OUR GALAXY

OUR GALAXY

ASTROPHYSICS| GALAXY| BLACK-HOLE| COSMOLOGY| SPACE

The main problem about trying to find out the shape of the Galaxy is that we live inside it; the situation is rather like that of a man who is standing in Piccadilly Circus and trying to work out the shape of London. Originally – and quite naturally – most people assumed that the Sun, with its planets, must lie near the center of the Galaxy; for example William Herschel found that star numbers are much the same all along the Milky Way, though admittedly some parts of it are richer than others.

 The first really reliable clue came from radio astronomy, during the 1940s. It was known that there is a great deal of thinly spread matter between the stars, and it was reasonable to assume that much of this must be hydrogen, which is by far the most plentiful of all the elements. 

In 1944, H. C. Van de Hulst, in Holland, predicted that clouds of cold hydrogen spread through the Galaxy should emit radio waves at one special wavelength: 21.1 centimetres. He proved to be right. The positions and the velocities of the hydrogen clouds were measured, and indicated a spiral structure – which was no surprise, inasmuch as many of the other galaxies are also spiral in form. 

By now we are in a position to draw up what we believe to be a reliable picture of the shape and structure of the Galaxy. It is about 100,000 light-years from one end to the other (some authorities believe this to be something of an overestimate), with a central bulge about 10,000 light-years across. 

The Sun lies between 25,000 and 30,000 light-years from the galactic center, not far from the main plane and near the edge of a spiral arm. Beyond the main system there is the galactic halo, which is more or less spherical, and contains objects which are very old, such as globular clusters and highly evolved stars. 

There are in fact two distinct ‘stellar populations’; the relatively young Population I, found in the nuclei of our Galaxy and others, and the older Population II, which is dominant in globular clusters and other halo objects.

We cannot look directly through to the center of the Galaxy, because of obscuring material; the center lies beyond the lovely star-clouds in Sagittarius. Infra-red radiations are not blocked in the same way, and the center was located as long ago as 1983 by IRAS, the Infra-Red Astronomical Satellite. 

It had long been suspected that there might be a massive central black hole, associated with a compact X-ray and radio source known as Sagittarius A* (pronounced Sagittarius A-star). This was confirmed in 2002 by observations made with the Yepun mirror of the VLT. 

Stars were detected in the central region, and one of these stars, lettered S2, was found to orbit the central object in a period of 15.2 years, approaching the object to a mere 17 light-hours (three times the mean distance between our Sun and Pluto). The orbital speed reached over 5000 kilometres (3100 miles) per second. 

This indicates that the black hole, Sagittarius A*, has a mass around 2.6 million times that of the Sun. Near it there are swirling gas-clouds and highly luminous stars.

We know that the Galaxy is rotating round its center, and that our Sun takes about 225 million years to complete one circuit. 

Yet the general rotation does not follow the expected pattern. Kepler’s Laws show that in the Solar System, bodies moving close to the center (in this case the Sun) move quicker than bodies which are further out, so that, for instance, Mercury moves at a greater rate than the Earth, while the Earth moves faster than Mars. 

In the Galaxy, this sort of situation does not arise, and the speeds are actually greater near the edge of the disk. The only explanation is that the main mass of the Galaxy is not concentrated near the center at all, and there must be a tremendous amount of material further out.

We cannot see it, and we do not know what it is – all we can say for certain is that it exists. The ‘missing mass’ problem is one of the most puzzling in modern astronomy. Nowadays the term ‘Milky Way’ is restricted to describing the luminous band in the sky, though it is true that we still often refer to the Milky Way Galaxy. 

Sweeping along it with binoculars or a wide-field telescope is fascinating, and it is not always easy to remember that each tiny speck of light is a true sun.

KNOWLEDGE 

▲ In our Galaxy, neutral hydrogen (in blue) is aggregated mostly along the four large spiral arms where also HII regions (in red) and massive molecular clouds (in black) are clustered. The galactic center contains numerous expanding regions of ionized hydrogen and giant molecular complexes. It is surrounded by a huge ring of radius about 5 kiloparsecs where a great quantity of atomic and molecular hydrogen is concentrated.

▲ Center of our Galaxy, in an image obtained by the VLT Yepun telescope in 2002. The two small arrows in the center of the picture mark the position of Sagittarius A*. The color of the stars in this image is related to their temperature, with the blue ones being hotter than the red. 

▲ The center of our Galaxy as seen from observations made by the Infra-Red Astronomical Satellite (IRAS). The infra-red telescope carried by IRAS sees through the dust and gas that obscures stars and other objects when viewed by optical telescopes. The bulge in the band is the center of the Galaxy. The yellow and green knots and blobs scattered along the band are giant clouds of interstellar gas and dust heated by nearby stars. Some are warmed by newly formed stars in the surrounding cloud, and some are heated by nearby massive, hot, blue stars tens of thousands of times brighter than our Sun.