The Milky Way

The Milky Way is the great band of light that is best seen in the summer from a dark site. The name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin: via lactea. The Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–370 BC) proposed the Milky Way might consist of distant stars. Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars

In the mid-eighteenth century, Thomas Wright and Immanuel Kant suggested that it could be explained if we were located within a flattened disk of stars. Kant even went on to propose that other fuzzy patches of light were similar distant galaxies coining the phrase “Island Universes” The Milky Way

Gemini Observatory

Trying to see the Milkyway The view in visible light show reflection, emission nebula as well as dark dust. Trying to decide the size and shape is hard when you are embedded in it.

Neutral Hydrogen Using the 21-cm band, the regions of neutral hydrogen may be mapped.

The Radio View

Infrared View

The first attempt In 1785, William Herschel counted all the stars he could see and produced a drawing of the band of light in the night sky From this, Herschel decided that the Sun was embedded in a thin band of stars He wasn’t too far off

The Milky Way Around 1900, Jacobus Kapteyn started a large project measuring motions of stars to determine 3D structure of Galaxy. By 1920, he had formulated a model in which:  the Sun was 650 pc from center of an oblate spheroid, about 5 times wider than it was thick, which had a radius of order 3000 pc.

Globular Clusters At about the same time, Shapley had formulated a very different picture from observations of globular clusters:  the galaxy was 100 Kpc across.  the Sun was 15 Kpc from center of the globular cluster distribution.

The Milky Way The modern picture is closer to Shapley's than Kapteyn's. The two were only reconciled when it was realized than interstellar extinction is very significant. This has the effect of making the distribution of stars appear more centered upon us than it really is (Kapteyn's model) as well as rather small. At the same time, it makes the globular clusters appear fainter and more distant so makes Shapley's model too large. Interstellar extinction is less important for the globular clusters than stars in the disk, and so Shapley was closest. Kapteyn realized that this was possible but could find no evidence for extinction; the phenomenon was only proved beyond doubt in the 1950s from studies of star clusters.

The Milky Way In the modern picture of our Galaxy, the Sun is 8 kpc from the center of the Galaxy. The Sun resides in a flattened circular disk which is of order 200 pc thick and extends out to about 15 Kpc. Thus the disk is of order 100,000 light-years across. At the center of the disk is the spheroid or bulge. This is probably elongated into a bar, making the Milky Way a barred spiral. The bulge does not have a well defined size, but dominates the inner kiloparsec of the Galaxy. This whole structure is surrounded by the halo, a fairly spherical distribution again centered upon the center of the Galaxy. The halo contains stars, globular clusters and dark matter. It is possible that the Galaxy is only a minor component located at a dense clump in a much larger distribution of dark matter that permeates the whole Universe. Finally at the center of the Galaxy is a region some 30 pc or so across known as the nucleus. Star densities in the nucleus are greater than 10 6 stars/pc 3, over a million times the density near the Sun.

Shape and Structure

The Milky Way: The Disk With a radius of 15 Kpc and a thickness of a few hundred parsecs, the disk is very flattened. Stars, gas and dust in the disk execute circular orbits around the center of the Galaxy under the influence of the gravitational attraction of the mass of the Galaxy. –At the Sun, the orbital speed is about 200 Km/sec, which means that one orbit takes 250 million years. The disk is where most of the action now takes place, with giant molecular clouds, spiral arms, dust clouds, stars, star forming regions and core-collapse supernovae. The disk is about years old. There are of order stars in the disk. The disk probably formed from the collapse of gas clouds with their spin or `angular momentum' more-or-less preserved during collapse. This is in contrast to the formation of the halo.

The Disk Neighborhood Of course we see individual (sometimes multiple) star systems --- Many of them! From the 'normal' To the spectacular

The Disk Neighborhood Then there are the vast Star Clouds --- Open clusters both older (Hyades) and newer (Pleades) and those still being born (Orion and Eagle nebulae)

The Disk Neighborhood There is a lot of gas - both hot (glowing) and cold (dark) There is a lot of dust And, probably, a great deal of 'Dark Matter'

The Milky Way: The Bulge Hidden behind the galactic plane to a large extent, the bulge is less well understood than the disk. Stellar orbits there are very elongated, there are many cool old stars there. It appears to have an elongated bar shape.

The Milky Way: The Halo The halo extends out to 25 Kpc. It is probably a flattened sphere in shape, although it is hard to tell. It contains relatively few stars - of order but a great deal of unseen or dark matter. It has little rotation and stars are on very elongated orbits. Thus nearby halo stars appear to move backwards relative to the motion of the disk. The halo probably formed very early in the history of the Galaxy from the merger of large star clusters. –This is a complex process, but means that stars is the halo are in general very old and therefore lacking in heavy elements cooked up in later supernovae. –Thus they can be spotted as stars with low element abundance as well as high speed (of order 200 km/s) motion. The globular clusters are part of the halo and may be remnants of the early clusters that formed it.

The Halo The halo has many globular clusters in orbit about the galactic center

The Milky Way: The Nucleus At the center of the Galaxy sits the nucleus. This is best seen at infrared and radio wavelengths which are less affected by dust than visual wavelengths. In recent years high resolution observations of stars near the Galactic center at IR wavelengths have revealed high speed motions. –The interpretation of these motions is that there is concentration of over 2 million solar masses within the inner few light-weeks. –It is thought that this might be a black-hole as no star cluster can survive for long at such high density. –It is not an active one in that gas is not accreting onto it.

The Galactic Nucleus These are near- infrared images of the galactic center

Our Current View: You are here

A Roadmap to Home

The Warp in the Milky Way Choosing distant stars from Hipparcos data, astronomers at the Turin Observatory and Oxford University confirm that the disk is not really flat. Outside the Sun's orbit, stars follow tilted orbits. The result is the curved shape of the disk shown here, resembling the brim of a hat. For clarity's sake, the extent of the warping is exaggerated here (X10).

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they circle the Galaxy, causing vibrations at certain frequencies when they pass through its edges. Note: The Magellanic galaxies have too small a mass by themselves to do the job, but using Dark Matter as an amplifier seems to account for everything. The Warp in the Milky Way