The Life Cycles of Stars (2024)

The Life Cycles of Stars

Imagine an enormous cloud of gas and dust many light-yearsacross. Gravity, as it always does, tries to pull the materials together.A few grains of dust collect a few more, then a few more, then more still.Eventually, enough gas and dust has been collected into a giant ball that,at the center of the ball, the temperature (from all the gas and dustbumping into each other under the great pressure of the surroundingmaterial) reaches 15 million degrees or so. A wondrous event occurs....nuclear fusion begins and the ball of gas and dust starts to glow. A newstar has begun its life in our Universe.

So what is this magical thing called "nuclear fusion" and whydoes it start happening inside the ball of gas and dust? It happens likethis..... As the contraction of the gas and dust progresses and thetemperature reaches 15 million degrees or so, the pressure at the centerof the ball becomes enormous. The electrons are stripped off of theirparent atoms, creating a plasma. The contraction continues and the nucleiin the plasma start moving faster and faster. Eventually, they approacheach other so fast that they overcome the electrical repulsion that existsbetween their protons. The nuclei crash into each other so hard that theystick together, or fuse. In doing so, they give off a great deal ofenergy. This energy from fusion pours out from the core, setting up anoutward pressure in the gas around it that balances the inward pull ofgravity. When the released energy reaches the outer layers of the ball ofgas and dust, it moves off into space in the form of electromagneticradiation. The ball, now a star, begins to shine.

New stars come in a variety of sizes and colors. They range from blueto red, from less than half the size of our Sun to over 20 times the Sun�ssize. It all depends on how much gas and dust is collected during thestar�s formation. The color of the star depends on the surface temperatureof the star. And its temperature depends, again, on how much gas and dustwere accumulated during formation. The more mass a star starts out with,the brighter and hotter it will be. For a star, everything depends on itsmass.

Throughout their lives, stars fight the inward pull of the force ofgravity. It is only the outward pressure created by the nuclear reactionspushing away from the star's core that keeps the star "intact".But these nuclear reactions require fuel, in particular hydrogen.Eventually the supply of hydrogen runs out and the star begins itsdemise.

II. Beginning of the End

After millions to billions of years, depending on their initialmasses, stars run out of their main fuel - hydrogen. Once the ready supplyof hydrogen in the core is gone, nuclear processes occurring there cease.Without the outward pressure generated from these reactions to counteractthe force of gravity, the outer layers of the star begin to collapseinward toward the core. Just as during formation, when the materialcontracts, the temperature and pressure increase. This newly generatedheat temporarily counteracts the force of gravity, and the outer layers ofthe star are now pushed outward. The star expands to larger than it everwas during its lifetime -- a few to about a hundred times bigger. The starhas become a red giant.

What happens next in the life of a star depends on its initial mass.Whether it was a "massive" star (some 5 or more times the massof our Sun) or whether it was a "low or medium mass" star (about0.4 to 3.4 times the mass of our Sun), the next steps after the red giantphase are very, very different.

III. The End

A. The Fate of Sun-Sized Stars: Black Dwarfs

Once a medium size star (such as our Sun) has reached the redgiant phase, its outer layers continue to expand, the core contractsinward, and helium atoms in the core fuse together to form carbon. Thisfusion releases energy and the star gets a temporary reprieve. However, ina Sun-sized star, this process might only take a few minutes! The atomicstructure of carbon is too strong to be further compressed by the mass ofthe surrounding material. The core is stabilized and the end is near.

The star will now begin to shed its outer layers as a diffuse cloudcalled a planetary nebula. Eventually, only about 20% of the star�sinitial mass remains and the star spends the rest of its days cooling andshrinking until it is only a few thousand miles in diameter. It has becomea white dwarf. White dwarfs are stable because the inward pull of gravityis balanced by the electrons in the core of the star repulsing each other.With no fuel left to burn, the hot star radiates its remaining heat intothe coldness of space for many billions of years. In the end, it will justsit in space as a cold dark mass sometimes referred to as a black dwarf.

B. The Fate of Massive Stars: Supernovae! and Then...

Fate has something very different, and very dramatic, in storefor stars which are some 5 or more times as massive as our Sun. After theouter layers of the star have swollen into a red supergiant (i.e., a verybig red giant), the core begins to yield to gravity and starts to shrink.As it shrinks, it grows hotter and denser, and a new series of nuclearreactions begin to occur, temporarily halting the collapse of the core.However, when the core becomes essentially just iron, it has nothing leftto fuse (because of iron's nuclear structure, it does not permit its atomsto fuse into heavier elements) and fusion ceases. In less than a second,the star begins the final phase of its gravitational collapse. The coretemperature rises to over 100 billion degrees as the iron atoms arecrushed together. The repulsive force between the nuclei overcomes theforce of gravity, and the core recoils out from the heart of the star inan explosive shock wave. As the shock encounters material in the star'souter layers, the material is heated, fusing to form new elements andradioactive isotopes. In one of the most spectacular events in theUniverse, the shock propels the material away from the star in atremendous explosion called a supernova. The material spews off intointerstellar space -- perhaps to collide with other cosmic debris and formnew stars, perhaps to form planets and moons, perhaps to act as the seedsfor an infinite variety of living things.

So what, if anything, remains of the core of the original star? Unlikein smaller stars, where the core becomes essentially all carbon andstable, the intense pressure inside the supergiant causes the electrons tobe forced inside of (or combined with) the protons, forming neutrons. Infact, the whole core of the star becomes nothing but a dense ball ofneutrons. It is possible that this core will remain intact after thesupernova, and be called a neutron star. However, if the original star wasvery massive (say 15 or more times the mass of our Sun), even the neutronswill not be able to survive the core collapse and a black hole willform!

IV. More about the Stellar Endpoints

A. White/Black Dwarfs

A star like our Sun will become a white dwarf when it hasexhausted its nuclear fuel. Near the end of its nuclear burning stage,such a star expels most of its outer material (creating a planetarynebula) until only the hot (T > 100,000 K) core remains, which thensettles down to become a young white dwarf. A typical white dwarf is halfas massive as the Sun, yet only slightly bigger than the Earth. This makeswhite dwarfs one of the densest forms of matter, surpassed only by neutronstars.

White dwarfs have no way to keep themselves hot (unless they accretematter from other closeby stars); therefore, they cool down over thecourse of many billions of years. Eventually, such stars cool completelyand become black dwarfs. Black dwarfs do not radiate at all.

Many nearby, young white dwarfs have been detected as sources of softX-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultravioletobservations enable astronomers to study the composition and structure ofthe thin atmospheres of these stars.

B. Neutron Stars

Neutron stars are typically about ten miles in diameter, haveabout 1.4 times the mass of our Sun, and spin very rapidly (one revolutiontakes mere seconds!). Neutron stars are fascinating because they are thedensest objects known. Due to its small size and high density, a neutronstar possesses a surface gravitational field about 300,000 times that ofEarth.

Neutron stars also have very intense magnetic fields - about1,000,000,000,000 times stronger than Earth's. Neutron stars may "pulse"due to electrons accelerated near the magnetic poles, which are notaligned with the rotation axis of the star. These electrons travel outwardfrom the neutron star, until they reach the point at which they would beforced to travel faster than the speed of light in order to stillco-rotate with the star. At this radius, the electrons must stop, and theyrelease some of their kinetic energy in the form of X-rays and gamma-rays.External viewers see these pulses of radiation whenever the magnetic poleis visible. The pulses come at the same rate as the rotation of theneutron star, and thus, appear periodic. Neutron stars which emit suchpulses are called pulsars.

C. Black Holes

Black holes are objects so dense that not even light can escapetheir gravity and, since nothing can travel faster than light, nothing canescape from inside a black hole. Nevertheless, there is now a great dealof observational evidence for the existence of two types of black holes:those with masses of a typical star (4-15 times the mass of our Sun), andthose with masses of a typical galaxy. This evidence comes not from seeingthe black holes directly, but by observing the behavior of stars and othermaterial near them!

Galaxy-mass black holes are found in Active Galactic Nuclei (AGN). Theyare thought to have the mass of about 10 to 100 billion Suns! The mass ofone of these supermassive black holes has recently been measured usingradio astronomy. X-ray observations of iron in the accretion disks mayactually be showing the effects of massive black holes as well.

The Electromagnetic Spectrum as a Probe of theUniverse

All objects in our Universe emit, reflect, and absorbelectromagnetic radiation in their own distinctive ways. The way an objectdoes this provides it special characteristics which scientists can use toprobe an object�s composition, temperature, density, age, motion,distance, and other chemical and physical characteristics. Astronomers cantime events (for instance, recording exactly when a binary star system iseclipsed and for how long), can obtain the energy distribution of a source(by passing its electromagnetic radiation through a prism or grating tobreak it into component colors), or can record the appearance of a source(such as taking an image of the source). These three methods are by nomeans exclusive of each other, but each reveals different aspects of asource and each method gives the astronomer slightly differentinformation.

While the night sky has always served as a source of wonder andmystery, it has only been in the past few decades that we have had thetools to look at the Universe over the entire electromagnetic (EM)spectrum and see it in all of its glory. Once we were able to usespace-based instruments to examine infrared, ultraviolet, X-ray, and gamma-ray emissions, we found objects that were otherwise invisible to us (e.g.,black holes and neutron stars). A "view from space" is criticalsince radiation in these ranges cannot penetrate the Earth's atmosphere.Many objects in the heavens "light up" with wavelengths tooshort or too long for the human eye to see, and most objects can only befully understood by combining observations of behavior and appearance indifferent regions of the EM spectrum.

We can think of electromagnetic radiation in several different ways:

• From a physical science standpoint, all electromagnetic radiation canbe thought of as originating from the motions of atomic particles. Gamma-rays occur when atomic nuclei are split or fused. X-rays occur when anelectron orbiting close to an atomic nucleus is pushed outward with suchforce that it escapes the atom; ultraviolet, when an electron is joltedfrom a near to a far orbit; and visible and infrared, when electrons arejolted a few orbits out. Photons in these three energy ranges (X-ray, UV,and optical) are emitted as one of the outer shell electrons loses enoughenergy to fall down to the replace the electron missing from the innershell. Radio waves are generated by any electron movement; even the streamof electrons (electric current) in a common household wire creates radiowaves ...albeit with wavelengths of hundreds of kilometers and very weakin amplitude.

• Electromagnetic radiation can be described in terms of a stream ofphotons (massless packets of energy), each traveling in a wave-likepattern, moving at the speed of light. The only difference between radiowaves, visible light, and gamma-rays is the amount of energy in thephotons. Radio waves have photons with low energies, microwaves have alittle more energy than radio waves, infrared has still more, thenvisible, ultraviolet, X-rays, and gamma-rays. By the equation
, energy dictates a photon�s wavelength and frequency.

Activities

This model shows the discrete stages that a low mass star goesthrough over billions of years, from its beginnings as a gas cloud, to itsdeath as a black dwarf.

Materials:

* tape

* tissue paper and cotton batting

* string of indoor Christmas lights with white, red, orange, and yellowbulbs

* different-sized spherical light globes either clear or white (rangingfrom 1 to 5 inches in

diameter; these can be found in any store selling light fixtures)

* opaque black ball (or you could paint a light globe)

Procedure:

1. Punch 6 holes in a piece of cardboard or cotton batting and insertone of the lights through each hole. You might need to tape them inplace.

2. To show the birth of a star as a hot gas cloud, wrap the outside ofa globe in cotton and place it over the first bulb of the string oflights.

3. For a newborn star, have an orange light inside a 3-inch globe.

4. For a steady star, have a yellow light inside a 2-inch globe.

5. For a red giant, have a red light inside a 5-inch globe.

6. For a planetary nebula, have a red light inside a 3-inch globe. Wrapcrumpled tissue paper around the outside of the globe.

7. For a white dwarf, have a white light inside a 1-inch globe.

8. For a black dwarf, have a 1 inch black opaque globe. No lightsshould be used for the black dwarf.

The globes used for the various stages are not to scale. Do a simplecalculation to see why...if a steady star is 1.4 million km in diameter(and represented by a 2-inch globe), how big would the red giant globehave to be on the same scale? You might need to refer back to theinformation in Section II to help you.

Model a Black Hole

This demonstration allows for a visual depiction of the effectof a large mass on the fabric of spacetime. In particular, what effect ablack hole does or does not have on the other stars around it and how thateffect depends on the mass of the black hole.

Materials:

* large latex balloon cut open and pulled flat, or sheet of latex

* round bowl, 4"- 5" in diameter

* tape

* package of small round beads

* 1" solid ball bearing (the eraser end of a pencil can be used asa replacement)

Procedure:

1. Tape the sheet of latex (this represents space-time) tightly acrossthe top of some round object...such as a bowl. The sheet should not be sotight that it will tear if stretched further, but should be taut enoughthat there are not any wrinkles!

2. Scatter a few beads on the sheet of latex (this represents matterthat is near the black hole).

Make sure they are spread out to all parts of the sheet.

3. Gently drop the ball bearing onto the sheet of latex (thisrepresents the black hole). Try not to let it bounce! If you don�t have aball bearing, gently push down on the center of the sheet with the eraserend of a pencil.

4. Explain what happened to the matter when the black hole was put intoplace. Why did this occur?

5. What would happen if the ball bearing was heavier (or if you pushharder on the pencil)? What physical analogy to the black hole may bemade?

These Stars are Classified

Annie Jump Cannon (1863 - 1941) was known as the world�s expertin the classification of stars. Her work laid the foundation for modernstellar spectroscopy.

Annie Jump Cannon entered Wellesley College in Massachusetts in 1880 tostudy astronomy. She became interested in stellar spectroscopy, theprocess of breaking light from stars down into its component colors so thevarious elements can be identified. After suffering from scarlet fever,which left her hearing impaired, she earned her master�s degree and thencontinued her studies at Radcliffe College. She became an assistant at theHarvard College Observatory, the first observatory to include women asstaff members. During her career, she observed, classified, and analyzedthe spectra of some five hundred thousand stars, assigning each one itsplace in the sequence O, B, A, F, G, K, and M. In 1911 she almost becamea faculty member at Harvard but the university officials refused topromote a woman to such high status. So she became the curator ofastronomical photographs, earning a salary of twelve hundred dollars ayear. Finally, in 1936, Harvard hired her as a permanent faculty member.She was seventy-three years old at the time.

Astronomers now realize that everything which appears to distinguishone star from another - temperature, luminosity, size, life span -- isdetermined almost entirely by one factor: the star�s mass. The mainsequence along the HR diagram is not a singular evolutionary path, as manyhad thought, but a portrait of the sky at one moment in time of stars withvarying masses.

Below is a version of the Hertzsprung-Russell diagram, which shows howthe size, color, luminosity, spectral class, and absolute magnitude ofstars relate. Each dot on this diagram represents a star in the sky whoseabsolute magnitude and spectral class have been determined. Notice thatthe data appear to clump naturally into groups: main-sequence stars,giants, supergiants, and white dwarfs.

1. Imagine that you are an astronomer and you have detected asource that has a temperature of about 3700 Kelvin, and a luminosity ofabout 0.1. Examine the H-R diagram; explain what luminosity class and typeof source this could be. In what part of its life cycle is thissource?

2. What if a source has a temperature of about 10,000 Kelvin, and aluminosity of about 10^-3. Explain what type of source thiscould be, and the part of its life cycle the source is enduring.

3. Make a line plot superimposed on the H-R diagram that would tracethe entire life cycle of our star, the Sun. Remember all of the stages ofthis main-sequence, low mass star.

4. What will be the final stage of evolution (black dwarf, neutronstar, or black hole) for each of the following: (Hint: reread the text inSections I, II, and III)

(a) Type O main sequence star

(b) Type A main sequence star

(c) Type G main sequence star

Suggested Extension:

Examine the difference between absolute magnitude and apparentmagnitude. Why is an understanding of this crucial to an astronomer�sability to describe the evolution of any given star?

Blackbody Radiation & Wien�s Law

A star is considered to be an example of a "perfectradiator and perfect absorber" called a black body. This is anidealized body that absorbs all electromagnetic energy incident on it. Ablack body is black only in the sense that it is absolutely opaque at allwavelengths; it need not look black. Its temperature depends only on thetotal amount of radiant energy striking it each second. Stars are goodapproximations to a black body because their hot gases are very opaque,that is, the stellar material is a very good absorber of radiation.

The energy emitted by black bodies was studied by the German physicistMax Planck. He derived an equation that gives the radiant energy emittedper second from 1 cm² of a black body�s surface. This equationis called Planck�s Radiation Law and can be written as

.

In this equation, T is the temperature in Kelvins, the wavelength in centimeters,c the speed of light, k is Boltzmann�s constant (1.37 x10^-18 erg/K), and h is Planck�s constant (6.626 x10^-27 erg sec). Calculus students can prove to themselves thatfor such a function there will be a single wavelength, , at which maximum light is emitted.In fact, we can determine that for wavelength in cm and temperature inKelvins,

.

This is known as Wien�s Law. This Law is very important toastronomers. It tells us that the wavelength at which a star emits itsmaximum light indicates the star�s temperature.

1. What are the spectral classes of stars that have the followingmaximum light wavelengths?

You will need to refer to the H-R Diagram!

(a) 3 x 10^-5 cm(b) 1.5 x 10^-5 cm(c)5.5 x 10^-5 cm(d) 1.25 x 10^-4 cm

2. In what region of the electromagnetic spectrum would objects withthe following temperatures be best observed by a scientist�sexperiment?

(a) .001 K(b) 800 K(c) 15,000 K(d) 1,750,000K

Suggested Extension:

Are there really objects in space that have a temperature of 0.001 K?What are you detecting at that temperature?

Bigger than a Breadbox?

The Universe is a very big place and it contains some very bigobjects. In many images scientists create from data, it is difficult tounderstand the actual sizes of the objects. In this activity, we want tounderstand the extent (or size) of some supernova remnants. We will dothis by applying a simple physics equation.

In physics, we know that velocity = distance traveled / time it takesto travel that distance.

or

For this activity, we know that the distance (d) traveled isequivalent to the distance from the initial or central star of thesupernova remnant, to the edge of the outer material of the remnant. Inaddition, we know the velocity (v) at which the material of eachremnant is expanding outward, and understand that as the remnant getsolder the velocity slowly decreases. Lastly, we know how long ago(t) the initial star blew up in a supernova explosion.

Use this information, and the following data to determine the biggestsupernova remnant among those listed below. Be careful with thedimensional analysis!

Data:

Now let us look at things from a different angle. What if you knew thata certain supernova is located about 3 kiloparsecs from Earth. Whenastronomers look at the remnant with their telescopes, they measure it tobe 8 arc minutes (480 arc seconds) in diameter. What is the radius of theremnant in kilometers? Scientists have also measured the expansionvelocity to be 4,800 km/sec. In what year did the supernova occur? Perhapsyou can look up in a library, or on the World Wide Web, information aboutsupernovae observed to occur in that year and find the common name of thissupernova remnant. Need a hint?... Johannes Kepler was a famousastronomer.

A Teaspoon of Starstuff

Subrahmanyan Chandrasekhar (1910-1995) was born in Lahore, apart of India that is now in Pakistan. He won a Government of Indiascholarship and entered Cambridge University in England to work on hisdoctorate. As he sailed from India to England, he thought a lot about thedeath of stars. Using Einstein�s theory of relativity, he calculated thatstars of a certain mass should not become white dwarfs when they died; hebelieved that they should keep on collapsing. He put aside this work,earned his doctorate in 1934, and only later actively returned to histheory. He calculated that stars with more than 1.44 times the mass of theSun (now known as the Chandrasekhar limit) would not become white dwarfs,but would be crushed by their own gravity into either a neutron star or ablack hole. His work was viciously criticized by Sir Arthur Eddington,then the leading authority on stellar evolution and someone greatlyadmired by Chandrasekhar. His standing diminished by Eddington�s attacks,he came to the United States and was hired to teach at the University ofChicago. There he continued his research, which produced significantadvances in the field of energy transfer in stellar atmospheres.Eventually, his calculations about white dwarfs were proven correct. Withthe recognition of the Chandrasekhar limit, the theoretical foundation forunderstanding the lives of stars was complete. He won the Nobel Prize inphysics in 1983.

There are indeed distinct differences in the states of matter containedin main sequence stars, white dwarfs, and neutron stars. The followingexercise will help you to understand just how different they are!

Look at the following chart and use the information you find there tocalculate how much a teaspoonful of each object would weigh here on Earth.Assume that a teaspoon will hold about 1.5 cubic-centimeters ofmaterial.

Can you now relate these numbers to materials you know here onEarth? How much does a teaspoon of water weigh? Or air? Or iron?

Crossing the Event Horizon

If a black hole has no size, how do scientists talk about itssurface? Well, we don�t really mean the physical surface of the black hole-- we mean the surface around the black hole at which the escape velocityis equal to the speed of light. In other words, if you are closer to theblack hole than the distance to this surface, you cannot escape. If youare further away from the black hole than this distance, then there isstill hope for you! The surface is called the event horizon, and itsradius is the Schwarzschild radius. (Named for Karl Schwarzschild, anastronomer who was a member of the German army in World War I and died ofillness on the Russian front in 1916. He applied the equations of generalrelativity to see what would happen to light near such a massive object.)It is important to keep in mind that the event horizon is not a physicalboundary, but for all intents and purposes is the surface of the blackhole. Once inside it, you are cut off from the rest of the Universeforever.

The relationship of the Schwarzschild radius to the black hole mass issimple:

This can be easily understood by looking at the equation for theescape velocity from any spherical body such as a planet or star,namely,, where M andR are the mass and radius of spherical object. For a black hole,the escape velocity is equal to c, the speed of light.

1. What would be the radius of a black hole with the mass of the planetJupiter?
2. How would the period of the Earth�s revolution change if the Sunsuddenly collapsed into a black hole? Note that this can neverhappen!
3. Suppose the Earth were collapsed to the size of a golf ball...becominga small black hole. What would be the revolution period of the Moon, at adistance of 381,500 km? Of a spacecraft that had been hovering 300 m abovea point on the surface of the Earth before its collapse? Of a fly orbitingat 0.5 cm?

About the Poster...

The images on the poster are a combination of actual images andartist�s alterations. The low mass star, low mass red giant, white dwarf,black dwarf, neutron star, and black hole images are all artist�srenditions. The neutron star is depicted to emphasize its powerfulmagnetic field. The black hole image shows the large accretion disk andjets surrounding the black hole, which cannot be seen. Actual images aredescribed below.

SNR: Einstein IPC image of the Cygnus Loop Supernova Remnant

Betelgeuse: HST image of Betelgeuse

Nebula: HST image of Orion Nebula

Solutions

If done to scale, the red giant light globe would be over 5meters in diameter!

Model a Black Hole

The heavy object representing the black hole will distortthe latex surface (representing spacetime) and cause the small objects onthe surface to be pulled in toward it... but not if you are too far away.A heavier ball bearing, however, would affect beads further out in thelatex sheet...just as a more massive black hole creates a largerdistortion in spacetime, thus affecting objects further away.

These Stars are Classified

Note that when it goes off this graph on the left side, itactually goes out to about T ~ 100,000 K or higher before turning andheading steadily down to the white dwarf stage.

4. (a) black hole; (b) neutron star; (c) black dwarf

Blackbody Radiation and Wien�s Law

1. Solve Wien�s Law for T, substitute in the values forwavelength. With the temperature you obtain, look on the H-R diagram forthe corresponding spectral class.

(a) 9656 K Class A; (b) 19,313 K Class B; (c) 5267.2 K Class G; (d)2317 K Class M

2. Substitute the temperatures into Wien�s Law and obtain thewavelengths of the peak emission. Look up on a chart of the EM spectrumwhich region the wavelength falls into.

(a) 289.7 cm radio; (b) 3.62x10^-4 cm infrared; (c)1.93x10^-5 cm ultraviolet;

(d) 1.65x10^-7 cm X-ray

Extension:

No astronomical objects are as cold as 0.001 Kelvin. The radio emissionwe observe is produced by electrons moving in magnetic fields (this iscalled synchrotron radiation).

Bigger than a Breadbox?

Using the equation: distance = velocity x time,

Cygnus: 9.14x10¹⁴ km; Crab: 4.46x10¹³ km; Tycho:6.96x10¹³ km; SN1006: 9.37x10¹³ km

The supernova occurred in the year 1604 and is known as Kepler�ssupernova. It was observed and documented by the astronomer JohannesKepler.

A Teaspoonful of Starstuff

Using the equation: mass = density x volume,

We are given that the volume of interest is 1.5 cm³. So whatis the density of each of the objects? Density equals mass/volume, and thevolume of a sphere is ⁴/₃ p r³, where r is the radius of the sphere.Plugging in the values for each of the types of stars, we find that ourteaspoon of the Sun would contain 2.1 grams; of the white dwarf wouldcontain 2.85x10⁶ grams; of the neutron star would contain9.75x10¹⁴ grams. By looking up the density of water, air, andiron, you can calculate that each would be 1,500 grams, 1.935 grams, and1.179x10⁴ grams, respectively.

Crossing the Event Horizon

1. Using the Schwarzschild equation, we input the mass ofJupiter (1.9x10²⁷ kg), the Gravitational constant (G =6.67x10^-11 m³/kg-sec) and the velocity of light(3x10⁸ m/sec) to see that the event horizon of a Jupiter-massblack hole would occur at 2.96 meters.

2. It would not change.

3. (a) The lunar orbit would take the same as it does now, ~ 27.3 days.The orbit of a spacecraft that had been hovering just over the surface ofthe Earth would be the same as the current rotation period of Earth, 24hours. The fly would be inside of the event horizon...so we have no ideawhat is happening to it!

Glossary

Absolute Magnitude - apparent magnitude a star would haveif placed at a distance of 10 parsecs from Earth

Accretion - gradual accumulation of mass

Accretion Disk - a disk of material falling in toward a massiveobject such as a neutron star or black hole (the disk shape is the resultof conservation of angular momentum)

Active Galactic Nuclei - galaxies whose central regions areemitting enormous amounts of electromagnetic radiation

Apparent Magnitude - a measure of observed light flux received froman object at the Earth

Arc Minutes - a unit of measurement used for very small angles;there are 60 arc minutes in one degree

Arc Seconds - a unit of measurement used for very small angles;there are 60 arc seconds in one arc minute

Black Dwarf - the presumed final state of evolution of a low massstar in which no radiation is emitted

Black Hole - region in space where the escape velocity is equal to,or greater than, the speed of light. Thus, nothing (including radiation)can escape from it

Electrical Repulsion - the force which acts between particles oflike electrical charge to repel them from each other

Electromagnetic Radiation - radiation consisting of periodicallyvarying electric and magnetic fields that vibrate perpendicular to eachother and travel through space at the speed of light

Electromagnetic Spectrum - the full range of electromagneticradiation spread out by wavelength, it consists of gamma-rays, X-rays,ultraviolet rays, optical light, infrared radiation, microwaves, and radiowaves

Electron - a negatively charged subatomic particle that normallymoves about the nucleus of an atom

Escape Velocity - minimum velocity an object must achieve to breakfree from the gravity of another body (in physics, it is achieved when theobject�s kinetic energy is equal to its gravitational potential energy)

Event Horizon (also known as Schwarzschild Radius) - thevirtual surface around a black hole (often considered as the surface ofthe black hole) within which gravitational forces prevent anything,including light, from escaping

Expansion Velocity - the outward material velocity away from thecentral point of an explosion, such as a supernova

General Relativity - the geometric theory of gravitationdeveloped by Albert Einstein, incorporating and extending the theory ofspecial relativity to accelerated frames of reference and introducing theprinciple that gravitational and inertial forces are equivalent

Gravitational Energy - energy that can be released by thegravitational collapse of a system

Hertzsprung-Russell Diagram - a plot of absolute magnitudeversus spectral type (or temperature) for a group of stars

Isotope - any of two or more forms of the same element, whoseatoms all have the same number of protons but different numbers ofneutrons

Kinetic Energy - energy associated with motion; the kineticenergy of an object is equal to one-half the product of its mass and thesquare of its velocity

Light-Year - the distance light travels in one Earth year, equalto 9.46 x 10¹² km

Luminosity - the rate of radiation of electromagnetic energy intospace by a star or other object

Main-Sequence - diagonal region of the Hertzsprung-Russell diagramin which most stars are located; generally these are stable stars duringthe bulk of their lives

Neutron - a subatomic particle with no electrical charge; one ofthe constituents of the atomic nucleus

Neutron Star - a star of extremely high density composed almostentirely of neutrons

Nuclear Reaction - a reaction, as in fission, fusion, orradioactive decay, that alters the energy, composition, or structure of anatomic nucleus

Parsec - unit of distance often used by astronomers, equal to3.2616 light-years (a kiloparsec is equal to 1,000 parsecs)

Photon - a unit of electromagnetic energy associated with aspecific wavelength or frequency

Planetary Nebula - a shell of gas ejected from, and expanding awayfrom, a star that is nearing the end of its life

Plasma - a hot ionized gas, that is, it is composed of a mix offree electrons and free atomic nuclei

Potential Energy - stored energy that can be converted into otherforms; especially gravitational energy

Proton - a subatomic particle that carries a positive charge, oneof the constituents of the atomic nucleus

Radioactive Isotope - an isotope of any element which decays (ordecomposes) through the spontaneous emission of subatomic particles andgamma-rays

Red Giant - a star that has greatly increased in size and has arelatively cool surface which glows red; such stars occupy the upper righthand corner of the Hertzsprung-Russell diagram

Resolution - degree to which fine details in an image can beresolved, or separated

Schwarzschild Radius - see Event Horizon

Spacetime - a system of three spatial coordinates and one temporalcoordinate with respect to which the time and location of any event can bespecified

Special Relativity - the physical theory of space and timedeveloped by Albert Einstein, based on the postulates that all the laws ofphysics are equally valid in all frames of reference moving at a uniformvelocity and that the speed of light from a uniformly moving source isalways the same, regardless of how fast or slow the source or its observeris moving

Spectral Class - a classification of a star according to thecharacteristics of its spectrum

Spectrum - array of colors or wavelengths obtained when light isdispersed, as in passing it through a prism or grating

Star - a self-luminous sphere of gas

Stellar Spectroscopy - breaking down the electromagnetic radiationfrom a star in order to study the different wavelengths individually

Supergiant - an old, high-mass star greatly expanded from itsoriginal size; larger and brighter than a giant star

Supernova - catastrophic explosion of a star which can cause it toshine brighter than a galaxy for a few weeks or so

Supernova Remnant - expanding cloud of radioactive material formedwhen the outer layers of an exploding star (supernova) are blasted away

White Dwarf - a star that has exhausted most or all of its nuclearfuel, collapsed into a size similar to the Earth; such a star is near thefinal stage of its evolution

Resources

Books-

Apfel, Necia, Nebulae: The Birth and Death of Stars, 1988,Lothrop, Lee and Shepard, ISBN 0-688-07229-1. Explains the life cycle ofstars to upper elementary school students and above.

Branley, Franklyn, Journey Into A Black Hole, 1986, Crowell,ISBN 0-690-04544-1. Explains the black hole stage of a massive star'slife cycle to elementary school students and above.

Branley, Franklyn, Superstar: The Supernova of 1987, 1990,HarperCollins, ISBN 0-690-04839-4. Explains the supernova stage of amassive star's life cycle to middle school students and above.

Levy, David H., A Nature Company Guide: Skywatching, 1995,Time-Life Books. This book provides a general overview and discussion ofastronomical objects, including the life cycle of stars. For students inmiddle school or above.

Mitton, Jacqueline & Simon, The Young Oxford Book ofAstronomy, 1995, Oxford University Press, Inc. This excellent bookexplains many concepts in astronomy from the Solar System to galaxies andthe Universe, including a nice section on the life cycle of stars.Intended for the middle or high school student.

Magazines-

Berstein, Jeremy, "The Reluctant Father of Black Holes", ScientificAmerican, June 1996, vol. 274, no. 6. Discusses the details of howEinstein's equations of gravity are the foundation of the modern view ofblack holes. Intended for the high school (and above) student who isinterested in science.

Kirshner, Robert P., "SN 1987A: The First Ten Years", Sky andTelescope, February 1997, vol. 93, no. 2. Discussion of the supernovathat has taught us much about stellar evolution. Intended for the highschool (and above) student who is interested in science.

Hurst, Guy M., "Searching for Outbursts", Astronomy Now, September1995, vol. 9, no. 10. Talks about how amateur astronomers can help in thesearch for supernovae. Intended for the high school (and above) studentinterested in science.

Web sites-

To get a colorful step-by-step overview of the life cycle of stars,examine the site "The Life Cycle of Stars" which is located on the WorldWide Web. The URL for this site is

http://astron.berkeley.edu/~bmendez/ay10/cycle/cycle.html

For further information on the various stages of the life cycle ofstars, examine the site "Imagine the Universe!" which is located on theWorld Wide Web. The URL for this site is http://imagine.gsfc.nasa.gov/

Video-

"Evolution of a Star", Starfinder Series #11, MarylandInstructional Technology, 1990. This video can be ordered from the COREcatalog**, or recorded from your local PBS station. It describes thebirth, life and death of low mass and massive stars. Intended for themiddle school (and above) student.

**Educators may request a catalog and order form by sending a requeston school letterhead to the following address:

NASA CORE

Lorain County JVS

15181 Route 58 South

Oberlin, OH 44074

216/774-1051 Ext. 293 or 294

(Mon-Fri) 8-4:00 p.m. E.S.T.

FAX 216/774-2144

Slide Set-

ASP Slide Set #AS238, Stellar Evolution by Dr. James Kaler, 27 slideswith captions, $32.95, 1-800-962-3412.