Evolution of Single StarsEvolution of Single Stars

Physics 360/Geol 360

John Swez

Massive stars evolve quicker than light stars. For these stars the rate of consuming hydrogen is very large (millions of years as compared to billions of years)

Stars shine because of nuclear fusion reactions in their core

Slight increases in mass produce large increases in the luminosities of stars

Stellar Formation

• Cloud collapses

• Angular momentum forms the cloud into a disk

Star Forming Regions

• Same as Solar System formation• Need a dark, cold gas cloud to start• New stars emit lots of energy into

nearby clouds– H II Regions (red)– Blue Reflection Nebula (dust reflection)– Dark Nebula (dust clouds)– EGGs (Enveloping Gaseous Globules)

Protostar JetsGas is ejected from a forming star

The Protostar Stage

“As a gas clump collapses it heats up because the gas particles run into each other. The energy the gas particles had from falling under the force of gravity gets converted to heat energy. The gas clump becomes warm enough to produce a lot of infrared and microwave radiation. At this stage the warm clump is called a protostar. The gas clump forms a disk with the protostar in the center. Other material in the disk may coalesce to form another star or planets. “

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

A Protostar will reach between 2000 deg and 300 deg Kelvin Temperature. Fusion starts in the core and the subsequent generated pressures keep the star from further collapse due to gravitation. Stars are observed to form in clusters.

Bipolar Outflow

All Stars follow the same basic sequence of steps

Gas Cloud — Main Sequence — Red Giant — (Planetary Nebula or Supernova) — Remnant

Exactly how long a star lasts in each stage, whether a planetary nebula forms or a spectacular supernova occurs, and also what type of remnant will form depends on the initial mass of the star.

Giant Molecular Cloud

Explanation and source on following slide

Why do Molecular Clouds begin to condense into stars?

“Fragments of giant molecular clouds with tens to hundreds of solar masses of material a piece will start collapsing for some reason all at about the same time. Possible trigger mechanisms could be a shock wave from the explosion of a nearby massive star at its death or from the passage of the cloud through regions of more intense gravity as found in the spiral arms of spiral galaxies. These shock waves compress the gas clouds enough for them to gravitationally collapse. Gas clouds may start to collapse without any outside force if they are cool enough and massive enough to spontaneously collapse. Whatever the reason, the result is the same: gas clumps compress to become protostars. “

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

Protostars

Main Sequence Stars

• Definition: Stars that burn Hydrogen in their core• Lifetime = fuel/(fuel consumption rate)

• T life time proportional to mass/luminosity M/L

• Recall L=M3 **

• Therefore t life time proportional to M/M3 1/ M2

• Or t life time = 1010 yrs (M sun/M)2

• High mass -> short life

** actually the exponent is between 3 and 4, closer to 3

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

What about the lifetime of a 5 solar mass star?

Stellar Interior Evolution

• Hydrogen burns into Helium in the core• Eventually the core H runs out• The star begins to collapse• Interior pressure and temperature rise• H burns in a shell around the core• If the core reaches 108K, Helium burns into

Carbon and Oxygen (normally H begins to burn at 7 million K)

Stellar Fuel Sequence• Fuel Source Product• H in core/shell -> Helium core/shell• He core/shell -> Carbon core/shell• C core/shell -> Ne, Mg, O in core/shell• O core/shell -> Ne in core/shell• Ne core/shell -> Si, Sulfur in core/shell• Si core/shell -> ‘Iron’ elements (no

heat released)

Stellar Fuel

• Each fuel source is less efficient than the last -> works for less time

• Stellar mass determines when the fuel sequence stops

• More mass -> more fuel sources

• For the Sun, Helium shell burning is the last phase

Stellar Exterior Changes

• The stellar exterior/atmosphere responds to changes in the interior

• Shell burning creates an instability that leads to huge expansion

• Swelling size -> red giant

• The Sun’s radius will be about 1.5 AU

Core Shrinkage as a Star Ages

SupernovaMassive Stars Only

• Iron has the highest energy-density of any element

• Fusing iron into heavier elements requires extra energy - it does not release energy

• Burning iron elements cools a starRapid collapse (~1/7 speed of light)

1. This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and

corrected version

“Observations of thousands of main sequence stars show that there is definite relationship between their mass and their luminosity. The more massive main sequence stars are hotter and more luminous than the low-mass main sequence stars. Furthermore, the luminosity depends on the mass raised to a power that is between three and four (Luminosity ~ Massp, where p is between 3 & 4). This means that even a slight difference in the mass among stars produces a large difference in their luminosities. “

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

For the rare massive stars (M* > 30 Msun), p = 3 and for the more common

low-mass stars (M* < 10 Msun), p = 4.

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected

version

Eventually the star becomes stable because hydrostatic equilibrium has been established. The star settles down to spend about 90% of its life as a main sequence star. It is fusing hydrogen to form helium in the core. Stars initially begin their lives near other stars in a cluster. After a few orbits around the galactic center, gravitational tugs from other stars in the galaxy cause the stars in the cluster to wander away from their cluster and live their lives alone or with perhaps one or two companions. The Pleiades are a young cluster of stars easily visible in the shoulder of the Taurus (the Bull) constellation during the winter months. They are about 80 million years old (compare that to our Sun which is 4,600 million = 4.6 billion years old!). The gas and dust around the stars may be residual material from their formation or simply interstellar clouds the cluster is passing through.

Main Sequence Starsfrom Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

After a star is born!

Main Sequence and Spectral Class

A star remains at its spectral class during its entire main sequence stage! An F-Star, formed as an F-star, will remain an F-star through its entire evolution in the main sequence. Sometimes, if binary stars have close companions then they may chance spectral types due to matter interaction between stars while in a binary configuration.

Sub-giants, Red Giants and SupergiantsAn example of a future Red Giant—Our Sun

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

Red Giants

As the star becomes bloated the energy is spread out and the protosphere of the star becomes cooler. It Stellar classification now changes from the Main Sequence. (Important) Because up to now it retained its stellar classification identity. Despite its cooler temperature the red giant will be very luminous because of its large surface area.

Red giants will have more solar winds than normal stars which will help dispel more mass than when it was a main sequence star. The Red Giant has extremes in temperature, its core will be very hot and high density whereas its surface is cool and low density.

Eventually the Red Giants Core can further compress and heat up (especially if the Red Giant is massive enough) and reach temperatures necessary for a Helium burn.

The Star’s Beginning of Old Age

• During Main Sequence a star’s compression by gravity is balanced out by the pressures of nuclear burning.

• Eventually the H becomes He and nuclear reactions stop.• Gravity takes over and the core shrinks• Layers (outside the core) and near the core begin to collapse – the

latter collapse more.• This compression leads to heating.• Now the shell outside the core (shell layer) gets hot enough to go into

a H nuclear burn.• Fusion is very rapid. The gas shell outside the shell layer begins to

expand because of the temperature and hence begins to cool—star forms into a Sub-Giant then a Red Giant. Outer layers cool to 2500 K – C and Si atoms form flakes. They are pushed outward by solar photons.

• Gas shell expands becomes transparent. Glowing shell called a planetary nebula (has nothing to do with planets)

Old Age, Less Massive Stars

Stars, Like the Sun

Will undergo a Helium Fusion Burn…but it will be very rapid inducing a Helium Flash…The star becomes more stable..produces much more energy output than when it was a Main Sequence Star..Until its fuel runs out. At least during this time hydrostatic equilibrium is restored.

These stars can go through pulsation stages. Hydrostatic equilibrium can be lost and the star can go through a stage again. Next stage, it will produce Carbon and perhaps Oxygen.

In fact Stars can pulsate—Pulsars.

Planetary Nebula Ejection (Medium Mass Stars Only)

• If a star forms into a large red giant, the outer atmosphere may cool a lot

• Dust grains form in the atmosphere

• Grains create pressure that strips the outer atmosphere

Planetary Nebula Ejection

Planetary NebulaNext to the last stage of a star’s life. Outer layers of the star are ejected as the core shrinks to a more compact state. Outer layers of the star are lost as mass is returned to the interstellar medium. For star masses of 0.08 to 5 solar masses the immense solar winds created will eject the carbon and silicon grains outward to form a planetary nebula. The hot exposed core emits ultraviolet light.

New! From December 2002 Issue of Astronomy Magazine. Heinze 3-401 a recent Hubble image of a planetary nebula. “Some astronomers think a companion star can create the jet-like streamers of gas while others believe strong magnetic fields funnel the gas into long outflows.

One nearby example is the Orion Nebula which you can see as the fuzzy patch in the sword part of the Orion constellation. It is about 1500 light years away and is 29 light years across. The nebula is lit up by the fluorescence of the hydrogen gas around a O-type star in the Trapezium cluster of four stars at the heart of the nebula. The O-type star is so hot that it produces a large amount of ultraviolet light. The ultraviolet light ionizes the surrounding hydrogen gas. When the electrons recombine with the hydrogen nuclei, they produce visible light. Several still-forming stars are seen close to the Trapezium stars. They appear as oblong blobs in the figure below with their long axis pointed toward the hot Trapezium stars.

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

More on Planetary Nebulas – The latest picture

For many years astronomers thought that typical planetary nebula eject in all directions to form a “bubble”

Astronomers knew of oddly shaped nebulas but thought them the exception

Much evidence know shows that most shells are not spherical but the gas has been ejected mainly along two directions to form oppositely directed cones. The nebulas that look circular are merely the ones whose rotational axis happens to point toward us so that we see their cones end on.

Shells are typically ¼ of a ly in diameter and expand at 20 km/sec

The core of the star remains behind as a tiny glowing ball called a white dwarf

White Dwarf Origin

Low-mass Star Evolution

More Massive Stars!Stars which are more massive can go through similar stages; however the cycles can continue up to more heavier elements such as iron. The creation of heavier elements from lighter elements is called stellar nucleosynthesis. Again, stars such as are sun will only nucleosynthesize up to carbon and oxygen.

“Hydrogen and helium and some lithium, boron, and beryllium were created when the universe was created. All of the rest of the elements of the universe were produced by the stars in nuclear fusion reactions. Up to the production of iron in the most massive stars, the nuclear fusion process is able to create extra energy from the fusion of lighter nuclei. But the fusion of iron nuclei absorbs energy. The core of the massive stars implodes and the density gets so great that protons and electrons are combined to form neutrons + neutrinos and the outer layers are ejected in a huge supernova explosion. The more common low-mass stars will have a gentler death, forming a planetary nebula.”**

** from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

Betelgeuse , the bright red star in the top left corner of the Orion constellation

from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version

The are supergiants even larger than Betelgeuse

Supernova

• Collapsing star hits dense core and bounces• Over 1047 joules are released!

– Approximately the energy released by all the stars in a galaxy in a year

Before After

Supernova 1987a Supernova 1987a was in another galaxy was in another galaxy (150,000 ly away) (150,000 ly away) and visible to the and visible to the naked eyenaked eye

Nucleosynthesis in summary

The formation of heavy elements by nuclear burning is called nucleosynthesis (proposed by E. Margaret Burbidge, Geoffrey R. Burbidge, William A. Fowler and Fred Hoyle. One of the triumphs of stellar evolution theory. This occurs (past Helium) in more massive stars only.

Hydrogen and helium and some lithium, boron, and beryllium were created when the universe was created. Heavier elements are created in more massive stars.

By the time a star is burning Si into Fe its core has shrunk to smaller than earth and the core temp is about 2 billion K!

A massive star (30 solar masses) may take less than 106 years to form an iron core. Star burns fuel rapidly to offset energy loss. The fusion of elements heavier than H yields less energy so more fuel must be burned to supply the same amount of energy.

From Thomas T. Arny, Explorations

Core collapse of massive stars

Formation of iron signals the end of a massive star’s life. Nuclear fusion stops with iron. The star is out of fuel! The shrinking core is transformed into a shell of dense neutrons. Star’s core pressure drops and it collapses fast from a core the size of the earth to a solid neutron ball six miles across in literally seconds! The outside of the star then collapses to the center since this is no outward hydrostatic pressure to support it. The collapse of the star impacts the core even more and the falling gas to billions of degrees. The pressure surges and lifts the outer layers into an explosion – The Supernova.

The supernova explosion mixes the nucleosynthesized internal elements with the outer gases into a spray flung outward at 10,000 km/s. Up to 10 or so solar masses may become this fate. Thus the last death event of a star creates building material for new stars. Interstellar gas is thus enriched with heavy elements.

But more important, the explosion creates free neutrons which combine with the star’s atoms to build up heavy and rare elements such as gold, etc.!

In a matter of a hours the dying star brightens into several billion solar luminosities. A supernova also generates large pulses of neutrinos.

Evolution of Low Mass and Hi Mass Stars

• Protostar (Gravity)

• Main Sequence (H core burn)

• Red Giant (H burns in shell-core contract-He flash

• Yellow Giant (pulsates)

• Red Giant (He burns in shell—outer layers ejected

• Planetary nebula (core cools)

• White Dwarf

• Protostar (Gravity)• Main Sequence (H core

burn, Core H consumed)• Yellow Supergiant (He

burns in core, pulsates)• Red Supergiant (heavier

elements burn in a series in the star’s core; Iron core builds up and collapses)

• Supernova explosion when core collapses; remnant a neutron star or a black hole

Supernova Remnants

Supernova in the galaxy NGC 4725 in late 1940. The arrow in the bottom pictures indicates the supernova. The top pictures shows the galaxy after the supernova has faded.

From Thomas T Arny’s text, Explorations, an Introduction to Astronomy page 409

High-mass Star Evolution

Stellar Interior Evolution

Gravity causes collapse

Fuel Runs Out

Increase in Central Pressure and Temperature

Atomic or Degenerate

Pressure Stops Collapse

Brown DwarfsWhite DwarfsNeutron Stars

Nuclear Fusion

Iron Fuses (energy drain)

Supernova

Element Less than Iron (energy

released)Hydrostatic Equilibrium

Mas

s?Element?

Single Star Evolution SUMMARY

• Mass determines a star’s evolution• All stars start with about the same composition

• A star uses nuclear fusion to sustain hydrostatic equilibrium

• When it runs out of fuel, it is dead• Planetary nebula ejections and supernova

explosions can drop a star’s mass fast

Determining the Age of a Cluster of Stars

From Thomas T Arny, Explorations, An Introduction to Astronomy page 414