Galaxies Quiz

Galaxies: A Comprehensive Summary

• A galaxy is a system of stars, interstellar gas, dust, and dark matter, bound together by gravity.

• Galaxies range in size from dwarfs with less than a hundred million stars, to supergiants with one hundred trillion stars.

• Most of the mass in a galaxy is in the form of dark matter, with only a few percent visible in the form of stars and nebulae.

• Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular.

• Supermassive black holes are a common feature at the centers of galaxies.

• The observable universe contains roughly 200 billion galaxies, each containing an estimated average of 100 million stars.

• The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter.

• Most galaxies are gravitationally organized into groups, clusters, and superclusters.

• The Milky Way is part of the Local Group, which it dominates along with the Andromeda Galaxy, and the group is part of the Virgo Supercluster.

• Galaxies were initially discovered telescopically and were known as spiral nebulae.

• The Milky Way was discovered to be a flattened disk of stars in the 18th century, and the true distances of other galaxies were not known until the 20th century.

• Modern research includes the discovery of microwave radiation from interstellar atomic hydrogen gas, the discovery of dark matter, and improved observations from the Hubble Space Telescope.Galaxies: Types, Morphology, and Characteristics

• The Hubble Deep Field revealed that there are approximately 125 billion galaxies in the observable universe, but improved technology has since detected more galaxies.

• Galaxies are classified by their appearance, with three main types: ellipticals, spirals, and irregulars.

• Elliptical galaxies have an ellipsoidal profile, little structure, and are dominated by older stars with low abundances of heavy elements.

• Type-cD galaxies are the largest and most luminous galaxies known, with a central elliptical nucleus and an extensive, faint halo of stars.

• Shell galaxies have stars arranged in concentric shells and are thought to develop when a larger galaxy absorbs a smaller companion galaxy.

• Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars and bright arms extending outward.

• Barred spiral galaxies have a linear, bar-shaped band of stars that extends to either side of the core, then merges into the spiral arm structure.

• Super-luminous spirals are very large, with a diameter of 437,000 light-years and a mass of 340 billion solar masses.

• Most galaxies are dwarf galaxies, relatively small compared to other galactic formations, and may be classified as elliptical, spiral, or irregular.

• Interactions between galaxies, including near misses, collisions, and mergers, can play a significant role in galactic evolution.

• Starburst galaxies form stars at an exceptional rate, consuming their reserve of gas in a relatively brief period.

• Active galaxies contain an active galactic nucleus (AGN), emitting a significant portion of the galaxy's total energy output, and may be classified as Seyfert galaxies, quasars, or blazars.Galaxies: Types, Measurements, and Properties

Types of Galaxies:

• Elliptical galaxies are the most common type of galaxy, accounting for 70% of all observed galaxies. They are typically red or yellow in color and have a smooth, featureless appearance.
• Spiral galaxies have a flat disk with a central bulge and spiral arms. They are typically blue in color due to the presence of young stars, and are less common than elliptical galaxies.
• Irregular galaxies have no particular shape and are often smaller than other types of galaxies. They are typically blue in color and have a high rate of star formation.

Measurements of Galaxies:

• Isophotal diameter is a conventional way of measuring a galaxy's size based on its apparent surface brightness.
• The half-light radius (or effective radius) is a measure based on the galaxy's overall brightness flux.
• The Petrosian magnitude is a mathematical model based on the galaxy's azimuthally averaged profile of brightness flux.
• The near-infrared method is used by 2MASS and involves measuring a galaxy's overall luminous flux.

Properties of Galaxies:

• Galaxies have magnetic fields of their own that are strong enough to be dynamically important.

• The typical average equipartition strength for spiral galaxies is about 10 μG (microgauss) or 1 nT (nanotesla).

• Current models of the formation of galaxies in the early universe are based on the ΛCDM model.

• Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, with the discovery of the galaxy IOK-1.

• The first halo stars (called Population III stars) appeared within protogalaxies, composed almost entirely of hydrogen and helium and may have been more massive than 100 times the Sun's mass.Galaxies: Evolution, Future Trends, Larger-Scale Structures, and Multi-Wavelength Observation

• Within a billion years of a galaxy's formation, key structures begin to appear, including globular clusters, central supermassive black holes, and a galactic bulge of metal-poor Population II stars.

• Galaxies undergo a major burst of star formation during their early epoch.

• The accumulated matter settles into a galactic disc over the following two billion years, allowing the formation of planets.

• The evolution of galaxies can be significantly affected by interactions and collisions, producing tidal tails.

• The Milky Way and the Andromeda Galaxy are moving toward each other at about 130 km/s and may collide in about five to six billion years.

• Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.

• The current era of star formation is expected to continue for up to one hundred billion years, after which the "stellar age" will wind down.

• Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past billion years are relatively scarce.

• Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy.

• Superclusters contain tens of thousands of galaxies, arranged into sheets and filaments surrounding vast empty voids.

• The dust present in the interstellar medium is opaque to visual light, but more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail.

• Radio telescopes can be used to observe neutral hydrogen, including the non-ionized matter in the early universe that later collapsed to form galaxies.