The Milky Way Galaxy :
   One of the Places we call ... HOME


side
 


At night, from a dark location, part of the clear sky looks milky. Until the invention of the telescope, nobody really knew what the "Milky Way" was. About 300 years ago telescopes caused a startling revelation: the Milky Way was made of billions and billions of stars. The above picture was taken by the COBE satellite and shows the plane of our Galaxy in infrared light.
 

OUR OWN MILKY WAY GALAXY (Galaxy=Milky in Greek)

HISTORY

1912 -- Henrietta Levitt studied Cepheid variables (Type I) in the Small Magellanic Cloud. Type I are metal-rich and Type II are metal-poor. Cepheids dim and brighten in a regular manner. The time between maximum brightness is called the period of the variable star. Levitt found that the period was related to the absolute luminosity of the Cepheid variable. Hence, she found another way to measure the distance to stars!

cepeheid

1915 -- Harlow Shapley measured the position of 93 globular clusters using RR Lyrae stars. These stars, like Cepheids, are variables stars and have absolute luminosities that depend upon the period of light variation. Shapley found that most of these clusters were located in one part of the sky, near Sagittarius. Late one night, he realized that the "star system" was much bigger than anyone had ever suspected and that we did not live in the center of the star system (recall Copernicus). At that moment Shapley called the only other person in the building - a cleaning lady - and explained to her that "They were the only two people on the Earth who knew that humanity lived in a galaxy".


anatomy

 FROM THE VERY SMALL TO THE VERY BIG
 We have taken a long journey; remember our first lecture!

STRUCTURE

On a clear night the milky way arches along the sky as a patchy band of fixed light. The light comes from huge clusters of individual stars the dark patches come from large clouds of big dust. The Milky Way has a flat disk of stars about 100,000 light years across and 1 or 2 light years thick; an even thinner layer of gas and dust cut across the middle of the disk. At the center there is a low lodge flatten bulge above 20,000 light years across. Our sun lies in the disk about half way to two-thirds from the galactic center. Globular star clusters and a sprinkling of stars are scattered in a spherical halo stretching out perhaps as far as 130,000 light years from the center of the galaxy.

Disk -- The disk component consists of all matter confined to the plane of rotation; yes our Milky Way galaxy rotates around the center of the galaxy. Our sun is suspected to lie half to two thirds of the way from the center and near the Sun the average density of stars is about 0.003 per cubic light year. The most prominent feature of the disk are the spiral arms.

spiral


 

Imagine now you are looking down onto the galaxy. Four spiral arms wind out from the bulge. They are marked out by groups of bright bluish young stars and pink clouds of, of course, glowing hydrogen gas. Closing in on the central bulge the stars are by contrast mainly red and orange.

Bulge -- The nuclear bulge is the dense cloud of stars that surrounds the center of our galaxy. In the galactic core there are about 10 million stars per cubic light year. These are old stars  packed in thousands of times closer than the Sun's region of the galaxy. At the heart of the bulge (the central region of about 15 light years) lies the nucleus of the galaxy. To explain the high-density at the center of our galaxy astronomers have suggested that a black hole of 3 million solar masses might inhabit that part of that space. The center of the galaxy is located near Sagittarius. 


 bulge


Halo -- The halo is a spherical cloud of thinly scattered stars and globular clusters. It contains only about 2 percent as many stars as the disk of the galaxy.


CHARACTERISTICS

Diameter-- Although difficult to estimate, the diameter of the Milky Way is about 100,000 light years. The Sun is about 28,000 light years from the galactic center and it takes about 250 million years - give or take a few million years - to make a complete orbit - around the center of the galaxy. The whole galaxy is turning but not like a rigid disk. Each star and gas cloud is in its own orbit. We call this differential rotation. The manner in which our galaxy rotates tells us that it is surrounded by a huge invisible corona; of course, gravitational forces determine the rotational properties of the galaxy. The galaxy may be 5 times larger from what it appears to be. The gravity of this dark matter is pulling on all the stars; at present nobody knows the origin or composition of the dark matter (see below).

Age -- From the oldest stars in the Galaxy - globular clusters that lie in the halo - we believe that the galaxy is about 15 billion years old. Note that there are about 200 billion stars in the Milky Way. If Earth is the only inhabited planet in the Galaxy, then each person "owns" about 40 stars!

Luminous Mass -- As a galaxy, the Milky Way is actually a giant, as its mass is probably around 500 billion solar masses.

A conversation with Jay Leno:


Excerpts from a REAL interview with High-School Students:

Leno: May I ask you a few questions about science?
HSS:  Sure

Leno: Do you know the name of our galaxy?
HSS:  Hmmm ... I don't remember

Leno: I will give you some help, it is the name of a candy bar
HSS:  BABE RUTH?

Leno: Do you know how many planets are there in the Solar System?
HSS:  I don't know - A HUNDRED?


STELLAR POPULATIONS
 
Population I Stars -- These stars are located in the disk and spiral arms of the galaxy, just like the Sun. They have circular orbits in the plane of the galaxy and are relatively young stars, from one to few billion years old. Population I stars are "metal-rich" containing from 2 to 3 percent of elements heavier than helium. Clearly, our Sun is a population I star - containing a rich concentration of heavy elements, such as Carbon and Oxygen.

Population II Stars -- These stars are located in the spherical component of the galaxy. They have randomly tipped elliptical orbits and are relatively old stars, around ten billion years old. Population II stars are "metal-poor" containing only traces of elements heavier than helium.


STAR CLUSTERS

Open Clusters -- These are loose groupings of stars. They contain just a few hundred stars in a volume with a radius between 7 and 20 light years. The stars are mainly young population I stars. A beautiful example of an open cluster is the Pleiades.

Globular Clusters -- These are spherical groupings of stars that can contain thousands to millions stars. These clusters have radii in the range of 40 to 160 light years. A good example is the Great Globular Cluster in Hercules (M13). The large number of stars causes a strong gravitational pull towards the center, which causes the stars to form tight spherical clusters. The stars are always old population II stars.



GAS AND DUST

Interstellar Dust -- The interstellar dust causes light from background stars to be reddened and dimmed. Sometimes the dust is so thick that light cannot get through and we call such a dust cloud a dark nebula. A beautiful example is the Horsehead nebula in Orion. Sometimes, however, the dust acts like a kind of mirror and reflects light that falls upon it creating reflection nebulas.

Interstellar Gas -- The interstellar gas can glow because of stars embedded in them. A spectacular example of one of these emission and reflection nebulas is the Great Nebula  in Orion.


DARK MATTER: One of the great mysteries of the Universe

The further an object is from a central mass region the slower it moves in its orbit about the central "object". This is a direct consequence of Kepler's Laws (actually Newton's law) and is certainly true for the planets that orbit the Sun; Pluto moves a lot slower than the Earth. However, the stars in the Milky Way, and indeed in other galaxies do not seem to obey this rule: as we move away from the galactic center the orbital speeds of stars do fall as expected; however, beyond about 8000 light years or so from the galactic center the orbital speeds of stars remain roughly constant. The orbital speed as a function of the radial distance from the galactic center is called the galactic rotation curve.

r

Some scientists have suggested that this observation implies that Newton's Law of Gravity is incorrect on large distance scales (nonsense!). Another explanation is that galaxies are cloaked in a large amount of unseen matter, called dark matter, that causes the additional gravitational effects. This is the currently the favored hypothesis. The nature of the dark matter, however, is unknown. It cannot be normal stars because they would produce light and we could see them. They could not be neutron stars and black holes because, although some of these objects are so dense that not even light can escape, we could detect the X-rays that matter around these objects would emit. We hope that in a few years we will be better informed as the true nature of dark matter from the many experiments being conducted worldwide.


FORMATION OF THE MILKY WAY AND ITS SPIRAL ARMS

The Formation of the Galaxy -- According to the traditional theory the Milky Way began as a spherical cloud of gas in which stars and star clusters formed. As the rotating gas cloud collapsed into a disk, the halo stars were left behind as a fossil of the early galaxy. This view clearly explains the orbits and heavy-element content of the various stars. Population II stars are old and some of them formed while the cloud was collapsing, therefore their random orientation and low-metal concentration. In contrast, Population I stars like the Sun, are "young" and formed once the galactic plane was defined. Moreover, they were made from supernova explosion of massive stars making them rich in heavy elements.



The Formation of the Spiral Arms  -- The most striking feature of the Milky Way is the system of spiral arms. The arms contain young, hot, blue stars and suggest that the spiral arms involve star formation. Most of our evidence about the structure of the spiral arms comes from radio maps. At present, the most popular theory of spiral-arm formation is the density wave theory.