Our galaxy and its centre


Our galaxy and its centre

In 1610, Galileo Galilei was the first to show with his telescope that the universe consists of many more stars than what is seen by the naked eye. The idea that the stars we see all belong to the same group was due to William Herschel, who also discovered Uranus and the infrared rays. He tried in 1793 to find the shape of this collection of stars by carefully counting the stars in all directions.

When he found that the numbers of stars were practically same in all the directions, he concluded that the solar system must be close to the centre. The shape envisioned by Herschel was that of a flattened disc, similar to a crocodile. His model was taken to be true for more than a century till Jacobus Kapteyn tried to improve it.

However, since there is a lot of dust in the skies which affect the light reaching us from stars, our view of the Milky Way is blocked in many directions and our view is limited to a universe that is really smaller than what it really is.

Our understanding of the galaxy underwent a major change when Harrow Shapley in 1918 found that the stars orbited a common centre many light years away. The galactic centre was determined to be in the direction of Sagittarius. The picture we have now of our galaxy is that it is spiral-shaped, with the centre close to 27,000 light years away from us. While Shapley thought that all the objects in the sky belong to our galaxy, Edwin Hubble and others showed that many of them are in fact external galaxies.
With time, we have learnt that our galaxy is only one of the possible billion galaxies in the universe. Our closest neighbours include the Large and Small Magellanic Clouds, and the Andromeda Galaxy. Along with some 50 other galaxies, these galaxies make up a cluster known as the Local Group which itself is one of many such groups in the Virgo

While galaxies can also be elliptical or irregular, our galaxy is a spiral one, arms. The spiral arms are termed Sagittarius Arm, Perseus Arm etc. The width of the galaxy is about one lakh light years. The sun is 27,000 light years from the centre. The central bulge has a diameter of 12,000 light years. Compared with other galaxies, our galaxy is a modest one, comprising around 200-400 billion stars.

Since only a fraction of the galaxy is visible to the telescopes and the fact that we are within the galaxy makes it difficult to determine the mass of the galaxy, which is the sum of masses of many objects including stars, black holes, gas clouds, dust, dark matter etc. For this purpose, the velocities of globular star clusters (spherical groups of stars) that orbit the Milky Way  were studied recently by Canadian scientists. The velocities depend on gravity and thus the mass of the galaxy.

They also gave a ‘mass profile’ of the Milky Way, an estimate of the mass contained within any distance from the galactic centre. The mass thus determined for the whole galaxy is about 700 billion solar masses. The knowledge of the mass helps in understanding how galaxies evolve with time. Since the visible mass (mostly stars) of the galaxy is about 60 billion suns, the dark matter contribution can be calculated as due to 88% of the total mass.

In the middle
Just like the centre of a city is the busiest place, the galactic centre is also crowded with various types of exotic celestial objects in a very small region. Because of enormous dust around the region, the galactic centre cannot be seen by optical telescopes. However, infrared telescopes like Hubble and Spitzer have given a wealth of information about the region.

The objects seen at the centre are:
*A Supermassive Black Hole (SMBH) called Sagittarius A*. Its diameter is about the size of the orbit of mercury (0.3 AU). By studying the motions of stars around the centre, its mass is found to be around 4.3 million solar masses. Thus, this is much smaller than the billion solar mass black holes at the centre of many active galaxies.

*An energetic supernova remnant — Sag A east
*Giant molecular clouds of mostly hydrogen
*A very dense star cluster

Cosmic Rays, discovered in 1912, are  mostly protons, with their energy spectrum extending to energies 100 million times higher than the ones produced in the accelerators like LHC. Lower energy cosmic rays could be coming from supernova remnants whereas the origin of the higher energy ones is still unknown .

Cosmic rays get deflected in the magnetic fields in space and thus arrive isotropically on earth’s atmosphere. Therefore to find the source of cosmic rays one has to look for gamma rays and neutrinos produced by the primary cosmic rays. It is with this aim the field of gamma ray astronomy came into existence several decades ago. The standard technique at these energies is to look for Cherenkov radiation (a kind of electromagnetic radiation) produced by the particles in our atmosphere.

While high energy gamma rays from several sources have been detected, these  can be explained as due to electrons and thus have no connection with cosmic rays.    The HESS observatory in Namibia had some indication of galactic centre region as a cosmic ray source in their early data a decade ago.

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