A galaxy is a collection of stars and interstellar material held together by gravity. The galaxy our Sun lives in is called the Milky Way or the Galaxy (note the capital 'G'). The name `Milky Way' comes from the band of light that is seen overhead on very dark nights. The ancients called it the Celestial River. Galileo showed that the band is actually an edge-on concentration of stars seen looking through the disk of our Galaxy from the inside.
That same band looks very different when imaged at different wavelengths. For example, below is an image of the sky in the near-IR, sensitive to giant stars and dust.
The same region imaged in gamma-rays shows where all the neutron stars and x-ray binaries are found
The x-ray picture of our Galaxy shows where the hot supernova remnants are found (notice the partial arcs)
A deep optical picture shows where the dark nebula are found near the axis of the disk.
An image in the far-IR shows the concentration of old stars in the center of the Galaxy (called the bulge)
An an image taken at the 21-cm wavelength of neutral hydrogen shows how neutral gas avoids the center of the Galaxy and is found mostly out in the arms.
Size of the Milky Way:
Mapping of the Galaxy using star counts was shown to be ineffective due to the extinction of starlight by the interstellar medium.
A Harvard astronomer, H. Shapley, mapped the distribution of globular clusters in the Galaxy's halo to see where the Sun was with respect to the Galactic center. The distance to a globular cluster is found by main sequence fitting, where a HR diagram of a cluster is made and `slide' up and down to match the globular cluster main sequence luminosity to the absolute luminosity of the main sequence of nearby stars. The difference between the apparent and absolute luminosity determines the distance to the globular cluster.
When Shapley did this for 75 globular clusters he had the following plot.
The globular clusters orbit the center of the Galaxy, so where their centroid is on the plot is the Galactic center. The Sun is shown to be off center from the Galactic center by about 18 kiloparsecs or 50,000 light-years.
Later mapping of variable stars, neutral hydrogen radio maps and star clusters gives us our current view of the shape of our Galaxy shown below.
The key components of our Galaxy is a bulge of old stars in the center, a disk of stars and gas and a halo of globular clusters. The disk of our Galaxy is whirlpool shaped with numerous spiral arms spanning out from the center of the Galaxy. In the very center of the bulge of our Galaxy lies a nucleus, possibly a million solar mass black hole.
Notice that the total size of the Milky Way is about 100,000 light-years across, with the Sun about 2/3'rds from the center. Since the Galaxy is similar in shape to the solar system, we use a Galactic coordinate system where the plane of the disk forms the galactic equator. Angular distance from the center of the Galaxy eastward is galactic longitude, angular distance above or below the plane is galactic latitude
Rotation Curve of Galaxy:
The orbital period of the Sun around the Galaxy gives us a mean mass for the amount of material inside the Sun's orbit. But a detailed plot of the orbital speed of the Galaxy as a function of radius reveals the distribution of mass within the Galaxy. The simplest type of rotation is wheel rotation shown below.
Rotation following Kepler's 3rd law is shown above as planet-like or differential rotation. Notice that the orbital speeds falls off as you go to greater radii within the Galaxy. This is called a Keplerian rotation curve.
To determine the rotation curve of the Galaxy, stars are not used due to interstellar extinction. Instead, 21-cm maps of neutral hydrogen are used. When this is done, one finds that the rotation curve of the Galaxy stays flat out to large distances, instead of falling off as in the figure above. This means that the mass of the Galaxy increases with increasing distance from the center.
The surprising thing is there is very little visible matter beyond the Sun's orbital distance from the center of the Galaxy. So the rotation curve of the Galaxy indicates a great deal of mass, but there is no light out there. We call this the dark matter problem, and states that the halo of our Galaxy is filled with a mysterious dark matter of unknown composition and type.
Spiral Structure in the Galaxy:
It is difficult to measure the structure of our Galaxy since we are inside it (trying to see forest from trees). One method is to plot the position of tracers, such as young stars or molecular clouds. One such plot is shown below, the position of nearby HII regions and young clusters of stars. Since their age is young, then they will not have drifted far from their formation places.
Interstellar extinction prevents a map much larger than the one above for optical tracers, but even this plot is enough to show that there are distinct arms of material in the Galaxy. Maps of neutral hydrogen show the global spiral pattern throughout the Galaxy.
It is somewhat surprising that we even see a spiral pattern since the Galaxy does not rotate as a solid body. After a few rotations, the spiral pattern would be wound up very tight, as shown in the diagram below. This is known as the winding dilemma.
One explanation for the winding dilemma is to use density waves. Imagine a scenario such as shown below.
Even though all the cars and trucks are moving at different velocities, there is an apparent overdensity near the slow moving truck. A similar explanation is proposed for spiral arms in our Galaxy, they exist because they exert a gravitational influence on stars and gas that orbit the Galaxy. In particular, gas clouds will orbit slower in the arms and, thus, the density goes up in this region. The spiral arms don't wind up because they are not made of material arms, but rather density patterns that shift like cars in traffic.
The concentration of gas in the spiral arms explains why neutral hydrogen maps trace spiral structure, but why do young stars occur in spiral arms. Higher density of gas means more gas clouds and cloud collisions. This sparks star formation, which leads to HII regions and young clusters. As the young stars age, they drift out of the spiral pattern.
Stellar Populations:
The key tool in understanding the star formation history of the stars in our Galaxy is the HR diagram. However, unlike the simple HR diagrams of globular clusters, the HR diagram for the local Galaxy disk is complicated. The diagram below is the results from the Hipparcos mission for several thousand stars near the Sun. For globular clusters, the stars are born at the same time and evolve in a uniform fashion. But the HR diagram for the local solar neighborhood displays a complex distribution of stars reflecting recent star formation in the Galactic disk.
For example, unlike globular clusters, there are numerous stars occupying the upper main sequence in addition to a well populated giant branch. This indicates that star formation has been an ongoing process in our Galaxy for the last 13 billion years, older stars now evolved to the red giant branch, newer stars filling the upper main sequence. The thicker portion of the main sequence brighter than the Sun represents the range of stellar masses leaving the main sequence.
Stellar age, as given by the epoch of their formation, is not the only constraint to the appearence of a stellar group. Chemical evolution also alters the temperature and luminosity of a star, shifting the red giant branch to hotter temperatures for lower metallicities.
Changes in the chemical composition of a star are due to the initial chemical composition of the gas cloud that it was born from. This heavy elements are mostly produced by supernova explosions, gas clouds become enriched by the ejecta of supernova. The larger the number of supernova near a cloud, the richer in heavy elements it will become.
As time passes, each of the gas clouds in the Galaxy will increase in the abundance of elements such as carbon, iron, etc. So the more recent a star has been formed, the richer in heavy elements it is. A plot of the distribution of globular clusters as a function of their metallicity (shown below) demonstrates that metal-poor clusters are distributed through out the Galactic halo, whereas metal-rich clusters are focused need the bulge.
This is a form of dating system for stars and we deduce that halo stars are the oldest stars in the Galaxy since they have the lowest chemical abundances. The disk stars are the youngest since they are the most metal rich.
A group of stars within the Galaxy that resemble each other in spatial distribution, chemical composition or age are called a stellar population. Stellar populations are not discrete in their properties, but rather have a continuum of characteristics that reflect the changes in star formation with time. Stellar populations are tracers of events in our Galaxy's past and formation.
There are basically three stellar populations in our Galaxy, corresponding to the three distinct dynamical components to the Galaxy; the disk population, the bulge population and the halo population. The disk population inhabits the rotating, flattened region of our Galaxy. The bulge population is restricted to the rounded, central region of the Galaxy, also rotating. And the halo population inhabits the far outer regions of the Galaxy, on long ellipisodal orbits that takes it into the disk and bulge.
The three components not only have distinct kinematic properties, but the types of objects in them also varied. The disk contains all the gas and young stars, although old stars are also found there. The bulge is dominated by old stars and a violent core. The halo contains very old stars and globular clusters. The reason for this separation of stellar types is a clue to how the Galaxy formed.
Once the distinct kinematic components of the Galaxy had been isolated, an interesting fact arose in that the chemical composition of the stars in those components also varied in a regular manner. Disk and bulge stars tend to be rich in heavy elements (above helium on the periodic table). Halo stars tend to be very poor in heavy elements.
Center of the Galaxy:
The center of the Galaxy is obscured from us by thick interstellar clouds of gas and dust. We can observe the Galactic bulge as an ellipse of stars above and below the Galactic plane. In the solar neighborhood, the stellar density is about one star per cubic parsec (one parsec is 3.26 light-years). At the Galactic core, around 100 parsecs from the Galactic center, the stellar density has risen to 100 per cubic parsec, crowded together because of gravity.
Very near the center of the Galaxy, the stellar densities rise to several hundreds of thousands of stars per cubic parsec. The stars are separated by light-weeks rather than years. Starlight at night is bright enough to read by, although there is alot of dust in the Galactic core and much of the energy is radiated in the IR.
As one approaches the Galactic center, not only does the number of stars increase, but a thin ring of gas and dust forms visible by its radio radiation. Streamers of gas are visible in the image below, suggesting an accretion disk over 10 parsecs in size.
The rotation speeds of this inner gas ring indicates that the object located at Sgr A is less than 13 A.U.s in size and masses over 1 million solar masses. Only a black hole of massive proportions would satisfy these requirements.
Given all above information, the following is a trip to the Galactic core.
Formation of Our Galaxy:
The key to understand how our Galaxy formed is the location, ages and chemical composition of the various stellar populations. The oldest stars are in the halo and bulge. The most metal rich stars are in the disk and bulge. From this we deduce that the halo formed first, followed by the bulge then disk. All the gas is located in the disk (which is rotating) because gas clouds can undergo inelastic collisions.
Inelastic collisions occur when two objects collide and share momentum as a single body. Stars are too small to collide within the Galaxy (their cross section is very, very low). But gas clouds are large and can 'stick' together.
Even the above facts, the formation and evolution of our Galaxy must have taken place through a series of continuous stages. First, the Galaxy began as a large single gas cloud a few hundred of thousand light-years across. Passage near other proto-galaxies caused this large cloud to spin. This rotation was far from organized as currents and smaller clouds formed within the proto-Galaxy.
Spheres of gas containing about a million solar masses of material collapsed first, this will become the future halo globular clusters. These first clouds were very weak in chemical abundance, but the first supernovae in the halo stars begins to enrich the interstellar medium.
Cloud-cloud collisions steadily eliminated those clouds with the greatest inclination and those moving in the opposite directions until the distribution of gas clouds became flatter and flatter. Most of the gas is directed to the bulge regions where the high densities produce a highly dense core region.
Lastly, the remaining gas settles into the disk where the rotation slows the formation of new stars until spiral density wave form to dominate the appearance of the Galaxy today.
