The Evolution and Death of Stars

Stars spend most of their life cycles on the Main Sequence

• Main Sequence stars are in hydrostatic equilibrium because nuclear fusion is turning hydrogen into helium and producing enough outward pressure to balance gravitational collapse.

• 90% of all stars are found on the Main Sequence• 90% of the whole life of all stars is spent on the

Main Sequence • BUT – What happens when the hydrogen runs out?

Stars Leave the Main Sequence

• The hydrogen atoms in the core of the star that fuse together to create helium, start to run out and fusion begins to slow down

• The system becomes out of balance• Something has to happen to keep the star from

collapsing in on itself

Out of balance• Start running out of hydrogen in the core, now the

outward pressure is less than the gravitational collapse

Out of balance• What will happen to the core?

When core hydrogen fusion ceases, a main-sequence star becomes a giant

• When hydrogen fusion ceases in the core, the star will collapse inward – this causes the layer just outside the core to become so hot and dense that hydrogen fusion will begin in this outer layer.

• The energy produced by hydrogen fusion in this layer just outside the core causes the rest of the star to expand into a giant star.

• Stellar burp!

Helium fusion begins at the core of a giant

• While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon and oxygen– core helium fusion– 3 He C + energy and C + He O + energy

Main Sequence Stars become Red Giants

Helium fusion

Hydrogen fusion

As stars evolve, stars move from

being main sequence stars to Red Giants where

they increase in

luminosity and brightness and

decrease in temperature

InterstellarCloud (gas and dust)

Main Sequence Star

Red Giant

The Life of a Star

Main Sequence Stars become Red Giants

Helium fusion

Hydrogen fusion

What happens after core helium fusion stops?

The shell and core equilibrium game continues!

Depending on the mass of the star, heavier elements are produced: carbon,

oxygen, neon, silicon, the heaviest element being iron.

We are all made of Star Stuff!!

So what happens after the giant phase?

It depends on the mass of the star!

Low Mass stars (< 8 M ) have a different fate from

High Mass stars (> 8 M )

• The core runs out of fuel!• Shell fusion begins outside the core.

Low Mass stars (< 8 M )

Example of a low-mass giant:its outer layers and core

• The core runs out of fuel!• Shell fusion begins outside the core. • Eventually the process shell fusion creates too

much outward pressure and energy which explosively pushes out the outer layers of the star and produce a planetary nebulaplanetary nebula.

Low Mass stars (< 8 M )

Main Sequence Star

Red Giant

PlanetaryNebula

Low mass stars (< 8 M )

InterstellarCloud (gas and dust)

Ring Nebula

The burned-out core of a low-mass star becomes a white dwarf

• Surrounding planetary nebula disperses leaving behind just the remaining WHITE DWARF

White Dwarf

• A core with remaining mass less than 1.4 M. • These tiny star remnants are approximately the

size of planet Earth • One cubic centimeter (like a sugar cube) of a

White Dwarf star would weigh several tons.

Ring Nebula

White Dwarf

Main Sequence Star

Red Giant

PlanetaryNebula

White Dwarf

Low mass stars (< 8 M )

InterstellarCloud (gas and dust)

What happens to white dwarfs? Do they just sit there??

If the white dwarfs are isolated, yes. They will cool down and become

BLACK DWARFS.

white dwarf

Sirius and its White Dwarf companion

BUT: White dwarfs are not always left alone. Sometimes they can have a companion star! As its companion evolves and gets bigger, the white dwarf can steal mass from it. The stolen matter forms an external layer which can quickly ignite and shine brightly creating a

Nova.

What’s a Nova?

• A nova occurs in binary systems where a white dwarf is pulling mass from its companion.

• A nova is a relatively gentle explosion of hydrogen gas on the surface of a white dwarf in a binary star system.

• This process does not damage the white dwarf and it can repeat.

Sometimes the mass transfer can be excessive. So excessive that the white dwarf will not be able to support the mass it gains. So, what would have been a nova becomes a SUPERNova!

Main Sequence Star

Red Giant

Planetary Nebula

White Dwarf

Pulling material off of a companion star

Nova

Supernova IaLeaves no remnant!

White Dwarf

Low mass stars (< 8 M )

Interstellar Cloud (gas and dust)

So what is the fate of out Sun?

• Since the Sun has a mass less than 8 M and since it is alone without a companion, it will become a White Dwarf and then slowly cool into a Black Dwarf

High Mass Giant Stars (> 8 M ) Have a Different Story

• Fusion in the core continues through many more stages than for low mass stars

• Heavier elements are produced: – carbon, – oxygen,– neon, – silicon, – and so on up to iron

• We’re all made of star stuff!!

A series of different types of fusion reactions occur in high-mass stars

Core runs out of fuel!Gravity ( ) wants to collapse the star!

High-Mass Stars (> 8 M )

• The core and outer layers run out of fuel. • The star then collapses, due to gravity. • The mass, however, is high enough that

nothing can balance the gravitational collapse and…..

Supernovae -Type II• The collapsing outer layers of the star will

collapse against and bounce outward off the compact collapsed core in an explosive event sending out a shockwave. This explosive event is called a Type II Supernova!!!

• During the Supernova, heavier elements are created from fusion events, like magnesium, lead, or gold.

A Supernova Type II occurred here before we did.

• The atoms that created our world and solar system come from nuclear fusion in stars and from Supernovae events!

• We are all made of star stuff!

Gravity ( ) wants to collapse the star

No outward pressure = implosion

Rebound of outer layers against the core = supernova

High-Mass Stars (> 8 M )

After

BeforeSupernovae can be as bright as a whole galaxy!

Big Main Sequence Star

Red Giant

Type II Supernova

High-Mass Stars (> 8 M )

InterstellarCloud (gas and dust)

A101 Slide Set:A101 Slide Set:From Supernovae to PlanetsFrom Supernovae to Planets

Drafted by Manning for the SOFIA Team

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Topic: Supernovase.

Concepts: Supernovae, planet formation, infrared observations

Missionb: SOFIA

Coordinated by: the NASA Astrophysics Forum

An Instructor’s Guide for using the slide sets is available at the ASP website https://www.astrosociety.org/education/resources-for-the-higher-education-audience/

The DiscoveryThe Discovery

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• Astronomers using data gathered by the Stratospheric Observatory for Infrared Astronomy (SOFIA) have found a massive dust cloud within the supernova remnant Sagittarius A East.

• The dust was created in the supernova explosion about 10,000 years ago that left behind the expanding, hot remains of the original star (the remnant).

• Astronomers estimate that the dust cloud contains enough material to create about 7,000 Earths.

• The discovery confirms that supernovae are capable of producing the material needed to form planets, and may be responsible for most of the dust found in young galaxies.

SOFIA data reveal warm dust (shown as white contour lines) surviving inside a supernova remnant (SNR) near the center of our galaxy. The SNR Sagittarius A East cloud is traced in X-rays (blue). Radio emission (red) shows expanding shock waves colliding with surrounding interstellar clouds (green). Credits: NASA/CXO/Herschel/VLA/SOFIA-FORCAST/Lau et al.

How was the Discovery Made?How was the Discovery Made?

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Supernova remnant dust detected by SOFIA (yellow contour lines) survives away from the hottest X-ray gas (purple). The red ellipse outlines the supernova shock wave. The inset shows a magnified image of the dust (orange) and gas emission (cyan). Credits: NASA/CXO/Herschel/VLA/SOFIA-FORCAST/Lau et al.

• Astronomers made detailed infrared observations of the Sagittarius A East supernova remnant using instruments aboard the SOFIA aircraft—measuring the long infrared wavelengths that can travel through intervening interstellar dust clouds to reveal activity in the center of the supernova remnant.

• They found infrared emission coming from a dust cloud, and measured the mass of the cloud based on the intensity of the emissions.

• The “contour map” of the dust cloud traces lines of equal intensity, with greater intensity lines in the center and lesser toward the outside. This maps the density of the cloud from the denser center to the less dense edges.

The Big PictureThe Big Picture

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The Crab Nebula supernova remnant, created by an exploding massive star in AD 1054. Credit: HST, NASA, ESA, J. Hester and A. Loll (Arizona State University)

• When massive stars end their lives in titanic explosions called supernovae, the chemical elements forged in the stars’ interiors-and created in the heat and pressure of the explosion--are released into space as a debris cloud of hot gas and dust.

• Scientists had evidence of such dust formation, but couldn’t be sure that the dust wasn’t destroyed in the “rebound” shock wave when the expanding supernova remnant collided with the interstellar medium of thinly scattered material, creating another shock wave traveling inward toward the source of the explosion.

• The new finding demonstrates that supernova-formed dust can survive rebound shock waves and spread into space to form part of that interstellar medium from which stars—and planets—can form.

•

What are the Implications?What are the Implications?

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Artist’s concept of a stellar system in formation, in which leftover gas and dust in the disk surrounding a newborn star clump together to form planets. Credit: NASA/JPL-Caltech..

• The SOFIA finding demonstrates that supernovas not only produce dust, but that the dust can survive the explosion to become raw material for the formation of other stars—and planets.

• This result supports the notion that most of the dust observed in distant young galaxies may have been made by supernova explosions of early massive stars, since no other known mechanism could have produced nearly as much dust.

• This result may provide the “missing link” between supernovas and planet formation, with dust produced by supernovas making possible planets like Earth!

•

What happens to the core after a supernova?

– the whole story depends on mass!• neutron star

– the really big ones: remaining mass of 1.4 M to about 3 M

• black hole – the really really big ones: remaining mass

greater than 3 M

Neutron Stars

• A core with remaining mass of 1.4 to 3 M, composed of tightly packed neutrons.

• These tiny stars are much smaller than planet Earth -- in fact, they are about the diameter of a large city (~20 km).

• One cubic centimeter (like a sugar cube) of a neutron star, would have a mass of about 1011 kg! (hundreds of billions of pounds!)

Neutron Star

Neutron Star

Supernova

Neutron Star Spins Neutron Star Spins Down Down

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Topic: Neutron stars

Concepts: Pulsars, magnetic fields, pulsar timing

Missions: Swift, Fermi

Coordinated by the NASA Astrophysics Forum

An Instructor’s Guide for using the slide sets is available at the ASP website https://www.astrosociety.org/education/resources-for-the-higher-education-audience/

The DiscoveryThe Discovery• X-ray observations by

NASA’s Swift satellite showed a sudden slow-down in the spin of a highly magnetized neutron star known as 1E 2259+586 .

• The discovery creates a puzzle for astronomers because usually they see neutron stars (the collapsed cores of massive stars that end their lives as supernovae) suddenly speed up – this is the first time they’ve seen one slow down.

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Neutron star 1E 2259+586 shines in blue-white in this X-ray image of the CTB 109 supernova remnant. (Credit: ESA/XMM-Newton/M. Sasaki et al.)

Supernova Remnant

Neutron star

How Was the How Was the Discovery Made?Discovery Made?

• The Gamma-ray Burst Monitor on NASA’s Fermi Gamma-ray Space Telescope observed a brief but intense outburst from IE 2259+586 on April 21, 2012.

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An artist’s rendering of an outburst from a highly-magnetized pulsar, which is a fast-spinning neutron star. (Credit: NASA/GSFC)

• Data from the X-ray Telescope on Swift showed that the spin rate of IE 2259+586 decreased abruptly on April 28, 2012 – just a week after the outburst.

• The spin rate of once every seven seconds decreased by 2.2 millionths of a second – a tiny but significant change for a neutron star.

The Big PictureThe Big Picture• Neutron stars contain twice the mass of our Sun

crushed into a sphere about 12 miles across.

• Pulsars are rapidly rotating neutron stars that emit a beam of radiation (directed along their magnetic poles) and sweep it across our line-of-sight. Much like a lighthouse, we observe a pulse every time the beam sweeps past us.

• Astronomers have observed “glitches” in many neutron stars – these are sudden increases in the rate of rotation of the pulsar.

• This new observation showed an “anti-glitch” – or a sudden decrease – in the rotational frequency of 1E 2259+586.

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Top: A neutron star has the mass of about two suns in a sphere the size of Manhattan. Credit: NASA's Goddard Space Flight CenterBottom: A pulsar emits light beams (magenta) that we see as pulses as it rotates. Pulsars also have super-strong magnetic fields (blue lines illustrate magnetic fields lies; the beams are emitted along the magnetic poles). Credit: NASA

How Does this How Does this Discovery Change our Discovery Change our

View?View?

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• The internal structure of neutron stars is a long-standing puzzle in astronomy. The current view is that a neutron star has:

• an interior containing oddities that include a neutron superfluid (which is a state of matter without friction)

• a crust of electrons and ions• a surface where streams of high-energy particles

are accelerated through the star’s powerful magnetic field

• Current theory holds that a glitch occurs when streaming particles drain energy from the crust causing the crust to spin down. The fluid inside the crust resists this change. Under the strain, the crust fractures, which results in an X-ray outburst and a speedup kick from the faster-spinning interior.

• Processes that would lead to a sudden slow-down in the spin pose a new theoretical challenge!

A composite optical (red) and X-ray (blue) image of the environment near the Crab Nebula pulsar (center), a pulsar from which multiple glitches have been observed. (Credit: NASA/HST/CXC/ASU/J. Hester et al.)

ResourcesResourcesPress releases

• NASA press release http://www.nasa.gov/mission_pages/swift/bursts/new-phenom.html

• Penn Sate press release http://science.psu.edu/news-and-events/2013-news/Kennea5-2013

Scientific article• Archibald, R. F., et al. 2013, Nature, 497, 591http://www.nature.com/nature/journal/v497/n7451/full/nature12159.html

(subscription required)

Multimedia:• Visit the Interactive Fermi Pulsar Explorer http://www.nasa.gov/externalflash/fermipulsar/

• Additional stories/visuals on pulsars available at NASA/Goddard Space Flight Center’s Science visualization Studio:

http://svs.gsfc.nasa.gov/cgi-bin/search.cgi?sortby=relevance&value=pulsar

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Pulsars – The discovery of rotating neutron stars

• First detected in 1967 by Cambridge University graduate student Jocelyn Bell.

• She found a radio source with a regular on-off-on cycle of exactly 1.3373011 seconds.

• Some scientists speculated that this was evidence of an alien civilization’s communication system and dubbed the source LGM (Little Green Men!!!)

• Today, we know pulsars are rapidly spinning neutron stars.

Lighthouse Model

Black Holes

• A remaining core with a mass of more than 3 M, will continue to collapse into an infinitely small location in space.

• We cannot observe what is left behind, directly. We can only detect its presence if it has a companion star, and it attracts material in an accretion disk.

Black Holes

A black hole is a collapsed stellar core. It is a location in space of enormous gravitational attraction. The gravitational attraction is so

strong that photons of light can not even escape (that’s why it’s black)!

Black Hole

To detect a black hole, we look for the x-rays given off by material as it falls toward the black hole.

Big Main Sequence Star

Red Giant

Type II Supernova

Neutron Star

High-Mass Stars (> 8 M )

Black HoleInterstellarCloud (gas and dust)

Tutorial: Stellar Evolution (p.83)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask

another group.

0/0 Cross-Tab Label

Black holes are formed by

1. a lack of any light in a region of space.

2. supernovae from the most massive stars.

3. supernovae from binary stars.

4. collapsed dark nebulae.

0/0 Cross-Tab Label

Which of the following lists, in the correct order, a possible evolutionary

path for a star?1. Red Giant, Neutron Star, White

Dwarf, nothing2. Red Giant, Type I Supernova, Black

Hole3. Red Giant, Type II Supernova,

Planetary Nebula, Neutron Star4. Red Giant, Planetary Nebula, White

Dwarf5. Red Giant, Planetary Nebula, Black

Hole