Lesson IX - Galaxies PDF

Lesson IX Properties, Types and Structures Learning Objectives At the end of the discussion the students should be able to: Define what is a galaxy. Identify the types, properties and structures of galaxies. Compare and contrast the different types, properties and structures of galaxies. ...

Lesson IX Properties, Types and Structures Learning Objectives At the end of the discussion the students should be able to: Define what is a galaxy. Identify the types, properties and structures of galaxies. Compare and contrast the different types, properties and structures of galaxies. Galaxies: Definition A galaxy is a large collection of stars, dust, and gas held together by gravity. The smallest galaxies are known as dwarf galaxies and can contain as few as 100 million stars, while on the other end of the scale are massive galaxies with trillions of stars. It is currently believed that there are 100 to 200 billion galaxies in the observable universe. There are four main categories of galaxies: elliptical, spiral, barred spiral, and irregular. These types of galaxies are further divided into subcategories while at the same time other types of galaxies exist based on their size and other unique features. Galaxies: Definition The most common type of galaxy found throughout the universe is the spiral galaxy. Around 77% of the galaxies observed by man are spiral galaxies. A good example of this type is the Andromeda galaxy. The rarest type of galaxies is the elliptical double-ringed galaxy. The largest galaxies are usually giant elliptical galaxies, containing a trillion or even more stars. They span as much as one million light-years across – 10 times as much as the Milky Way. Galaxies: Formation Modern cosmological models of the early universe are based on the Big Bang theory. Around 300.000 years after the Big Bang, it is theorized that atoms of hydrogen and helium began to form. This event is called recombination. At the time, most hydrogen atoms were neutral – non-ionized – and readily absorbed light. Density fluctuations in this primordial matter led to the creation of larger structures. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures would eventually become the galaxies we see today. Galaxies: Number Most galaxies are between 1.000 to 100.000 parsecs / 3.000 to 300.000 light-years in diameter. Recent calculations of the observable universe suggest that there are over 2 trillion galaxies that contain an estimated 1×1024 stars. Galaxies: Habitability Probably the most powerful argument for life to exist elsewhere besides our Solar System is the sheer vastness of the universe in its unimaginable number of galaxies. All of these galaxies contain innumerable stars, and these stars may contain innumerable exoplanets orbiting them. From this perspective at least, life should almost be guaranteed to exist somewhere else as well. Types and Classification of Galaxies Spiral Galaxies They have a flat, spinning disk with a central bulge surrounded by spiral arms. The spinning motion reaches speeds of up to hundreds of kilometers/miles per second. This usually can cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel. The bulge located in the center is made up of older, dimmer stars, and is thought to usually contain a supermassive black hole. Types and Classification of Galaxies Elliptical Galaxies Elliptical galaxies have an elongated spherical shape and lack a nucleus or bulge at the center. Although there is no nucleus, the galaxy is still brighter in the center and becomes less bright toward the outer edges of the galaxy. Their light is dominated by older reddish stars. They appear to also lack spiral arms. The stars, gases and other materials are spread throughout an elliptical galaxy. Elliptical galaxies can be nearly round or long and cigar-shaped. Types and Classification of Galaxies Irregular Galaxies Irregular galaxies have no definite shape, though they are in constant motion like all other galaxies. They have a chaotic appearance as they don’t seem to possess a nuclear bulge or traces of spiral arms. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust Irregular galaxies are commonly small, about one-tenth the mass of the Milky Way galaxy. Types and Classification of Galaxies Until now, three major types of irregular galaxies have been established: Irr I – they feature some structure but not enough to be placed into the Hubble sequence. Irr II – they do not appear to feature any structure. DIrr III – dwarf irregular galaxy. They tend to have low metallicity and relatively high levels of gas. They are also believed to be similar to the earliest galaxies that populated the Universe. Types and Classification of Galaxies Barred Spiral Galaxies Barred spiral galaxies share the same features and functions as regular spiral galaxies, but they also have a bar of bright stars that lie along the center of the bulge and extend into the disk. The bright bulge has very little activity here and contains mostly older, red stars. Types and Classification of Galaxies The perfect example of a barred spiral galaxy is our own galaxy, the Milky Way. They are further divided into three categories: Barred spiral galaxy type A – abbreviation SBa – they feature tightly bound arms. Barred spiral galaxy type B – abbreviation SBb – they feature both tightly and loosely bond arms. Barred spiral galaxy type C – abbreviation Sbc – they have loosely bound arms. Around two-thirds of spiral galaxies contain a bar structure through their center. Types and Classification of Galaxies Lenticular Galaxies Lenticular galaxies are a type of galaxy that shares similarities with both spiral and elliptical galaxies. They possess ill-defined spiral arms with an elliptical halo of stars. It’s Trivia Time! Milky Way was classified as a spiral galaxy for a long time. However, observations only recently reclassified it as a barred type spiral galaxy. One of the youngest galaxies ever discovered was the I Zwicky 18 galaxy. It was estimated to be around just 500 million years old though recent observations have discovered old stars in this galaxy. Whether this galaxy is young or not is still debated. Sometimes nicknamed the zombie galaxy, MACS 2129-1 is a disk-shaped galaxy which spins twice as fast as our Milky Way. This galaxy hasn’t made any stars for around 10 billion years. One of the brightest galaxies in the universe is W2246-0526. This galaxy is “stealing” the mass of three nearby galaxies at once. Little Cub is a dwarf galaxy that remained dormant since the Big Bang. It might still contain unchanged molecules since just moments after the rapid expansion of the universe around 13.7 billion years ago. Most galaxies are being pulled by an unknown force named the Great Attractor. Video Links Click on the link below. https://www.youtube.com/watch?v= https://www.youtube.com/watch?v= wsCedw3iBU0 sdt3hKIP2VQ