Astronomy Exam 3 (Pray for me, please)

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What do we mean when we say that the Sun is in gravitational equilibrium? A) The hydrogen gas in the Sun is balanced so that it never rises upward or falls downward. B) The Sun maintains a steady temperature. C) This is another way of stating that the Sun generates energy by nuclear fusion. D) There is a balance within the Sun between the outward push of pressure and the inward pull of gravity. E) The Sun always has the same amount of mass, creating the same gravitational force.

Higher temperature would cause the rate of nuclear fusion to rise, which would increase the internal pressure, causing the core to expand and cool until the fusion rate returned to normal.

Suppose that, for some unknown reason, the core of the Sun suddenly became hotter. Describe what would happen.


The core of the Sun is A) at the same temperature and density as the surface. B) at the same temperature but denser than the surface. C) hotter and denser than the surface. D) constantly rising to the surface through convection. E) composed of iron.


At the center of the Sun, fusion converts hydrogen into A) hydrogen compounds. B) plasma. C) radiation and elements like carbon and nitrogen. D) radioactive elements like uranium and plutonium. E) helium, energy, and neutrinos.

The Sun formed from a cloud of gas. As it contracted, its gravitational potential energy was converted to thermal energy. The Sun continued to contract until the core became hot enough to sustain nuclear fusion.

Briefly explain how the Sun became hot enough for nuclear fusion.


A star’s luminosity is the A) apparent brightness of the star in our sky. B) surface temperature of the star. C) lifetime of the star. D) total amount of light that the star will radiate over its entire lifetime. E) total amount of light that the star radiates each second.

Surface temperature on the horizontal axis and luminosity on the vertical axis

What is plotted on the axes of a Hertzsprung-Russell diagram?


Which of the following is true about low-mass stars compared to high-mass stars? A) Low-mass stars are cooler and less luminous than high-mass stars. B) Low-mass stars are hotter and more luminous than high-mass stars. C) Low-mass stars are cooler but more luminous than high-mass stars. D) Low-mass stars are hotter but less luminous than high-mass stars. E) Low-mass stars have the same temperature and luminosity as high-mass stars.


On the main sequence, stars obtain their energy A) from chemical reactions. B) from gravitational contraction. C) by converting hydrogen to helium. D) by converting helium to carbon, nitrogen, and oxygen. E) from nuclear fission.


By mass, the interstellar medium in our region of the Milky Way consists of A) 70% Hydrogen, 30% Helium. B) 70% Hydrogen, 28% Helium, 2% heavier elements. C) 70% Hydrogen, 20% Helium, 10% heavier elements. D) 50% Hydrogen, 50% Helium. E) 50% Hydrogen, 30% Helium, 20% heavier elements

Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.

What is interstellar reddening?


What happens to the visible radiation produced by new stars within a molecular cloud? A) It escapes the cloud completely. B) It is absorbed by dust grains and heats up the cloud. C) It is reflected back onto the protostar, heating it up further. D) The blue light is absorbed and the red light transmitted. E) It shoots out in bright jets.


What prevents the pressure from increasing as a cloud contracts due to its gravity? A) As the cloud becomes denser, gravity becomes stronger and overcomes the pressure buildup. B) The pressure is transferred from the center of the cloud to its outer edges where it can dissipate. C) Thermal energy is converted to radiative energy via molecular collisions and released as photons. D) Excess pressure is released in jets of material from the young stars. E) Once the cloud reaches a critical density, the pressure becomes degenerate and independent of temperature

In cold, dense molecular clouds, gravity brings material together. As gas moves inwards it converts gravitational potential energy to thermal energy and warms up. Once the cloud becomes so dense that the thermal radiation cannot escape, the temperature rises rapidly, nuclear fusion begins and the dense core becomes a protostar. As the cloud has collapsed from a large size to a small size, it must spin very fast to conserve angular momentum. This results in the formation of a protostellar disk around the protostar. Planets may form in this disk as the star continues to grow. Eventually stellar winds and jets clear away the surrounding gas and a newly formed star emerges.

Briefly describe how a star forms


What do astronomers mean when they say that we are all "star stuff"? A) that life would be impossible without energy from the Sun B) that Earth formed at the same time as the Sun C) that the carbon, oxygen, and many elements essential to life were created by nucleosynthesis in stellar cores D) that the Sun formed from the interstellar medium: the "stuff" between the stars E) that the Universe contains billions of stars

Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.

Why does a star grow larger after it exhausts its core hydrogen?


Which of the following statements about degeneracy pressure is not true? A) Degeneracy pressure varies with the temperature of the star. B) Degeneracy pressure can halt gravitational contraction of a star even when no fusion is occurring in the core. C) Degeneracy pressure keeps any protostar less than 0.08 solar mass from becoming a true, hydrogen-fusing star. D) Degeneracy pressure arises out of the ideas of quantum mechanics. E) Degeneracy pressure supports white dwarfs against gravity.

(1) Thermal pressure occurs when the particles inside a star are heated enough so that their random motions cause an outward pressure. The two energy sources of internal thermal pressure are gravitational contraction, found in protostars and when a star has used up a fusionable material in its core, and nuclear fusion, which can occur in the core or in a shell of a star.
(2) Degeneracy pressure arises from the idea of quantum mechanics that two electrons (or neutrons) cannot occupy the same state. Degeneracy pressure occurs in the cores of low-mass stars before a helium flash, maintains equilibrium in white dwarfs and neutron stars, and may be present immediately before a supernova event.
(3) Radiation pressure exists only in massive stars where fusion rates are so high that photons transfer momentum to the surrounding gas and apply a third kind of pressure.

What are the three types of pressure that can push against the inward force of gravity? Explain what causes each pressure and where it would be likely to occur.


White dwarfs are so called because A) they are both very hot and very small. B) they are the end-products of small, low-mass stars. C) they are the opposite of black holes. D) it amplifies the contrast with red giants. E) they are supported by electron degeneracy pressure.


Which of the following statements about novae is not true? A) A star system that undergoes a nova may have another nova sometime in the future. B) A nova involves fusion taking place on the surface of a white dwarf. C) Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now. D) When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova. E) The word nova means "new star" and originally referred to stars that suddenly appeared in the sky, then disappeared again after a few weeks or months.

As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.

What causes the radio pulses of a pulsar?

As he approached the black hole, he would be stretched by tidal forces, his time would run slow, and light coming from him would be redshifted. The closer he got to the event horizon, the slower time would run. You would never see him cross the event horizon, but he would disappear from view when his light became redshifted out of the range of detection.

Briefly describe what you would see if your friend plunged into a black hole.


If you were to come back to our Solar System in 6 billion years, what might you expect to find? A) a red giant star B) a white dwarf C) a rapidly spinning pulsar D) a black hole E) Everything will be pretty much the same as it is now


What makes up the interstellar medium? A) open clusters B) O and B stars C) K and M stars D) gas and dust E) all of the above

A star made of only helium and hydrogen would have to be among the first generation of stars ever born, arising out of the primordial mix of elements that came from the Big Bang. The oldest stars we know about are over 12-15 billion years old—a star made of only helium and hydrogen would have to be at least this old. (No such star has ever been discovered.)

Suppose you discovered a star made purely of hydrogen and helium. How old do you think it would be? Explain.

The star-gas-star cycle gradually enriches the interstellar medium with heavy elements. Therefore, stars that formed early in the history of the galaxy were formed before much enrichment from supernova events could take place. Stars that formed more recently were formed from material that had been enriched by the many previous generations of stars.

Briefly explain why stars that formed early in the history of the galaxy contain a smaller proportion of heavy elements than stars that formed more recently.


What produces the 21-cm line that we use to map out the Milky Way Galaxy? A) atomic hydrogen B) ionized hydrogen C) molecular hydrogen D) carbon monoxide E) helium

gravitational contraction

The process of the Sun gaining energy by contracting in size; proposed by astronomers in the late 1800s.

gravitational equilibrium

The process of gravity pulling inward and pressure pushing outward.


Appear as dark splotches on the Sun’s surface.


The Sun’s total power output.

solar wind

A stream of charged particles continuously blown outward in all directions from the Sun.


The outermost layer of the Sun’s atmosphere that extends several million kilometers above the visible surface of the Sun.


The middle layer of the solar atmosphere and the region that radiates most of the Sun’s UV light.


The lowest layer of the Sun’s atmosphere that is the visible surface of the Sun.

convection zone

Where energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection.

radiation zone

Where energy moves outward primarily in the form of photons of light. The temperature rises to almost 10 million K, and your spacecraft is bathed in X rays trillions of times more intence than the visible light at the solar surface.

solar core

The source of the sun’s energy: nuclear fusion transforming hydrogen into helium.

nuclear fission

The process of splitting the nucleus into two smaller nuclei.

nuclear fusion

The process of combining nuclei to make a nucleus with a greater number of protons or nuetrons.

strong force

Binds protons and neutrons together in atomic nuclei and is the only force in nature that can overcome the electromagnetic repulsion between two positively charged nuclei.

proton-proton chain

The sequence of steps that occurs in the sun during nuclear fusion; begins with the collissions between individual protons.


A subatomic particle with a very tiny mass. Produced in step 1 of the proton-proton chain.

nuclear fusion

Converts four hydrogen nuclei to one helium nucleus.

radiative diffusion

The slow, outward migration of photons in the sun.


to spread out


refers to photons of light or radiation.

mathematical models

The primary way we learn about the interior of the sun.

solar neutrino problem

The disagreement between model predictions and actual observations of the sun.

solar activity

The changing of sunspots over time.

magnetic field lines

What we draw to represent invisible magnetic fields.

solar prominences

Loops of trapped gas in the sun’s chromosphere and corona.

solar flares

Short-lived, but intense storms on the sun.

coronal holes

Regions of the corona that barely show up in x-ray images.

coronal mass ejections

Highly energetic particles from the sun’s corona that travel outward from the sun in huge bubbles.

sunspot cycle

A cycle in which the average number of sunspots on the sun gradually rises and falls.

apparent brightness

The brightness of a star as it appears to our eyes.


The total amount of power that a star emits into space.

inverse square law for light

Relates to the apparent brightness, luminosity, and distance of any light source.

1 parsec

The distance to an object with a parallax angle of 1 arcsecond.

magnitude system

A system describing stellar brightness by using numbers, called magnitudes, based on an ancient Greek way of describing the brightness of stars in the sky.

magnitude system

This system uses apparent magnitude to describe a star’s apparent brightness and absolute magnitude to describe a star’s luminosity.

apparent magnitudes

They describe how bright different stars appear in the sky.

absolute magnitudes

The apparent magnitude a star would have if it were at a distance of 10 parsecs.

spectral type

A way of classifying a star by the lines that appear in its spectrum; it is related to surface temperature. The types are designated by a letter (OBAFGKM with O for the hottest star and M for the coolest) and are subdivided with the numbers 0 through 9.

binary star systems

Systems in which two stars continually orbit one another.

eclipsing binary

A pair of stars that orbit in the plane of our line of sight.

spectroscopic binary

A binary star system whose binary nature is revealed because we detect the spectral lines of one or both stars alternately becoming blueshifted and redshifted as the stars orbit each other.

visual binary

A pair of stars we see distinctly (with a telescope) as the stars orbit each other.

Hertzsprung-Russel Diagram

A graph plotting individual stars as points with stellar luminosity on the vertical axis and spectral type (or surface temperature) on the horizontal axis.

main sequence

The prominent streak running from the upper left to the lower right of the H-R diagram.


Stars in the upper right of the H-R diagram because they are very large and very bright.


Just below the supergiants on the H-R diagram because they’re somewhat smaller in radius and lower in luminosity but still much larger and brighter than main-sequence stars of the same spectral type.

white dwarfs

The stars near the lower left of the H-R diagram that are small in radius and appear white in color because of their high temperatures.

luminosity class

Describes the region of the H-R diagram in which a star falls. Class I represents supergiants, III represents giants, and V represents main-sequence stars; classes II and IV are intermediate to the others.

main-sequence lifetime

The limited time a star can remain as a hydrogen-fusing, main sequence star before it runs out of its limited supply of core oxygen.

the Sun

Most ancient thinkers believed it was a lump of fire that was burning coal or wood.


He was the first to propose the hypothesis of gravitational contraction.

Some of the gravitational potential energy of gas particles far from the cloud center is converted into thermal energy as the gas moves inward.

Why does a shrinking gas cloud heat up?

gravitational contraction

Where 19th century astronomers believed the Sun’s energy came from.

They believed gravitational contraction could keep the Sun fueled for 25 million years, but they soon realized the solar system was much older than that.

Why did astronomers realize that gravitational contraction can’t be the source of the Sun’s energy, even though it would make sense if the Sun was slowly contracting each year to convert gravitational potential energy into more thermal energy?

Einstein’s special theory of relativity disclosed the information by stating that mass itself contains potential energy, and it simply needs a stimulus to convert the energy to thermal energy to power the Sun.

How did scientists finally realize that the Sun could produce its own energy for billions of years?

The bottom person supports the weight of everybody above him, so his arms must push upward with enough pressure to support all this weight. At each higher level, the overlying weight is less, so it’s easier for each additional person to hold up the rest of the stack.

Explain how a stack of acrobats provides a simple example of gravitational equilibrium.

internal gas pressure

Where the outward push of gravity in the Sun’s gravitational equilibrium comes from.

The Sun’s internal pressure balances gravity at every point within it, thereby keeping the Sun stable in size. Because the weight of the overlying layers is greater as we look deeper in the Sun, the pressure must increase with depth. In the core, the pressure makes the gas hot and dense enough to sustain nuclear fusion. The energy released by fusion, in turn, heats the gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity.

Explain how gravitational equilibrium works inside the Sun.

the energy released by fusion

Where the pressure in gravitational equilibrium comes from.

4.5 billion

How many years ago the Sun was born.

10 billion years

How many years the Sun will be able to survive with the amount of hydrogen it was born with.

gravitational contraction in its core will begin once again.

What will happen when the Sun runs out of hydrogen to fuse?


The Sun is a giant ball of this.


A gas in which many of the atoms are ionized because of high temperatures.

It can create and respond to magnetic fields

How does a plasma behave differently from a regular gas?

solar wind

Helps shape the magnetosphere of planets and blows back the material that forms the plasma tails of comets.


Where you’ll find sunspots, regions of intense magnetic fields that would cause your compass needle to spin wildly.


Caused by the Sun’s seething, churning appearance.

Strong force is the only force in nature that can overcome the electromagnetic repulsion between two positively charged nuclei. It overpowers the electromagnetic force over very small distances but is insignificant when the distances between particles exceed the typical sizes of atomic nuclei. So, it has to push the positively charged nuclei close enough together to outmuscle the electromagnetic repulsion.

Explain how nuclear fusion works.

a helium nucleus containing two protons and two neutrons

What does fusion transform four individual hydrogen protons into?

The Sun regulates its own heat with its balances between both pressure & gravity and the flow of energy through the Sun. If the core gets too hot, fusion rates increase and increase the pressure in the core to make it expand. After it expands, it has to cool down, which ultimately caused fusion to slow down back to normal and the size to decrease back to normal. If the temperature in the core dropped, nuclear fusion would decrease, which would cause pressure to drop and the core to shrink. Subsequently, temperature would rise again and the fusion rate and core size would regulate.

Explain how the Sun works as a solar thermostat.

Because 4 hydrogen nuclei only converts to 1 helium nucleus, the number of independent particles falls with time, which causes the core to shrink and the temperature and fusion rate to increase. Thus, as fusion increases, brightness increases as well.

How is the Sun gradually brightening?


How much as solar luminosity increased since the Sun was born?

random walk

The haphazard way a photon travels through the Sun’s interior as it bumps into electrons.

because hot gas is less dense than cool gas

Why does convection occur in the convection zone?

1. mathematical models
2. solar vibrations
3. solar neutrinos

The three ways we can study the Sun’s interior.

Neutrinos come in 3 types: electron neutrinos, muon neutrinos, and tau neutrinos. However, fusion only creates electron neutrinos, and until recently, solar neutrino detectors could only electron neutrinos. Now, we think some electron neutrinos transform into other neutrinos as they travel to Earth, explaining why past experiments counted fewer (because they were only counting electron neutrinos)

How do scientists think neutrinos hide?

They have strong magnetic field lines that suppress convection within the sunspot and prevent surrounding plasma from entering it.

Why do sunspots remain so much cooler than the remaining gas around them?

when the magnetic field lines in a sunspot become so twisted and knotted that they can no longer bear tension and suddenly snap to reorganize themselves; in the process, it releases energy, which generates x-rays and accelerates charged particles to the speed of light.

When do solar flares occur?

They hamper radio communications, disrupt electrical power delivery, and damage electronic components in orbiting satellites.

How do coronal mass ejections hinder Earth?

solar maximus

The time when sunspots are most numerous and we see dozens of sunspots on the Sun at one time.

22 years (two sunspot cycles)

How long does a complete magnetic cycle on the Sun take?

core, radiation zone, convection zone, photosphere, chromosphere, corona

Name the Sun’s layers from the inside out.

5,778 K

The Sun’s surface temperature.

432,169 mi

The Sun’s radius.


The most direct way to measure a star’s distance.


The small annual shifts in a Star’s apparent position caused by Earth’s motion around the Sun.

magnitude system

Originally classified stars by how bright they look to the human eye.

A star’s color and spectrum

The two ways we determine surface temperature.

By comparing a star’s apparent brightness in two different colors of light.

How do astronomers measure surface temperature from color?


The spectral types.


What spectral type are the hottest blue stars?


The luminosity class for supergiants.


The luminosity class for main-sequence stars.


The luminosity class for giants.


Represents stars with radii larger than those of main-sequence stars but not quite large enough to qualify them as giants.

white dwarfs

These fall out of the luminosity classes and are assigned the class "wd."

1. spectral type, which tells surface temperature and color
2. luminosity class, which tells us about radius

The two different ways to categorize stars.

G2 V

The spectral type and luminosity class of the Sun.


The most important attribute of a hydrogen-burning star.

The weight of a star’s outer layers determines the nuclear fusion rate in its core. More weight means the star must sustain a higher nuclear fusion rate in order to maintain gravitational equilibrium.

Why does luminosity depend on mass?

They start out with more hydrogen and end up fusing it faster.

Why do massive stars have shorter lives?

Most have already died due to their rapid fusion and they are born in smaller numbers to begin with.

Why are massive stars so rare?

variable star

Any star that varies significantly in brightness with time.

pulsating variable stars

They lie between the main sequence and the red giants on the H-R Diagram on the instability strip.

star clusters

Groups of stars that formed from the same interstellar cloud.

globular cluster

Most of these clusters are found in the halo, and their stars are some of the oldest in the universe.

globular cluster

Can contain more than a million stars concentrated in the shape of a ball.

main-sequence turnoff

the precise point on the H-R diagram at which a cluster’s main sequence diverges from the standard main sequence.

The age is equal to the lifetimes of stars at its main-sequence turnoff point.

How do we find the age of a cluster?

interstellar medium

The gas and dust that fill the spaces between stars within a galaxy.

molecular clouds

Cold and dense clouds that allow atoms to combine together into molecules and eventually form stars.

interstellar dust

Tiny, solid grains found in molecular clouds.

1. Individual gas clumps within the cloud move at substantially different speeds, indicating that the overall gas motion is turbulent.
2. Magnetic fields can help the cloud resist gravity.

How do massive clouds resist the crush of gravity?

Gravity increases as cloud size shrinks. The contraction of the cloud gives gravity a growing advantage in the battle against pressure, which in turn means that a smaller total amount of mass is necessary for gravity to win the battle. Because molecular clouds are turbulent and lumpy, there are plenty of small, dense clumps within a contracting cloud that are soon able to shrink on their own. The accelerating nature makes it inevitable that a large molecular cloud will split into numerous individual cloud fragments.

Why does a large molecular cloud form many individual stars instead of a single extremely massive star?

The molecular clouds that formed them only had hydrogen, so the stars did not have carbon monoxide to cool them off. Thus, they were huge when they were formed, and they died off quickly from their rapid fusion.

Why don’t we find any first generation stars in the universe?

The first generation of stars were huge, so they were able to create heavier elements.

Why were heavy elements created so quickly?


A clump of gas that will become a new star.

protostellar disk

Similar to the spinning disk of gas from which the planets formed; gas that settles into a disk due to the rapid rotation of a protostar during star formation.

protostellar wind

An outward flow of particles similar to the solar wind.


High-speed streams of gas that many young protostars fire into space.

Gravitational interactions between the binary pair of protostars and other protostars and gas clumps in their vicinity can remove angular momentum from the binary system. Then, their orbit gets smaller, and they end up quite close to one another.

How are close binary systems formed?

degeneracy pressure

Pressure that halts gravitational contraction before hydrogen burning can begin; occurs when contracting clouds with too low a mass never become stars because their central temperatures never climb above the 10 million K threshold needed for efficient nuclear fusion.

degeneracy pressure

Type of pressure that depends only on density and not on temperature.

degeneracy pressure

Type of pressure that occurs when the electrons are so tightly packed that they can barely move around; occurs in clouds with low masses.

brown dwarf

Failed stars that form as a result of a protostar that was restricted from starting nuclear fusion due to degeneracy pressure.

brown dwarfs

These objects occupy the fuzzy gap between planets and stars because their degeneracy pressure will never succumb to gravity, so it will continue to exist.

brown dwarfs

Despite their names, these objects radiate in the infrared and look magenta or red in color.

radiation pressure

Pressure caused by light that determines the maximum mass of a star.

Radiation pressure is so strong that gravity often can’t resist it, so stars grow too big, the radiation pressure blows away their extra layers until gravitational equilibrium is achieved once more.

Why do stars have maximum masses?

They count the number of low-mass stars in the immediate vicinity of the Sun and use this count to estimate how many such stars have been made in the entire history of our galaxy. The results show that newly formed star clusters have far more low mass stars than high mass stars.

How do astronomers determine the average mass of newborn stars?

low-mass starts

Stars born with less than 2 times the mass of our sun.

intermediate-mass stars

Starts with birth weights between 2 and 8 solar masses.

high-mass starts

Stars born with masses greater than about 8 solar masses.

internal temperature and mass

When does the depth of a star’s convection zone depend on?

almost to the core

How thick is the convection zone in extremely low-mass stars?

flare star

A small, spectral type M star that displays particularly strong flares on its surface.

The Sun’s outer layers will expand outward as the hydrogen in the outer shells fuses, and the core will shrink under the crush of gravity. It will grow to become a subgiant, and its luminosity will increase substantially. Within a billion years, the Sun will become a red giant. It will be 100 times larger in radius and 1000 times brighter in luminosity. Gravitational equilibrium diminishes, so the fusion never balances out, and the core continues to shrink. The huge radius weakens the pull of gravity, and large amounts of mass escape. Degeneracy pressure prevents the core from getting hot enough to fuse helium on its own.

What will happen when the Sun runs out of hydrogen to fuse?

hydrogen shell burning

Hydrogen fusion in a shell around the core.

helium fusion

Occurs only when nuclei slam into one another at much higher speeds than those needed for hydrogen fusion (because the protons are more positively charged and therefore repel each other with a stronger force), which means it requires much higher temperatures than hydrogen fusion.

Three helium nuclei are converted into one carbon nucleus.

Explain the helium fusion process.

helium flash

The event that marks the sudden onset of helium fusion in the previously inert helium core of a low-mass star.

The helium flash releases an enormous amount of energy into the core. The temperature rises so much that thermal pressure becomes dominant and degeneracy pressure is no longer relevant. The thermal pressure pushes back against gravity, and the core begins to expand. It pushes the hydrogen burning shell outward, which slows its fusion rate. The outer layers contract, and the star turns from red to yellow, becoming a helium-burning star.

Explain how helium fusion starts in a low-mass star.

horizontal branch

The line of helium-burning stars on the H-R diagram that all differ in surface temperature but have the same luminosity.

It will expand once again, which will be triggered by the helium fusion in the outer shells. The hydrogen shell will still burn on top of the helium layer. Both shells will contract along with the core, which will drive fusion rates so high that the star will expand to an even greater size and luminosity than it had as a red giant. The fusion rate spikes every few thousands of years due to thermal pulses. Degeneracy stops the collapse of the core, so it never heats up enough to fuse carbon. It may become a carbon star when thermal pulses bring carbon from the core to the surface. The outer layers will be ejected. The exposed core will emit radiation, which will make it glow brightly as a planetary nebula. The nebula will eventually dissappear, and all that will remain is the star’s core, which is now a white dwarf.

How does a low mass star die after exhausting its helium fusion?

planetary nebula

The glowing cloud of gas ejected from a low-mass star at the end of its life.

In 3-4 billion years, the brightening of the Sun will doom Earth. It will have a runaway greenhouse effect. The oceans will boil away, and living things will likely die. Temperatures will rise drastically. The sun will shrink, but a 100 million years later, it will expand again and engulf all of the planets.

Explain what will happen to the Earth when the Sun stops fusing hydrogen.

CNO cycle

The cycle of reactions by which intermediate stars fuse hydrogen into helium.

It develops a hydrogen-burning shell, and its outer layers begin to expand outward, ultimately turning it into a supergiant star. The core contracts, which released energy that increases the core temperature until it becomes hot enough to fuse helium into carbon. After the helium is completely fused, the core shrinks, so its temperature and pressure rises. A helium-burning shell forms between the core and the hydrogen-burning shell. The core will start to fuse even heavier elements until the end of the star’s life.

What happens when a high-mass star runs out of hydrogen in its core?

helium-capture reactions

Reactions in which a helium nucleus fuses into some other nucleus.

It looks like an onion because it has fused so many different kinds of elements that it has tons of fusing layers.

What does a high-mass star look like near the end of its life and why?

It can’t generate any kind of nuclear energy.

Why is iron unique?

The star has no way to make energy anymore, so when the core and shells are full of iron, even degeneracy pressure can’t do anything, so gravity is crushing it; the only thing that can save it is a supernova explosion.

Why is the buildup of iron catastrophic for the star?

The electrons and protons in the star’s core fuse together, forming neutrons. Thus, without electrons, degeneracy pressure can’t exist, and the core releases neutrinos. Gravity has free reign, and the core full of iron collapses into a ball of neutrinos. The outer layers of the star are thrown into space, creating the supernova. The ball of neutrons left behind forms a neutron star. In some cases, the mass may be so large that it becomes a black hole.

How does a supernova occur?


A titanic explosion of a high-mass star’s outer layers projecting into space after the collapse of the iron core.

neutron star

The ball of neutrons left behind after a supernova from the collapse of a high-mass star’s core.

black hole

A bottomless pit in spacetime created after a supernova explosion and the collapse of a high-mass star’s neutron star.

supernova remnant

An expanding cloud of debris from a supernova explosion.

Supernova 1987 A

A supernova witnessed on Earth in 1987; it was the nearest supernova to Earth in nearly 400 years and helped astronomers refine theories of supernovae.

The two stars in close binary–one a high-mass star that seems to be living longer than its low-mass companion that’s already a subgiant–and they exert tidal forces on one another, The gravity of each star attracts the near side of the other star more strongly than it attracts the far side. The stars therefore stretch into football-like shapes rather than remaining spherical. They are also tidally locked and always show the same face to one another. The high-mass star has spilled its outer layers onto its companion, so the companion star gained mass instead.

Explain the Algol paradox.

mass exchange

The process in which tidal forces in close binary systems cause matter to spill from one star to a companion star in a close binary system.

degeneracy pressure

The source of pressure that pushes against gravity in a white dwarf.

electron degeneracy pressure

Degeneracy pressure asserted by electrons in white and brown dwarfs.

white dwarf limit

The maximum possible mass for a white dwarf, which is about 1.4 solar masses.

black dwarf

What a white dwarf will become as it ages.

accretion disk

The whirlpool-like disk around a white dwarf that forms when its companion star in a binary spills mass onto it as it spins to conserve angular momentum.

accretion disks

When they surround white dwarfs, they emit X-rays or ultraviolet radiation, which provides energy to the white dwarf.

If its companion drops material on its surface, a hydrogen layer will form, and it will fuse until it burns off in a nova. The process can start again if the companion drops more material on the surface.

How can a white dwarf in a binary restart fusion temporarily.

The white dwarf will explode in a white dwarf supernova

What happens if an accreting white dwarf in a binary system continues to gain mass until it approaches the white dwarf limit?

There are no hydrogen lines in a white dwarf supernova, and the brightness of a white dwarf supernova is less complicated than the brightness of a massive star supernova.

How can astronomers distinguish between white dwarf supernovas and massive star supernovas?


A neutron star from which we see rapid pulses of radiation as it rotates.

A huge amount of energy would be released, which makes an extremely luminous and hot accretion disk. The neutron star radiates powerful x-rays. Emissions pulsate rapidly as the neutron star spins. In a spike of luminosity, the neutron star erupts in an x-ray burst, which results from the sudden ignition of nuclear fusion. Steady fusion occurs, and a layer of hydrogen fusion sits upon a layer of helium fusion. Soon, heavier elements are formed until an X-ray burst ends the process. It can start over again though.

What happens if matter falls into a neutron star in a binary system?

event horizon

The boundary between the inside of a black hole and the universe outside.


The inseparable, 4-D combination of space and time.

Scwarz-schild radius

The radius of the event horizon.

studying X-ray binaries

Where does strong evidence for black holes come from?

gamma rays

Light with very short wavelengths; shorter than X-rays.

gamma ray bursts

Sudden bursts of gamma rays from deep space; come from distant galaxies, but their precise mechanism is unknown.

the formation of black holes or the collision of a neutron star and a black hole in a binary system

Where do gamma ray bursts seem to come from?


The type of galaxy that is the Milky Way.

the disk

Where do most of the Milky Way’s bright stars reside?

200 globular clusters

Where are the most prominent stars in the halo found?


How many light-years thick is the disk?

a circular path going in the same direction on the same plane

How do stars in the disk orbit?

in randomly oriented orbits above and below the disk

How do stars in the bulge and halo orbit?

In the halo; not in the center

Where is most of the Milky Way’s mass located?

star-gas-star cycle

The process of galactic recycling in which stars expel gas into space where it mixes with the interstellar medium and eventually forms new stars.

1. stellar winds that blow away their mass throughout their entire lives
2. the death events of planetary nebulae and supernovae

The two basic ways stars return their original mass into interstellar space.


An expanding shell of hot, ionized gas driven by stellar winds or supernovae; inside it, the gas is very hot and has a very low density.

local bubble

The result of a supernovae detonating within the Sun’s neighborhood.

cosmic rays

Particles such as electrons, protons, and atomic nuclei that zip through interstellar space at close to the speed of light.

cosmic rays

They can cause genetic mutations in living organisms


Cavities of hot gas that arise when many individual bubbles combine.


When a superbubble breaks out of the disk and nothing remains to slow its expansion, so hot gas erupts from the disk, spreading out as it shoots upward into the galactic halo.

galactic fountain

Refers to a model for the recycling of gas in the Milky Way Galaxy in which fountains of hot, ionized gas rise from the disk into the halo and then cool and form clouds as they sink back into the disk.

atomic hydrogen gas

Cool gas in which hydrogen atoms remain neutral rather than becoming ionized.

ionization nebulae

Colorful, wispy blobs of glowing gas found near hot stars.

spiral density waves

Gravitationally driven waves of intense density that move through a spiral galaxy and are responsible for maintaining its spiral arms.

disk population

Contains both young stars and old stars, all of which have heavy-element proportions of about 2%, like our Sun.

spherodial population

Consists of stars in the halo and bulge, both of which are roughly spherical in shape; stars in this population are always old and therefore low in mass.

Our galaxy began as a giant protogalactic cloud containing all the hydrogen and helium gas the galaxy eventually turned into stars. Gravity caused the cloud to contract and fragment. Stars in the spheroidal population formed first. Later, the remaining gas settled into a flattened, spinning disk as it contracted under the force of gravity because of conservation of angular momentum. So, stars in that disk orbitted in a routine pattern.

How did the Milky Way galaxy form?

the bulge and halo

What the spheroidal population consists of.

Swirling clouds of gas and a cluster of several million stars; also, there’s a source of magnetic fields called Sagittarius A*, which contains a few million solar masses, so it probably contains a massive black hole.

What is in the center of the galaxy?

Stars are born from gravitational collapses of gas clumps in molecular clouds. Massive stars explode as supernovae when they die, creating hot bubbles in the interstellar medium that contain new elements made by these stars. Eventually, this gas cools and mixes into the surrounding interstellar medium, turning into atomic hydrogen and then cooling further, producing molecular clouds. These molecular clouds then form stars, completing the star-gas-star cycle.

How is gas recycled in our galaxy?

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