Armagh Observatory: School Student Essays
The Evolution Of Galaxies
Lara Martin, Strathearn School, Belfast
Introduction
The Universe has been a subject of great interest to man since ancient times. In the Neolithic era, sophisticated observatories were erected to aid man in his understanding of solar and lunar science, star activity and construction of the ancient calendar. In the 6th century B.C, Democritus suggested that the Universe was populated with never ending worlds such as the earth, although it would be another 2,500 years until any evidence could be found to support his statement. During the Renaissance, Copernicus claimed that the sun was at the centre of our solar system and that the stars and sun appeared to move because the Earth rotated daily about its axis. He also speculated that stars might be distant suns. In the 16th century, Giordano Bruno suggested that the universe had no limits; it was infinitely massive and perhaps contained other suns, planets and life forms. His experimentation also led to a speculation that perhaps stars shine brightly or dimly according to their distance from the observer. It was Galileo Galilei who first used the optical telescope to look at the sky, through a suitable combination of convex and concave lenses, which allowed for the discovery of the four satellites orbiting Jupiter; living proof that the sun lay at the centre of a solar system with planets orbiting around it. Galileo’s discoveries also showed the Milky Way as a starry cluster, deep in space.
Throughout history there have been many theories and speculations on how the Universe was formed and how it operates, but it is only that the technology is available to help us accurately assess the origins of the Universe and the matter within it.
The Big Bang
In recent years the Big Bang theory has become the most widely accepted explanation of the birth and development of the Universe. It is thought the Universe began as a small, hot, dense state consisting of pure energy and subatomic particles in a violently exploding space. Shortly after the Big Bang, the Universal matter was an extremely hot mixture of particles, consisting of negatively charged electrons, positively charged protons and neutrally charged neutrons. After some time, the fusion of two neutrons with two protons created helium nucleii. After one million years, electrons had combined with protons and helium nucleii to produce hydrogen atoms and helium atoms. Thus two of the commonest and most basic elements known to man were born. Most protons were not affected by this process so one million years later the Universe was made up of 94% hydrogen atoms, 6% helium atoms and energy. Heavier elements such as oxygen, carbon and iron were produced later by nuclear reactions in stars that condensed from the hydrogen and helium gas.
The Formation and Classification of Galaxies
Eventually galaxies came into being. Before any stars were formed, the galaxy would have been an extended cloud of gas much larger than it is at present, falling together under a gravitational pull. Astronomers believe that in a low density Universe composed mainly of dust and gas, gravitational instability would exist, leading to the creation of regions in which the random velocities of the particles would usually be less than the total escape velocity. During this collapse, globular clusters – the oldest structures in the universe – must have condensed out into their tight aggregations of millions of stars. These would form a halo around the galaxy with individual clusters constantly plunging in and out in memory of the original collapse. The remaining gas in the original cloud would continue to collapse and gradually spin faster, probably due to the tidal attraction from nearby protogalaxies. The cloud would then collapse and form a rotating disc, where star formation was to occur. It is thought that at this stage of development, our Galaxy would look very similar to the way it does now.
It has recently been speculated that the mechanism responsible for causing clouds of gas to form stars, is a spiral compression wave that is driven by the gravitational attraction of the stars and travels around the galaxy, generating the spiral arms we can see on the Andromeda nebula, as well as our own galaxy.
The Andromeda Galaxy
Throughout the Universe there are billions of isolate star systems separated by massive distances. Two centuries ago these star systems were given the name of nebulae, a Latin term meaning cloud, due to their misty appearance when viewed from a simple telescope. Today these nebulae are generally known as galaxies, ‘the "atoms" of our universe’.
The Orion Nebula: A drawing made by Lord Rosse after observing the sky through his telescope
Galaxies are classified into three main groups: elliptical; spiral; and irregular. Both elliptical and spiral galaxies began as a roughly spherical cloud of gas that collapsed due to gravity. If the star formation process was efficient, then the gas was used up quickly and the protogalaxy collapsed to form an elliptical galaxy. If the star formation process was slow, then leftover gas formed a disc that rotated about the galactic centre of the protogalaxy, where it later contributed to the formation of new stars, resulting in a spiral galaxy. Elliptical galaxies range in shape from spherical to seemingly flattened spheroids and range in size from small dwarf ellipticals to large giant ellipticals. A spiral galaxy such as the Milky Way is defined by stars and virtually all interstellar matter being imbedded in a sphere of old stars that strongly resembles an elliptical galaxy with a less dense region of stars. An irregular galaxy is the term giving to all the remaining galaxies that cannot be classified by their shape. Occasionally a fourth classification of galaxies called lenticular is used. A lenticular galaxy is intermediate in shape between the ellipticals and spirals.
Hubble's Law
In 1929, five years after he had determined galactic distances, Hubble discovered that the red shifts of galaxies were proportional to their distances. Hubble's red shift theory comes from the understanding and application of the Doppler Effect. A dark line spectrum is formed when radiation is emitted from a star. The dark line patterns correspond to elements in the stars’ atmospheres and occur at specific, well known wavelengths. If the star is moving away from the observer, the dark line pattern shifts towards the long wavelength end of the spectrum, a displacement commonly referred to as the red shift. Hubble noticed that fainter galaxies generally had larger red shifts and concluded that because an object seems fainter at a distance, the more distant galaxies are receding faster.
Hubble's measurements of galactic red shifts led to the following relation:
v =c(Dl/ l0) = cz = H0r
If you plot apparent recessional velocity against distance, the Hubble Constant is the slope of a straight line through the data
Hubble's Constant, H0, is expressed in units of kilometres per second per Megaparsec. One parsec equals approximately one third of a light year, therefore one Megaparsec equals about 3.2 million light years. The Hubble constant is used to explain that for every 3.2 million light years you look out into space, the objects appear to be receding from you at a rate of H0 kilometres per second. Measuring the exact value of Hubble’s constant is a difficult job and can be attempted in several different ways. These methods include the study of supernovae with optical telescopes, study of the period-luminosity relationship and by combining X-ray images and microwave astronomy measurements to study physical processes in distant clusters of galaxies; the Sunyaev-Zeldovich effect. According to recent measurements using these various methods, the value of H0 is between 55 and 75 km/s/Mpc. This means that the objects appear to recede between 55km/s and 75km/s for every 3.2 million light years you look out into space. If scientists can make an accurate measurement of the Hubble Constant, then we would be much closer to determining the age and eventual fate of the Universe. If the Hubble Constant is found to be very large, then the deceleration parameter, q0, would have to be negative, meaning that the Universe is not slowing down, but rather expanding at a greater rate.
However we know the universe does not expand freely and the mutual gravitational attraction of all matter pulls on retreating galaxies and restrains their dispersal. Thus in the early 1930s it was concluded that the rate of expansion of the universe is growing but it was much faster in the past.
Now it is known that past galaxies were closer together the further back in time we go but there is clearly a limit as to how close these galaxies can get. Eventually the matter of the universe would become infinitely dense and further crowding would be impossible therefore the universe must have had a beginning; this statement supports the aforementioned Big Bang theory.
The Hubble Deep Field
In 1995 a scientific milestone was reached with the making of the "Hubble Deep Field." A region of uncluttered sky, with a high galactic latitude and no bright stars, galaxies or strong radio signals present, was chosen, the Ursa Major, and the Hubble Space Telescope was focused on this area for ten days, photographing it in four different colours. These colours had central wavelengths of 300nm, 450nm, 606nm and 814nm. The finalized image was a composite consisting of sections with shorter exposure times. In 1998 the telescope was used to determine corresponding images within the southern hemisphere in the opposite direction in the celestial sphere. The results showed a similar distribution of galaxies. The dimmest galaxies visible are the most distant galaxies and represent what the Universe looked like in the past. Because the most distant objects are also among the dimmest, the image allows us to look into the past to witness the early formation of galaxies, perhaps less than one billion years after the universe's birth in the Big Bang. Scientists hope that the Hubble Deep Field will reveal when galaxies first formed and how they have evolved over time, providing some insight into the age of the Universe
The Fate Of The Universe
The orange curve represents a high density universe which will continue to expand for several billion years before turning around and collapsing under its own weight. The green curve represents a flat universe where the rate of expansion will gradually decease through time. During the time of diminishing size, galaxies would rush together into a dense conglomeration with a radius that would theoretically equal zero. The universe would eventually crush everything into high energy light and several unusual forms of mass and thus the Big Crunch theory would be in effect and the current universal form would be destroyed.
The blue curve represents a high density, open universe where the rate of expansion through time will also decease but less slowly than a flat universe due to a weaker pull of gravity. The red curve represents a universe consisting of dark matter, causing the rate of expansion of the universe to gradually speed up.
Despite great technological advances in recent times, much remains unknown about the birth, development and future of our galaxy. The fate of our Universe is largely dependant on the amount of dark matter in the Universe. Based on 50 years of observations of the motions of the galaxies and the expansion of the Universe, most astronomers believe that as much as 90 percent of the stuff constituting the universe may be objects or particles that cannot be seen. This means that most of the Universe’s matter does not radiate; does not provide a glow that is detectable in the electromagnetic spectrum. This invisible matter is referred to as "dark matter" and some astronomers estimate that as much as 90% of the matter in our Universe is "dark matter". The lack of knowledge regarding dark matter has made it difficult to understand how much mass our Universe contains, how galaxies were formed and whether or not the Universe will continue expanding forever, however recent observation of distant supernova has implied that the speed of universal expansion is increasing, as shown in the above red curve therefore suggesting the presence of dark energy, otherwise known as the cosmological constant. The presence of dark energy suggests that the universe will continue expanding infinitely.
Many cosmologists are now agreeing that the universe is spatially flat, as the total density of matter appears to be equal to the critical density of matter. It is estimated that of this matter, three tenths is low pressure dark matter, and seven tenths is negative pressure dark energy i.e. the cosmological constant. If these observations are proven to be true, it is likely that the universe will continue to expand forever as depicted in the above red curve.