THE GALAXY AND ITS CENTRE - PALAHALLI VISHWANATH (Deccan Herald)

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)

----------------------------------------------------------------------------------------------------

Pic 1 : Our Milky way Galaxy   2.milky way with galactic centre

THE COOLER UNIVERSE (THE IR SKY) - PALAHALLI VISHWANATH IinDeccanHerald

THE COOLER UNIVERSE

Palahalli R Vishwanath 

Article in DeccanHerald 8 nov 2016

 Pics are Comaprison in INR/Visible light  of (1) Andromeda galaxy (2) ORION constellation

 

(Many objects in the universe which are much too cool and faint to be detected in visible light, can be detected in the infrared . Unlike X and Gamma ray astronomies which deal with death of stars, ,Infrared astronomy is concerned with their birth. IR studies are also contributing significantly to the the exciting field of Exo Planets )

The great astronomer William Herschel who discovered Uranus also did a simple and beautiful experiment in1800 in which white light was sent through a prism and temperatures measured for different colors of the ensuing spectrum. He reasoned that the unexpected excess in temperature in the control region next to red is due to to light which cannot be seen and was eventually named Infra Red (IR) rays. While this was the first non visible light , whole of electromagnetic spectrum was eventually discovered, the others being Ultraviolet, Radio, X rays and Gamma rays . By early 1900s IR rays had been detected from moon and several objects in the solar system

A piece of coal when heated produces at first heat which is due to IR rays. In fact, any body which has any temperature is a source of IR rays. The wavelength of IR extends from 700 nanometers (just beyond red) to ~ 1 millimeters. The study of infrared , depending upon the instruments, is divided into three regions , near-infrared, mid-infrared and far-infrared. While most big optical telescopes in sites with less moisture can be used for studies in the near IR region, instruments above atmosphere are needed for other regions of IR. The important satellite IR missions have been the IRAS, the ISO , SPITZER , HERSCHEL, SOFIA etc. Significant results keep coming from both SPITZER and SOFIA which are expected to function for at least 10 more years. Lyman Spitzer was the first person to propose the idea of placing a large telescope in space and was the driving force behind the development of the Hubble Space Telescope.

Discoveries in IR astronomy

1) BIRTH OF STARS : IR rays give important information about birth of stars since they are suited to study proto-stars and star formation regions which are at of lower temperatures . Large number of stars which are too cool to emit visible light or are hidden behind obscuring dust have been detected by this method. Brown dwarfs are an odd set of objects that are neither planets nor stars. The best hope for finding brown dwarfs is in using infrared telescopes, which can detect the heat from these objects. IR studies seem to indicate that there is 1 brown dwarf star for every 6 stars in

our galaxy. Also SOFIA , a modified Boeing 747 jetliner, with a 2.5 meter IR telescope which can cruise near the edges of the atmosphere observed very recently (October 2016) the collapse of few interstellar clouds on their way to becoming new stars. Detecting such infall in proto-stars which happens very fast is very difficult to observe, but is critical to confirm our overall understanding of star formation. This aircraft can also travel to almost any point on the Earth's surface, allowing observation from the northern and southern hemispheres.

2. GALAXIES :The expansion of the universe was discovered by study of the redshift of external galaxies by Edwin Hubble in the last century. However, expansion stretches light further and the wavelengths are shifted down into the infrared. As a result of this Doppler effect, at large redshifts, visible light from distant sources is shifted into the infrared part of the spectrum. Therefore IR studies give us much information about the very young, distant galaxies. Our Universe is about 14 billion years old, and HUBBLE/SPITZER spotted in March 2016 a galaxy born only 400 million years after the Big Bang. This galaxy is small , about 25 times smaller than the Milky Way galaxy . Apart from the distant ones, many galaxies ( more than 20000) have been detected only in the infrared. Many of these are star-burst galaxies -with formation of enormous numbers of new stars, and are thus extremely bright in the infrared. Also Interstellar matter radiates strongly in the infrared. Due to all thee reasons IR pictures reveal the structure of our galaxy much better than visible pictures can. The center of our galaxy is one of the brightest infrared sources in the sky. It is the IR studies which show the rapid rotation of stars and gases near the center , thus pointing to the existence of a super massive black hole.

3) PLANETARY STUDIES : Quite a few of the important information about the solar system has come from IR studies like (a) IR detection from the moon, as early as 1856 (b) the composition of Venusian atmosphere, (c) possible internals source of energy in Jupiter (d) the Methane atmosphere in Titan etc. An enormous ring around Saturn that had remained hidden earlier has also been detected . HERSCHEL telescope detected water in Comets and also the dwarf planet Ceres giving credence to the theory that part of water on earth has come from comets and asteroids.

4) EXO PLANETS : The exciting field of Exo planets (Extra- Solar Planets) really started by observation of IR emitting dust round stars. After this initial fillip from IR astronomy, several thousands of these exoplanets have been found by ground based telescopes and KEPLER satellite. These detections are indirect in the sense they look for regular dip in the in the light level given out by the system . However SPITZER recently became the first telescope to directly detect light of such planets outside of our Solar System, It captured directly the warm

infrared glows of two previously detected "hot Jupiter" planets - gas giants that zip closely around their parent stars and shine brightly in infrared wavelengths. Since the star-planet contrast is more favorable in IR because the planet emits its own light, it is easier to directly detect such planets . Further since molecules in the atmospheres of exoplanets have the largest number of spectral features in IR wavelengths, the temperatures, winds, and atmospheric compositions on these distant planets can be obtained. Last year SPITZER confirmed a very close (21 Light Years) rocky planet by finding its density . More details on the closest exo planet, the one in Proxima Centauri, will also be available soon from IR studies

Finally because the majority of the stars in the Galaxy are low-mass and predominantlyIR emitters , if aliens exist and ever do visit us, they’ll probably have infrared vision! Our own eyes evolved to make maximum use of the Sun’s light, which peaks in the visible. but the eyes of such aliens would have evolved to use their home star’s infrared light !