Lecture Notes

Arny Chapter 15 & 16

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You should first read Overview Six

Chapter 15 The Milky Way Galaxy
We will cover just sections 1, 2, 3, 7, and 8 of chapter 15.
Skip sections 4, 5, and 6.

Chapter 16 Galaxies
We will cover just sections 1, 2, and 3 of chapter 16.
Skip sections 4, 5, and 6.

Section 15.1 Discovering the Milky Way
The term "Milky Way" originally just described a band or streak of light across the nighttime sky, see figure 15.1 on page 446.
Recall Galileo discovered that the Milky Way was just billions of faint stars.

So what does our galaxy look like?
That is a surprisingly difficult question to answer.
This section does a good job of describing how the shape of our galaxy was determined and the history of size estimates for the galaxy.
It's like standing at a street corner in Bakersfield and trying to make a complete map of the city.
It would be a lot easier if you were in a helicopter above the city.
We're trapped inside our galaxy, so it's hard for us to make the map.

Section 15.2 Overview of the Milky Way
The structure of our Milky Way galaxy is like this:

Side View: [See figures 15.6 and 15.8]
[disk, nuclear bulge or core, globular clusters, "halo"]

Top View: [See figures 15.7 and 15.8]
[spiral arms, disk, bulge, Sun]

The Milky Way is a mostly flat
disk with a spiral structure.

The Milky Way contains over
100 billion stars.
The disk has a diameter of
about 100,000 light-years
and a thickness of about 1000 light-years.
Our Sun is about 30,000 light-years from
the center, about half-way from the center
to edge.

All the stars in the galaxy are moving.
Gravity causes the stars to move in orbits, the gravitational pull from all the other stars.
The orbits all go around the Galaxy's center in giant ellipses.
Our Sun makes a near-circular orbit every 240 million years.
[Since it was born, 4.5 billion years ago, the Sun has made about 19 revolutions around the galaxy, or you could say it is about 19 galactic-years old.]

Section 16.3 Dark Matter
Here's a surprise: the Sun is orbiting the Galaxy faster than it should be!
The Sun orbits because of all the mass in the galaxy inside its orbit (the mass outside of its orbit cancels out).
But the mass we see is not as much as the gravitational pull indicates.
There seems to be more mass than that we can see.

Astronomers work this out by
making a rotation curve.
See figure 16.17 on page 495.

The "expected" line is the orbiting
speed of stars we expect based on
the stars (and gas and dust) we see.
The "observed" is how the stars in
our Galaxy (and others) actually move.

This is telling us that there is more mass
in our Galaxy than that we see in stars.

Stars appear to make up only a small fraction of our Galaxy's mass!
What is the rest of our galaxy made of?

Brown dwarfs and white dwarfs?
Black holes?
Neutrino particles?
Something new and unexpected?

Astronomers have not yet figured it out.
This unseen component of the Galaxy is called dark matter.
It appears to exist in all other galaxies as well.
From the way stars move (from the rotation curve), astronomers can tell that much of this dark matter is in the "halo" of the Galaxy, a spherical region beyond most of the stars (see figure 15.8 on page 450).

Some recent searches indicate that some of the dark matter is dim stars, white dwarfs, red dwarfs, and the like, but at best this explains only a fraction of the dark matter.
We don't know what most (90%) of our Galaxy is made of! (95%?)
This is currently one of the biggest unsolved problems in astronomy.

Stars in the Milky Way fall naturally into two groups based on their composition.
Called Population I and Population II stars.

Population I Stars:
Usually found in the disk of the galaxy.
Usually include young stars, less than a few billion years old.
Groups of Pop. I stars appear overall blue in color.
Note: Blue because contain blue supergiants which can only be young.
The stars contain a higher-than-normal proportion of heavy elements ("metals").

Population II Stars:
Usually found in the bulge and halo of the galaxy (core and globular clusters).
Older stars, often approaching 10 billion years old.
Groups of Pop. II stars appear overall red or yellow in color.
Note: Old stars are red or yellow giants or red dwarfs, hence the color.
These stars contain basically no elements heavier than H and He.

Globular Clusters:
[See Fig. 15.10 on page 456]
Contain Population II stars, from 100,000 to over 1,000,000 of them.
The Milky Way has more than 150 globular clusters.
Orbit the center of the Milky Way in highly elliptical orbits.

Open Clusters:
Contain Population I stars, typically a few hundred stars.
The spiral arms are composed mostly of open clusters.


Section 15.8 History of the Milky Way
Our model for how the Galaxy formed is similar to our model for how stars and the solar system form, just on a bigger scale.
The Milky Way started as a huge gas cloud (maybe a million light-years across) containing a few trillion (10^12 ) solar masses of H and He, no metals.

This huge cloud collapsed due to gravity.
Small initial spin magnified, shaped cloud into disk.
The cloud fragments into hundreds of billions of smaller clouds, these go on to form individual stars.
The mass ends up spread out and all orbiting the common center.

Globular clusters are groups of stars which formed very early on when the cloud was still spherical (the globulars still occupy a spherical volume).
The Milky Way is about 10 billion years old (maybe 12 or even 15 billion).
Our Sun and solar system has been around only for the second half of this lifetime.

Stars formed early in the bulge and halo, that's why we see Pop. II stars there.
Early on there was only H and He, so that's what Pop. II stars are made of.

Most of the gas and dust in the galaxy is in the spiral arms.
This is the only place new stars are being formed.
Much of the gas and dust is from past supernova explosions, and contains heavier elements produced by the previous generation of stars.
This is why Pop. I stars are young, have metals, and appear in spiral arms.

Population II stars are first-generation stars.
Population I stars are second-generation stars.
If astronomers had known this from the start, they would've swapped the I and II.

Section 15.7 The Galactic Center
Our solar system formed from a big gas cloud.
Most of the gas fell into the center to form the Sun.
For galaxies, the mass is more spread out but there is still a concentration of mass ending up at the center.

The center of the galaxy (bulge or core) is densely packed with stars.
And at the very center is a gigantic black hole!
A black hole with maybe a million solar masses of material.

All large galaxies appear to contain super-massive black holes.
[Determined from pictures and motions of stars near the center.]
Early in the life of galaxies, when there is still lots of loose gas and dust falling into the black hole, galaxies have super-bright centers and appear as quasars.
[More discussion on this topic can be found in section 16.4.]

Section 16.1 Discovering Galaxies
The words `galaxy' and `universe' used to mean the same thing.
It wasn't until 1924 that some nebulae in the sky were recognized as huge collections of stars (galaxies) and not just nearby gas clouds.
Edwin Hubble did this by spotting Cepheid variables.
The Milky Way (our galaxy) is just one of many in the universe.

Types and properties of galaxies:

Spirals:
Disks with central nucleus and surrounding "arms" of stars.
Types Sa, Sb, Sc, Sd
Sa = bright nucleus, tight arms =>

Sd = small nucleus, open arms =>
Sb, Sc = in-between
See figures 16.3, 16.8, 16.9

Barred Spirals:
Types SBa, SBb, SBc, SBd
These have a "bar" of stars which passes through the nucleus.

Recent results suggest that our
Galaxy is a barred spiral, type SBb.
Previously thought to be Sb.
(Keep in mind, we can only see
our galaxy from inside.)
A type SBb galaxy looks like the "Top View" diagram done earlier for the Milky Way.
See figures 16.6 and 16.9.

Spirals and Barred Spirals:
Large. Typically 90,000 LY in diameter.
Blue disk (young stars).
Red halo and nucleus (old stars).
Contain about 15% gas and dust.


Ellipticals:
Categories E0 to E7
E0 = spherical
E7 = cylinder, football, or pancake shaped (probably due to greater initial spin).
Contain older stars, overall color more red.
Little gas and dust (this explains lack of new stars).

Giant Ellipticals:
150,000 LY in diameter.
Largest, brightest type of galaxy.

Dwarf Ellipticals:
30,000 LY in diameter.
Small and dim.


Irregulars:
Neither elliptical nor spiral.
Triangular, Cone, Crescent, Weird, ...
Small, 20,000 LY in diameter.
Young stars, lots of gas and dust, blue color.
50% of all galaxies???

Do galaxies evolve and change types?
Do spirals merge and become ellipticals? (currently favored)
Are irregulars due to disruptive tidal forces skewing ellipticals? That is, maybe irregulars are the result "collisions" or close encounters. (also currently believed)
These questions have not been fully answered.
No convincing model of how all galaxies evolve has been found.



Section 16.2 Measuring the Properties of Galaxies

Distances to Galaxies
We've never been to another galaxy, not even close.
Nothing launched from Earth has ever even visited another star, we haven't explored anything other than our own solar system.
How can we determine the distance to another galaxy?
You need telescopes just to see them.
We're not even sure about the size of our own galaxy.

Most our distance estimates involve the same idea.
If we know the luminosity of an object (how much light it emits), we can determine the distance.
Just combine that luminosity with the observed brightness and use the inverse-square law.
This is called the method of standard candles.

1. Cepheid Variables
Cepheid variables are a special type of giant star.
They are pulsating variable stars, their luminosity varies with time.
We learned about them back in section 13.5.
Cepheid variables are regular, they get brighter than dimmer than brighter than dimmer with a regular period.
Cepheid variables have periods that range from days to months.

The important property of Cepheid variables is that their period is related to their maximum luminosity.
The bigger, more luminous Cepheids vary in brightness over a longer period of time.
This is called the period-luminosity relationship.

Cepheid variables are a wonderful tool.
They are bright stars, we can see them even in other galaxies.
Watch it, measure how long its period is (time from one max brightness to the next).
From the period, we can use the period-luminosity relationship to get the luminosity.
Knowing the luminosity, we can figure out the distance.

Cepheid variables are our most accurate tool for determining distances to other galaxies.
Unfortunately, we can only see Cepheids in nearby galaxies.
This is one of the main tasks the Hubble Space Telescope is being used for, it can pick out Cepheid variable stars even in fairly distant galaxies.
But not really distant galaxies, for them we use:

2. Supergiants, Planetary Nebula, Novas, Globular Clusters, Supernovae
We have a rough idea how luminous each of these things are and they are all bright enough to be seen in very distant galaxies.
Observed brightness + guessed luminosity => distance.

These methods are not very accurate, because we don't really know how luminous the objects are.
Having multiple things to measure does improve the accuracy.

Best are Type I Supernovae (white dwarf pushed over the Chandrasekhar limit by a companion red giant).
Type I Supernovae all have the same luminosity (because they are explosions in every case of essentially identical objects).
But supernovas are extremely rare, and Type I's even more so.
We can only use this technique for galaxies in which we happen to have a supernova occur.

Tully-Fisher Method
The luminosity of a galaxy depends on the number of stars.
The number of stars is related to the total mass of the galaxy.
The total mass is related to how fast the galaxy rotates.
The galaxy's rotation affects the light reaching us (Doppler shifts).
So from the arriving light we can estimate the actual mass and luminosity.
Then we can calculate the distance.

Graininess of Galaxy's Picture
The closer a galaxy is, the more detail we should be able to see in its picture.
Judging our ability to resolve stars can be used to estimate the distance.

Galaxy Size
If we know (or can guess) the actual size of a galaxy, we can determine its actual distance from how big it appears.
On page 493 of the text, this process is done in reverse.
There they use the distance and apparent size to figure out the actual size but it can also be done in reverse.

V = velocity moving away from us (in km/s)
D = distance away from us (in megaparsecs, Mpc)
A megaparsec = million parsecs = 3.26 x 10^6 light-years
H = Hubble's constant

Hubble's constant has a value of roughly 65 (km/s)/Mpc
That is, for every million parsecs of distance of galaxies (or anything) away from us, they move away from us with an additional 65 km/s (about 145,000 mph).

We are uncertain about the value of Hubble's constant because we are uncertain of the exact distance to other galaxies.
Some astronomers continue to argue that H is as low as 50 or as high as 80.

We can use Hubble's Law to calculate distances.
Look at a galaxy, measure the redshift of its spectrum, calculate the recessional velocity, use Hubble's Law to calculate the distance.

End chapters 15 and 16.