Chapter 23 · 2012. 5. 17. · The Andromeda Galaxy, our closest spiral neighbor, probably...

Lecture Outlines

Astronomy Today

7th Edition

Chaisson/McMillan

© 2011 Pearson Education, Inc.

Chapter 23


Chapter 23The Milky Way Galaxy


23.1 Our Parent Galaxy

23.2 Measuring the Milky Way

Early “Computers”

23.3 Galactic Structure

23.4 The Formation of the Milky Way

23.5 Galactic Spiral Arms

Density Waves

23.6 The Mass of the Milky Way Galaxy

23.7 The Galactic Center

Units of Chapter 23


From Earth, we see few stars when looking out of our galaxy (red arrows) and many stars when looking in (blue arrows). Milky Way is what our galaxy appears as in the night sky.



Our galaxy is a spiral galaxy. The Andromeda Galaxy, our closest spiral neighbor, probably resembles the Milky Way fairly closely.



Here are two other spiral galaxies, one viewed from the side and the other from the top:



One of the first attempts to measure the Milky Way was done by Herschel using visible stars.

Unfortunately, he was not aware that most of the galaxy, particularly the center, is blocked from view by vast clouds of gas and dust.



We have already encountered variable stars—novae, supernovae, and related phenomena. These are called cataclysmic variables.

There are other stars whose luminosity varies in a regular way, but much more subtly. These are called intrinsic variables.

Two types of intrinsic variables have been found: RR Lyrae stars and Cepheids.



The upper plot is an RR Lyrae star. All such stars have essentially the same luminosity curve with periods from 0.5 to 1 day.

The lower plot is a Cepheid variable; Cepheid periods range from about 1 to 100 days.



The variability of these stars comes from a dynamic balance between gravity and pressure—they have large oscillations around stability.



The usefulness of these stars comes from their period–luminosity relation:



This allows us to measure the distances to these stars:

• RR Lyrae stars all have about the same luminosity; knowing their apparent magnitude allows us to calculate the distance.

• Cepheids have a luminosity that is strongly correlated with the period of their oscillations; once the period is measured, the luminosity is known and we can proceed as above.



We have now expanded our cosmic distance ladder one more step:



Many RR Lyrae stars are found in globular clusters. These clusters are not all in the plane of the galaxy, so they are not obscured by dust and can be measured.

This yields a much more accurate picture of the extent of our galaxy and our place within it.



Discovery 23-1: Early “Computers”

Much of the early detailed work in astronomical research was done by analysis of photographs; this work was mostly done by women (called “computers,” as they did computations). Several of these women went on to become well-known astronomers in their own right. Their work enabled the advances made by Shapley as well as Hertzsprung and Russell, among others.


Discovery 23-1: Early “Computers”

Some of the discoveries and innovations made by these women include:

• The spectral classification system (OBAFGKM)

• The period–luminosity relationship of Cepheid variable stars


This artist’s conception shows the various parts of our galaxy, and the position of our Sun:



The galactic halo and globular clusters formed very early; the halo is essentially spherical. All the stars in the halo are very old, and there is no gas and dust.

The galactic disk is where the youngest stars are, as well as star formation regions— emission nebulae and large clouds of gas and dust.

Surrounding the galactic center is the galactic bulge, which contains a mix of older and younger stars.



This infrared view of our galaxy shows much more detail of the galactic center than the visible-light view does, as infrared is not absorbed as much by gas and dust.



Stellar orbits in the disk move on a plane and in the same direction; orbits in the halo and bulge are much more random.



Any theory of galaxy formation should be able to account for all the properties shown to the right:



The formation of the galaxy is believed to be similar to the formation of the solar system, but on a much larger scale:



Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form:



The spiral arms cannot rotate at the same speed as the galaxy; they would “curl up”.



Rather, they appear to be density waves, with stars moving in and out of them such as cars move in and out of a traffic jam:



As clouds of gas and dust move through the spiral arms, the increased density triggers star formation. This may contribute to propagation of the arms. The origin of the spiral arms is not yet understood.



Discovery 23-2: Density WavesThe persistence of the spiral arms as density waves, rather than as structures made up of particular stars, may be understood using a traffic jam as an analogy. The jam persists even though particular cars move in and out of it, and it can persist long after the event that triggered it is over.


The orbital speed of an object depends only on the amount of mass between it and the galactic center:

23.6 The Mass of the MilkyWay Galaxy


Once all the galaxy is within an orbit, the velocity should diminish with distance, as the dashed curve shows.

It doesn’t; more than twice the mass of the galaxy would have to be outside the visible part to reproduce the observed curve.



What could this “dark matter” be? It is dark at all wavelengths, not just the visible.

• Stellar-mass black holes?

Probably no way enough of them could have been created

• Brown dwarfs, faint white dwarfs, and red dwarfs?

Currently the best star-like option

• Weird subatomic particles?

Could be, although no direct evidence so far



A Hubble search for red dwarfs turned up too few to account for dark matter; if enough existed, they should have been detected.



The bending of spacetime can allow a large mass to act as a gravitational lens. Observation of such events suggests that low-mass white dwarfs could account for as much as 20% of the mass needed. The rest is still a mystery.



This is a view toward the galactic center, in visible light: the two arrows in the inset indicate the location of the center; it is entirely obscured by dust.



These images—in infrared, radio, and X-ray—offer a different view of the galactic center:



The galactic center appears to have:


• A stellar density a million times higher than near Earth.• A ring of molecular gas 400 pc across• Strong magnetic fields• A rotating ring or disk of matter a few parsecs across• A strong X-ray source at the center


Apparently, there is an enormous black hole at the center of the galaxy, which is the source of these phenomena.

An accretion disk surrounding the black hole emits enormous amounts of radiation.



These objects are very close to the galactic center. The orbit on the right is the best fit; it assumes a central black hole of 3.7 million solar masses.



Process of Science/

Concept Checks

Chapter 23

Astronomy Today

7th Edition

Chaisson/McMillan



The Milky Way is the thin plane of the Galactic disk, seen from within. When our line of sight lies in the plane of the Galaxy, we see many stars blurring into a continuous band. In other directions, we see darkness.


No, because even the brightest Cepheids are unobservable at distances of more than a kiloparsec or so through the obscuration of interstellar dust.


Because the compositions, ages, and orbits of the two classes of stars are quite different from one another.


The halo formed early in our Galaxy’s history, before gas and dust had formed a spinning, flattened disk. Disk stars are still forming today. Consequently, halo stars are old and move in more or less random three-dimensional orbits, while the disk contains stars of all ages, all moving in roughly circular orbits around the Galactic center.


Because differential rotation would destroy the spiral structure within a few hundred million years.


Scientists are unwilling to suggest new physics to explain observations, and competing theories of galactic rotation curves and “missing mass” on large scales do exist. However, despite their reservations, most astronomers have concluded that dark matter, introduced as material with gravity but no interaction with electromagnetic radiation, best fits the observed facts. Several theories exist to explain how dark matter might have been formed in the early universe, and they lead to experimental tests that should some day detect dark matter if it exists.


Observations of rapidly moving stars and gas, and the variability of the radiation emitted suggest the presence of a roughly 4 million-solar-mass black hole.

Lecture Outlines

Astronomy Today

7th Edition

Chaisson/McMillan


Chapter 24


Chapter 24Galaxies


24.1 Hubble’s Galaxy Classification

24.2 The Distribution of Galaxies in Space

24.3 Hubble’s Law

24.4 Active Galactic Nuclei

Relativistic Redshifts and Look-Back Time

24.5 The Central Engine of an Active Galaxy

Units of Chapter 24



This pair of images shows the Coma cluster of galaxies. Almost every object visible is a galaxy.


Spiral galaxies are classified according to the size of their central bulge



Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest.

Type Sa tends to have the most tightly bound spiral arms with Types Sb and Sc progressively less tight, although the correlation is not perfect.

The components of spiral galaxies are the same as in our own galaxy: disk, core, halo, bulge, and spiral arms.




The Sombrero galaxy, with its large central bulge, is a type Sa. We cannot see the spiral arms, as they are edge-on.


Similar to the spiral galaxies are the barred spirals



Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars.

Ellipticals also contain very little, if any, cool gas and dust, and they show no evidence of ongoing star formation.

Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy.



Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated)



S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas



The irregular galaxies have a wide variety of shapes. The small and large Magellanic Clouds are close neighbors to our own Milky Way.



Here are several other irregular galaxies: AM 0644-741 and its neighbors on the left, and NGC 1569 on the right.



A summary of galaxy properties by type



Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning



Cepheid variables allow measurement of galaxies to about

25 Mpc away

24.2 The Distribution of Galaxiesin Space


However, some galaxies have no Cepheids, and most are farther away than 25 Mpc. New distance measures are needed.

• Tully-Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity.



Type I supernovae all have about the same luminosity, as the process by which they happen doesn’t allow for much variation. They can be used as “standard candles”—objects whose absolute magnitude is known, and which can therefore be used to determine distance using their apparent magnitude.



With these additions, the cosmic distance ladder has been extended to about 1 Gpc



This is the Local Group of galaxies, about 45 galaxies within about 1 Mpc of the Milky Way



There are three spirals in this group—the Milky Way, Andromeda, and M33. These and their satellites—about 45 galaxies in all—form the Local Group.

Such a group of galaxies, held together by its own gravity, is called a galaxy cluster.



A nearby galaxy cluster is the Virgo Cluster; it is much larger than the Local Group, containing about 3500 galaxies.




This image shows the Abell 1689 cluster of galaxies, a very large cluster almost 1 billion parsecs away


Universal recession: all galaxies (with a couple of nearby exceptions) seem to be moving away from us, with the redshift of their motion correlated with their distance.

24.3 Hubble’s Law


These plots show the relation between distance and recessional velocity for the five galaxies in the previous figure, and then for a larger sample

24.3 Hubble’s Law


The relationship (slope of the line) is characterized by Hubble’s constant H0:

recessional velocity = H0 × distance

The currently accepted value for Hubble’s constant is

H0 = 70 km/s/Mpc

Measuring distances using Hubble’s law actually works better on farther away objects; random motions are overwhelmed by the recessional velocity.

24.3 Hubble’s Law


This puts the final step on our distance ladder

24.3 Hubble’s Law


About 20–25% of galaxies don’t fit well into the Hubble scheme—they are far too luminous.

Such galaxies are called active galaxies. They differ from normal galaxies in both the luminosity and type of radiation they emit.



The radiation from these galaxies is called nonstellar radiation.

Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor. These galaxies are called starburst galaxies, and we will discuss them later.

The galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center.



This active galaxy has star-formation rings surrounding a very luminous core



Active galaxies are classified into three types: Seyfert galaxies, radio galaxies, and quasars. Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more luminous.



The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact



Radio galaxies emit very strongly in the radio portion of the spectrum. They may have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy.



Radio galaxies may also be core dominated



Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles



Many active galaxies have jets, and most show signs of interactions with other galaxies.



Quasars—quasi-stellar objects—are starlike in appearance, but have very unusual spectral lines



Eventually it was realized that quasar spectra were normal, but enormously redshifted



Solving the spectral problem introduces a new problem—quasars must be among the most luminous objects in the galaxy, to be visible over such enormous distances.



More Precisely 24-1: Relativistic Redshifts and Look-Back Time

The redshift of a beam of light is its fractional increase in wavelength. Redshifts are measured directly; distances are calculated from them using Hubble’s constant, which is uncertain. Astronomers therefore prefer to quote redshifts rather than distances.

The look-back time is the time when light was emitted from a distant object; for very distant objects it is less than the redshift would indicate, as the object has receded in the meantime.


Active galactic nuclei have some or all of the following properties:

• high luminosity

• nonstellar energy emission

• variable energy output, indicating small nucleus

• jets and other signs of explosive activity

• broad emission lines, indicating rapid rotation



This is the leading theory for the energy source in an active galactic nucleus: a black hole, surrounded by an accretion disk. The strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis.



In an active galaxy, the central black hole may be billions of solar masses. The accretion disk is whole clouds of interstellar gas and dust; they may radiate away as much as 10–20% of their mass before disappearing.




This pair of images shows evidence for a black hole at the center of NGC 4261.



The central portion of M87 shows rapid motion and jets characteristic of material surrounding a black hole.


One might expect the radiation to be mostly X and gamma-rays, but apparently it is often “reprocessed” in the dense clouds around the black hole and reemitted at longer wavelengths.



Particles will emit synchrotron radiation as they spiral along the magnetic field lines; this radiation is decidedly nonstellar.



Process of Science/

Concept Checks

Chapter 24

Astronomy Today

7th Edition

Chaisson/McMillan



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


Because distance-measurement techniques ultimately rely upon the existence 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 out into intergalactic space.


It doesn’t use the inverse-square law. The other methods all provide a way of determining the 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.


It means that the energy source cannot simply be the summed energy of a huge number of stars—some other mechanism must be at work.


When quasars were discovered, they were thought to be faint, relative 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 redshifts, it became clear that quasars were actually among the most distant—and hence the most luminous—objects in the entire universe.


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.

Chapter 23 · 2012. 5. 17. · The Andromeda Galaxy, our closest spiral neighbor, probably...

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