This was published on 28 June 2016 in De ccan Herald
OUR GALAXY AND ITS CENTRE
(RECENT RESEARCH SHOWS THAT OUR GALAXY HAS A MASS OF 700 BILLION
SUNS
AND THE SUPERMASSIVE BLACK HOLE AT THE GALACTIC CENTRE IS A
SOURCE OF
COSMIC RAYS)
In 1610, Galileo 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
Infra red 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 center. The shape envisioned by Herschel was that of a a
flattened disc similar to that of a crocodile . His model was taken
to be true for for more than a century till Kapetyn 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
center many light years away from the center .The galactic center
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 center
~ 27000 light years 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 neighbors 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
Supercluster.
While galaxies can also be elliptical or irregular, our galaxy
is a spiral galaxy with several spiral arms. The spiral arms are
termed Sagittarius Arm, Perseus Arm etc. . The width of the galaxy is
about 1 lakh light years. The sun is 27000 light years from the
center. The central bulge has a a diameter of 12000 light years.
Compared to other galaxies, our galaxy is a modest one comprising ~
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 (May 2016) 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 center. The mass
thus determined for the whole galaxy is 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.
TEH GALACTIC CENTER
Just like the center of a city is the busiest place, the
galactic center is also crowded with various types of exotic
celestial objects in a very small region. Because of enormous dust
around the region the galactic center cannot be seen by optical
telescopes. However Infra red telescopes like Hubble and Spitzer have
given a wealth of information about the region. The objects seen at
the centre are : (a) 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
center, its mass is found to be ~ 4.3 million solar masses . Thus
this is much smaller than the billion solar mass black holes st the
center of many Active Galaxies. (b) An energetic supernova remnant
- Sag A east (c) Giant molecular clouds of mostly hydrogen up to
150-200 PC from the centre ( d) A very dense ( ~10 million stars)
star cluster ( Hubble's IR data - March 2016)
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 Cerenkov radiation
produced by the particle sin 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. A very recent analysis (
March 2016) of the gamma ray data from the GC region for 12 years
shows a source coinciding with Sagittarius A*; as the researchers put
it "Somewhere within the central 10 parsec of the Milky Way
there is an astrophysical source capable of accelerating protons to
energies of about one thousand Terra electron volt...the SMBH at the
galactic center is the most plausible source of these ultra high
energy protons," The gamma rays detected in the experiment are
from the interaction of the high energy protons with the hydrogen in
molecular cloud surrounding the center. Thus the age old question
of the source of cosmic rays seems to have some answers (Palahalli R
Vishwanath)
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Pic 1 : Our
Milky way Galaxy 2.milky way with galactic centre
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