4A3 The Universe and Its Stars Flashcards
Explore how stars evolve through distinct stages, and describe how the origins of the universe are explained through the Big Bang Theory.
What is a star?
A massive ball of hot gas undergoing nuclear fusion.
Stars are primarily composed of hydrogen and helium and emit light and heat due to nuclear fusion occurring in their cores.
What is the difference between a star and a planet?
A star emits light; a planet reflects light.
Stars generate energy through nuclear fusion, while planets do not produce their own light but reflect the light of nearby stars.
What are the stages of a star life cycle?
- Nebula
- Protostar
- Main sequence
- Red giant
- Planetary Nebula or Supernova
- White Drawf, Neutron Star, or Black Hole
Average size stars will become red giants, who then become planetary nebulas, and eventually white dwarf stars. Large or massive stars will become red supergiants, who become supernova, who then become either a neutron star or black hole.
Fill in the blank:
A star spends the majority of its life in the _____ ________ phase.
main sequence
The main sequence phase is the longest in a star’s life, where hydrogen nuclei fuse into helium in the core, producing energy.
What is the first stage in the life cycle of a star?
Nebula
A nebula is a massive cloud of gas and dust in space, primarily hydrogen and helium, where stars begin to form due to gravitational collapse.
What triggers a nebula to collapse and form a protostar?
Gravitational forces
Gravitational forces cause regions of higher density within a nebula to collapse, forming a protostar as pressure and temperature increase.
What is a protostar?
An early stage of a forming star.
A protostar is a hot, dense core that forms as a nebula collapses, though nuclear fusion has not yet begun.
True or False:
Nuclear fusion begins in a protostar.
False
Nuclear fusion starts only after the protostar evolves into a main sequence star, when core temperatures reach sufficient levels.
What determines the lifespan of a star?
Its mass.
High-mass stars burn fuel quickly and have shorter lifespans, while low-mass stars burn fuel slowly and live longer.
What happens when hydrogen runs out in a main sequence star?
The core contracts and the outer layers expand.
This marks the transition into the next phase, such as becoming a red giant or supergiant, depending on the star’s mass.
Fill in the blank:
A star much larger than the Sun becomes a/an _______ at the end of its life cycle.
supergiant
Massive stars expand into supergiants as they burn heavier elements in their cores near the end of their lives.
What is a white dwarf?
The remnant of a low or medium-mass star.
A white dwarf forms after a star ejects its outer layers, leaving behind a hot, dense core that slowly cools over time.
True or False:
A supernova occurs when a star runs out of helium.
False
A supernova happens when massive stars run out of nuclear fuel and their cores collapse, releasing enormous energy.
What can form from the core of a supernova?
A neutron star or black hole.
The outcome depends on the star’s original mass: neutron stars form from medium-mass cores, while black holes form from very massive cores.
Fill in the blank:
A ______ _____ is a region of space where gravity is so strong that nothing, not even light, can escape.
black hole
Black holes are formed from the remnants of massive stars that collapse under their own gravity after a supernova.
What is the event horizon of a black hole?
The boundary beyond which nothing can escape.
The event horizon is the point where the gravitational pull becomes so strong that escape is impossible.
What is a neutron star?
A dense core of neutrons.
Neutron stars are the remnants of supernovae, formed from collapsed stellar cores where protons and electrons combine to form neutrons.
True or False:
All stars eventually become black holes.
False
Only the most massive stars become black holes; smaller stars end as white dwarfs or neutron stars.
What determines the mass of a star?
The amount of matter in the star during its formation.
A star’s mass is determined by the gas and dust available in the region where it forms.
True or False:
A star’s color depends on its temperature.
True
Cooler stars appear red, while hotter stars appear blue. The color corresponds to the surface temperature of the star.
Fill in the blank:
The ________ of a star measures its total energy output per second.
luminosity
Luminosity depends on both the star’s size and temperature and is a key measure of its brightness.
What is the Hertzsprung-Russell (H-R) diagram?
A graph linking a star’s luminosity and temperature.
The H-R diagram organizes stars by their properties, such as main sequence, giants, and white dwarfs.
Which stars are in the upper right of the Hertzsprung-Russell diagram?
Red giants and supergiants.
These stars are large and cool but very luminous due to their size.
True or False:
All stars eventually become white dwarfs.
False
Only low to medium-mass stars end up as white dwarfs. High-mass stars may explode as supernovae or collapse into black holes.
What do white dwarfs represent in stellar evolution?
The end stage of low to medium-mass stars.
White dwarfs are the remnants of stars that have shed their outer layers and no longer undergo fusion.
What happens to the brightness of a star if its size increases?
It increases.
Larger stars have greater surface areas, emitting more light and increasing their luminosity.
What are binary stars?
Two stars that orbit a common center of mass.
Binary stars are common and help astronomers calculate stellar masses through their orbital dynamics.
Fill in the blank:
The Sun is classified as a _____-type star.
G
The Sun is a G-type main-sequence star, with a moderate temperature and yellowish color.
True or False:
A red giant is cooler than a blue star.
True
Red giants are cooler, appearing red, but they are much larger and more luminous than blue stars.
How is temperature of a star related to its spectral type?
Spectral types rank stars by temperature, from O (hottest) to M (coolest).
The sequence O, B, A, F, G, K, M corresponds to decreasing temperature and specific color ranges.
What role does mass play in a star’s life cycle?
It determines the star’s path and end state.
High-mass stars may end as black holes, while low-mass stars become white dwarfs.
Fill in the blank:
The faintest stars are typically ______ dwarfs.
red
Red dwarfs are small, cool, and emit low levels of light, making them the faintest main-sequence stars.
True or False:
Neutron stars are formed from the collapse of low-mass stars.
False
Neutron stars result from the supernova collapse of high-mass stars, not low-mass stars.
What element is primarily formed in stars like the Sun through nuclear fusion?
Helium
The Sun primarily fuses hydrogen into helium through the proton-proton chain reaction.
What role does gravity play in nuclear fusion?
It provides the pressure necessary to initiate fusion.
The immense gravitational pressure at a star’s core forces atoms to collide, triggering nuclear fusion.
True or False:
Nuclear fusion only occurs in the core of stars.
True
Fusion happens in the star’s core where temperatures and pressures are high enough to initiate the process.
What is the significance of carbon in the process of nuclear fusion?
It acts as a catalyst in the fusion of heavier elements.
In stars with masses greater than the Sun, carbon is used in the CNO cycle, a fusion process that generates energy in massive stars.
Fill in the blank:
The process where helium nuclei fuse to form carbon is called the _______ cycle.
triple-alpha
In this process, three helium nuclei (alpha particles) fuse to form a carbon atom.
At what stage of a star’s life is iron primarily produced?
During silicon burning in massive stars’ cores before supernova.
Iron is formed in the cores of massive stars just before they explode in a supernova, where it cannot undergo fusion to release energy.
True or False:
Nuclear fusion stops once iron is formed in a star’s core.
True
Fusion ceases at iron because it requires more energy than it produces, causing core collapse and potentially leading to a supernova or black hole formation.
What fusion process fuses hydrogen into helium using carbon, nitrogen, and oxygen?
CNO cycle
This cycle occurs in massive stars and is responsible for a significant portion of energy generation.
What is the universe?
Everything that exists, including matter, space, stars, and galaxies.
It is constantly expanding and contains all physical entities and phenomena.
True or False:
The universe is finite and has clear boundaries.
False
The universe is believed to be infinite or at least unbounded, continuously expanding since the Big Bang.
Define:
Galaxy
A system of stars, gas, dust, and dark matter.
Galaxies are bound by gravity and vary in size and shape.
What are the types of galaxies?
- Spiral
- Elliptical
- Irregular
Spiral galaxies have well-defined arms, elliptical galaxies are smooth and oval-shaped, while irregular galaxies lack a defined structure and often have an uneven appearance.
Fill in the blank:
The Milky Way is a ______ galaxy.
spiral
The Milky Way is a barred spiral galaxy with a central bulge surrounded by a flat, rotating disk containing spiral arms that extend outward.
What does dark matter do in galaxies?
It helps galaxies stay intact by providing additional gravity.
It cannot be seen, but its presence is inferred through its gravitational effects.
True or False:
The Milky Way is the largest galaxy.
False
While the Milky Way is large, galaxies such as IC 1101 are much larger, with IC 1101 being one of the largest known galaxies, vastly outscaling the Milky Way in size and mass.
Fill in the blank:
Most galaxies have a ________ black hole at their center.
supermassive
Supermassive black holes are millions of times heavier than the Sun.
What is the Cosmic Microwave Background?
(CMB)
Radiation left over from the Big Bang.
The CMB is the faint afterglow of the Big Bang, providing crucial evidence for the early universe’s conditions and supporting the Big Bang theory by showing the universe’s cooling over time.
What is the observable universe?
The part of the universe we can observe.
It extends about 93 billion light-years across.
What is the difference between a star cluster and a galaxy?
- A star cluster is a group of stars.
- A galaxy is a system of stars, gas, dust, and dark matter.
Galaxies contain billions of stars and often include other celestial objects like nebulae and black holes.
True or False:
All galaxies are moving toward each other.
False
Most galaxies are moving away from each other due to the universe’s expansion.
Fill in the blank:
The largest galaxy in the Local Group is the _________ galaxy.
Andromeda
The Andromeda Galaxy is the largest in the Local Group and is also the closest large galaxy to the Milky Way, located about 2.5 million light-years away.
What is the Local Group?
A collection of over 80 galaxies, including the Milky Way and Andromeda.
The Local Group spans ~10 million light-years and includes numerous dwarf galaxies alongside three large spirals—Andromeda (largest), Milky Way (second-largest), and Triangulum (third-largest).
What is the difference between elliptical and spiral galaxies?
Ellipticals lack star formation and contain older stars, while spirals form stars in their gas-rich arms.
Elliptical galaxies are smoother and redder, with older stars and little star formation. Spiral galaxies have blue star-forming regions and a mix of young and old stars.
True or False:
The Milky Way is colliding with the Andromeda galaxy.
True
The two galaxies will merge in about 4 billion years.
What is redshift?
The shifting of light to longer wavelengths as objects move away.
Redshift occurs when light stretches to longer wavelengths as space itself expands between distant objects—a key observation supporting cosmic expansion (Hubble’s Law).
How old is the universe according to the Big Bang theory?
Approximately 13.8 billion years.
This estimate comes from observations of cosmic background radiation and the rate of expansion.
Fill in the blank:
The expansion of the universe is observable through the _______ of distant galaxies.
redshift
Redshift occurs when light from distant objects stretches, indicating they are moving away from us.
What term describes galaxies moving away from each other?
The expansion of the universe.
This phenomenon was first observed by Edwin Hubble and is evidence of the Big Bang.
What event started the universe, according to the Big Bang theory?
It began from an extremely hot and dense state.
This event marked the start of the universe’s expansion, which continues today.
True or False:
The universe’s expansion is slowing down.
False
Observations show that the expansion is accelerating due to dark energy.
What is the Hubble constant?
A value that represents the rate of expansion of the universe.
It helps determine the age of the universe and the distance between galaxies.
Fill in the blank:
The redshift of distant galaxies suggests that the universe is _______.
expanding
As light from distant galaxies shifts to longer wavelengths, it shows that they are moving away from us.
True or False:
The Big Bang theory explains the creation of all elements in the universe.
False
The Big Bang theory explains the formation of the lightest elements, such as hydrogen and helium. Heavier elements were formed later in stars through nuclear fusion and supernovae.
Who first observed that galaxies are moving away from Earth?
Edwin Hubble
While Vesto Slipher initially measured redshifts of galaxies, Edwin Hubble confirmed their proportional relationship to distance (Hubble’s Law) in 1929, solidifying the concept of an expanding universe.
Fill in the blank:
The Big Bang theory was proposed in the early _____ century.
20th
The theory emerged in the early 20th century, with Georges Lemaître proposing cosmic expansion in 1927 and Edwin Hubble providing observational confirmation in 1929.
True or False
The universe was once much hotter and denser than it is now.
True
According to the Big Bang theory, the universe began in a very hot, dense state before expanding and cooling.
What is the role of dark energy in the universe’s expansion?
It accelerates the expansion.
Dark energy drives the universe’s accelerated expansion by counteracting gravity on large scales, accounting for about 68–72% of its total energy density.
What key observation supports the Big Bang theory about the universe’s composition?
The abundance of light elements such as hydrogen and helium.
These elements were formed during the first few minutes of the universe’s existence in a process called Big Bang nucleosynthesis.
True or False:
The Big Bang theory is the only explanation for the universe’s origin.
False
While alternative theories like the steady-state model or oscillating universe exist, they lack the strong observational evidence, such as redshift, CMB radiation, and primordial element abundances, that support the Big Bang theory.
