4A3 The Universe and Its Stars

Question 1

What is a star?

Answer 1

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.

Question 2

What is the difference between a star and a planet?

Answer 2

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.

Question 3

What are the stages of a star life cycle?

Answer 3

1. Nebula
2. Protostar
3. Main sequence
4. Red giant
5. Planetary Nebula or Supernova
6. 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.

Question 4

Fill in the blank:

A star spends the majority of its life in the _____ ________ phase.

Answer 4

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.

Question 5

What is the first stage in the life cycle of a star?

Answer 5

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.

Question 6

What triggers a nebula to collapse and form a protostar?

Answer 6

Gravitational forces

Gravitational forces cause regions of higher density within a nebula to collapse, forming a protostar as pressure and temperature increase.

Question 7

What is a protostar?

Answer 7

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.

Question 8

True or False:

Nuclear fusion begins in a protostar.

Answer 8

False

Nuclear fusion starts only after the protostar evolves into a main sequence star, when core temperatures reach sufficient levels.

Question 9

What determines the lifespan of a star?

Answer 9

Its mass.

High-mass stars burn fuel quickly and have shorter lifespans, while low-mass stars burn fuel slowly and live longer.

Question 10

What happens when hydrogen runs out in a main sequence star?

Answer 10

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.

Question 11

Fill in the blank:

A star much larger than the Sun becomes a/an _______ at the end of its life cycle.

Answer 11

supergiant

Massive stars expand into supergiants as they burn heavier elements in their cores near the end of their lives.

Question 12

What is a white dwarf?

Answer 12

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.

Question 13

True or False:

A supernova occurs when a star runs out of helium.

Answer 13

False

A supernova happens when massive stars run out of nuclear fuel and their cores collapse, releasing enormous energy.

Question 14

What can form from the core of a supernova?

Answer 14

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.

Question 15

Fill in the blank:

A ______ _____ is a region of space where gravity is so strong that nothing, not even light, can escape.

Answer 15

black hole

Black holes are formed from the remnants of massive stars that collapse under their own gravity after a supernova.

Question 16

What is the event horizon of a black hole?

Answer 16

The boundary beyond which nothing can escape.

The event horizon is the point where the gravitational pull becomes so strong that escape is impossible.

Question 17

What is a neutron star?

Answer 17

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.

Question 18

True or False:

All stars eventually become black holes.

Answer 18

False

Only the most massive stars become black holes; smaller stars end as white dwarfs or neutron stars.

Question 19

What determines the mass of a star?

Answer 19

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.

Question 20

True or False:

A star’s color depends on its temperature.

Answer 20

True

Cooler stars appear red, while hotter stars appear blue. The color corresponds to the surface temperature of the star.

Question 21

Fill in the blank:

The ________ of a star measures its total energy output per second.

Answer 21

luminosity

Luminosity depends on both the star’s size and temperature and is a key measure of its brightness.

Question 22

What is the Hertzsprung-Russell (H-R) diagram?

Answer 22

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.

Question 23

Which stars are in the upper right of the Hertzsprung-Russell diagram?

Answer 23

Red giants and supergiants.

These stars are large and cool but very luminous due to their size.

Question 24

True or False:

All stars eventually become white dwarfs.

Answer 24

False

Only low to medium-mass stars end up as white dwarfs. High-mass stars may explode as supernovae or collapse into black holes.

Question 25

What do white dwarfs **represent** in stellar evolution?

Answer 25

The **end** stage of low to medium-mass stars. ## Footnote *White dwarfs* are the remnants of stars that have shed their outer layers and no longer undergo fusion.

Question 26

What **happens** to the brightness of a star if its size increases?

Answer 26

It increases. ## Footnote Larger stars have greater surface areas, emitting more light and increasing their luminosity.

Question 27

What are **binary stars**?

Answer 27

Two stars that orbit a **common center of mass**. ## Footnote Binary stars are common and help astronomers calculate stellar masses through their orbital dynamics.

Question 28

# Fill in the blank: The Sun is **classified** as a \_\_\_\_\_-type star.

Answer 28

G ## Footnote The Sun is a G-type main-sequence star, with a moderate temperature and yellowish color.

Question 29

# True or False: A red giant is **cooler** than a blue star.

Answer 29

True ## Footnote Red giants are cooler, appearing red, but they are much larger and more luminous than blue stars.

Question 30

How is temperature of a star **related** to its spectral type?

Answer 30

Spectral types rank stars by **temperature**, from O (hottest) to M (coolest). ## Footnote The sequence O, B, A, F, G, K, M corresponds to decreasing temperature and specific color ranges.

Question 31

What **role** does mass play in a star’s life cycle?

Answer 31

It **determines** the star’s path and end state. ## Footnote High-mass stars may end as black holes, while low-mass stars become white dwarfs.

Question 32

# Fill in the blank: The **faintest** stars are typically \_\_\_\_\_\_ dwarfs.

Answer 32

red ## Footnote *Red dwarfs* are small, cool, and emit low levels of light, making them the faintest main-sequence stars.

Question 33

# True or False: Neutron stars are **formed** from the collapse of low-mass stars.

Answer 33

False ## Footnote Neutron stars result from the supernova **collapse** of high-mass stars, not low-mass stars.

Question 34

What **element** is primarily formed in stars like the Sun through nuclear fusion?

Answer 34

Helium ## Footnote The Sun primarily fuses hydrogen into helium through the proton-proton chain reaction.

Question 35

What role does **gravity** play in nuclear fusion?

Answer 35

It provides the **pressure** necessary to initiate fusion. ## Footnote The immense gravitational pressure at a star’s core forces atoms to collide, triggering nuclear fusion.

Question 36

# True or False: Nuclear fusion **only** occurs in the core of stars.

Answer 36

True ## Footnote Fusion happens in the star's core where temperatures and pressures are high enough to initiate the process.

Question 37

What is the significance of **carbon** in the process of nuclear fusion?

Answer 37

It acts as a **catalyst** in the fusion of heavier elements. ## Footnote In stars with masses greater than the Sun, carbon is used in the CNO cycle, a fusion process that generates energy in massive stars.

Question 38

# Fill in the blank: The **process** where helium nuclei fuse to form carbon is called the \_\_\_\_\_\_\_ cycle.

Answer 38

triple-alpha ## Footnote In this process, three helium nuclei (alpha particles) **fuse** to form a carbon atom.

Question 39

At what **stage** of a star's life is iron primarily produced?

Answer 39

During silicon **burning** in massive stars' cores before supernova. ## Footnote **Iron** is formed in the cores of massive stars just before they explode in a supernova, where it cannot undergo fusion to release energy.

Question 40

# True or False: Nuclear fusion **stops** once iron is formed in a star's core.

Answer 40

True ## Footnote 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.

Question 41

What fusion process **fuses** hydrogen into helium using carbon, nitrogen, and oxygen?

Answer 41

CNO cycle ## Footnote This cycle occurs in massive stars and is responsible for a significant portion of energy generation.

Question 42

What is the **universe**?

Answer 42

**Everything that exists**, including matter, space, stars, and galaxies. ## Footnote It is constantly *expanding* and contains all physical entities and phenomena.

Question 43

# True or False: The universe is **finite** and has clear boundaries.

Answer 43

False ## Footnote The universe is believed to be **infinite** or at least unbounded, continuously expanding since the Big Bang.

Question 44

# Define: Galaxy

Answer 44

A **system** of stars, gas, dust, and dark matter. ## Footnote Galaxies are bound by *gravity* and vary in size and shape.

Question 45

What are the **types** of galaxies?

Answer 45

* Spiral * Elliptical * Irregular ## Footnote **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.

Question 46

# Fill in the blank: The **Milky Way** is a \_\_\_\_\_\_ galaxy.

Answer 46

spiral ## Footnote The Milky Way is a barred **spiral** galaxy with a central bulge surrounded by a flat, rotating disk containing spiral arms that extend outward.

Question 47

What does dark matter **do** in galaxies?

Answer 47

It helps galaxies stay **intact** by providing additional gravity. ## Footnote It cannot be seen, but its presence is inferred through its gravitational effects.

Question 48

# True or False: The Milky Way is the **largest** galaxy.

Answer 48

False ## Footnote 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.

Question 49

# Fill in the blank: Most **galaxies** have a \_\_\_\_\_\_\_\_ black hole at their center.

Answer 49

supermassive ## Footnote **Supermassive** black holes are millions of times heavier than the Sun.

Question 50

What is the **Cosmic Microwave Background**? | (CMB)

Answer 50

**Radiation** left over from the Big Bang. ## Footnote 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.

Question 51

What is the **observable** universe?

Answer 51

The **part** of the universe we can observe. ## Footnote It extends about 93 billion light-years across.

Question 52

What is the **difference** between a star cluster and a galaxy?

Answer 52

* A **star cluster** is a group of stars. * A **galaxy** is a system of stars, gas, dust, and dark matter. ## Footnote Galaxies contain billions of stars and often include other celestial objects like nebulae and black holes.

Question 53

# True or False: All galaxies are moving **toward** each other.

Answer 53

False ## Footnote Most galaxies are moving **away** from each other due to the universe’s expansion.

Question 54

# Fill in the blank: The **largest** galaxy in the Local Group is the \_\_\_\_\_\_\_\_\_ galaxy.

Answer 54

Andromeda ## Footnote 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.

Question 55

What is the **Local Group**?

Answer 55

A **collection** of over 80 galaxies, including the Milky Way and Andromeda. ## Footnote 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).

Question 56

What is the **difference** between elliptical and spiral galaxies?

Answer 56

**Ellipticals** lack star formation and contain older stars, while **spirals** form stars in their gas-rich arms. ## Footnote 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.

Question 57

# True or False: The Milky Way is **colliding** with the Andromeda galaxy.

Answer 57

True ## Footnote The two galaxies will **merge** in about 4 billion years.

Question 58

What is **redshift**?

Answer 58

The shifting of light to **longer** wavelengths as objects move away. ## Footnote Redshift occurs when light stretches to longer wavelengths as space itself expands between distant objects—a key observation supporting cosmic expansion (Hubble's Law).

Question 59

How **old** is the universe according to the Big Bang theory?

Answer 59

Approximately 13.8 billion years. ## Footnote This estimate comes from *observations* of cosmic background radiation and the rate of expansion.

Question 60

# Fill in the blank: The expansion of the universe is **observable** through the \_\_\_\_\_\_\_ of distant galaxies.

Answer 60

redshift ## Footnote Redshift occurs when light from distant objects stretches, indicating they are moving away from us.

Question 61

What term **describes** galaxies moving away from each other?

Answer 61

The **expansion** of the universe. ## Footnote This phenomenon was first observed by Edwin Hubble and is evidence of the Big Bang.

Question 62

What **event** started the universe, according to the Big Bang theory?

Answer 62

It **began** from an extremely hot and dense state. ## Footnote This event marked the start of the universe's expansion, which continues today.

Question 63

# True or False: The universe’s expansion is **slowing** down.

Answer 63

False ## Footnote Observations show that the expansion is **accelerating** due to dark energy.

Question 64

What is the **Hubble constant**?

Answer 64

A value that **represents** the rate of expansion of the universe. ## Footnote It helps determine the age of the universe and the distance between galaxies.

Question 65

# Fill in the blank: The redshift of distant galaxies **suggests** that the universe is \_\_\_\_\_\_\_.

Answer 65

expanding ## Footnote As light from distant galaxies shifts to longer wavelengths, it shows that they are moving away from us.

Question 66

# True or False: The Big Bang theory **explains** the creation of all elements in the universe.

Answer 66

False ## Footnote 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.

Question 67

Who first **observed** that galaxies are moving away from Earth?

Answer 67

Edwin Hubble ## Footnote 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.

Question 68

# Fill in the blank: The Big Bang theory was **proposed** in the early \_\_\_\_\_ century.

Answer 68

20th ## Footnote The theory emerged in the early *20th century*, with Georges Lemaître proposing cosmic expansion in 1927 and Edwin Hubble providing observational confirmation in 1929.

Question 69

# True or False The universe was **once** much hotter and denser than it is now.

Answer 69

True ## Footnote According to the Big Bang theory, the universe *began* in a very hot, dense state before expanding and cooling.

Question 70

What is the **role** of dark energy in the universe's expansion?

Answer 70

It **accelerates** the expansion. ## Footnote Dark energy drives the universe's accelerated expansion by counteracting gravity on large scales, accounting for about 68–72% of its total energy density.

Question 71

What key **observation** supports the Big Bang theory about the universe's composition?

Answer 71

The **abundance** of light elements such as hydrogen and helium. ## Footnote These elements were formed during the first few minutes of the universe's existence in a process called Big Bang nucleosynthesis.

Question 72

# True or False: The Big Bang theory is the **only** explanation for the universe's origin.

Answer 72

False ## Footnote 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.