes, where it is eventually emitted by the synchrotron process in the form of radio waves

Briefly describe the leading model for the central engine of an active galaxy

The leading theory for the energy source in active galactic nuclei holds that these objects are powered by material accreting onto a supermassive black hole. As matter spirals toward the hole, it heats up, producing large amounts of energy. At the same time, high-speed jets of gas can be ejected perpendicular to the accretion disk, forming the jets and lobes observed in many active objects. The jets transport magnetic fields generated in the disk out to the radio lobe, where they play a crucial role in producing the observed radiation.

Name 2 basic differences between normal galaxies and active ones

The luminosities of active galaxies are extremely large and their spectra differ slightly. Also, their energy emission is mostly nonstellar.

What is the Virgo Cluster

The nearest large galaxy cluster to the Local Group. It houses 2500 galaxies, much more than our own.

Describe the 4 rungs in the distance-measurement ladder used to determine the distance to a galaxy lying 5 mpc away.

The techniques show in the the bottom 4 rungs of the Extragalactic Distance Ladder are: - radar ranging - stellar parallax - spectroscopic parallax - variable stars And they take us as far as the nearest galaxies. To go further, new techniques, such as the Tully-Fisher relation, must be used.

Compare/contrast similarities and differences between elliptical galaxies and the halo of our own galaxy

They are similar in that the orbits of stars in both are disordered as they exhibit little or no overall rotation. They differ in that ellipticals have amounts of very hot interstellar gas distributed throughout their interiors, often extending well beyond the visible portions of the galaxies.

How is Hubble's law used by astronomer's to measure distances to galaxies?

Using Hubble's Law, we can derive the distance to a remote object simply by measuring the object's recessional velocity and dividing by Hubble's Constant (70km/s/mpc)

How did the determination of quasar distances change astronomers' understanding of these objects?

When quasars were discovered, they were thought to be faint, relatively nearby stars, or starlike objects, although their unusual spectra posed problems for astronomers. Once astronomers realized that their odd spectra actually meant that they had very large red shifts, it became clear that quasars were actually among the most distant- and most luminous- objects in the entire universe

within 30 Mpc of the Sun, there are about

a few thousand galaxies

If a galaxy were smaller and spinning more slowly, then, in order to represent it correctly, the figure should be redrawn to show

a narrower combined line

quasar spectra

are strongly redhshifted

According to galaxy energy spectra, active galaxies

emit large amounts of energy at all wavelengths

Astronomers classify elliptical galaxies by

how flattened they appear

Young stars in a galactic disk are

mostly found in the spiral arms

Using the methods of standard candles, we can find the distance to a campfire if we know

the number of logs used

If the light from a galaxy fluctuates in brightness very rapidly, the region producing the radiation must be

very small