15 The Milky Way Galaxy

1. Our Parent Galaxy

A galaxy is a huge collection of stellar and interstellar matter (stars, gas, dust, NSs, BHs etc) isolated in space and held together by its gravity.
Millions of galaxies exist beyond our own "Milky Way" (use it with capital Galaxy).

(a) Galactic Plane (artist’s conception): Gazing from Earth toward the Galactic center (white arrow) we see countless stars stacked up within the thin band of light known as the Milky Way.

Looking in the opposite direction (blue arrow), we still see the Milky Way band, but now it is fainter because our Sun lies far from the Galactic center, so we see more stars when looking toward the center than when looking in the opposite direction.

Looking perpendicular to the disk (red arrows), we see far fewer stars.

Disk and bulge are embedded in a roughly spherical ball of "faint - old" stars known as the Galactic Halo (not drawn in the picture).

(a) The Andromeda Galaxy, our sister galaxy, is about 800 kpc away and it probably resembles fairly closely the overall layout of our own Milky Way Galaxy. The disk and bulge are clearly visible in this image, which is about 30,000 pc across.

The faint stars of the halo, completely surrounding the disk and bulge, cannot be seen here. The white stars sprinkled all across this image are foreground stars in our own Galaxy about a thousand times closer.

2. Measuring the Milky Way

Before 20 cc:
- our Galaxy ≡ the universe
- the Sun is at the center of Galaxy
- Galaxy is at the center of universe
William Herschel (18 cc):
- He counted the stars in different directions of the sky. He assumed that all stars are equal in brightness.
- He concluded that Galaxy was flattened (sun is being at the center), roughly disk-shaped collection of stars.
Early 20 cc:
- Some refinements in counting: width=10 kpc, thickness=2 kpc
Why it is so different than the current view?
- Observation was done in Visible band.
- They didn't take into account of ISM (only after 1930s).

Variable Stars: A New Yardstick

We have already encountered variable stars (novae, supernovae, and related phenomena): These are called cataclysmic variables.
The 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 variability of these stars comes from a dynamic balance between gravity and pressure. They have large oscillations around stability.
Why do they pulsate?
- Opacity increases ➤
- radiation trapped ➤
- internal pressure increases ➤
- star "puffs up" ➤
  - star brightens ➤ surface becomes hot ➤ radius shrinks
  - luminosity decreases ➤ star expands & cools
Pulsating variable stars are found in the instability strip of the H–R diagram.
- As a high-mass star evolves through the strip, it becomes a Cepheid variable.
- Low-mass horizontal-branch stars in the instability strip are RR Lyrae variables.

RR Lyrae Period: 0.5 - 1 days

Cepheid Period: 1 - 100 days

Distance Calculation

Pulsation period and luminosity are quite tightly correlated.
To calculate the distance:
- Observe the period.
- Calculate the luminosity (using the plot/relation).
- Calculate the distance: apparent brightness ∝ L / d2 .

Understanding Period vs Luminosity Graph

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;
- once the period is measured, the luminosity is known and we can proceed as above.

Variable Stars on Distance Ladder

Application of the period–luminosity relationship of Cepheid variable stars allows us to determine distances out to about 25 Mpc with reasonable accuracy.

3. Galactic Structure

Infrared view of Milky Way

It 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 Populations in Our Galaxy

Artist’s conception of a (nearly) edge-on view of the Milky Way Galaxy (The brightness and size of our Sun are greatly exaggerated for clarity). It schematically shows the distributions of

young blue stars,
open clusters,
old red stars,
globular clusters.

The galactic halo:

It and globular clusters formed very early
The halo is spherical.
All the stars in the halo are very old
There is no gas and dust.

The galactic disk:

It contains the youngest stars
It also contains star formation regions (emission nebulae and large clouds of gas and dust).

The galactic bulge:

It surrounds the galactic center.
It contains a mix of older and younger stars.

Population Classes

Population-I:
- YOUNG + ON THE DISK
- Place of Star Forming Regions
- Contains stars of all ages
Population-II:
- OLD + OFF THE DISK (i.e HALO)
- Contains old stars
- Less abundant in heavy (> He) elements

Orbital Motion

Disk stars (blue curves) move in orderly (circular orbits).
Halo stars (orange curves) have orbits with largely random orientations and eccentricities.
Bulge stars (yellow curves) are intermediate between disk and halo stars.
- They don't share the disk's well defined rotation.

Around the Sun:

Small Scale (within a few 10s of pc):
- RANDOM.
Large Scale (100s of 1000s of pc):
- ORDERED.

Entire Galactic Disk rotating "differentially" about Galactic Center.
- @ 8 kpc away, Sun rotates with 220 km/s.
- Period of the Sun:
  - Galactic Year = 225 Million years.

4. Formation of Milky Way

(a) The Milky Way Galaxy possibly formed through the merger of several smaller systems.

(b) In early stages, our Galaxy was irregularly shaped, with gas distributed throughout its volume.

When stars formed during this stage, there was no preferred direction in which they moved and no preferred location in which they were found. Their orbits carried them throughout an extended three-dimensional volume surrounding the newborn Galaxy.

(c) In time, the gas and dust fell to the Galactic plane and formed a spinning disk. The stars that had already formed were left behind in the halo.

(d) New stars forming in the disk inherit its overall rotation and so orbit the Galactic center on ordered, circular orbits.

Cloud's angular momentum effecting galactic evolution

Cloud Density effecting galactic evolution

5. Galactic Spiral Arms

Radio Maps

Interstellar gas prevents us to observe beyond the disk.
Visible spectrum is not enough to reach the largest scale of our Galaxy.
Since Hydrogen is the most abundant element in ISM:
- 21-cm radio emission line from atomic Hydrogen
- It can be used in mapping the whole structure.

Galactic Model

A mathematical model of the Galaxy can be constructed by using the Radio Maps which uses simple circular orbits.
Galaxy has differential rotation:
- Velocity of clouds depend on their distance.
- Strength of the signal is a measure of the density of gas.

Spiral Structure

Centers of two main structures at the Galactic Center coincides in Radio maps.
- Gas Distribution
- Globular Cluster System
Radio emitting gas:
- It extends up to 50 kpc from the Galactic Center.
- The central part of the gas (20 kpc) is confined within about 100 pc on the disk.
- Beyond this distance the gas is warped.
Therefore, due to radio studies:
- Milky Way ≡ A spiral galaxy

Milky Way Spiral Structure. An artist’s conception of our Galaxy seen face-on.

Persistence of the Spiral Arms

Inner parts of the Galactic Disk rotates faster than outer.
- This creates differential rotation.
- Therefore, stars change positions relative to others.
- Therefore, any potential large scale structures will not last long enough on the Galactic Disk.
Thus, spiral arms cannot simply be counted as dense star forming regions orbiting along with the rest of the disk.

Spiral Density Waves

They are coiled waves of gas compression that move through the Galactic disk
Gas squeezes interstellar gas clouds and triggers star formation as they move.
The spirals we see are merely patterns moving through disk, NOT great masses of matter being transported from place to place.
Even though the rotation rate of the disk material varies with distance from the Galactic Center, the wave itself remains intact, defining the Galaxy's spiral arms.
Gas Distribution:
- Gas Motion is indicated in red; Arm Motion is indicated in white.
- BEHIND & FAST: Highest density of gas (marked with dust lanes).
- FRONT & SLOW: Spiral density wave.
- IN BETWEEN: Gas enters the arm from behind; gas is compressed and forms stars.
The spiral galaxy in the inset is NGC1566 which shows very similar behavior described above.

Illustration of Density Wave

The persistence of the spiral arms as density waves may be understood using a traffic jam as an analogy.
The jam persists even though particular cars move in and out of it.
It can persist long after the event that triggered it is over.

6. Mass of the Milky Way

Mass can be calculated by studying motion of gas clouds and stars in the Galactic Disk.
Kepler's Law links Orbital Period, Orbit Size and Masses of any two objects.
For the Sun: P=225 Myr, a=8 kpc and therefore MMW = 1011 M☉ .

This mass is concentrated at the center of the Galaxy. However Galactic Matter is distributed along the Galaxy.

Dark Matter

To determine true mass of the Galaxy, gas motion way beyond the Sun has to be studied.
This can only be done in radio wavelength.
Radio observations give Galaxy's rotation rate at various distances from the Galactic Center.
The resultant plot of rotation speed vs. distance from the center is called Galactic rotation curve.
Note that at the far end of the Galaxy (15 kpc from the center) there exists something (detected in radio) which still rotates with respect to the center.
Therefore
- Luminous portion of the Milky Way (outlined by globular clusters and spiral arms) is visible [aka. tip of the iceberg] and detected by normal means.
- Luminous region is surrounded by an extensive, invisible dark halo which extends well beyond observed boundary.
Since it is not visible (not only visually but in all wavelengths), the content is named as dark matter.
- It is not Hydrogen gas
- It is not made up of star stuff.

7. The Galactic Center

Photograph of stellar and interstellar matter in the direction of the Galactic center.
Because of heavy obscuration, even the largest optical telescopes can see no farther than one-tenth the distance to the center.
Note the location of M8 nebula at extreme top center.
The field is roughly 20° vertically across. The overlaid box outlines the location of the center of our Galaxy.
The inset shows an adaptive-optics infrared view of the dense stellar cluster surrounding the Galactic center, whose very core is indicated by the twin arrows.

Galactic Center Close-Up

(a) An infrared image of part of the Galactic plane shows many bright stars packed into a relatively small volume surrounding the Galactic center (white box). The average density of matter in this boxed region is estimated to be about a million times that in the solar neighborhood.

(b) The central portion of our Galaxy, as observed in the radio part of the spectrum. This image shows a region about 100 pc across surrounding the Galactic center (which lies within the bright blob at the bottom right). The long-wavelength radio emission cuts through the Galaxy’s dust, providing a view of matter in the immediate vicinity of the Galaxy’s center.

(c) A recent Chandra image showing the relation of a hot supernova remnant (red) and Sgr A*, the suspected black hole at the very center of our Galaxy.

(d) The spiral pattern of radio emission arising from Sagittarius A itself suggests a rotating ring of matter only a few parsecs across.

The Galactic Center

A stellar density a million times higher than near Earth.
A ring of molecular gas 400 pc across
Strong magnetic fields
A rotating ring 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.

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