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Home , Crab Nebula, Egg Nebula, Orion Nebula

Eagle Formation Region

AST 303: Chapter 17 1 The Formation of (2)

• A cloud of gas and dust must collapse if stars are to be formed. • The self-gravity of the cloud will tend to cause it to collapse. • pressure from nearby hot stars may do the same. • The passage of a from a nearby blast or some other source (such as galactic shock waves) may do the same. – Note: The “sonic boom” of a jet plane is an example of a shock wave. • When two clouds collide, they may cause each other to collapse.

AST 303: Chapter 17 2 Trifid Nebula

AST 303: Chapter 17 3 Trifid Nebula Stellar Nursery Revealed

AST 303: Chapter 17 4 Young Starburst Cluster Emerges from Cloud

AST 303: Chapter 17 5 The Formation of Stars (3)

• The gas in the collapsing cloud probably becomes turbulent. • This would tend to fragment the collapsing gas, producing condensations that would be the nuclei of new stars. • There is abundant evidence that shows that the stars in a cluster are all about the same age. For a young cluster, many stars have not yet reached the : !

Isochron

"Temperature

AST 303: Chapter 17 6 The Formation of Stars (4)

• The evolutionary paths of young stars on the H-R diagram look like this. Note the T Tauri stars, long thought to be young stars. • Theory says that these stars use convection as the main method of transporting to their surfaces.

! T Tauri Stars Luminosity

"Temperature

AST 303: Chapter 17 7 The Search for Stellar Precursors

• Astronomers have long been fascinated by very dark, dense regions seen outlined against bright gas, called globules. • These may be the first visible evidence of . • When a star begins to collapse, it will heat up, but it will still be fairly low temperature. Furthermore, it may be wrapped up in dust that makes it impossible for us to see optically. • For this reason, astronomers turn to observations to look for stellar precursors and very young stars.

AST 303: Chapter 17 8 Stellar Nurseries

AST 303: Chapter 17 9 Stellar Nurseries

AST 303: Chapter 17 10 The Nebula

• The (M 42) is the nearest (1500 years) and most prominent region of star formation. • It contains regions of emission and absorption, and is a strong microwave (OH, H2O) and infrared emitter. Several regions of strong infrared emission are the BN (Becklin- Neugebauer) object and the KL (Kleinman-Low) nebula. • It’s center is dominated by 4 very bright stars (the Trapezium), including an O4.5 star, ϑ1C Orionis, about 50 solar . This star is extremely hot (emits a lot of ) and very young. The ultraviolet emission from this star is largely responsible for exciting the gas of the Orion Nebula to glow. • This glowing gas is an H II region.

AST 303: Chapter 17 11 Orion Nebula (WFPC-2 Portrait)

AST 303: Chapter 17 12 The Orion Nebula (2)

• A model of the Orion Complex:

Dark molecular cloud

Visible nebula

Trapezium

AST 303: Chapter 17 13 Star Formation in Orion Nebula

AST 303: Chapter 17 14 Closeup of Star Being Born

AST 303: Chapter 17 15 Evolution of the

• Eventually, the Sun’s core will become depleted of and turned entirely into . • At this point, no energy can be generated in the core. The core will contract under gravity and heat up. Ironically, the atmosphere swells up and the star becomes a red giant

Helium core

Hydrogen envelope

(Not to scale)

AST 303: Chapter 17 16 Evolution of the Sun (2)

• Sometime later, hydrogen will start to burn into helium in a shell around the helium core. This happens when the contracting core gets hot enough at the boundary between the helium and hydrogen layers.

Helium core

Hydrogen-burning shell

Hydrogen envelope

(Not to scale)

AST 303: Chapter 17 17 Evolution of the Sun

AST 303: Chapter 17 18 The Fate of the

AST 303: Chapter 17 19 Evolution of the Sun (4)

• The position of the point on the H-R diagram that represents the luminosity and surface temperature of a star will trace out an evolutionary track on the H-R diagram with time.

106

104 D 2 E 10 F B 1 C A: Main sequence A 10-2 B: Shell burning C: Fully convective

Solar -4 D: Helium flash 10 E, F: Later evolution

10-6 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 20 Evolution of the Sun (5)

• Eventually the core becomes so hot that helium can fuse into . • The process that burns helium into carbon is called the triple alpha process. It requires the near-simultaneous collision of three helium nuclei, and since the electrical repulsion of helium nuclei is much stronger than the electrical repulsion of hydrogen nuclei, it requires much higher temperatures (100 million K) and pressures. 4 He 4 2 2 He ! ray

8 ( Be)* 4 12 4 6 C 2 He AST 303: Chapter 17 21 Helium Burning Phase

AST 303: Chapter 17 22 Pulsating Stars

• The point representing the Sun moves back down and to the left on the H-R diagram. This time it enters a region called the instability strip. At this point, the star becomes somewhat unstable and pulsates. It has turned into a Cepheid variable.

106 Instability 104 Strip

102

1

10-2

Solar luminosities 10-4

10-6 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 23 Pulsating Stars (2)

• A Cepheid pulsates very regularly, with a period of days to weeks. At first, the star gets smaller, heating up its interior. It overshoots the equilibrium point (like a child on a swing overshooting the bottom of the swing), so eventually the pressure in the interior starts to make it expand again. Again it overshoots the equilibrium point. This happens as long as the star is in the instability strip. !

Luminosity

Time !

AST 303: Chapter 17 24 A Cepheid Variable

AST 303: Chapter 17 25 Light Curve of Cepheid Variable

AST 303: Chapter 17 26 Pulsating Stars (3)

• Other types of pulsating stars are δ Scuti stars, whose masses are similar to the Sun, and RR Lyrae stars, of lower mass than Cepheids and with periods about a day. • All of these stars are important for measuring the distance scale in the , because their luminosities can be related to the pulsational period. This makes them useful standard candles.

AST 303: Chapter 17 27 Dying Stars

• Eventually, a star can no longer burn any kind of nuclear fuel. • It dies as a nuclear energy source. Most stars end up as stars. • White dwarfs are about the size of the Earth, but have mass comparable to that of the Sun. • Their densities are about a million times that of the Sun.

Surface of Sun

Earth White dwarf

AST 303: Chapter 17 28 Evolution to White Dwarf

AST 303: Chapter 17 29 Structure of White Dwarf

AST 303: Chapter 17 30 Dying Stars (2)

• The first white dwarf discovered was a companion to the star Sirius. Friedrich Bessel discovered that Sirius “wobbled” back and forth in the sky. The wobble was not due to parallax. Bessel concluded that Sirius had a companion, but he couldn’t see it. • Visually detected in 1862. It was only 10-4 as luminous as Sirius, and after the orbit was calculated, it turned out to be as massive as the Sun. • A single cubic centimeter of matter from this star would weigh over a ton! • White dwarfs have no internal nuclear reactions; they simply cool off, emitting heat that is left over from when they were a star. Their surface area is very small.

AST 303: Chapter 17 31 Dying Stars (4)

• S. Chandrasekhar showed that there is a relationship between the mass and diameter of white dwarfs. No white dwarf can have a mass over 1.44 times the mass of the Sun (the ).

30,000

! 20,000

Chandrasekhar Limit 10,000 Diameter (km)

0 0.0 0.5 1.0 1.5 Mass (solar masses) !

AST 303: Chapter 17 32 Planetary Nebulae

• In the later stages of evolution (red giants, formation, novae and supernovae), stars may lose large amounts of mass. This mass is recycled into the . • Since new stars are formed from the recycled material, and since like the earth are formed form the heavier elements in it, this means that we ourselves are made of stuff that used to be in the interior of stars.

AST 303: Chapter 17 33 Planetary Nebula

AST 303: Chapter 17 34 Planetary Nebula

AST 303: Chapter 17 35 Planetary Nebula

AST 303: Chapter 17 36 Egg Nebula

AST 303: Chapter 17 37 Hourglass Nebula

AST 303: Chapter 17 38 NGC 6543

AST 303: Chapter 17 39 Planetary Nebulae (3)

• As the nebula dissipates, the star cools down. The star becomes a white dwarf; here is the evolutionary track.

106

4 G: Central star of 10 G planetary nebula H: White dwarfs 102

1 H 10-2

Solar luminosities 10-4

10-6 80000 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 40 Evolution of Massive Stars

• Massive stars (say more massive than 5 solar masses) have a different history. • They produce energy through the CNO Cycle, in which carbon, and catalyze the conversion of hydrogen into helium. • Their cores are convective, and energy is transported through their envelopes by radiation. Radiation pressure is much more important in these stars.

AST 303: Chapter 17 41 Evolution of Massive Stars (2)

• The evolutionary track of a massive star on the H-R diagram looks like this:

6 10 Multiple shell sources 104

2 10 Helium ignition

1

10-2

Solar luminosities 10-4

10-6 80000 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 42 Evolution From Helium Burning to Carbon Ignition

AST 303: Chapter 17 43 Evolution of Massive Stars (3)

• As the evolution of the star progresses, one can have more than one shell burning nuclear fuel. Here, hydrogen and helium are burned in shells.

Hydrogen envelope Hydrogen-burning shell Helium layer Helium-burning shell Carbon-Oxygen core

AST 303: Chapter 17 44 Three Layers...

AST 303: Chapter 17 45 Four Layers...

AST 303: Chapter 17 46 Seven Layers...

AST 303: Chapter 17 47 Another Artist's View

AST 303: Chapter 17 48 Evolution of Massive Stars (4)

• Eventually there can be many shells, each burning a different kind of nuclear fuel. • However, there is an end: cannot be fused to even heavier elements with the release of energy. It takes energy in to make heavier elements than iron.

Iron core Silicon Magnesium, silicon, sulphur, oxygen, , etc.

Carbon and oxygen Helium Hydrogen envelope

AST 303: Chapter 17 49 Observational Evidence for (2)

• In very young open clusters, the lowest mass stars will not even have reached the main sequence.

106

104

102

1

10-2

Solar luminosities 10-4

10-6 80000 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 50 Observational Evidence for Stellar Evolution (4)

• Here we see how the turnoff point is found for a particular cluster at a later stage in evolution.

106 Evolved 104 stars

102

1 Turnoff point

10-2

Solar luminosities 10-4

10-6 80000 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 51 Observational Evidence for Stellar Evolution (5)

• Here we see how the location of the turnoff point varies with the age of the cluster.

106

4 6 10 10 7 102 10 8 Ages (years) 10 9 1 10 10 10 10-2

Solar luminosities 10-4

10-6 80000 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 52 Observational Evidence for Stellar Evolution (6)

• Evolution off the main sequence is very rapid. The positions of stars trace out the evolutionary tracks of the stars. Agreement of the observations with computer calculations is very good. 106

104

102 Evolutionary 1 track for 1 Globular cluster star 10-2 H-R diagram Solar luminosities 10-4

10-6 40000 20000 10000 5000 2000 Surface temperature (Kelvin)

AST 303: Chapter 17 53 Novae

• Occasionally a star will briefly appear where none was seen or noticed before, rising quickly to a maximum brightness and then slowly fading away again. • The early astronomers called them “novae” (Latin for “new”). The ancient Chinese astronomers called them “guest stars.” • Whatever the culture, novae were perceived as portents of the future. • Now we know that they are not new at all, but are actually certain old stars passing through a particular phase of their evolution.

AST 303: Chapter 17 54 Novae (2)

• Novae eject mass • Their spectra show blueshifted absorption lines, indicating matter in a shell of gas around the star moving towards us rapidly (we talked about this earlier).

Star

Expanding shell of gas

• After a time, if the is near enough to the Sun, we may actually see the expanding shell of gas.

AST 303: Chapter 17 55 Nova Persei

AST 303: Chapter 17 56 Nova Cygni 1992

AST 303: Chapter 17 57 Novae (3)

• A nova’s luminosity may increase in a day or two by a factor of over 100,000. • At its brightest, it may be 1,000,000 times as bright as the Sun, one of the most luminous objects in our . • Although we cannot predict in advance that a particular star will “go nova” for the first time, often the star will have been observed prior to the event. • This can enable astronomers to learn more about the processes that cause a star to become a nova.

AST 303: Chapter 17 58 Novae (4)

• The star in the center of a nova is typical of the central stars of planetary nebulae; however, planetary nebulae are not former novae. • The process that forms planetary nebulae is gradual; novae are explosions. • Merle Walker (1954) discovered that the nova DQ Herculis is actually a close binary system. • We know now that most novae are binaries. Probably all are. • The models that most astronomers believe describe novae assume that they are in fact binaries.

AST 303: Chapter 17 59 Supernovae

• Supernovae are a much rarer phenomenon. They are also much more spectacular. • Famous supernovae are Tycho’s star (1572), Kepler’s star (1604), the supernova of 1054 that produced the Nebula (observed in China but not noted in Europe), S Andromedae (which blew up in the M31 in 1885) and the SN 1987 A, in the Large Magellanic Cloud (a galaxy in the southern hemisphere). • At their brightest, supernovae have been visible in broad daylight.

AST 303: Chapter 17 60 Supernovae (2)

• In a supernova explosion, the luminosity of the star can increase by a factor of billions (much more than for a nova). • The star itself can be as luminous as its entire galaxy. That is 1011 times as luminous as the Sun. • Supernovae in a given galaxy are rare. The last one seen in our galaxy was Kepler’s star. • There may have been a supernova in 1667 that left behind the radio source ; however, it was not noticed visually at the time. • However, there are many other , and we can study supernovae in them. Most of what we know about supernovae comes from studying supernovae in external galaxies, and from studying supernova remnants like the and Cassiopeia A in our own galaxy.

AST 303: Chapter 17 61 Supernovae (3)

• There are two main types of supernovae. • Type I supernovae have very typical light curves. The luminosity increases steadily for several weeks and then declines. • The spectrum shows weak hydrogen lines. • They appear very old and not very massive. • The best model is somewhat similar to that of a nova: a binary system, with a giant star dumping mass onto a white dwarf. However, the white dwarf is a carbon-oxygen white dwarf with degenerate matter. As the mass increases, the temperature and pressure in the interior increase until the degenerate core detonates—burning carbon and oxygen.

AST 303: Chapter 17 62 Supernovae (4)

• This produces large amounts of nickel 56 (which is radioactive). The nickel 56 decays to cobalt 56 which decays to iron 56. A great deal of energy is released in the form of rays, that heat the nebula around the star. • Evidence for this is that the decay curve of a Type I supernova declines at just the rate that nickel 56 turns into iron. • Computer calculations also predict that this is what will happen.

AST 303: Chapter 17 63 Supernovae (5)

• Type II supernovae are much less uniform. They rise more quickly to a maximum and decay more quickly too. Their spectra contain prominent hydrogen lines. • The best model for a Type II supernova is as the explosion of a highly evolved, massive star with the “onion” structure of many shells like we already saw. • By slow neutron capture, elements up to bismuth can be formed. Heavier elements are radioactive and would decay too rapidly for significant amounts to be formed. • This process of slow neutron capture is called the S process. • During the supernova explosion, heavier elements still can be formed by rapid neutron capture. This is called the R process.

AST 303: Chapter 17 64 Supernova Stage

AST 303: Chapter 17 65 Eta Carinae, a Possible Supernova Precursor

AST 303: Chapter 17 66 Supernovae (6)

• The “onion” structured star turns out to be unstable at the end. • Because of the high pressures, the reaction + proton → neutron + neutrino is favored over the reverse reaction. This “soaks up” . But the electrons were the particles that were providing the pressure that held the star up against gravity. So all of a sudden, the core of the star collapses. • Enormous amounts of energy are released. The neutrinos stream outwards through the dense stellar material. Because the envelope of the star is so dense—1015 times that of water, the neutrinos apply pressure to eject it into space. • These neutrinos were actually detected by the neutrino observatories when SN 1987 A (a Type II supernova) blew up.

AST 303: Chapter 17 67 The Field Around SN 1987A

AST 303: Chapter 17 68 SN 1987A

AST 303: Chapter 17 69 SN 1987A

AST 303: Chapter 17 70 SN 1987A Rings

AST 303: Chapter 17 71 Changes in SN 1987A's Ring

AST 303: Chapter 17 72 SN 1987A Spectra

AST 303: Chapter 17 73 Supernova in M 51

AST 303: Chapter 17 74 Supernovae (7)

• When the core collapses completely, the pressure is so great that it “bounces”, producing an outwardly moving shock wave. • This shock wave also assists in blowing off the star’s envelope. • According to the calculations, a very dense ball of neutrons remains. Gravity binds the ball of neutrons together. • Theoretically, this ball of neutrons would be about 10-20 km in diameter, no bigger than a medium-sized city. Yet it would contain approximately a solar mass of material. • The star would become a . In 1934 and Fritz Zwicky suggested that such objects might remain behind after a supernova exploded.

AST 303: Chapter 17 75

AST 303: Chapter 17 76 Supernova Remnant

AST 303: Chapter 17 77 Crab Nebula (1054 A.D.)

AST 303: Chapter 17 78 Crab Nebula

AST 303: Chapter 17 79 and Neutron Stars

• In 1967, Jocelyn Bell, then a graduate student at Cambridge University, noticed a radio source in the sky that pulsated in a regular fashion. She found that the pulsations were very rapid—the period was only 1.33728 seconds (and the period was very precise). • Some people speculated that perhaps intelligent beings were transmitting some sort of message (the LGM theory). • , of Cornell University, suggested instead that it might be a rapidly rotating neutron star. The pulsations corresponded to the rotational period, in his model.

AST 303: Chapter 17 80 Pulsars and Neutron Stars (2)

• Astronomers investigated known supernova remnants, such as the Crab Nebula. • There was an object in the center of the Crab Nebula that Zwicky had long before suggested must be the supernova remnant. • It was found to pulsate, both optically and in the radio, with a period of 0.033 seconds (corresponding to rotating 30 times per second).

AST 303: Chapter 17 81 Crab (Optical)

AST 303: Chapter 17 82 Pulsars and Neutron Stars (3)

• This clinched the rotating neutron star theory. • No other known object could rotate so fast without flying apart. Even a white dwarf would not do—the surface would be travelling faster than the , which would contradict the theory of relativity. • The pulsar’s rotation rate was slowing down ever so slightly—and when you calculated the amount of energy loss, it equaled the luminosity of the pulsar—if it was a neutron star.

AST 303: Chapter 17 83 Pulsars and Neutron Stars (4)

• Finally, everything fit together. • The supernova phenomenon. • The existence of supernova remnants. • The prediction that the remnants would be neutron stars. • The agreement with the pulsar data—the neutron star is the powerhouse that is the source of the pulsar’s energy.

AST 303: Chapter 17 84 Pulsars and Neutron Stars (5)

• The best model of a pulsar says that there is a strong magnetic field frozen into the neutron star. As the star rotates, the magnetic field rotates with it, which transfers energy to a disk of material around the star. • The disk of material generates which is beamed away from the star in a narrow beam, just like a searchlight.

Magnetic field

Gas

Radiation

AST 303: Chapter 17 85 Pulsar Model (T. Gold)

AST 303: Chapter 17 86 Pulsars and Neutron Stars (6)

• Theoretically, a neutron star has a core, whose composition involves matter at such high densities and pressures that we can only speculate about it. There is a crystalline “crust”, and an atmosphere only 1-2 cm. thick.

Core Crystalline crust (unknown)

Atmosphere

10-20 km

AST 303: Chapter 17 87 Structure of Neutron Star

AST 303: Chapter 17 88 Pulsars and Neutron Stars (7)

• Evidence of a solid crust comes from the fact that occasionally, the pulsation rate will suddenly speed up. We call this a “glitch,” and it is believed to be the star’s way of releasing strain in the crust, a sort of an “starquake.”

AST 303: Chapter 17 89 Pulsars and Neutron Stars (8)

• Exciting discoveries included the binary pulsar, a pulsar that has a stellar companion. The orbital motion of the two stars changes in a way that indicates that it is radiating gravitational radiation. • There is also the millisecond pulsar, which rotates some 642 times per second. It is possible that this object once had less mass, but acquired mass from a companion. This would speed it up • Finally, evidence for planets around one pulsar was announced. (Another such case turned out to be a false alarm, due to systematic errors that weren’t properly accounted for.)

AST 303: Chapter 17 90 The Bottom Line: Black Holes

• Just as with white dwarfs, it is the degenerate neutron material in a neutron star that supports the star against the force of gravity. This means that there is a “Chandrasekhar limit” for neutron stars. • No neutron star can have a mass greater than this limit, which is about 2 solar masses. • If a supernova were to go off, but left behind a remnant with more than this amount of mass we would have something even more extreme • It turns out that nothing could stop the collapse of the remnant. One would get a black hole, from which nothing, not even light, could escape.

AST 303: Chapter 17 91 The Bottom Line: Black Holes (2)

• We know that gravitational fields can affect electromagnetic radiation. For one thing, electromagnetic radiation that escapes from a massive body is redshifted. This has been observed in white dwarf stars as well as experimentally on Earth.

• For another thing, electromagnetic radiation that goes past a massive object will be bent. This has also been observed. Apparent position of distant galaxy

Actual position of distant galaxy Nearby galaxy AST 303: Chapter 17 92 Gravitational Lens

AST 303: Chapter 17 93 The Bottom Line: Black Holes (3)

• With a black hole, light that attempts to escape will be infinitely redshifted. This means that the light can never escape from the black hole. And if light cannot escape, nothing else can either. • Outside the black hole, the gravitational field looks just like the gravitational field of a star. Things are not “sucked in,” they orbit normally. Particles going directly into the hole are captured. Black hole

Particle that is captured

Particle that orbits without being captured AST 303: Chapter 17 94 The Bottom Line: Black Holes (4)

• A black hole of the mass of the Sun would be about 3 km in (effective) diameter. • The larger the mass, the larger the black hole. • If no light can escape from a black hole, how could we detect one? • By gravitational light bending if a star passed behind a black hole? • Another way that black holes might be detected is if they have a companion that is dumping mass onto them. The matter would be heated so strongly that it would emit X- rays. Such objects have been detected, and it is tempting to believe that they might be black holes.

AST 303: Chapter 17 95 Black Hole Disk

AST 303: Chapter 17 96 The Bottom Line: Black Holes (5)

• X-1 is an X-ray emitting binary object. Careful studies show that the mass of the collapsed companion to the star is at least 8 solar masses. • We also detect X-rays from the center of our galaxy. There is believed to be a black hole there. • Similarly, some globular clusters may have black holes in their centers, because they, too, are X-ray sources.

AST 303: Chapter 17 97 Statistical Issues

• A huge amount of information can be gotten from the H-R diagram of star clusters, by looking at the isochrons from particular stellar evolution codes. But traditionally, picking the “best” isochron has been done rather crudely, for example by eyeball or simple chi-square methods. • The problem is complicated by contaminating factors: If a star is an undetected binary, its position in the H-R diagram will be altered significantly. A star may be a field star, and such objects should be excluded. The lower end of the H-R diagram is truncated by the limiting magnitude of the used, and so forth. This is a rich area for modern statistical methods.

AST 303: Chapter 17 98 Statistical Issues

• Also, there are many competing stellar evolution models (implemented as complex computer codes). Choosing between models is an interesting but difficult problem in model selection • The two dozen neutrinos detected from SN 1987a are an interesting example. Many astronomers tried to make use of this sparse data set. Many failed because they used inappropriate methods.

AST 303: Chapter 17 99