2.2 Structure of the universe: galaxies, stars, and planets
3 min read•july 22, 2024
Galaxies come in various shapes and sizes, from spirals like our Milky Way to ellipticals and irregulars. Each type has unique characteristics, influencing the stars and planets within them. Understanding galactic structure helps us grasp the cosmic context of potential life-bearing worlds.
Stars and planets form intricate systems, with stellar evolution playing a crucial role. From birth in nebulae to death as white dwarfs or black holes, stars shape their surroundings. Planetary systems, including our own, showcase diverse worlds, some potentially habitable.
Galactic Structure and Characteristics
Types and characteristics of galaxies
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- Spiral galaxies
- Characterized by spiral arms extending from a central bulge containing a mix of young and old stars (Milky Way, Andromeda galaxy)
- Elliptical galaxies
- Smooth, elliptical shape with no distinct features primarily composed of older, red stars
- Classified based on their ellipticity from E0 to E7
- Irregular galaxies
- Lack a distinct shape or structure often smaller than spiral and elliptical galaxies
- May result from galactic collisions or gravitational interactions
- Barred spiral galaxies
- Subtype of spiral galaxies characterized by a central bar-shaped structure composed of stars
- Lenticular galaxies
- Transitional type between spiral and elliptical galaxies containing a central bulge and disk but lacking spiral arms
Stellar Evolution and Planetary Systems
Life cycle of stars
- Star formation
- Begins in molecular clouds or nebulae where gravitational collapse of gas and dust leads to the formation of protostars
- Main sequence
- Stars fuse hydrogen into helium in their cores with stability maintained by equilibrium between gravity and outward pressure
- Post-main sequence
- Low-mass stars (< 8 solar masses)
- phase: Expansion and cooling of the star's outer layers
- : Ejection of the star's outer layers
- : Exposed core of the star with no further fusion
- High-mass stars (> 8 solar masses)
- phase: Significant expansion and cooling
- : Explosive death of the star
- or : Remnants of the collapsed core
- Neutron stars are extremely dense and composed primarily of neutrons
- Black holes have a gravitational pull so strong that not even light can escape
- Low-mass stars (< 8 solar masses)
Structure of planetary systems
- Terrestrial planets (Earth, Mars, Venus, Mercury)
- Rocky composition with relatively small size and mass and few or no moons
- Gas giants (Jupiter, Saturn)
- Primarily composed of hydrogen and helium with large size and mass and many moons and ring systems
- Ice giants (Uranus, Neptune)
- Composed mainly of water, methane, and ammonia ices and smaller than gas giants but larger than terrestrial planets
- Extrasolar planets (exoplanets)
- Planets orbiting stars other than our Sun with a diverse range of sizes, masses, and compositions
- Detection methods include radial velocity, transit, direct imaging, and gravitational microlensing
Habitable zones for potential life
- Definition
- Range of distances from a star where liquid water can exist on a planet's surface dependent on the star's luminosity and the planet's atmospheric composition
- Importance for life
- Liquid water is considered essential for life as we know it so planets within the habitable zone are more likely to support life
- Factors affecting habitability
- Planetary mass and composition, atmospheric pressure and greenhouse gases, and stellar activity and variability
- Circumstellar habitable zone
- Region around a single star where planets can maintain liquid water
- Galactic habitable zone
- Region within a galaxy where conditions are favorable for the development of life considering factors such as star formation rate, metallicity, and frequency of potentially sterilizing events (supernovae, gamma-ray bursts)
Key Terms to Review (28)
Barred spiral galaxy: A barred spiral galaxy is a type of galaxy characterized by its spiral arms that extend from a central bar-shaped structure made up of stars. These galaxies exhibit a distinct pattern, where the bar acts as a conduit for star formation and gas dynamics, influencing the overall structure and evolution of the galaxy.
Black hole: A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. Formed from the remnants of massive stars that have undergone gravitational collapse, black holes play a crucial role in stellar evolution and have significant implications for the structure of the universe, influencing galaxies and the distribution of matter within them.
Carl Sagan: Carl Sagan was an influential American astronomer, astrophysicist, and science communicator known for popularizing science and advocating for the search for extraterrestrial life. His work spans multiple disciplines, bridging the gap between astronomy, biology, and the humanities, influencing how we think about life in the universe and our place within it.
Cosmic microwave background: The cosmic microwave background (CMB) is the afterglow radiation from the Big Bang, filling the universe with a nearly uniform glow of microwave radiation. This relic radiation provides a snapshot of the early universe, around 380,000 years after the Big Bang, when protons and electrons combined to form hydrogen atoms, allowing photons to travel freely. The CMB serves as crucial evidence for the Big Bang theory and helps us understand the structure of the universe, including galaxies, stars, and planets.
Dark matter: Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects. It plays a crucial role in shaping the universe's structure, influencing the formation of galaxies and the movement of celestial bodies. Understanding dark matter helps explain the distribution of mass in the cosmos and supports the Big Bang theory, which posits that the universe expanded from an extremely hot and dense state.
Dwarf planet: A dwarf planet is a celestial body that orbits the Sun, has enough mass for its self-gravity to overcome rigid body forces and assume a nearly round shape, but has not cleared its neighboring region of other debris. Dwarf planets are essential for understanding the diversity of objects within our solar system and help shed light on the formation and evolution of planetary bodies.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in establishing the field of extragalactic astronomy and demonstrated that the universe is expanding. His observations led to the formulation of Hubble's Law, which states that galaxies are moving away from us at speeds proportional to their distance, providing critical evidence for the Big Bang theory and shaping our understanding of cosmic evolution and the structure of the universe.
Elliptical galaxy: An elliptical galaxy is a type of galaxy characterized by its smooth, featureless light profile and an ellipsoidal shape, often containing older stars and little interstellar matter. These galaxies are typically more rounded than other types and can vary greatly in size, ranging from small dwarfs to massive giants. Their structure has implications for understanding the formation and evolution of galaxies within the broader context of the universe's structure.
Exoplanet: An exoplanet, or extrasolar planet, is a planet that orbits a star outside our solar system. The discovery of exoplanets has profound implications for understanding the structure of the universe and the potential for extraterrestrial life, as they are considered crucial in the search for habitable worlds beyond Earth.
Galactic Collision: A galactic collision occurs when two or more galaxies come into close proximity and interact gravitationally, often leading to significant structural changes in their shapes and compositions. These cosmic events can result in the merging of galaxies, the formation of new stars, and the redistribution of gas and dust, affecting the overall structure of the universe.
Hubble's Law: Hubble's Law is a fundamental observation in cosmology stating that the farther away a galaxy is, the faster it appears to be moving away from us. This relationship provides critical evidence for the expansion of the universe, linking it to the Big Bang theory and demonstrating how cosmic evolution has shaped the structure of galaxies and their distribution throughout the universe.
Irregular galaxy: An irregular galaxy is a type of galaxy that lacks a distinct regular shape or structure, often appearing chaotic and disorganized. These galaxies are generally smaller than their spiral and elliptical counterparts and can contain a mix of young and old stars, as well as significant amounts of gas and dust. Irregular galaxies often result from gravitational interactions or collisions with other galaxies, leading to their unique formation and evolution.
Kepler's Laws: Kepler's Laws are three fundamental principles that describe the motion of planets around the Sun, formulated by Johannes Kepler in the early 17th century. These laws provide a framework for understanding the orbits of celestial bodies, explaining how planets move in elliptical paths, relate their orbital periods to their distances from the Sun, and how they sweep out equal areas in equal times. Kepler's work was crucial in advancing our knowledge of planetary motion, influencing both astronomy and our understanding of gravity.
Lenticular Galaxy: A lenticular galaxy is a type of galaxy that possesses a central bulge and a disk-like structure, similar to spiral galaxies, but lacks significant spiral arms. This unique structure places lenticular galaxies in a transitional category between elliptical and spiral galaxies, highlighting their mixed characteristics and importance in understanding galactic evolution.
Light-year: A light-year is the distance that light travels in one year in a vacuum, approximately 5.88 trillion miles or about 9.46 trillion kilometers. This measurement is crucial for understanding the vastness of space, as it provides a standard unit to express distances between celestial objects like galaxies, stars, and planets. By using light-years, astronomers can more easily communicate and comprehend the immense scales involved in the universe.
Main sequence star: A main sequence star is a type of star that is in a stable phase of stellar evolution, where it fuses hydrogen into helium in its core. This process generates energy that balances the gravitational forces trying to collapse the star, resulting in a long-lasting period of stability. Main sequence stars represent the majority of stars in the universe and are classified based on their mass, temperature, and luminosity.
Moon: A moon is a natural satellite that orbits a planet, providing gravitational stability and influencing various planetary processes. Moons can vary in size and composition, with some being large enough to be spherical, while others may be irregularly shaped. They play a crucial role in the dynamics of their parent planets and can impact aspects such as tides, axial tilt, and even biological cycles on the surface.
Neutron star: A neutron star is a highly dense remnant of a massive star that has undergone a supernova explosion, resulting in a core primarily composed of neutrons. These celestial objects are the smallest and densest stars known, often exhibiting strong magnetic fields and rapid rotation. Their formation and characteristics offer critical insights into the life cycle of stars and the fundamental forces that shape the universe.
Newton's Law of Gravitation: Newton's Law of Gravitation states that every point mass attracts every other point mass in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This fundamental principle not only describes the gravitational pull between two objects but also explains the motions of celestial bodies, helping to establish our understanding of the structure and behavior of galaxies, stars, and planets.
Nuclear fusion: Nuclear fusion is a process where two light atomic nuclei combine to form a heavier nucleus, releasing a significant amount of energy in the process. This reaction powers stars, including our Sun, and is responsible for the energy produced in galaxies and the formation of heavier elements in the universe. It plays a crucial role in stellar evolution and the lifecycle of stars.
Parsec: A parsec is a unit of distance used in astronomy, equivalent to about 3.26 light-years or approximately 19 trillion miles. This measurement is important for determining the distances between stars, galaxies, and other celestial objects, allowing astronomers to map the structure of the universe more effectively.
Planetary nebula: A planetary nebula is a glowing shell of gas and dust ejected from a star during the late stages of its evolution, particularly when it has exhausted its nuclear fuel and expelled its outer layers. This process usually occurs in stars that are similar in size to our Sun and marks the transition from the red giant phase to the final stage, where the core remains as a white dwarf. The vibrant colors and structures observed in planetary nebulae are due to ionized gas that emits light as it interacts with ultraviolet radiation from the remaining hot core.
Red giant: A red giant is a late stage in the life cycle of a star, characterized by an increase in size and a cooler surface temperature, resulting in a reddish appearance. As stars exhaust their hydrogen fuel in the core, they begin to fuse helium and other heavier elements, causing them to expand significantly. This phase is crucial for understanding stellar evolution and the fate of stars in the universe.
Red supergiant: A red supergiant is a massive star that has exhausted the hydrogen in its core and has expanded to a very large size, exhibiting a reddish hue due to its cooler surface temperature. These stars are among the largest in the universe and play a crucial role in stellar evolution, particularly in the formation of supernovae and the enrichment of the interstellar medium with heavy elements.
Spiral galaxy: A spiral galaxy is a type of galaxy characterized by its distinct spiral arms that wind outwards from the center, creating a flat, rotating disk. These galaxies often contain a mix of young and old stars, along with gas and dust, which contribute to star formation in the arms. The structure of spiral galaxies plays an important role in the overall architecture of the universe and is a key aspect of understanding how galaxies, stars, and planets are organized.
Stellar nucleosynthesis: Stellar nucleosynthesis is the process by which elements are created through nuclear reactions in the cores of stars during their life cycles. This process is crucial for understanding the chemical composition of the universe, as it leads to the formation of elements heavier than hydrogen and helium, contributing to the diversity of matter found throughout galaxies, stars, and planets.
Supernova: A supernova is a powerful and luminous explosion that occurs at the end of a star's life cycle, resulting in a dramatic increase in brightness that can outshine entire galaxies. This explosive event is crucial in the evolution of stars, as it marks the transition from massive stars to neutron stars or black holes, and plays a significant role in the chemical enrichment of the universe.
White dwarf: A white dwarf is a small, dense stellar remnant that forms when a star exhausts its nuclear fuel and expels its outer layers, leaving behind a hot core. These objects are typically composed mainly of carbon and oxygen and are the final evolutionary stage of stars like our Sun. White dwarfs represent a critical phase in the life cycle of stars and play a significant role in the overall structure of galaxies.