Astronomy - Galaxies - Quick Guides | Astronomy

Preview (6 of 17 Pages)

100%

Astronomy - Galaxies - Page 1 preview image

Loading page ...

Study GuideAstronomy–Galaxies1.Clusters of GalaxiesGalaxies are rarely found alone. Just like stars often form groups, galaxies tend to gather in smallgroups or very large clusters.Some clusters are small and contain only a few galaxies. A famous example isStephan’s Quartet,which has just four galaxies. Other clusters are enormous. TheVirgo Cluster, for example, containsabout3,000 galaxiesand has a total mass of about10¹⁵times the mass of the Sun.Small clusters usually have galaxies scattered in irregular patterns across a relatively small region ofspace. In contrast, very large clusters tend to look more organized. Their galaxies are spread outsymmetrically around a center. This orderly shape suggests that the galaxies’ motions are balancedby the cluster’s gravity. In other words, gravity is strong enough to hold the cluster together.1.1The Large-Scale Structure of the UniverseFor a long time, astronomers thought clusters were the largest structures in the universe. Modernobservations show that the universe is even more complex.Galaxies are arranged in hugesheets or wallsthat surround vast regions calledvoids, where veryfew galaxies exist. Clusters and long chains of galaxies appear where these walls intersect. Whenastronomers map this structure, it looks a lot like the inside of a sponge—dense regions surroundinglarge empty spaces.1.2The Local GroupOur home neighborhood in space is called theLocal Group. It contains about30 galaxiesspreadacross roughly1 megaparsec (Mpc)of space.Most of the mass in the Local Group is concentrated in two large spiral galaxies: theMilky WayandtheAndromeda Galaxy. These two galaxies are close enough to each other that they are bestdescribed as abinary galaxy system.Each of these large galaxies is surrounded by many smallerdwarf galaxies, sometimes called theircoronas. In addition, about six smaller galaxies are spread more widely and are not clearly tied toeither the Milky Way or Andromeda.

Astronomy - Galaxies - Page 2 preview image

Loading page ...

Study GuideThere is also a small collection known as theIC 342 group, named after its largest galaxy. Modelssuggest this group may have been flung away from the Local Group by Andromeda’s gravity millionsof years ago.1.3Giant Clusters of GalaxiesAt the largest scale, we findgiant clustersof galaxies.TheVirgo Clusteris a good example. It contains thousands of galaxies spread loosely over severalmegaparsecs. Most of these galaxies are spirals, but the two largest galaxies at the center areelliptical galaxies.On the other end of the spectrum is theComa Cluster, which is much more compact. It also containsthousands of galaxies, including about300 large elliptical galaxies.The types of galaxies found in a cluster depend on how crowded the environment is.Ellipticalgalaxiesare common where galaxies are packed closely together.Spiral galaxiesare morecommon in loose, spread-out clusters. This pattern supports the idea that elliptical galaxies may formwhen smaller disk-shaped galaxies merge over time.1.4Measuring the Mass of GalaxiesOne way astronomers measure a galaxy’s mass is by studying how fast it rotates. By observing themotion of stars across the visible part of a galaxy (up to about20 kiloparsecs), scientists canestimate a typical mass of around10¹¹ solar massesusing the formula:M(R) = V²(R)R / GIn the 1970s, radio telescopes made it possible to detect rotation farther out, using neutral hydrogengas. These measurements extended to40–50 kiloparsecs.Surprisingly, galaxy rotation speeds stay almost constant at large distances. This means that whenthe distance doubles, the mass also doubles. However, the extra mass is not hydrogen gas, showingthat something unseen must be present.1.5Binary GalaxiesGalaxies that orbit each other can also be used to estimate mass. Because their orbital periods areextremely long, astronomers cannot directly apply Kepler’s laws to individual pairs.Instead, they study many galaxy pairs and average their distances and velocity differences. Thismethod reveals mass out to about80 kiloparsecs, suggesting even more mass—around4 × 10¹¹solar massesper galaxy.

Astronomy - Galaxies - Page 3 preview image

Loading page ...

Study Guide1.6Galaxy Clusters and the Virial TheoremFor very large clusters, astronomers use theVirial Theorem. This principle connects the gravitationalforces 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 effectThe virial theorem then allows them to infer the average galaxy mass. These studies suggest typicalgalaxy masses of about8 × 10¹¹ solar massesspread over150 kiloparsecs.This leads to a surprising conclusion:about seven-eighths of a galaxy’s mass lies outside itsvisible region. This unseen material does not emit detectable electromagnetic radiation.This puzzle became known as themissing mass problem, now more commonly called thedarkmatter problem.1.7Why Dark MaƩer Is a Big ProblemNormal 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.1.8Possible ExplanaƟons for Dark MaƩerOne idea is thatneutrinosmake up dark matter. Neutrinos are produced in large numbers in starsand during the early universe.If neutrinos have even a tiny mass, their huge numbers could make them a major contributor to theuniverse’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 unlikelyexplanation for dark matter.

Astronomy - Galaxies - Page 4 preview image

Loading page ...

Study Guide1.9Black HolesAnother idea is that dark matter could consist of black holes. These would not be the black holesformed by dying stars—there are far too few of those.Instead, they would be extremely dense remnants from the early universe that somehow avoidedexpansion. How such objects formed and survived is still unexplained.1.10MACHOsAstronomers are also searching forMACHOs(Massive Compact Halo Objects). These could besmall, 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 notrevealed any completely new type of object.1.11Cold and Hot Dark MaƩerEven without knowing exactly what dark matter is, astronomers can study how it behaves.•Cold dark mattermoves slowly and clumps easily under gravity, helping galaxies form.•Hot dark mattermoves quickly and resists clumping, slowing galaxy formation.Computer simulations show that the universe we observe today is best explained by a mix of bothtypes. However, the final answer to the dark matter mystery is still unknown—and remains an activearea of research.2. The Origin and EvoluƟon of GalaxiesGalaxies 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 galaxiesform has changed a lot as new observations and theories have improved.2.1Early Ideas About Galaxy FormaƟonAt first, astronomers thought galaxies formed when huge clouds of gas collapsed under their owngravity in the early universe. As the gas cloud shrank, it flattened into a disk shape. During thiscollapse, the gas broke into smaller pieces, and stars beganto form. Star formation continued evenafter the disk was fully formed.

Astronomy - Galaxies - Page 5 preview image

Loading page ...

Study GuideA galaxy was considered “finished” when gravity and motion balanced each other, creating a stablestructure. Scientists also believed that the type of galaxy depended mainly on how much angularmomentum (rotational motion) the gas had at the beginning:•High angular momentumled to rotatingdisk galaxies.•Low angular momentumcaused the gas to quickly turn into stars, formingellipticalgalaxies.2.2A More Complex Picture EmergesLater research showed that galaxy formation is far more complicated than these early ideassuggested.First, star formation is actually not very efficient. Much of the gas doesnotimmediately 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 amajor role in shaping galaxy types.2.3How Galaxy InteracƟons Shape Different Types•Violent collisions between disk galaxiescan scramble the motion of stars and quickly turngas into new stars. After the gas is used up, the result is often agas-free elliptical galaxy.•Galaxies that grow without major collisionstend to remain disks and slowly evolve into themany types ofspiral galaxieswe see today.•Gentle encounters between gas-rich disk galaxiescan remove much of the gas withoutdisrupting the overall shape. This process creates relatively rare, flat, gas-free galaxies calledS0 galaxies.2.4The Turbulent Early Universe and Black HolesScientists now believe that the early universe was a much more chaotic place. As galaxies wereforming and settling into stable shapes, massive black holes grew at their centers. When largeamounts of matter fell into these black holes, enormous energy wasreleased. We observe this energyasquasars.

Astronomy - Galaxies - Page 6 preview image

Loading page ...

Study GuideOnce a galaxy reached equilibrium and stopped feeding material into its center, the quasar phaseended. However, if new material later falls into the central black hole, the galaxy can become activeagain. This process explains theactive galactic nucleiobserved in some galaxies today.SummaryGalaxy formation is a slow, dynamic process influenced by gravity, motion, gas, and interactions withother galaxies. Rather than forming in simple ways, galaxies evolve through collisions, mergers, andinternal changes. These processes have created the wide variety of galaxy types we observe acrossthe universe today.3. Galaxies Types and ClassificaƟons3.1Discovering Other GalaxiesFor a long time, astronomers thought the Milky Way was the entire universe. This changed in1924,whenEdwin HubblediscoveredCepheid variable starsin nearby objects calledMessier 31(Andromeda Galaxy)andMessier 33.Cepheid stars are special because their brightness changes in a predictable way. By using thePeriod–Luminosity Relation, astronomers calculated their distances and proved that these objectsliefar outside the Milky Way. This confirmed that they areseparate galaxies, similar in size to ourown.Further studies revealed that galaxies come in many sizes and masses:•Giant galaxiescan be over150,000 parsecs wideand have masses up to10¹³ times theSun’s mass.•Dwarf galaxiesmay be only1,000 parsecs acrosswith masses as low as10⁶solarmasses.Although dwarfs are small, they are themost common type of galaxy. With modern telescopes,astronomers estimate thatabout 100 billion galaxiesare observable in the universe.3.2Hubble’s Galaxy ClassificaƟon SystemHubble noticed that most galaxies fall into asmall number of shapes. He classified galaxies basedon theirappearance (morphology). This system divides galaxies intofour main types:

Preview Mode

This document has 17 pages. Sign in to access the full document!

Report

Astronomy - Galaxies

Study Now!

Document Details

Related Documents

Astronomy - The Universe

Astronomy - The Sun a Representative Star

Astronomy - The Structure of Stars

Astronomy - The Solar System

Astronomy - The Science of Astronomy

Astronomy - The Milky Way Galaxy

Astronomy - The Hertzsprung-Russell Diagram

Astronomy - Other Planets of the Solar System

Astronomy - Observing the Sky

Astronomy - Observational Properties of Stars

Company

Explore

Study Tools