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Astronomy - Galaxies

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    Study Guide Astronomy – Galaxies 1 . Clusters of Galaxies Galaxies are rarely found alone. Just like stars often form groups, galaxies tend to gather in small groups or very large clusters. Some clusters are small and contain only a few galaxies. A famous example is Stephan’s Quartet , which has just four galaxies. Other clusters are enormous. The Virgo Cluster , for example, contains about 3,000 galaxies and has a total mass of about 10¹ ⁵ times the mass of the Sun . Small clusters usually have galaxies scattered in irregular patterns across a relatively small region of space. In contrast, very large clusters tend to look more organized. Their galaxies are spread out symmetrically around a center. This orderly shape su ggests that the galaxies’ motions are balanced by the cluster’s gravity. In other words, gravity is strong enough to hold the cluster together. 1.1 The Large - Scale Structure of the Universe For a long time, astronomers thought clusters were the largest structures in the universe. Modern observations show that the universe is even more complex. Galaxies are arranged in huge sheets or walls that surround vast regions called voids , where very few galaxies exist. Clusters and long chains of galaxies appear where these walls intersect. When astronomers map this structure, it looks a lot like the inside of a sponge — dense regions surrounding large empty spaces. 1.2 The Local Group Our home neighborhood in space is called the Local Group . It contains about 30 galaxies spread across roughly 1 megaparsec (Mpc) of space. Most of the mass in the Local Group is concentrated in two large spiral galaxies: the Milky Way and the Andromeda Galaxy . These two galaxies are close enough to each other that they are best described as a binary galaxy system . Each of these large galaxies is surrounded by many smaller dwarf galaxies , sometimes called their coronas . In addition, about six smaller galaxies are spread more widely and are not clearly tied to either the Milky Way or Andromeda.

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    Study Guide There is also a small collection known as the IC 342 group , named after its largest galaxy. Models suggest this group may have been flung away from the Local Group by Andromeda’s gravity millions of years ago. 1.3 Giant Clusters of Galaxies At the largest scale, we find giant clusters of galaxies. The Virgo Cluster is a good example. It contains thousands of galaxies spread loosely over several megaparsecs. Most of these galaxies are spirals, but the two largest galaxies at the center are elliptical galaxies . On the other end of the spectrum is the Coma Cluster , which is much more compact. It also contains thousands of galaxies, including about 300 large elliptical galaxies . The types of galaxies found in a cluster depend on how crowded the environment is. Elliptical galaxies are common where galaxies are packed closely together. Spiral galaxies are more common in loose, spread - out clusters. This pattern supports the idea that elliptical galaxies may form when smaller disk - shaped galaxies merge over time. 1.4 Measuring the Mass of Galaxies One way astronomers measure a galaxy’s mass is by studying how fast it rotates. By observing the motion of stars across the visible part of a galaxy (up to about 20 kiloparsecs ), scientists can estimate a typical mass of around 10¹¹ solar masses using the formula: M(R) = V²(R)R / G In the 1970s, radio telescopes made it possible to detect rotation farther out, using neutral hydrogen gas. These measurements extended to 40 – 50 kiloparsecs . Surprisingly, galaxy rotation speeds stay almost constant at large distances. This means that when the distance doubles, the mass also doubles. However, the extra mass is not hydrogen gas, showing that something unseen must be present. 1.5 Binary Galaxies Galaxies that orbit each other can also be used to estimate mass. Because their orbital periods are extremely long, astronomers cannot directly apply Kepler’s laws to individual pairs. Instead, they study many galaxy pairs and average their distances and velocity differences. This method reveals mass out to about 80 kiloparsecs , suggesting even more mass — around 4 × 10¹¹ solar masses per galaxy.

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    Study Guide 1.6 Galaxy Clusters and the Virial Theorem For very large clusters, astronomers use the Virial Theorem . This principle connects the gravitational forces in a system to the random motions of its members. Astronomers can measure: • Galaxy positions in the sky • Velocities along the line of sight using the Doppler effect The virial theorem then allows them to infer the average galaxy mass. These studies suggest typical galaxy masses of about 8 × 10¹¹ solar masses spread over 150 kiloparsecs . This leads to a surprising conclusion: about seven - eighths of a galaxy’s mass lies outside its visible region . This unseen material does not emit detectable electromagnetic radiation. This puzzle became known as the missing mass problem , now more commonly called the dark matter problem . 1.7 Why Dark Ma Ʃ er Is a Big Problem Normal matter — such as stars, gas, and dust — always emits some kind of electromagnetic radiation. We can detect it at different wavelengths depending on its temperature and density. Dark matter does not emit radiation. That means it cannot be made of ordinary matter as we know it. Explaining what dark matter is has become one of the biggest challenges in modern astrophysics. 1.8 Possible Explana Ɵ ons for Dark Ma Ʃ er One idea is that neutrinos make up dark matter. Neutrinos are produced in large numbers in stars and during the early universe. If neutrinos have even a tiny mass, their huge numbers could make them a major contributor to the universe’s mass. They also interact very weakly with other matter, making them almost invisible. However, neutrinos move too fast. Because of this, they cannot clump together easily under gravity. They would escape galaxies and clusters instead of forming them. This makes neutrinos an unlikely explanation for dark matter.

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    Study Guide 1.9 Black Holes Another idea is that dark matter could consist of black holes. These would not be the black holes formed by dying stars — there are far too few of those. Instead, they would be extremely dense remnants from the early universe that somehow avoided expansion. How such objects formed and survived is still unexplained. 1.10 MACHOs Astronomers are also searching for MACHOs ( Massive Compact Halo Objects ). These could be small, dense objects made of normal matter that emit very little radiation. So far, searches for MACHOs have improved our understanding of the Milky Way’s halo but have not revealed any completely new type of object. 1.11 Cold and Hot Dark Ma Ʃ er Even without knowing exactly what dark matter is, astronomers can study how it behaves. • Cold dark matter moves slowly and clumps easily under gravity, helping galaxies form. • Hot dark matter moves quickly and resists clumping, slowing galaxy formation. Computer simulations show that the universe we observe today is best explained by a mix of both types. However, the final answer to the dark matter mystery is still unknown — and remains an active area of research. 2. The Origin and Evolu Ɵ on of Galaxies Galaxies did not appear fully formed. Scientists believe they developed over long periods of time, shaped by gravity, motion, and interactions with other galaxies. Our understanding of how galaxies form has changed a lot as new observations and theories ha ve improved. 2.1 Early Ideas About Galaxy Forma Ɵ on At first, astronomers thought galaxies formed when huge clouds of gas collapsed under their own gravity in the early universe. As the gas cloud shrank, it flattened into a disk shape. During this collapse, the gas broke into smaller pieces, and stars began to form. Star formation continued even after the disk was fully formed.

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    Study Guide A galaxy was considered “finished” when gravity and motion balanced each other, creating a stable structure. Scientists also believed that the type of galaxy depended mainly on how much angular momentum (rotational motion) the gas had at the beginning: • High angular momentum led to rotating disk galaxies . • Low angular momentum caused the gas to quickly turn into stars, forming elliptical galaxies . 2.2 A More Complex Picture Emerges Later research showed that galaxy formation is far more complicated than these early ideas suggested. First, star formation is actually not very efficient. Much of the gas does not immediately turn into stars. Because of this, elliptical galaxies could not form in the simple way scientists once believed. Instead, galaxy formation usually creates disk galaxies that still contain large amounts of interstellar gas. Second, galaxies do not evolve in isolation. Over billions of years, they interact with one another. Galaxies can collide, merge, and even absorb smaller neighboring galaxies. These interactions play a major role in shaping galaxy types. 2.3 How Galaxy Interac Ɵ ons Shape Di ff erent Types • Violent collisions between disk galaxies can scramble the motion of stars and quickly turn gas into new stars. After the gas is used up, the result is often a gas - free elliptical galaxy . • Galaxies that grow without major collisions tend to remain disks and slowly evolve into the many types of spiral galaxies we see today. • Gentle encounters between gas - rich disk galaxies can remove much of the gas without disrupting the overall shape. This process creates relatively rare, flat, gas - free galaxies called S0 galaxies . 2.4 The Turbulent Early Universe and Black Holes Scientists now believe that the early universe was a much more chaotic place. As galaxies were forming and settling into stable shapes, massive black holes grew at their centers. When large amounts of matter fell into these black holes, enormous energy was released. We observe this energy as quasars .

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    Study Guide Once a galaxy reached equilibrium and stopped feeding material into its center, the quasar phase ended. However, if new material later falls into the central black hole, the galaxy can become active again. This process explains the active galactic nuclei observed in some galaxies today. Summary Galaxy formation is a slow, dynamic process influenced by gravity, motion, gas, and interactions with other galaxies. Rather than forming in simple ways, galaxies evolve through collisions, mergers, and internal changes. These processes have created the wi de variety of galaxy types we observe across the universe today. 3. Galaxies Types and Classi fi ca Ɵ ons 3.1 Discovering Other Galaxies For a long time, astronomers thought the Milky Way was the entire universe. This changed in 1924 , when Edwin Hubble discovered Cepheid variable stars in nearby objects called Messier 31 (Andromeda Galaxy) and Messier 33 . Cepheid stars are special because their brightness changes in a predictable way. By using the Period – Luminosity Relation , astronomers calculated their distances and proved that these objects lie far outside the Milky Way . This confirmed that they are separate galaxies , similar in size to our own. Further studies revealed that galaxies come in many sizes and masses: • Giant galaxies can be over 150,000 parsecs wide and have masses up to 10¹³ times the Sun’s mass . • Dwarf galaxies may be only 1,000 parsecs across with masses as low as 10 ⁶ solar masses . Although dwarfs are small, they are the most common type of galaxy . With modern telescopes, astronomers estimate that about 100 billion galaxies are observable in the universe. 3.2 Hubble’s Galaxy Classi fi ca Ɵ on System Hubble noticed that most galaxies fall into a small number of shapes . He classified galaxies based on their appearance (morphology) . This system divides galaxies into four main types :

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