chapter 14 the milky way galaxy

Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 14The Milky Way

Galaxy

Copyright © 2010 Pearson Education, Inc.

Chapter 14The Milky Way Galaxy


Units of Chapter 14Our Parent GalaxyMeasuring the Milky WayGalactic StructureThe Formation of the Milky WayGalactic Spiral ArmsThe Mass of the Milky Way GalaxyThe Galactic Center


The Milky Way• 200+ billion stars• 100k light years across• 10k light years thick at

the galactic bulge• 1k light years thick in

the disc.• 1 of 100s of billions


Women In Astronomy• Williamina Fleming

– Catalog of brightness and spectra• Antonia Maury

– Stellar spectra leading to H-R Diagram• Annie Cannon

– Spectra classification system• Henrietta Leavitt

– Cepheid variable stars


Williamina Fleming

Maid to Curator of Astronomical Photographs

http://en.wikipedia.org/wiki/File:Williamina_Paton_Stevens_Fleming_circa_1890s.jpg


Antonia Maury

Classification of stellar spectra leading to H-R Diagram


Annie Cannon

spectral classes O, B, A, F, G, K, M

http://en.wikipedia.org/wiki/File:Annie_Jump_Cannon_1922_Portrait.jpg


Henrietta Leavitt

'period-luminosity relationship‘ of Cepheids


Human Computers

Harvard Observatory 1910


Cepheid and RR Lyrae Stars

• Two stars played a key role in creating a model of the universe.

• Variable stars allowed measurement of luminosity and thus distance.

luminosityApparent brightness

distance2


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

Measuring the Milky Way


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.



The usefulness of these stars comes from their period–luminosity relationship.



a) measuring distances with Cepheid variable stars.

b) identifying the mass of the Galaxy’s central black hole.

c) determining the masses of stars in an eclipsing binary system.

d) using spectroscopic parallax to measure distances to stars.

Question 4

The period – luminosity relationship is a crucial component of


a) measuring distances with Cepheid variable stars.

b) identifying the mass of the Galaxy’s central black hole.

c) determining the masses of stars in an eclipsing binary system.

d) using spectroscopic parallax to measure distances to stars.

Question 4

The period – luminosity relationship is a crucial component of

Cepheid variable stars with longer periods have higher

actual luminosities; short-period Cepheids are dimmer.


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.



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



a) supernova remnants.b) white dwarf stars in the spiral arms.c) red giant variable stars in globular

clusters.d) bright O and B stars in open clusters.e) X-ray sources.

Question 1

The location of the galactic center was identified using


a) supernova remnants.b) white dwarf stars in the spiral arms.c) red giant variable stars in globular

clusters.d) bright O and B stars in open clusters.e) X-ray sources.

Question 1

The location of the galactic center was identified using

Harlow Shapley used pulsating RR-Lyrae variables as distance indicators

to the globular clusters. He then deduced the distance and direction of the Milky Way’s center.


a) about 30 Kpc from the center in the halo.

b) 30,000 light-years from the center in a globular cluster.

c) at the outer edge of the galactic disk, in the plane.

d) about halfway from the center, in the spiral arms.

e) in the bulge, near the Orion arm.

Question 2

Our Sun is located in the Milky Way Galaxy


Question 2

Our Sun is located in the Milky Way Galaxy

The Sun orbits the center of the Galaxy within the disk,

taking about 225 million years to complete one orbit.

a) about 30 Kpc from the center in the halo.

b) 30,000 light-years from the center in a globular cluster.

c) at the outer edge of the galactic disk, in the plane.

d) about halfway from the center, in the spiral arms.

e) in the bulge, near the Orion arm.


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

Our Parent Galaxy


a) a spiral galaxy.b) a barred spiral galaxy.c) an elliptical galaxy.d) a quasar.e) an irregular galaxy.

Question 3

Detailed measurements of the disk suggest that our Milky Way is


a) a spiral galaxy.b) a barred spiral galaxy.c) an elliptical galaxy.d) a quasar.e) an irregular galaxy.

Question 3

Detailed measurements of the disk suggest that our Milky Way is

Measurements of stellar motion in and near the bulge imply that it is football shaped, about half as wide as it is

long, characteristic of a barred spiral galaxy.


Andromeda which can be seen with the naked eye. 2.5Mly away

Spiral Galaxies

M101

NGC 4565

M31


Finding Andromeda

near Pegasus


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 – which 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.



Overlay of Images


This artist’s conception shows the various parts of our Galaxy, and the position of our Sun.

Galactic Structure


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

Galactic Structure


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

Galactic Structure


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

Galactic Structure


Any theory of galaxy formation should be able to account for all the properties below.

The Formation of the Milky Way


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

The Formation of the Milky Way


a) the spiral arms formed first.b) the globular clusters formed first.c) the disk component started out thin

and grew.d) spiral density waves formed first.e) the bar in the bulge formed first.

Question 5

In the formation of our Galaxy


a) the spiral arms formed first.b) the globular clusters formed first.c) the disk component started out thin

and grew.d) spiral density waves formed first.e) the bar in the bulge formed first.

Question 5

In the formation of our Galaxy

Globular clusters contain very old stars, no gas or

dust, and orbit around the center randomly.


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

Galactic Spiral Arms


The spiral arms cannot rotate along with the galaxy; they would “curl up.”



Rather, they appear to be density waves, with stars moving in and


out of them much 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.



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

The Mass of the Milky Way Galaxy


a) the Sun’s mass and velocity in orbit around the galactic center

b) the rotation of the bulge and disk components

c) the Sun’s age and age of globular cluster stars

d) the motion of spiral arms and the mass of the central black hole

e) the Sun’s orbital period and distance from the center

Question 6

What two observations allow us to estimate the Galaxy’s mass?


Question 6

What two observations allow us to estimate the Galaxy’s mass?

Use the modified form of Kepler’s law to find the mass:

Total mass = (orbital size)3 / (orbital period)2

a) the Sun’s mass and velocity in orbit around the galactic center

b) the rotation of the bulge and disk components

c) the Sun’s age and age of the globular cluster stars

d) the motion of spiral arms and mass of the central black hole

e) the Sun’s orbital period and distance from the center


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.



Question 7

a) 21-cm maps of the spiral armsb) the rotation curve of the outer edges

of the Galaxyc) orbits of open clusters in the diskd) infrared observations of new star-

forming regionse) X-ray images of other galaxies

What suggests that the mass of our Galaxy extends farther than its visible disk?


a) 21-cm maps of the spiral armsb) the rotation curve of the outer edges

of the Galaxyc) orbits of open clusters in the diskd) infrared observations of new star-

forming regionse) X-ray images of other galaxies

Question 7

What suggests that the mass of our Galaxy extends farther than its visible disk?

The outer edges of the Galaxy’s disk rotate much faster than they should. Most of the mass of the

Galaxy must be dark matter.


What could this “dark matter” be? It is dark at all wavelengths, not just the visible.• Stellar-mass black holes?

Probably no way enough 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 evidence so far



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 about half 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.

The Galactic Center


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

The Galactic Center

Infrared

Radio X-ray

Radio


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

The Galactic Center


a) tidal forces from the Andromeda Galaxy.b) accretion disks around neutron stars.c) gamma-ray bursts.d) gravitation from globular clusters.e) a supermassive black hole.

Question 8

High-speed motion of gas and stars near the Milky Way Galaxy’s center is explained by


Question 8

High-speed motion of gas and stars near the Milky Way Galaxy’s center is explained by

Recent observations estimate the black hole to be

4 million solar masses.

a) tidal forces from the Andromeda Galaxy.b) accretion disks around neutron stars. c) gamma-ray bursts.d) gravitation from globular clusters.e) a supermassive black hole.


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.

The Galactic Center


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.

The Galactic Center

Evidence

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