The Milky Way - our galaxy

Team: Reiterer Martin, Reiterer Stefan, Dinhobl Erhard

Introduction

If you look up into the sky on a clear dark night, far from the intrusive glow of streetlights, you will see a faint band of light running across the heavens. This is the Milky Way, our own galaxy. It contains about two hundred billion stars and countless other objects besides. The Milky way viewed through binoculars is a magnificent sight, with thousands of stars in each field of view. The image on the right was taken at the Anglo Australian Observatory and shows the centre of the Milky Way, in the constellation of Sagittarius.

Like the constellations and planets, the Milky Way was important in mythology, and many stories were told to explain it.
Our galaxy is only one of billions of other galaxies in the universe, but it is special to us, because it is home to Earth and our solar system. The Milky Way has been studied extensively by astronomers, and today our ideas about it's nature are set on a more firm foundation. Here we set out to examine what this faint band of light is, where it came from, and where Earth and the Sun fit into the picture.

 

The birth of our Galaxy

Long before the Sun and the solar system formed, before the galaxy existed, the universe was filled with gas - mainly hydrogen, with some helium. This gas was eventually to be turned into stars, planets and people. But before these things could happen, the galaxy had to form.

Astronomers believe that the galaxy formed out of a large, fairly spherical cloud of cold gas, rotating slowly in space. At some point in time, the cloud began to collapse in on itself, or condense, in the same way that the clouds which formed individual stars also condensed. Initially, some stars may have formed as the gas cloud began to fragment around the edges, with each fragment condensing further to form a star or group of stars. Because the cloud was spherical at that time, we do see some very old stars distributed in a spherical halo around the outside of the galaxy today. At such early times, these stars consisted only of the hydrogen and helium gas which made up the cloud.

The cloud continued to collapase, with more and more stars being formed as it did so. Since the cloud was rotating, the spherical shape began to flatten out into a disc, and the stars which were formed at this time filled the disc regions. Once again we see this shape today in the main body of the galaxy. As the formation of new stars continued, some of those which had been created earlier had enough time to evolve to the end of their active lifetimes, and these stars began to shed their atmospheres or explode in huge supernova events. In the process, these older citizens of the still young galaxy enriched the gas in the cloud with the new, heavier elements which they had formed, and the new stars being created in the disc regions contained the heavier elements. Astronomers call these younger, enriched stars population 1 stars, and the older stars population 2.

This process of star formation, then manufacturing heavier elements inside stars and finally returning these elements into the gas between the stars (called the interstellar medium) continued, as it still does today, all the time enriching the medium so that today the gas within our galaxy consists not only of Hydrogen and Helium, but also the other elements which are needed to form the Earth and the rest of the universe we see around us.

 

The Milky Way today

The Milky Way galaxy in which we now live is a very different place to the cold gas from which it formed over 16 billion years ago. No longer is it a spherical mass of hydrogen; today astronomers with radio telescopes have charted the clouds of gas and have found that the Milky Way is a Spiral Galaxy, one of countless others. Whilst we can't "stand back" and see our galaxy as a whole, we can look out into space and see other galaxies which we think may be similar to our own.

 


The spiral galaxy NGC2997 (C) the AAO & The edge-on galaxy NGC4565 (C) KPNO

The images above show two different galaxies. Both are spirals like our own, but whilst the first galaxy is seen "face on", displaying it's spiral arms to full effect, the second system is edge on. We cannot see the spiral arms, but instead this galaxy shows the central "bulge" of the nucleus, and the disc structure described above - becomming thinner towards the edge of the galaxy. The image also shows dark streaks, particularly noticeable through the central bulge. These streaks are in fact lanes of dust which absorb light and prevent us from seeing the stars behind. The significance of these images is that they show different aspects of spiral galaxies, and they are very close in appearance to the Milky Way as it must look to an observer in another galaxy.

The photograph below also shows NGC2997, with the size of the Milky way shown, along with the approximate position of the Sun and solar system, which are located about two thirds of the way out from the centre of the galaxy. Since the galaxy rotates, the Sun "orbits" the centre, taking 200 million years to complete one circuit.

 

Not shown on these pages is the halo which is a spherical region, centred on the nucleus, with a radius of about 50000 light years. This halo contains very old stars, produced early on when the galaxy was still forming. Most of these stars are in vast collections called globular clusters.

 

The Milky Way is the galaxy which is the home of our Solar System together with at least 200 billion other stars (more recent estimates have given numbers around 400 billion) and their planets, and thousands of clusters and nebulae including at least almost all objects of Messier's catalog which are not galaxies on their own (the only possible exception may be M54 which may belong to SagDEG, a small galaxy which is currently in a close encounter with the Milky Way, and thus our closest known intergalactic neighbor). All the objects in the Milky Way Galaxy orbit their common center of mass, called the Galactic Center (see below).

As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Radio astronomial investigations of the distribution of hydrogen clouds have revealed that the Milky Way is a spiral galaxy of Hubble type Sb or Sc. Therefore, out galaxy has both a pronounced disk component exhibiting a spiral structure, and a prominent nuclear reagion which is part of a notable bulge/halo component. It is still not clear if it has a bar structure (so that it would be type SB) or not, but an increasing number of investigations has given some evidence for this, so that the Milky Way may look like M61 or M83, and is perhaps best classified as SABbc.

The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest ( after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer: Many of the dwarf Local Group members are satellites or companions of the Milky Way. The closest of all is above-mentioned SagDEG at about 80,000 light years from us and some 50,000 light years from the Galactic Center, followed by the more conspicuous Large and Small Magellanic Cloud at 179,000 and 210,000 light years, respectively.

The spiral arms of our Milky Way contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from this matter. On the other hand, the bulge component consists of old stars and contains the globular star clusters; our galaxy has probably about 200 globulars, of which we know about 150. These globular clusters are strongly concentrated toward the Galactic Center: From their apparent distribution in the sky, Harlow Shapley has concluded that this center of the Milky Way lies at a considerable distance (which he overestimated) in the direction of Sagittarius and not rather close to us, as had been thought previously.

Our solar system is thus situated within the outer regions of this galaxy, well within the disk and only about 20 light years above the equatorial symmetry plane but about 28,000 light years from the Galactic Center. Therefore, the Milky Way shows up as luminous band spanning all around the sky along this symmetry plane, which is also called the "Galactic Equator". Its center lies in the direction of the constellation Sagittarius, but very close to the border of both neighbor constellations Scorpius and Ophiuchus. The distance of 28,000 light years has recently (1997) been confirmed by the data of ESA's astrometric satellite Hipparcos. Other investigations published consequently have disputed this value and propose a smaller value of some 25,000 light years, based on stellar dynamics; a recent investigation (McNamara et.al 2000, based on RR Lyrae variables) yields roughly 26,000 light years. These data, if of significance, wouldn't immediately effect values for distances of particular objects in the Milky Way or beyond.

The solar system is situated within a smaller spiral arm, called the Local or Orion Arm, which is merely connection between the inner and outer next more massive arms, the Sagittarius Arm and the Perseus Arm.
Similar to other galaxies, there occur supernovae in the Milky Way at irregular intervals of time. If they are not too heavily obscurred by interstellar matter, they can be, and have been seen as spectacular events from Earth. Unfortunately, none has yet appeared since the invention of the telescope (the last well observed supernova was studied by Johannes Kepler in 1604).

Milky Way pictures are wide-field exposures. Besides being attractive and often colorful, they are often suited to view the Milky Way objects (including nebulae and star clusters) in their celestial surroundings of field stars.

 

 

 

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