chapter 23 the milky way galaxy - university of texas … 23 the milky way galaxy 23.1 our parent...

Chapter 23The Milky Way Galaxy

23.1 Our Parent Galaxy

23.2 Measuring the Milky Way XXEarly “Computers”

23.3 Galactic Structure

23.4 The Formation of the Milky Way

23.5 Galactic Spiral Arms XXDensity Waves

23.6 The Mass of the Milky Way Galaxy

23.7 XXThe Galactic Center

Units of Chapter 23

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


Our galaxy is a spiral galaxy. The AndromedaGalaxy, our closest spiral neighbor, probablyresembles the Milky Way fairly closely.


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


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

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

23.2 Measuring the Milky Way

We have already encountered variablestars—novae, supernovae, and relatedphenomena.

There are other stars whose luminosityvaries in a regular way, but much moresubtly. These are called intrinsic variablestars.

Two types of intrinsic variables havebeen found: RR Lyrae stars andCepheids.


The upper plot is an RRLyrae star. All such starshave essentially the sameluminosity curve withperiods from 0.5 to 1 day.

The lower plot is aCepheid variable; Cepheidperiods range from about1 to 100 days.


The variability of thesestars comes from adynamic balancebetween gravity andpressure—they havelarge oscillationsaround stability.


The usefulness of these stars comes fromtheir period–luminosity relation:


This allows us to measure the distances tothese stars:

• RR Lyrae stars all have about the sameluminosity; knowing their apparent magnitudeallows us to calculate the distance.

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


We have nowexpanded ourcosmic distanceladder one morestep:


Many RR Lyrae stars arefound in globularclusters. These clustersare not all in the plane ofthe galaxy, so they arenot obscured by dust andcan be measured.

This yields a much moreaccurate picture of theextent of our galaxy andour place within it.


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


The galactic halo and globular clusters formedvery early; the halo is essentially spherical. Allthe stars in the halo are very old, and there isno gas and dust.

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

Surrounding the galactic center is the galacticbulge, which contains a mix of older andyounger stars.


Stellar orbits in thedisk move on aplane and in thesame direction;orbits in the haloand bulge aremuch morerandom.


Any theory of galaxy formation should beable to account for all the properties below:


The formation of thegalaxy is believed tobe similar to theformation of thesolar system, but ona much larger scale:


Measurement of the position and motion of gasclouds shows that the Milky Way has a spiralform:

23.5 Galactic Spiral Arms

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


Rather, they appear to be density waves, withstars moving in and out of them such as carsmove in and out of a traffic jam:


As clouds of gas and dust move through thespiral arms, the increased density triggers starformation. This may contribute to propagationof the arms. The origin of the spiral arms is notyet understood.


The orbital speed of an object depends only onthe amount of mass between it and thegalactic center:


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

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


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

• Stellar-mass black holes?

Probably no way enough of them could have been created

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

This was the best star-like option, but comes up short by asignificant margin.

• Subatomic particles?

•Recall the solution to the “solar neutrino problem”, that neutrinos havea tiny bit of mass. Neutrinos turn out to be the most abundant particlein the universe (more than even photons). But falls short by an order ofmagnitude.

A “weird subatomic particle” is the most (only?) favored candidate. Butwhatever they are, they are too massive to observe in present-dayparticle accelerators (takes too much energy to create them).


Conclusion: Our galaxy (and others) consist mostly of “dark matter,” but wehave no idea what that is, other than it exerts a gravitational force.

A Hubble search for red dwarfs turned up toofew 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 agravitational lens. Observation of such events suggests that low-mass white dwarfs could account for as much as 20% of the massneeded. The rest is still a mystery.


• A galaxy is stellar and interstellar matter boundby its own gravity.

• Our galaxy is spiral.

• Variable stars can be used for distancemeasurement through the period–luminosityrelationship.

• The true extent of the galaxy can be mappedout using globular clusters.

• Star formation occurs in the disk, but not in thehalo or bulge.

Summary of Chapter 23

• Spiral arms may be density waves.

• The galactic rotation curve shows largeamounts of undetectable mass at large radiicalled dark matter.

• Activity near galactic center suggests presenceof a 2 to 3 million solar-mass black hole

Summary of Chapter 23 (cont.)

chapter 23 the milky way galaxy - university of texas … 23 the milky way galaxy 23.1 our parent...

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