Chapter 23: The Milky Way Galaxy

Where is the vast majority of mass in the solar system located? What objects or objects account for most of this mass?

The Sun.

Some variable stars, including novae, supernovae, and similar phenomenon are:

cataclysmic variables

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

intrinsic variables.

There is a theory that there

is a supermassive black hole in the center of our Galaxy.

RR Lyrae stars all have approximately the same luminosity (~100 solar luminosities);

knowing their apparent magnitudes allows us to calculate the distance to the star.

The formation of the Galaxy is believed to be similar to the formation of the Solar System, but on a:

much larger scale.

To understand the dimension of our Galaxy, as well as our place in it, we needed a new distance measuring technique. We finally found one that involves

variable stars.

The spiral arms rotate more slowly than the disk as spiral density waves,

with stars moving in and out of the spiral arms (like cars move in and out of a traffic jam.)

The galactic disk is where the

youngest stars and open clusters are, as well as star-forming regions (emission nebulae and large clouds of gas and dust)

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

So, there must exist some kind of dark matter in the outer regions of our Galaxy. But what could this dark matter be? It is dark at all wavelengths, not just in the visible.

- There are not enough stellar black holes. - MACHOs can only account for ~20% of the dark matter in the Galaxy. - WIMPs are among the leading candidates for dark matter.

What are Massive astrophysical compact halo objects (MACHOS)?

- This is a general name for any kind of astronomical body that might explain the presence of dark matter in galaxy halos. - A macho is made of "normal" matter and emits little to no radiation. - Examples might be brown dwarfs, faint white dwarfs, red dwarfs, and neutron stars.

Measurements of the positions and motions of gas clouds and stars show that the Milky Way has a spiral form.

The Milky Way is actually a barred spiral galaxy (with Earth 8 kpc from the center).

The planets only add a very tiny amount of mass to the total amount of mass in the Solar System.

The Sun is where almost all of the solar system's mass is located.

Surrounding the galactic center is the galactic bulge.

The galactic bulge contains a mixture of old and young stars.

All the stars in the galactic halo are very old, and thus contains red stars.

There is also no gas and dust.

Which planets would experience an incrase in gravitational force and speed from a large amount of added mass between Jupiter and Saturn?

[ Saturn, Uranus, and Neptune ] The extra mass is interior to these planets' orbits, so only these planets will experience an increase in their orbital speeds and in the gravitational forces they feel.

A galaxy is

a collection of stellar and interstellar matter - stars, gas, dust, neutron stars, black holes - isolated in space and held together by gravity.

The gravitational forces felt by the Milky Way Galaxy's stars must be

greater than we expected since the stars are orbiting faster than we expected.

RR Lyrae stars have

the same luminosity (or light) curve and have periods of ~0.5 - 1 day.

The sun is about

25,000 ly away from the Milky Way's center

Earth is located on the Milky Way's Galactic disk at about:

8 kpc, or 2/3 of the way, from the center of the Milky Way.

Most variable stars were once:

A- type main sequence stars.

How much additional mass would need to be added outside of the disk of visible material to produce the observed curve?

Additional mass equivalent to more than twice the mass of the Galaxy would need to be added.

What is self-propagating star formation?

As clouds of gas and dust move through the spiral arms, the increased density triggers star formation.

How many galaxies exist?

Billions! (We are not alone!)

By observing the period of their brightness variations, we can determine their luminosity with great confidence.

From the star's luminosity, we can determine its distance.

Apparently, there is a supermassive black hole in the center of our Galaxy.

It is called Sagittarius A and has a mass of 4 million solar masses. (The event horizon of Sag A is 0.2.)

The Sun orbits at ~220 km/s around the galactic center.

It takes 225 million years for one "Galactic year."

Which of the following correctly lists objects from largest to smallest?

Local Group, Galaxy, Nebula, Star

Where do you expect most mass in galaxy to be?

Most of the mass is in the center of the galaxy.

Once all of the Galaxy is within objects' orbits', velocity should decrease with increasing distance from the Galactic center, as shown by the dashed line.

Our galaxy does not.

How do the orbit speeds of planets farther from the Sun compare to the orbital speeds of planets closer to the Sun?

Planets farther from the Sun orbit the Sun slower than planets closer to the Sun.

There are two types of intrinsic variable stars:

RR Lyrae stars and Cepheids.

If the galaxy had more mass at its center than anywhere else, then we would expect to see stars orbiting at slower and slower speeds farther from the center, just like planets farther from the Sun orbit the Sun slower than planets closer to the Sun.

Since we observe stars far from the center all orbiting at the same speed, the galaxy's mass cannot be concentrated at its center.

The Milky Way is actually a barred spiral galaxy.

The Andromeda Galaxy, our closest spiral neighbor, probably resembles the Milky Way closely.

The 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.

This self-propagating star formation may contribute to the formation of the arms.

The origin of the spiral arms is not yet understood.

How come galactic arms don't get twisted up and merge together?

The spiral arms must rotate at different speeds from the disk of our Galaxy, which rotates such that closer disk material takes less time to orbit the galactic center.

There are seven planets inside Neptune's orbit and zero planets inside Mercury's orbit. However, the interior mass for Neptune is approximately the same as the interior mass of Mercury. Neptune is much farther from the Sun than Mercury.

Therefore, the gravitational force exerted on Neptune is weaker than the force exerted on Mercury. As a result, Neptune has an orbital speed that is much slower than the orbital speed of Mercury.

Variable stars really "open up the universe to us."

They allow us to measure distances out to ~25 Mpc, which is much farther than the previous methods we have discussed.

A planet close to the Sun experiences a larger gravitational force between itself and the Sun than a planet farther from the Sun experiences.

This is because the gravitational force gets stronger as the distance between objects gets smaller, and because the mass inside planets' orbits are all about the same, since most of the solar system's mass is in the Sun.

What are Weakly interacting massive particles (WIMPS)?

This term refers to a candidate dark matter particle that was produced by falling out of thermal equilibrium with the hot dense plasma of the early universe. They're basically some type of weird subatomic particle.

One of the first attempts to measure the Milky Way was done by William Herschel. He counted the number of visible stars he saw when looking in different directions in the sky. Shown is the map of stars he saw.

Unfortunately, he was unaware that the majority of the Galaxy, especially the center, is blocked by gas and dust.

Where is our Sun located in the Milky Way Galaxy?

about halfway between the center and the outer edge

From Earth, we see few stars when looking out of the disk of the Galaxy

and many stars when looking in toward the center

Luminosity gives us the

distance to the star.

The variability of Cephids and RR Lyrae stars (intrinsic variable stars) come from a

dynamic balance between gravity and pressure.

The galactic halo and globular clusters

formed very early.

Many RR Lyrae stars are found in

globular clusters.

Orbits in the halo and bulge are

much more random.

The 4 million solar masses of Sagittarius A* is

obtained by measuring how quickly stars orbit it.

Variable stars are useful because:

of their period-luminosity relation.

Cepheids have luminosities that are tightly correlated with the periods of their oscillations (or brightness changes);

once the period is measured, we can determine the luminosity. ( Then we can observe the star and measure its apparent magnitude, and ultimately determine the distance to the star.)

Cepheid variables have

periods that range from 1 - 100 days.

Stellar orbits in the disk move on a plane and in the

same direction.

We use Kepler's third law to measure that there are 90 billion solar masses between

the Sun and the Galactic center!

The orbital speed of an object only depends on

the amount of mass between it and the Galactic center.

Thus, Cephids and RR Lyrae stars (intrinsic variable stars) are plotted on

the instability strip on the HR diagram.

Thus, the galactic disk appears blue, even though:

there are also old stars.

In order for stars far from the center of the galaxy to all orbit at the same speed,

there must be more mass throughout the entire galaxy, including the disk and halo, than we originally expected.

Globular clusters are found in the halo of the Milky Way so:

they are not obscured by dust. (Harlow Shapley first used globulars to learn the extent of our Galaxy, where the center is, and where we are within it)

Cephids and RR Lyrae stars (intrinsic variable stars) don't have a true hydrostatic equilibrium;

they have large oscillations around stability.