Galaxy Structure, Dynamics, and Interstellar Medium: A Comprehensive Study
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213 Terms
What is a galaxy?
A large system of stars, gas, and dust held together by gravity
What is the range of stars a galaxy can contain?
From about 1 million to 10 trillion
What force holds a galaxy together?
Gravity
What do stars in a galaxy do?
Orbit a common center
Approximately how many stars are in the Milky Way?
About 100 billion
What galaxy do we live in?
The Milky Way
Is the Sun a special star in the galaxy?
No, it is just one of many stars
How does the Milky Way appear from Earth?
A faint band of light across the sky
Why is our view of the Milky Way obscured?
Dust clouds absorb visible light
What is the interstellar medium (ISM)?
Gas and dust between stars
Why is the interstellar medium (ISM) important?
It is the material from which stars form
Is the galaxy a rigid object?
No, each star moves independently
What determines a star's orbit in the galaxy?
Its location in the galaxy
How do stars in the disk move?
In nearly circular orbits in the same direction
How do stars in the bulge and halo move?
In random, elliptical orbits above and below the disk
How long does the Sun take to orbit the galaxy?
About 200-230 million years
How long is one vertical "bob" of the Sun?
About 10 million years
Why does the galactic disk have thickness?
Due to stars moving slightly above and below the plane
What is centripetal force?
The inward force required for circular motion
What provides centripetal force for stars in a galaxy?
Gravity
What happens if a star orbits faster?
It requires a stronger centripetal force
How do we calculate galaxy mass?
From orbital speeds of stars
What does orbital speed tell us?
The total mass inside the orbit
What is the estimated mass inside the Sun's orbit?
About 10^11 solar masses
Why are astronomers surprised by star speeds?
They move faster than expected from visible matter
What is the unseen mass in the galaxy called?
Dark matter
What does the Milky Way's structure include?
Disk, bulge, and halo
Where are most stars and gas located in the galaxy?
In the disk
What is found in the galactic halo?
Sparse stars and globular clusters
How did astronomers find our position in the galaxy?
By mapping globular clusters
Where are we located in the Milky Way?
In the outskirts of the disk
Why did early astronomers think we were at the center of the galaxy?
Dust blocked their view of the galaxy
What is the 'Star-gas-star cycle'?
The process by which gas from old stars is recycled into new star systems.
What physical force triggers the birth of stars?
Gravity causing the collapse of cold, dense molecular clouds.
What process allows stars to shine for millions to billions of years?
Nuclear fusion.
What are the two primary ways dying stars return material to interstellar space?
Stellar winds and death events (planetary nebulae for low-mass stars and supernovae for high-mass stars).
What is the 'interstellar medium' (ISM)?
The gas and dust spread throughout the galaxy.
How do high-mass stars affect the ISM during their lives?
They produce strong stellar winds that blow back the ISM, forming bubbles of hot ionized gas.
What distinguishes supernova bubbles from stellar wind bubbles?
Supernova bubbles expand much more rapidly, often faster than the speed of sound in the ISM.
What forms the walls around a supernova bubble?
Shock fronts of high-pressure gas.
Why do supernova remnants emit X-rays?
The shocked gas is hot enough to emit X-rays.
How can astronomers determine the composition of gas in a supernova remnant?
By analyzing the emission lines in the gas spectrum.
What is a 'superbubble'?
An enormous bubble formed by the combination of individual supernova bubbles from stars in a cluster.
Describe the 'Galactic Fountain' model.
Hot gas blows out of the galactic disk into the halo, cools and condenses into clouds, and then rains back down onto the disk.
What is the temperature trend as matter transitions from plasma to molecular gas?
The temperature decreases.
What are the three main phases of gas in the ISM?
Molecular gas, atomic gas, and ionized gas (plasma).
What is the primary composition of warm atomic gas in the ISM?
Mostly hydrogen, with some helium and trace amounts of heavier elements.
Where do molecular clouds tend to settle within the galaxy?
Toward the center layer of the galactic disk.
What defines the 'molecular gas' phase of the ISM?
It is the coldest and densest gas in the ISM.
What gives spiral arms their characteristic blue color?
The presence of numerous young, hot stars.
Why do spiral arms not move with the individual stars in a galaxy?
Because spiral arms are density waves, not fixed structures; stars enter and exit these regions of high density.
What is the analogy used to explain how stars move through spiral arms?
Individual cars entering and exiting a traffic jam.
What happens to the elements created by high-mass stars after they die?
They are mixed into the ISM and spread throughout the galaxy.
What is the relationship between spiral arms and star formation?
Spiral arms are regions of high gas density where active star formation occurs.
What are the primary visible components of the Milky Way galaxy?
The disk, the bulge, and the halo.
How is the halo of the Milky Way described?
A large, spherical region surrounding the disk and bulge that contains very sparse stars.
What are the primary characteristics of the Disk Population (Population I) stars?
They have a composition similar to the Sun, contain about 2% heavy elements, and include a mix of young and old stars.
What are the primary characteristics of the Halo Population (Population II) stars?
They are old, low-mass stars with fewer heavy elements and orbits at random inclinations.
Why do new stars not form in the halo of the Milky Way?
The halo lacks the cold, dense molecular clouds required for star formation; it contains only hot, sparse gas.
What does the low heavy-element content in halo stars suggest about their age?
It suggests they formed early in the galaxy's history, before supernovae had enriched the interstellar medium with heavy elements.
What was the starting point for the formation of the Milky Way according to the basic model?
A large protogalactic cloud of hydrogen and helium, surrounded by a larger cloud of dark matter.
In the basic model of galaxy formation, which stars formed first?
The halo stars, which formed while the gas was still in a cloud-like state.
How did the disk of the Milky Way form?
The remaining gas settled into a spinning disk due to angular momentum, similar to the formation of the solar system.
What is the primary difference in orbital motion between disk stars and halo stars?
Disk stars orbit in the same direction within the plane, while halo stars have random positions and motions.
What is a limitation of the simple model of galaxy formation regarding heavy elements?
The model predicts that heavy-element content should vary based on distance from the center, but observations show no such dependency in halo stars.
What is the 'multiple protogalactic clouds' theory for the formation of the Milky Way?
The idea that the galaxy formed from the merger of several smaller clouds, which explains the mixing and variation of stars in the halo.
What are the Magellanic Clouds in relation to the Milky Way?
They are dwarf galaxies that are currently on a collision course with the Milky Way.
Why is visible light ineffective for observing the center of the Milky Way?
The view is obscured by dense clouds of gas and dust.
What type of light is best used to observe the center of the Milky Way?
Infrared light, which can pass through gas and dust clouds.
What evidence indicates the presence of a massive object at the center of the Milky Way?
The orbital periods and paths of stars near the galactic center.
What is the estimated mass of the central object at the center of the Milky Way?
Approximately 4 million times the mass of the Sun.
What is the maximum estimated diameter of the central object at the center of the Milky Way?
Approximately 70 AU.
Why must the object at the center of the Milky Way be a black hole?
Because a mass of 4 million solar masses packed into a space no larger than 70 AU is too dense to be anything else.
What phenomenon provides X-ray evidence for a supermassive black hole at the galactic center?
Occasional X-ray flares caused by tidal forces ripping apart matter as it falls into the event horizon.
What happens to the cool gas that was once in the halo?
It has sunk into the disk over time.
What is the 'star-gas-star cycle'?
The process that enables ongoing star birth throughout the disk of the galaxy over billions of years.
What is the definition of cosmology?
The study of the structure and evolution of the Universe.
Approximately how many galaxies are estimated to exist in the observable Universe?
Over 1 trillion.
What are the three main types of galaxies?
Spiral, elliptical, and irregular galaxies.
What are the two primary components of a spiral galaxy?
The disk component and the halo component.
How does the disk component of a spiral galaxy differ from the halo component?
The disk contains stars of all ages and gas clouds, while the halo (including the bulge) is spherical with little gas and mostly old stars.
What color are regions of active star formation in spiral galaxies, and why?
They appear blue-white because they contain hot, young stars.
What distinguishes a barred spiral galaxy from a regular spiral galaxy?
Barred spiral galaxies have a straight bar of stars running through the center.
What are the defining characteristics of lenticular galaxies?
They possess disk and halo components like spirals but lack spiral arms, serving as an intermediate class between spirals and ellipticals.
Why do elliptical galaxies typically appear red-yellow in color?
They contain an older star population and lack the cool gas necessary for significant new star formation.
How do elliptical galaxies differ from spiral galaxies in terms of structure?
Elliptical galaxies only contain a halo component and lack a significant disk.
What is the relationship between galaxy size and elliptical galaxies?
Elliptical galaxies exhibit a wider range of sizes and masses than any other galaxy type, ranging from dwarfs to giants.
What are the characteristics of giant elliptical galaxies?
They are the most massive galaxies, often containing up to 10^12 stars and spanning hundreds of thousands of light-years.
What are dwarf elliptical galaxies?
Small, common galaxies that resemble large globular clusters and are often found near larger spiral galaxies.
What defines an irregular galaxy?
They are neither spiral nor elliptical in shape and often show scattered patches of active star formation.
What does the blue-white color of irregular galaxies indicate?
Ongoing, active star formation.
Who devised the classification system for galaxies based on their shape?
Edwin Hubble.
What criteria are used to classify elliptical galaxies in Hubble's tuning fork?
Their degree of flatness, indicated by a number from E0 (spherical) to E7 (very elongated).
What criteria are used to classify spiral galaxies in Hubble's tuning fork?
The size of the central bulge and the tightness of the spiral arms (labeled a, b, or c).
Did astronomers originally believe the Hubble tuning fork represented an evolutionary sequence?
Yes, but that theory has since been proven incorrect.
What is a common cause for the formation of giant elliptical galaxies?
They are often the result of galaxy mergers.
In what type of environment are spiral galaxies typically found?
They are often found in loose groups, typically containing 10 to 30 galaxies.
Where are elliptical galaxies most commonly found?
In huge clusters containing 100 to 1,000 galaxies.
What is gravitational lensing in the context of galaxy clusters?
A phenomenon where the total mass of a large cluster is so high that it distorts the light of background galaxies into arcs.
Why can't we use the same distance measurement techniques for all objects in the universe?
Techniques for measuring distances within our galaxy do not directly work for other galaxies, requiring a chain of interlocking methods where each step calibrates the next.