Astronomy Midterm 2
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What type of galaxy is the Milky Way?
A barred spiral galaxy.
Where is the Sun located in the Milky Way?
In the disk, away from the center, in one of the spiral arms.
How long does it take the Sun to orbit the Milky Way's center?
About 230 million years.
What is the galactic bulge?
A rounded central area containing mostly old stars, with little gas and dust.
What is the galactic disk?
A flattened area containing young stars, gas, and dust where star formation occurs.
What are spiral arms?
Overdense regions in a galaxy's disk that contain more stars and active star formation.
What is a supermassive black hole?
A black hole with millions or billions of times the Sun's mass, usually found at a galaxy's center.
What was the Great Debate?
A 1920 debate about whether spiral nebulae were inside or outside the Milky Way.
Who proved that other galaxies exist outside the Milky Way?
Edwin Hubble, by finding Cepheid variable stars in other galaxies and measuring their distances.
What is a Cepheid variable star?
A star whose changing brightness can be used to measure distance.
What does the Hubble sequence classify?
Galaxies based on their shapes and structures; it is not a strict ranking or evolutionary sequence.
What are the main types of galaxies?
Spiral, barred spiral, elliptical, lenticular, and irregular.
What are spiral galaxies generally like?
They have flattened disks, spiral arms, gas and dust, young blue stars, and ongoing star formation.
What are elliptical galaxies generally like?
They are rounded or oval, contain mostly old red stars, and have very little gas, dust, or star formation.
What are lenticular galaxies?
Galaxies with disks but no spiral arms and little gas or ongoing star formation.
What are irregular galaxies?
Galaxies without a clear spiral or elliptical shape; many are small and actively forming stars.
What is a galaxy merger?
When two galaxies collide and combine; mergers can transform spiral galaxies into elliptical galaxies.
What is a galaxy group?
A collection of a few dozen galaxies held together by gravity.
What is a galaxy cluster?
A large gravitationally bound collection of hundreds or thousands of galaxies.
What is a galaxy rotation curve?
A graph showing how a galaxy's rotational speed changes with distance from its center.
What did astronomers expect galaxy rotation curves to do?
They expected rotational speeds to decrease farther from the galaxy's center.
What did astronomers actually observe in galaxy rotation curves?
Rotational speeds stayed relatively flat even far from the center.
What does a flat galaxy rotation curve suggest?
There is extra unseen mass, called dark matter, providing additional gravity.
What is dark matter?
Matter that does not emit or absorb light but has gravity.
What percentage of the Universe's mass is dark matter?
About 85%.
What is a dark matter halo?
An extended region of dark matter surrounding a galaxy.
What are MACHOs?
Massive Compact Halo Objects, such as dim stars or black holes, once proposed as dark matter candidates.
What are WIMPs?
Weakly Interacting Massive Particles, hypothetical particles that could make up dark matter.
What are the three ways scientists search for WIMPs?
Direct detection, indirect detection, and creating particles in colliders.
What is the cosmic web?
A network of dark matter filaments where galaxies and galaxy clusters form, separated by large empty regions called voids.
Where do galaxies and galaxy clusters form in the cosmic web?
At intersections of dark matter filaments, where gravity pulls matter together.
What is cold dark matter?
Dark matter made of slow-moving particles that helps explain the structure of the Universe.
What is a star?
A hot ball of plasma held together by gravity.
What is hydrostatic equilibrium?
The balance between inward gravity and outward pressure inside a star.
What is nuclear fusion?
The process of combining light atomic nuclei into heavier nuclei and releasing energy.
What is nuclear fission?
The process of splitting heavy atomic nuclei into smaller nuclei and releasing energy.
Why does nuclear fusion release energy?
The fused nucleus has slightly less mass than the original particles; the missing mass is released as energy.
Why is iron important in stellar evolution?
Fusion up to iron can release energy, but fusing iron does not provide energy to support a star.
What is the proton-proton chain?
The main process of hydrogen fusion in low-mass stars.
What is the CNO cycle?
A process that uses carbon, nitrogen, and oxygen to help fuse hydrogen in high-mass stars.
What are the main parts of a star?
The core, the areas where energy moves outward, and the photosphere.
What happens in a star's core?
Nuclear fusion produces energy.
How does energy travel through a star?
Through radiation and convection.
What is a photosphere?
The thin outer layer of a star from which light is emitted.
Where do stars form?
In giant molecular clouds made of cold interstellar gas.
What is the main sequence?
The stage when a star spends most of its life fusing hydrogen in its core.
What does an H-R diagram show?
The relationship between a star's luminosity and its temperature or color.
What does the main sequence look like on an H-R diagram?
A diagonal band running from hot, bright stars to cool, dim stars.
How are massive main-sequence stars different from smaller ones?
Massive stars are generally hotter, bluer, and brighter.
What happens when a low-mass star runs out of hydrogen in its core?
Its core contracts while its outer layers expand, turning it into a red giant.
What will happen to the Sun in about 5 billion years?
It will run out of hydrogen in its core and expand into a red giant.
What happens to stars with initial masses below about 8 solar masses?
They eventually end as white dwarfs.
What happens to stars with initial masses greater than about 8 solar masses?
They can develop iron cores, collapse, and explode as supernovae.
Why does an iron core collapse?
Iron fusion cannot provide energy to support the core against gravity.
What is a white dwarf?
The dense leftover core of a low- or medium-mass star, about the size of Earth.
What is a neutron star?
An extremely dense stellar remnant made mostly of neutrons, formed after a massive star's core collapses.
What is a black hole?
An object whose gravity is so strong that nothing, not even light, can escape from inside its event horizon.
What is an event horizon?
The boundary around a black hole beyond which nothing can escape.
What is a singularity?
The central region of a black hole where matter is compressed to an extremely small volume.
What happens to a black hole's event horizon when its mass increases?
The event horizon becomes larger.
What is Sagittarius A*?
The supermassive black hole at the center of the Milky Way.
How do astronomers know Sagittarius A* exists?
They observe stars orbiting an unseen, compact object with enormous mass.
Why do astronomers use infrared light to study the Milky Way's center?
Infrared light can pass through dust that blocks much of the visible light.
What is an accretion disk?
Hot gas and dust swirling around a black hole as matter falls inward.
What is an active galactic nucleus (AGN)?
A very bright region at a galaxy's center powered by matter falling toward a supermassive black hole.
What is a quasar?
An extremely bright active galactic nucleus powered by a supermassive black hole.
What is a standard candle?
An object with known brightness that astronomers use to measure distances.
What did Edwin Hubble discover about the Universe?
That the Universe is expanding.
What did astronomers use Cepheid variable stars for?
Measuring distances to galaxies.
What is the Big Bang?
The beginning of the expanding Universe a little over 13 billion years ago.
What did Type Ia supernova observations reveal?
The expansion of the Universe is accelerating rather than slowing down.
What is dark energy?
The unknown form of energy thought to be responsible for the accelerating expansion of the Universe.
What are the three possible futures of the Universe?
Big Crunch, Big Rip, or Big Freeze.
What are gravitational waves?
Ripples in spacetime produced by accelerating massive objects, such as two orbiting black holes.
What is LIGO?
A detector that measures tiny changes in distance caused by passing gravitational waves.
How are photons, neutrinos, and gravitational waves different?
Photons are particles of light, neutrinos are nearly non-interacting particles, and gravitational waves are ripples in spacetime.
How can orbital motion reveal invisible objects or matter?
Gravity affects the motion of visible objects, allowing astronomers to detect unseen mass.