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Interstellar Medium

Gas between the stars
Space is not empty but has at least some gas and dust everywhere.

What elements make up our galaxy? (Two main elements of universe...)

70% H, 28% He, 2% heavier elements in our region of Milky Way

How do we determine the composition of interstellar gas?

We can determine the composition of interstellar gas from its absorption lines in the spectra of stars

Where do stars form? (where are they born?)

Stars are born in interstellar clouds that are particularly cold and dense.

These clouds are called molecular clouds.

What happens when there is interstellar reddening? How does this occur?

Long-wavelength infrared light passes through a cloud more easily than visible light

Observations of infrared light reveal stars on the other side of the cloud

Observing newborn stars? (is it possible?)

Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed
Observing the infrared light from a cloud can reveal the newborn star embedded inside it

Why do stars form?

Gravity vs. Pressure...
Gravity can create stars only if it can overcome the force of thermal pressure in a cloud

Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons
This allows gravity to contract the cloud.

How do clouds resist gravity long enough to grow so big?

Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud

Resistance to Gravity

A cloud must have even more mass to begin contracting if there are additional forces opposing gravity

Fragmentation of a Cloud

Gasses within the cloud are very turbulent.

Clouds contain many dense clumps.

Gravity can therefore overcome pressure in smaller dense pieces of the cloud, causing it to break into multiple fragments, each of which may go on to form a star

Fragmentation of a Cloud creates what?

Each lump in the cloud in which gravity can overcome pressure can become a star.

A large cloud can make a whole cluster of stars.

Protostar

Early form of a star; This stage lies between the collapsing of dust and gas and the beginning of nuclear fusion

Main sequence star

A normal star that is undergoing nuclear fusion of hydrogen into helium. Our sun is a typical main sequence star., a star that falls into the main sequence category on the H-R diagram; this category contains the majority of stars and runs diagonally from the upper left to the lower right on the H-R diagram, A normal star that is undergoing nuclear fusion of hydrogen into helium. Our sun is a typical main sequence star.

Where do stars form?

Stars form in dark, dusty clouds of molecular gas with temperatures of 10-30 K
These clouds are made mostly of molecular hydrogen (H2) but stay cool because of emission by carbon monoxide (CO)

Why do stars form?

Stars form in clouds that are massive enough for gravity to overcome thermal pressure (and any other forms of resistance)
Such a cloud contracts and breaks up into pieces that go on to form stars

Trapping of Thermal Energy means that ____ will happen...

As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape

Thermal energy then begins to build up inside, increasing the internal pressure

Contraction slows down, and the center of the cloud fragment becomes a protostar.
This marks the first stage of star formation.

What is the first stage of star formation?

Protostar

Growth of a Protostar

Matter from the cloud continues to fall onto the protostar until either the protostar or a neighboring star blows the surrounding gas away

What is the role of rotation in star birth?

Random motions of gas particles inevitably give a gas cloud some small, overall rotation.

As the cloud shrinks, what happens? (ice skater)

It rotates faster

Flattening

Collisions between particles in the cloud cause it to flatten into a disk

Formation of Jets

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

Rotation causes mass of Protostar to build

Particles in the inner part of the disc are slowed as they collide with slower moving particles on the outside of the disc.

This causes particles to fall into the protostar.

From Protostar to Main Sequence

Protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion

Contraction must continue until the core becomes hot enough for nuclear fusion

Contraction stops when the energy released by core fusion balances the push of gravity.

What slows the contraction of a star-forming cloud?

The contraction of a cloud fragment slows when thermal pressure builds up because infrared and radio photons can no longer escape

What is the role of rotation in star birth?

Conservation of angular momentum leads to the formation of disks around protostars.
Increases the mass of the star.

How does nuclear fusion begin in a newborn star?

Nuclear fusion begins when contraction causes the star's core to grow hot enough for fusion (10 million K)

Gravitational equilibrium:

Energy provided by fusion maintains the pressure
(gravity pushing in, pressure pushing out)

Gravitational contraction: what did it provide for the sun? When did it stop?

Provided the energy that heated the core as Sun was forming


Contraction stopped when fusion began

Sun's Layers:

Corona
Chromosphere
Photosphere
Convection Zone
Radiation Zone
Core

Solar wind:

A flow of charged particles from the surface of the Sun

Corona:

Outermost layer of solar atmosphere
~1 million K
Emits abundant X-rays
Low density gas

Chromosphere:

Middle layer of solar atmosphere
~ 10,000 K
Emits abundant UV light

Photosphere:

Visible surface of Sun
~ 6,000 K
Less dense than Earth's atmosphere
Active "boiling" surface with sunspots

Convection Zone:

~6000K (slightly less)
Convection: Energy transported upward by rising hot gas

Radiation Zone:

~10 million K
Energy transported upward by photons

Core:

~ 15 million K
Energy generated by nuclear fusion
Density ~100x greater than water
Pressure 200 billion times greater than atmospheric pressure on Earth

Why was the Sun's energy source a major mystery?

Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years

Why does the Sun shine?

The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

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

How does the energy from fusion get out of the Sun?

Convection (rising hot gas) takes energy to surface (Convective Zone)

Bright blobs on photosphere are where hot gas is reaching surface

Why do sunspots stay cooler than their surroundings?

Sunspots are areas on the surface of the sun where strong magnetic fields keep charged particles trapped. Matter on the surface convects (gets heated, rises to the surface, cools, and sinks down only to be reheated and continue the cycle) but not the matter trapped in these magnetic fields. It can't sink back down once it cools off, which is why it looks black from Earth. These spots are still 3000 degrees kelvin (essentially Celsius) which is really hot but colder than the surrounding 5800 kelvin surface.

C Convection currents arise to the surface cooling the plasma underneath

Stellar Mass and Fusion

The mass of a main-sequence star determines its core pressure and temperature.

Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived.

Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes.

Star Clusters and Stellar Lives

Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations.

Star clusters are particularly useful because they contain stars of different mass that were born about the same time.

How does a star's mass affect nuclear fusion?

A star's mass determines its core pressure and temperature and therefore determines its fusion rate.
Higher mass stars have hotter cores, faster fusion rates, greater luminosities, and shorter lifetimes.

How does the sun release energy? Specific...

The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.

How do low mass stars fuse hydrogen?

The proton-proton chain is how hydrogen fuses into helium in Sun.

How long does a star remain a main sequence star?

A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.

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

B. The core shrinks and heats up.

What happens to a star when its time on the main sequence is over?

Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.

Red Giants: Broken Thermostat

As the core contracts, H begins fusing to He in a shell around the core.

Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting.

Does helium fusion begin right away when hydrogen fusion stops?

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion.

Fusion of two helium nuclei doesn't work, so helium fusion must combine three helium nuclei to make carbon.

What happens in a low-mass star when core temperature rises enough for helium fusion to begin?
Hint: Degeneracy pressure is the main form of pressure in the inert helium core.

C. Helium fusion rises very sharply.

Helium Flash

the explosive ignition of helium fusion in the core of a low-mass, giant star

The thermostat of a low-mass red giant is broken because degeneracy pressure supports the core.

Core temperature rises rapidly when helium fusion begins.

Helium fusion rate skyrockets until thermal pressure takes over and expands the core again.

Life Track after Helium Flash

Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.

Thermostat is "fixed" means that... (what do helium stars do?)

Helium-burning stars neither shrink nor grow because core thermostat is temporarily fixed.

What happens when the low mass star's core runs out of helium?

Helium fuses in a shell around the core.

Double Shell Burning

stage after helium fusion...
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—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.

Red giants whose photospheres become especially carbon rich are called Carbon stars.

Planetary Nebulae

Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula.

The core left behind becomes a white dwarf.

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 (some helium fuses to carbon to make oxygen).

Degeneracy pressure supports the white dwarf against gravity.

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

Hydrogen fusion in core (main sequence)
Hydrogen fusion in shell around contracting core (red giant)
Helium fusion in core (horizontal branch)
Double shell burning (red giant)

How does a low-mass star die?

Ejection of hydrogen and helium in a planetary nebula leaves behind an inert white dwarf.

CNO Cycle

High-mass main- sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.

Greater core temperature enables hydrogen nuclei to overcome greater repulsion.

Life Stages of High-Mass Stars

Late life stages of high-mass stars are similar to those of low-mass stars:
Hydrogen core fusion (main sequence)
Hydrogen shell burning (supergiant)
Helium core fusion (supergiant)
No Helium flash. Thermal pressure is high enough so that Helium fusion begins gradually.

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

Helium capture reactions

what elements did the big bang make?

Big Bang made 75% H, 25% He; stars make everything else.

What can helium fusion make in low mass stars?

Helium fusion can make CARBON in low-mass stars.

What can the CNO cycle change carbon into?

CNO cycle can change carbon into nitrogen and oxygen.

What allows helium to fuse with other elements?

High core temperatures allow helium to fuse with heavier elements.

What does helium capture do?

Helium capture builds carbon into oxygen, neon, magnesium,
and other elements.

Why is there no fusion after iron?

Iron is a dead end for fusion because nuclear reactions involving iron do not release energy.

(This is because iron has lowest mass per nuclear particle.)

Evidence for helium capture:

Higher abundances of elements with even numbers of protons

How does a high-mass star die?

Iron builds up in core until degeneracy pressure can no longer resist gravity.

The core then suddenly collapses, creating a supernova explosion.

Supernova Explosion

Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos.

What creates a neutron star?

Neutrons collapse to the center, forming a neutron star.

Energy and neutrons released in a supernova allow __________ to form?

Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including gold and uranium.

Supernova Remnants do what?

Energy released by the collapse of the core drives the star's outer layers into space.

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

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

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

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

How does a high-mass star die?

Its iron core collapses, leading to a supernova

How does a star's mass determine its life story?

A star's mass determines its entire life story because it determines its core temperature.
High-mass stars with > 8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.
Low-mass stars with < 2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.
Intermediate-mass stars can make elements heavier than carbon but end as white dwarfs

Low-Mass Star Summary (stages)

Main sequence: H fuses to He in core.
Red giant: H fuses to He in shell around He core.
Helium core burning:
He fuses to C in core while H fuses to He in shell.
Double shell burning:
H and He both fuse in shells.
5. Planetary nebula leaves white dwarf behind.

Reasons for Life Stages

Core shrinks and heats until it's hot enough for fusion.
Nuclei with larger charge require higher temperature for fusion.
Core thermostat is broken while core is not hot enough for fusion (shell burning).
Core fusion can't happen if degeneracy pressure keeps core from shrinking.

Life Stages of High-Mass Star

Main sequence: H fuses to He in core.
Red supergiant: H fuses to He in shell around He core.
Helium core burning:
He fuses to C in core while H fuses to He in shell.
Multiple shell burning:
Many elements fuse in shells.
5. Supernova leaves neutron star behind.

How are the lives of stars with close companions different?

matter can flow from the subgiant onto the main-sequence star. (matter can move from one star to another)
The star that is now a subgiant 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 does a star's mass determine its life story?

Mass determines how high a star's core temperature can rise and therefore determines how quickly a star uses its fuel and what kinds of elements it can make.

How are the lives of stars with close companions different?

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

What is a white dwarf?

White dwarfs are the remaining cores of dead stars

What supports white dwarfs against gravity?

Electron degeneracy pressure supports them against gravity

Size of a White Dwarf?

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

The White Dwarf Size Limit

Quantum mechanics says that electrons must move faster as they are squeezed into a very small space

As a white dwarf's mass approaches 1.4MSun, its electrons must move at nearly the speed of light

Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4MSun, the white dwarf limit (or Chandrasekhar limit)

What happens to white dwarfs over time?

White dwarfs cool off and grow dimmer with time

What can happen to a white dwarf in a close binary system? (a white dwarf near other stars?)

Star that started with less mass gains mass from its companion

Eventually the mass-losing star will become a white dwarf


What happens next?

Accretion Disks

Mass 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

What causes the disk to heat up and glow?

Friction between orbiting rings of matter in the disk transfers angular momentum outward and causes the disk to heat up and glow
The loss of energy in the inner disk causes the gas of the inner disk to fall into the white dwarf

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

It would orbit indefinitely.

What causes Nova?

The temperature of accreted matter eventually becomes hot enough for hydrogen fusion

Fusion begins suddenly and explosively, causing a nova
The white dwarf's strong gravity compresses this hydrogen gas into a thin surface layer. Both the predure and temperature rixe as the layer builds up with more accreting gas. When the temperature at the bottom of the layer reaches about 10 million K, hydrogen fusion suddenly ignites. Hydrogen Flash shines for a few glorious weeks.

A Nova does not blow it's core away. In fact the gravity from the white dwarf may continue to steal matter from it's companion star and go Nova again and again.

What does the Nova look like?

The nova star system temporarily appears much brighter

The explosion drives accreted matter out into space

What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?

It explodes

Two Types of Supernova?

Massive star supernova:
White dwarf supernova:

White dwarf supernova:

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

Massive star supernova:

Iron core of massive star reaches
white dwarf limit and collapses into a
neutron star, causing explosion

How do you tell different types of supernova apart?

One way to tell supernova types apart is with a light curve showing how luminosity changes with time

Nova or Supernova?

Supernovae are MUCH MUCH more luminous!!! (about 10 million times)

Nova: H to He fusion of a layer of accreted matter, white dwarf left intact

Supernova: complete explosion of white dwarf, nothing left behind

Supernova Type: Massive Star or White Dwarf?

Light curves differ

Spectra differ (exploding white dwarfs don't have hydrogen absorption lines)

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

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

What is a neutron star?

A neutron star is the ball of neutrons left behind by a massive-star supernova.

What supports a neutron star against gravity?

Degeneracy pressure of neutrons supports a neutron star against gravity

When and why does electron degeneracy pressure go away?

Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos

Neutrons collapse to the center, forming a neutron star

How were neutron stars discovered?

Pulsars: 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

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

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

Yes

What is a neutron star?

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

Neutron Degeneracy Pressure

neutrons packed closely together that provide the outward pressure to balance the inward push of gravity in neutron stars

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

What is a black hole?

A black hole is an object whose gravity is so powerful that not even light can escape it.

What happens to the escape velocity from an object if you shrink it?

it increases

"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 event horizon is _________ for ________black holes...

Event horizon is larger for black holes of larger mass

A black hole's _____ strongly warps space and time in vicinity of the __________

A black hole's mass strongly warps space and time in vicinity of event horizon

No Escape

Nothing can escape from within the event horizon because nothing can go faster than light.

No escape means there is no more contact with something that falls in.

Neutron Star Limit

Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3 Msun

Some massive star supernovae can make black hole if enough mass falls onto core
, Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3Msun
Some massive star supernovae can make black hole if enough mass falls onto core
Beyond the limit no known force can resist the crush of gravity

Singularity

Beyond the neutron star limit, no known force can resist the crush of gravity.


As far as we know, gravity crushes all the matter into a single point known as a singularity.

How does the radius of the event horizon change when you add mass to a black hole?

A. Increases
B. Decreases
C. Stays the same

What happens to time near a black hole?

Time passes more slowly near the event horizon

Is it easy or hard to fall into a black hole?
Hint: A black hole with the same mass as the Sun wouldn't be much bigger than a college campus

Hard

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