ASTR 104 Exam 2 (Ch 13-15)

Radio Waves

- 21cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into the disk -Radio waves from carbon monoxide show the locations of molecular clouds

From cloud to protostar

- Cloud fragments eventually become dense enough to trap radiation, so central temperature and pressure increase significantly -Pressure slows gravitational contraction. The dense center of the cloud fragment is a protostar

Discovery of Neutron Stars

- Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky. -The pulses were coming from a spinning neutron star—a pulsar.

A neutron star or a black hole in a close binary with a companion star

-A neutron star, or a black hole, with a nearby binary companion star can also gravitationally attract material from the companion, forming accretion disk -The material swirling around the neutron star, or a black hole, shines brightly in x-rays - x-ray binary -When the neutron star accretes enough material, the layer of accreted material on the surface of the neutron star may begin to fuse H and He. - x-ray burster (neutron star remains in tact)

What happens when black holes merge?

-A powerful burst of gravitational waves is produced

Pulsars

-A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis -The radiation beams sweep through space like lighthouse beams as the neutron star rotates -Pulsars spin fast because the spin of the massive star's core speeds up as it collapses into a neutron star

What can happen to a white dwarf in a close binary system?

-A star that started with less mass gains mass from its companion -Eventually the mass-losing star will become a white dwarf

Summary: Role of Mass

-A star's mass determines its entire life story because it determines its core temperature -High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions -Low-mass stars have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs

What is a white dwarf?

-A white dwarf is the inert core of a dead star. -Electron degeneracy pressure balances the inward pull of gravity.

Where do stars tend to form in our galaxy?

-Active star-forming regions contain molecular clouds, hot stars, and ionization nebulae. -Much of the star formation in our galaxy happens in the spiral arms.

Double Shell Burning

-After core helium fusion stops, helium fuses into carbon in a shell around the carbon core, and hydrogen fuses to helium in a shell around the helium layer. -This double shell-burning stage never reaches equilibrium- for low-mass stars carbon fusion will never happen in the core. The core never contracts enough and degeneracy pressure will halt the collapse of the core before it gets hot enough for carbon fusion -The fusion rate periodically spikes upward in a series of thermal pulses. -With each spike, convection dredges carbon up from core and transports it to surface

Low and High-Mass Stars have the same beginning

-All stars form from the gravitational collapse of a cold, dense dusty gas cloud in the interstellar medium -Next, all stars become main sequence stars, fusing H to He in the core -In a high-mass star, the steps to fuse H to He is called the CNO cycle -Third, all stars run out of H in their cores, and begin fusing H to He in a shell around an inert He core -For low mass stars, this is the giant stage -For high mass stars, this is the supergiant stage -Fourth, all stars fuse He to C in their cores, while H is fusing to He in a shell -Fifth, all stars run out of He in the core. They undergo double shell fusion. Around the inert C core, there is a shell of H fusion

A Black Hole

-An object where gravity is so strong that nothing can escape -The event horizon occurs at the Schcwarzschild radius, which is the radius at which the escape velocity equals the speed of light. The Schcwarzschild radius depends on the mass of the black hole -At the center of the black hole is a singularity. All the mass is confined to the infinitely small point

Star Birth

-As gravity forces a cloud fragment to become smaller, it begins to spin faster -The material flattens into a disk due to collisions between particles

Cooling and Cloud Formation

-Atomic hydrogen gas forms at hot gas cools, allowing electrons to join with protons. We can detect atomic hydrogen using radio observations. -Matter remains in atomic hydrogen form for millions of years. But as the temperature drops, atoms to combine into molecules and molecular clouds form next.

How were neutron stars discovered?

-Beams of radiation from a rotating neutron star sweep through space like lighthouse beams, making them appear to pulse. -Observations of these pulses were the first evidence for neutron stars.

Helium "Flash"

-Before He fusion in the core, the thermostat is broken in a low-mass red giant because the pressure fighting gravity in the core is degeneracy pressure -The core temperature rises rapidly when helium fusion first begins, but the initially core doesn't increase in size because degeneracy pressure does not depend on the temperature -The helium fusion rate skyrockets (a helium flash) until thermal pressure dominates over degeneracy pressure, at which time the core expands again

Solar Thermostat

-Decline in core temperature causes fusion rate to drop, so core contracts and heats up. -Rise in core temperature causes fusion rate to rise, so core expands and cools down

Broken Thermostat

-For a Sun-like star, once hydrogen is exhausted in the core, there is inert helium at the center -As the inert He core contracts, H begins fusing to He in a shell around it -Luminosity increases because the "stellar thermostat" is broken- the increasing fusion rate in the shell does not stop the core from further contraction

The CNO Cycle

-For high-mass stars, the main H fusion process proceeds through the CNO cycle -Also fuses 4 H atoms into 1 He atom but uses a carbon atom as part of the reaction -High-mass main sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts -A greater core temperature enables H nuclei to overcome greater repulsion

Rank the size from largest to smallest:

-Galaxy Cluster -Spiral Galaxy -Molecular Cloud -Globular Cluster -Giant Star -The Sun -Jupiter -White Dwarf -Neutron Star -Black Hole

What causes gamma-ray bursts?

-Gamma-ray bursts are among the most powerful explosions in the universe and probably signify the formation of black holes -At least some gamma-ray bursts come from supernova explosions in distant galaxies

How is gas recycled in our galaxy?

-Gas from dying stars mixes new elements into the interstellar medium, which slowly cools, making the molecular clouds where stars form. -Those stars will eventually return much of their matter to interstellar space.

What happens when black holes merge?

-General relativity predicts that massive accelerating objects would disrupt spacetime and waves of distorted spacetime would move away from the source -The merger of two black holes produces a burst of gravitational waves

General Relativity

-Gravity arises from curvature of spacetime. Stronger gravity means greater spacetime curvature -Objects follow the straightest possible path allowed by the curvature of spacetime

Completing the Cycle

-Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle -Radiation from newly formed stars is eroding these star-forming clouds

What are the life stages of a low-mass star?

-H fusion in core (main sequence) -H fusion in shell around contracting inert Hr core (red giant) -He fusion in core, H fusion in shell (horizontal branch) -Double shell burning, inert C core, He fusion in shell, H fusion in shell (red giant again)

What do halo stars tell us about our galaxy's history?

-Halo stars are old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first

Helium Fusion

-Helium fusion does not begin right away because it requires much higher temperatures than hydrogen fusion-larger charge leads to greater repulsion -Helium fusion combines three helium nuclei to make carbon

What are the life stages of a high-mass star?

-High-mass stars are stars with masses > 8M -High-mass stars form in the same way as low-mass stars, out of a molecular cloud that fragments and contracts due to the gravity, creating a protostar -H fusion begins when temperatures are high enough in the core

Hydrogen Fusion

-High-mass stars release energy by fusing four hydrogen nuclei into one helium nucleus -This radiative energy mostly comes out in form of gamma-ray photons -The CNO cycle is how hydrogen fuses in helium in high-mass stars

How does a high-mass star die?

-Iron builds up in core until degeneracy pressure can no longer resist gravity -core then suddenly collapses, creating supernova explosion

A White Dwarf

-Is the exposed core of a former low-mass star -Made of carbon (and oxygen) and supported by electron degeneracy pressure -A white dwarf with the mass of the Sun is roughly the size of the Earth

Massive Star or White Dwarf?

-Light curves will be different -Spectra will be different -Exploding white dwarfs are made of Carbon and Oxygen, so they don't have any Hydrogen absorption lines

A Star's Mass Determines Its Fate

-Low-mass stars are born with <2M of material. -Intermediate-mass stars born with 2-8M of material High-mass stars are born with >8M of material -Lower end of high-mass stars are born with 8-25M of material -Higher end of high-mass stars are born with >25M of material

Gas from Dying Stars

-Lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae -High-mass stars have strong stellar winds, and supernovae produce bubbles of hot gas in the interstellar medium. -Multiple supernovae create huge hot bubbles that can blow out of disk -Gas clouds cooling in the halo can rain back down on disk

Summary: Life Stages of High-Mass Stars

-Main Sequence: H fuses to He in core. -Supergiant: H fuses to He in shell around He core -Helium Core Fusion: He fuses to C in core while H fuses to He in shell -Multiple Shell Fusion: many elements fuse in shells -Supernova leaves neutron star behind

Two types of supernovae

-Massive star supernova -White dwarf supernova -One way to tell supernova types apart is with a light curve showing how luminosity changes with time

Accretion Disks

-Material falling toward a white dwarf from its close binary companion has some angular momentum -The matter therefore orbits the white dwarf in an accretion disk -Friction between orbiting rings of matter in the disk removes orbital energy from the inner region of the disk and causes the disk to heat up and glow

x-ray bursts

-Matter accreting onto a neutron star can eventually become hot enough for helium fusion. -The sudden onset of fusion produces a burst of X rays.

What can happen to a neutron star in a close binary system?

-Matter falling toward a neutron star forms an accretion disk , just as in a white-dwarf binary -Binaries with an accreting neutron star are called x-ray binaries

What can happen to a white dwarf in a close binary system?

-Matter from its close binary companion can fall onto the white dwarf through an accretion disk -Accretion of matter can lead to novae and white dwarf supernovae

Gravity in a black hole is so strong that

-Nothing that passes the event horizon can ever escape -Light cannot escape the event horizon

How did our galaxy form?

-Our galaxy formed from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk.

Protostar to Main Sequence

-Protostar contracts and heats until the core temperature is sufficient for hydrogen fusion. -Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface. -It takes 30 million years for a star like the sun (less time for more massive stars)

Neutron Star Limit

-Quantum mechanics says that neutrons in the same place cannot be in the same state. -Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3M. -Some massive star supernovae can make a black hole if enough mass falls onto core.

Life Track after Helium Flash

-Red giants shrink and become less luminous after the helium flash, once the "thermostat" is temporarily restored -Stars settle on a "mini-main sequence" while they fuse helium in the core -This is called a helium core fusion star, or horizontal branch star

Reflection Nebulae

-Reflection nebulae scatter the light from stars -Why do reflection nebulae look bluer than the nearby stars? Interstellar dust grains scatter blue light more easily than red light

Do black holes really exist?

-Some X-ray binaries contain compact objects too massive to be neutron stars—they are almost certainly black holes.

How do stars form?

-Stars are born in cold, relatively dense molecular clouds. -As a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas. -The protostar may also fire jets of matter outward along its poles.

How massive are newborn stars?

-Stars greater than about 150M would be so luminous that radiation pressure would blow them apart. - Very massive stars are rare -Degeneracy pressure stops the contraction of objects <0.08M before fusion starts. - Low-mass stars are common

Close Binaries

-Stars in Algol-type systems are close enough that matter can flow from the red giant onto the main-sequence star -The star that is now a red giant was originally more massive -As it reached the end of its life and started to grow, it began to transfer mass to its companion (mass exchange) -Now the companion star is more massive

How do stars orbit in our galaxy?

-Stars in the disk all orbit in the same direction with a little up-and-down motion. -Orbits of stars in the halo have random orientations. -Some bulge stars orbit like halo stars while others orbit like disk stars

Summary of Galactic Recycling

-Stars make new elements by fusion. -Dying stars expel gas and new elements, producing hot bubbles (~106 K). -Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K). -Further cooling permits molecules to form, making molecular clouds (~30 K). -Gravity forms new stars (and planets) in molecular clouds.

Size of a black hole

-Stellar-mass black holes (with masses of several solar masses) result from the death of massive stars -The Schwarzschild radius for a black hole with 3x the mass of the Sun would only be a bit smaller than a neutron star of similar (but slightly smaller) mass

X-ray Binaries

-Strong observational evidence for stellar-mass black holes come from x-ray binaries. Material is accreted from a companion star onto the black hole, and produces strong x-rays -Cygnus-X1 is an x-ray binary, located in the Milky Way. It is a 15M black hole that is orbited by the star hde 226868

Nova or Supernova

-Supernovae are much more luminous than novae (about 10 million times) -Novae: H to He fusion of a layer of accreted matter,white dwarf left intact (for now) -(White Dwarf) Supernova: complete explosion of white dwarf, nothing left behind

What can happen to a neutron star in close binary system?

-The accretion disk around a neutron star gets hot enough to produce x-rays, making the system an x-ray binary -Sudden fusion events periodically occur on the surface of an accreting neutron star, producing x-ray bursts

A Neutron Star

-The ball of neutrons left behind after a massive-star supernova -Supported by neutron degeneracy pressure -Is about the size of a city, but about twice the mass of the Sun

What Next?

-The core and shell continue to contract and heat up, while the star as a whole grows larger and more luminous -At some point, the temperature of the He core reaches 100 million K and He fusion begins in the core

Which of the following is a difference between the disk population and the halo population of stars in the Milky Way?

-The disk population contains younger stars than the halo population -The halo stars are low-mass stars -The halo stars have much lower fractions of heavy elements than the disk stars

Crab Nebula Pulsar

-The pulsar at center of Crab Nebula pulses 30 times per second. Two thousand years from now, it will spin less than half as fast

What would it be like to visit a black hole?

-Time runs slower in regions of stronger gravity -gravitational redshift: light coming out of a strong gravitational field should show a redshift (not due to the Doppler effect) -Tidal forces near the event horizon of a 3M black hole would be lethal to humans -Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

Why do we think that clouds of gas and dust form stars?

-We see young star clusters with gas and dust around them -We see glowing regions in star-forming clouds with infrared telescopes -Computer models predict that if a cloud has enough mass it will contract from the pull of gravity, heat up, and form a star.

size of white dwarf

-White dwarfs with same mass as Sun are about same size as Earth. -Higher-mass white dwarfs are smaller.

What would it be like to visit a black hole?

-You can orbit a black hole like any other object of the same mass—black holes don't suck! -Near the event horizon, time slows down and tidal forces are very strong.

time dilation

-a clock moving past you runs more slowly than a clock that is at rest

Open Clusters

-a few thousand loosely packed young stars. Found in the galactic disk. Typically about 30 light years across

The Event Horizon Telescope

-a world-wide network of radio telescopes working together -better resolution

Main Sequence Stage

-all low-mass stars ( < 2M) go through similar life stages -Main sequence stars share one common property: stable fusion of H into He in their cores -Longest life stage: 10 billion years

LIGO results

-black hole mergers are more common than we thought -when black holes merge, there is no associated light. -the merging of neutron stars will produce an EM counterpart

gamma ray bursts

-brief bursts of gamma rays coming from space were first detected in the 1960s -observations show that at least some gamma-ray bursts are produced by supernova explosions that result in neutron stars and black holes -short gamma-ray bursts may come from collisions between neutron stars and/ or black holes

Supernova Explosion

-core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos -Neutrons collapse to the center, forming a neutron star(or if mass is large enough, a black hole) -neutron degeneracy pressure prevents neutrons from getting closer, which causes the core to rebound slightly. The rebound slams into overlying material that is still falling inward and there is a tremendous impart of energy outward

planetary nebulae

-double shell burning ends with a pulse that ejects the outer layers into space as a planetary nebula -the core left behind becomes a white dwarf

Disk Stars

-orbit in nearly the same plane and direction -both young and old stars -have a larger proportion of heavy elements

Halo Stars

-orbit with random orientations -old ages and are low mass stars -have a smaller proportion of heavy elements

White Dwarfs

-remaining cores of dead stars (no fusion) -electron degeneracy pressure supports them against gravity -cool off and grow dimmer with time

Neutron Star

-the ball of neutrons left behind after the collapse of the core of a massive-star (supernova) -Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos -Neutrons collapse to the center, forming a neutron star -The degeneracy pressure of neutrons can then support the new object against gravity

Supernova 1987 A

-the closest and brightest supernova in the last four centuries was seen in 1987 -it took place in the large Magellanic cloud -energy and neutrons released in a supernova explosion enable elements heavier than Fe to form, including Au & U.

Supernova Remenant

-the energy released by the collapse of the core drives the outer layers into space - the debris from the supernova carries away heavier elements produced during fusion. Later, these elements may be part of a new generation of stars. -the crab nebula is the remnant of a supernova explosion seen in 1054

Nova

-the temperature of accreted matter eventually becomes hot enough for hydrogen fusion -Fusion begins suddenly and explosively, causing a nova -The nova star system temporarily appears much brighter -The explosion drives accreted matter out into space

Globular clusters

-up to a million or more old stars in a dense ball typically 60-150 light years across. Found in the galactic halo

What is the lowest mass that an object can have and still be called a star?

0.08M

Summary of Star Birth

1. Gravity causes gas cloud to shrink and fragment. 2. Core of shrinking cloud heats up. 3. When core gets hot enough, fusion begins and stops the shrinking. 4. New star achieves long-lasting state of balance

Imagine that you could travel at the speed of light. Starting from Earth, how long would it take you to travel to the center of the Milky Way Galaxy?

27,000 years

What is a neutron star?

A ball of neutrons left over from a massive star supernova and supported by neutron degeneracy pressure

What is a black hole?

A black hole is a massive object whose radius is so small that the escape velocity exceeds the speed of light.

Protostar (30 million years)

A star system forms when a cloud of interstellar gas collapses under gravity

Red Giant Star (1 billion years)

After core hydrogen is exhausted, the core shrinks and heats. Hydrogen fusion begins around the inert helium core, causing the star to expand

Low-Mass Stars

After double-shell fusion, low mass stars cast off their outer layers in a planetary nebula and leave behind a white dwarf

High-Mass Stars

After double-shell fusion, there will be repeated core fusion, followed by more shell fusion, and so on until iron is left in the core -Iron does not produce energy by fusion. With no pressure to fight gravity, the result is a massive star supernova -Lower end a neutrons star is left -Higher end a black hole is left

Black Hole

An object in space whose gravity is so powerful that not even light can escape. -Anything can become a black hole, if its mass can be compressed to withing a Schwarzschild radius

Life Track after Main Sequence

As a red giant, the Sun's core will shrink, but its outer layers will expand. The Sun will be more than 100 times as large in radius and more than 1000 times as luminous as it is today

Strong Force

At low temperatures protons have small kinetic energy (low velocity) -The electromagnetic force repels the protons and prevents fusion At high temperatures, protons have large kinetic energy (high velocity) -Protons get close enough for the strong force to bind them (fusion takes place)

How do high-mass stars make the elements necessary for life?

Big Bang made all the H and most of the He we see in the Universe today. The stars made heavier elements -Hydrogen fusion makes Helium -Helium fusion makes Carbon -The CNO cycle can change C into N and O

white dwarf supernova

Carbon fusion suddenly begins as a white dwarf in close binary system reaches white dwarf limit, causing total explosion.

Helium Capture

Core temperatures are high enough (at least 600 million K) Nuclear reactions the final stages of a high-mass star's life are complex, with many different reactions at once. The simplest sequence is known as helium capture reactions -Higher abundances of elements with even numbers of protons -builds C into O, Ne, Mg -advanced reactions in these massive stars make elements like Si, S, and Fe

Advanced nuclear fusion

Core temperatures in stars with > 8M allow fusion of elements as heavy as iron

What stops the contraction of a brown dwarf?

Degeneracy pressure

Thermal Pressure

Depends on heat content. The main form of pressure in most stars

Star-Forming Clouds

Early in the star formation process , the contradicting gas quickly radiates away much of the energy, preventing pressure buildup. The fragment can continue to collapse.

How does a low-mass star die?

Ejection of H and He in a planetary nebula leaves behind an inert white dwarf.

Which of the following is not a consequence of special relativity?

Everything is relative

End of Fusion

Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements -Once all Helium in the core has fused, the core is made mostly of Carbon and Oxygen -Electron degeneracy pressure supports the white dwarf against gravity. -The white dwarf remains at a fixed size and cools off slowly, becoming increasingly dimmer

Gamma Rays

Gamma rays show the collision of particles from supernovae with atomic nuclei in gas clouds

gravitational equilibrium

Gravity pulling in balances pressure pushing out

Where do stars tend to form?

Halo: No ionization/ reflection nebulae or blue stars -No star formation Disk: Ionization and reflection nebulae, blue stars -Star formation Much of the star formation in the disk happens in spiral arms. Spiral arms contain both young stars and material needed to make new stars

What happens when the core runs out of helium?

Helium fuses in a shell around the core

Double Shell Fusion Red Giant (30 million years)

Helium fusion begins around the inert carbon core after the core helium is exhausted. There is now fusion in both a hydrogen shell and a helium shell.

Helium Core Fusion Star (100 million years)

Helium fusion begins when the core becomes hot enough to fuse helium into carbon. The core then expands, slowing the rate of hydrogen fusion in a shell and allowing the star's outer layers to shrink

life stages of a high mass star

High-mass stars progress through the stages more quickly - Hydrogen core fusion (main sequence) - Hydrogen shell burning (supergiant) - Helium core fusion (no helium flash) -Helium shell and hydrogen shell burning -Eventually, the core will become hot enough to fuse carbon into heavier elements. The star will undergo cycles of core fusion, followed by shell fusion. • After a high-mass star leaves the main sequence, it is a supergiant.

When a low-mass star can no longer fuse hydrogen into helium in its core, then

Hydrogen fusion will begin in a shell around the core

What happens in a high-mass star after it stops core hydrogen fusion?

Hydrogen shell fusion starts

Main Sequence Star (10 billion years)

In the core of a low-mass star, four hydrogen nuclei fuse into a single helium nucleus

Optical Light

In visible light, much of our view of the galaxy is blocked by opaque dust clouds

Ionization Nebulae

Ionization nebulae are found around short-lived high-mass stars, signifying active star formation

massive star (core-collapse) supernova

Iron core of massive star does not undergo fusion, gravity wins, collapses into a neutron star, and causes explosion

End of Fusion HM

Iron is a dead end for fusion because nuclear reactions involving iron do not release energy -Iron has the lowest mass per nuclear particle

What happens to the escape velocity from an object if you shrink it (but the mass is kept the same)?

It increases

What would gas in the disk do if there were no friction?

It would orbit indefinitely

What happens when a star can no longer fuse hydrogen to helium in its core?

Its core shrinks and heats up

What would happen to a contracting cloud fragment if it were not able to radiate away it kinetic (thermal) energy?

Its internal pressure would increase

Degeneracy Pressure

Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place

Where will the Milky Way gas be in 1 trillion years (when the Universe is 70 times older than now)?

Locked into brown dwarfs, white dwarfs, and low-mass stars

Long-wavelength Infrared Light

Long-wavelength infrared emission shows where young stars are heating dust grains

Orbital Velocity Formula

M= (r * v^2)/G -v: orbital speed -r: radius -M: mass within orbit

Which of the following lists, in the correct order, a possible evolutionary path for an isolated star?

Main Sequence Star, Red Giant, Planetary Nebula, White Dwarf

The size (Schwarzchild radius) of a black hole depends on its...

Mass

Special Relativity

Measurements of motion (of time and distance) make sense only when describe whom or what they are being measured relative to Two absolutes -The laws of nature are the same for everyone -The speed of light is the same for everyone

Upper Limit on a Star's Mass

Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart Observations have not found stars more massive than about 150MSun

Black Hole Verification

Need to measure mass by: -Using orbital properties of a companion -Measuring the velocity and distance of orbiting gas -It's a black hole if it's not a star and its mass exceeds the neutron star limit (~3MSun)

What would stars be like if hydrogen, rather than iron, had the lowest mass per nuclear particle?

Nuclear fusion would be impossible so stars would not shine after they had gravitationally contracted to their final state

Stars we see in the halo of our galaxy formed even before the Milky Way collapsed into a disk. Since we see them now, they must be

Old and Low mass

Multiple Shell Burning

Once the core is depleted of a certain element, the core shrinks, heats up allowing another fusion reaction -A new type of shell fusion begins between the core and the overlying shells. Get a series of nested shells

What is the evidence for a black hole at our galaxy's center?

Orbits of stars near the center of our galaxy indicate that it contains a black hole with 4 million times the mass of the Sun

What does our galaxy look like?

Our galaxy consists of a disk of stars and gas, with a bulge of stars at the center of the disk, surrounded by a large spherical halo. -Length: 100,000 light-years -Width: 1,000 light-years

Which of the following is an observational result a merger of two black holes?

Powerful gravitational waves

Formation of Jets

Rotation causes jets of matter to shoot out along the rotation axis.

Short-wavelength Infrared Light

Short-wavelength infrared light reveals stars whose visible light is blocked by gas clouds

What nearby system has a white dwarf?

Sirius

If Sirius B has a higher surface temperature than Sirius A, which of the two will emit more X-rays per square meter?

Sirius B

Sirius B is a white dwarf with a mass of 0.98M. What would happen if Sirius A became a red giant and transferred up to 0.46M of material to it?

Sirius B would explode as a white-dwarf supernova

Star Clusters

Stars are born from giant collapsing gas clouds. Since the gas clouds can contain enough material to form many stars, usually stars form in clusters. -all the stars in a cluster lie at about the same distance -All the stars in a cluster formed at about the same time.

How are the lives of stars with close companions different?

Stars with close companions can exchange mass, altering the usual life stories of stars

surface of a black hole

The "surface" of a black hole is the radius at which the escape velocity equals the speed of light. This spherical surface is known as the event horizon. The radius of the event horizon is known as the Schwarzschild radius. The Schwarzschild radius increases as the mass of the black hole increases

The binary star Algol consists of a 3.7 M main sequence star and a 0.8M star that has evolved off the main sequence. What's strange about this pairing?

The 3.7M star, being more massive, should have evolved off the main sequence before the 0.8M star

Why does the Sun shine?

The Sun shines by converting mass into energy through nuclear fusion

The structure of the Milky Way galaxy

The disk, which contains the spiral arms, the halo, which is home to the globular clusters, and the bulge, in which the center of our galaxy lies.

Planetary Nebula (10,000 years)

The dying star expels its outer layers, leaving behind the exposed inert core

Interstellar medium

The gas and dust that fills the space between stars in a galaxy.

Why do orbits of bulge stars swoop above and below the disk?

The gravity of disk stars pulls them toward the disk

How does a high-mass star die?

The iron core collapses, leading to a supernova

The binary star Algol has a 3.7 solar mass main sequence star and a 0.8 solar mass red giant. How could that be?

The lower mass star used to be a more massive main sequence star, but when it became a giant some of its mass went onto the other star.

White Dwarf (Indefinite)

The remnant is made primarily of carbon and oxygen because the core of a low mass star never grows hot enough to produce heavier elements

How would you expect the temperature of the collapsing cloud fragment to change?

The temperature increases

What do we find in a stellar graveyard?

There are three possible ends to the life of a star, depending on its mass: -White dwarf (for stars with less than 8M) -Neutron Star (for stars between 8 and 25M) -Black hole (for stars above 25M)

Energy Balance

Thermal energy released by fusion in core balances radiative energy lost from surface.

What are the life stages of a high-mass star?

They are similar to the life stages of a low-mass star.

Imagine that you are observing a region of space where a cloud of gas and dust is about to form a star. Due to the gravitational force how will particles in the cloud move?

They will move towards each other

A white dwarf in a close binary with a companion star

When the white dwarfs accretes enough material, one of the two things can happen 1. The layer of accreted material on the surface of the white dwarf may begin to fuse H. -A nova (the white dwarf remains in tact) 2. If the white dwarf mass exceeds 1.4M, then carbon fusion begins suddenly throughout the white dwarf causing the white dwarf and the accreted material to explode -A white dwarf supernova (nothing is left behind)

X-Rays

X-rays are observed from hot gas above and below the Milky Way's disk

size of neutron star

a neutron star is about the size of Bryan/College Station, but has about twice the mass of the Sun

At the end stages of its life, the Sun will shed much of its atmosphere in

a planetary nebula

Brown Dwarf

a starlike object that is not massive enough to sustain hydrogen fusion in its core -luminosity gradually declines with time as it loses thermal energy

Is it easy or hard to fall into a black hole?

hard -black holes don't suck

Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.

have a greater abundance of heavy elements

What is the main product of the CNO cycle?

helium

How do high-mass stars make the elements necessary for life?

higher masses produce higher core temperatures that enable fusion of heavier elements

Orion Nebula

is a nearby site of star formation

Fission

large nuclei split into smaller pieces -nuclear power plant

What happens to a brown dwarf as it cools off?

none of the above

Long-wavelength infrared light is brightest from...

regions where many stars are currently forming

Fusion

small nuclei stick together to make a bigger one -Sun and stars

Sirius

the brightest star in the night sky

Protostar

the clump of gas that will become a star -not true stars -no nuclear fusion

length contraction

the length you measure an object to have depends on how the object is moving -the faster it moves, the shorter its length along its direction of motion -only noticeable at high speeds, comparable to the speed of light

Singularity

the mass of a black hole is confined to an infinitely small point

What affects the average orbital speed of a star in our galaxy?

the mass of the galaxy inside the star's orbit and the size of the star's orbit

interstellar medium

the material between stars

The White Dwarf Limit

the maximum possible mass for a white dwarf, which is about 1.4M

Where are most star-forming regions in the Milky Way?

the spiral arms

White Dwarf Supernova

when a white dwarf accretes enough matter to reach the 1.4M limit, it explodes by violent fusion of carbon

Could there be neutron stars that appear as pulsars to other civilizations but not to us?

yes