Module 12: When Stars Die: The Exotic Remains

Cygnus X-1: A Black Hole

(A) Cygnus X-1 in the constellation Cygnus is a famous X-ray binary with a stellar-sized black hole. Cygnus X-1 has about 15 times the mass of the Sun and is in orbit with a massive blue companion star. (B) An optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box.

Fundamentals of a neutron star

1. heavy liquid interior composed mostly of neutrons with other particles 2. Solid crust thickness~1.6 km (1 mi) 3. Diameter~ 19.3 km (12 mi) Interior (slide 28)

(1) What is a white dwarf and (2) what balances it from the inward pull of gravity?

1.A white dwarf is an inert stellar core. 2.Electron degeneracy pressure balances the white dwarf from the inward pull of gravity.

Pulsar Geometry

A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis.

Spacetime around a neutron star

Einstein describes gravity as a deformation of spacetime around a massive object. The more massive an object, the greater its gravity and the more it warps spacetime.

Evidence for black holes

Evidence for black holes is indirect—since we cannot "see" something that is not in our universe. Black holes in close binaries will produce accretion disks that emit hot, energetic X-rays that we can detect. Cygnus X-1 is a likely black hole candidate because it is an X-ray binary composed of two high-mass objects.

GRB's and Black Holes

Gamma-ray bursts are often associated with supernovas and the creation of black holes. (A) An example of a long gamma-ray burst. The long ones are widely thought to be cataclysmic explosions signaling the end of stars 50 to 100 times more massive than the Sun. When such a behemoth star explodes, it leaves behind a black hole and beams the news across the cosmos on a wave of gamma rays.

Is it easy or hard to fall into a black hole?

Hard A black hole with the same mass as the Sun wouldn't be much bigger than a college campus.

Tidal Forces Near a Black Hole

Here we see what happens to a circular object as it approaches a black hole. As the object slides into the gravity well, the side nearest the well experiences more acceleration than the side further away. The tidal forces just above the black hole severely distorts the object. A black hole acts as a gravity well. As an object approaches close to a black hole, the leading edge of the object will experience more gravity and thus more acceleration than the trailing edge. As a object slides into the black hole it is severely distorted by the tidal forces.

What prevents our Sun from becoming a black hole?

Our Sun has too little mass

Surface of a black hole

The "surface" of a black hole is the radius at which the escape velocity equals the speed of light. This spherical surface is known as the event horizon. The radius of the event horizon is known as the Schwarzschild radius.

Masses in stellar graveyard

The masses of stellar remnants are measured in many different ways. This graphic shows the masses for black holes detected through electromagnetic observations (purple); the black holes measured by gravitational-wave observations (blue); neutron stars measured with electromagnetic observations (yellow yellow); and the masses of the neutron stars that merged in an event called GW170817, which were detected in gravitational waves (orange). The remnant of GW170817 is unclassified and labeled as a question mark.

Neutron stars & gravitational waves

Two neutron stars rotating rapidly around one another gradually lose energy by emitting gravitational radiation. As they lose energy, they revolve about each other more quickly and more closely to one another.

Merger of two black holes

the merger of two black holes surrounded by disks of hot gas. The black holes orbit each other for hundreds of millions of years before they merge to form a single, more massive black hole, sending out intense gravitational waves in the process.

How can black holes emit radiation

Astronomers estimate that up to a quarter of the total radiation in the universe emitted since the Big Bang comes from material falling towards stellar black holes and supermassive black holes, including those powering quasars, the brightest known objects. (slide 128)

Supernova Remnant Cassiopeia A

(A) Artist's illustration shows a simplified picture of the inner layers of the star that formed Cas A just before it exploded, with the predominant concentrations of different elements represented by different colors: iron in the core (blue-1), overlaid by sulfur and silicon (green-2), then magnesium, neon and oxygen (red-3). (B) This X-ray image from NASA's Chandra X-ray Observatory uses the same color scheme to show the distribution of iron, sulfur, and magnesium in the supernova remnant.

Interior of a White Dwarf

(A) Cross-section of a white dwarf showing thickness and mass of its layers. (B) Planetary nebula Menzel 2 is located in the southern constellation Norma 7,710 l-y (2,364 pc) distant. In Menzel 2, the nebula forms a winding blue cloud that perfectly aligns with two stars at its center. The star at the upper right is the white dwarf central star of the nebula, and the star to the lower left is probably a true physical companion of the central star. Carbon and Oxygen Helium Hydrogen

Roche Lobe

(A) In a binary star system, the Roche Lobe is the volume around a star within which matter is gravitationally bound to that star. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point. If a star in a close binary system evolves to the point at which it "fills" its Roche lobe, material from this star will overflow onto the companion star (via the L1 point). Thus mass can be transferred from one to the other. (B) In situations of mass transfer, the star's companion can be a compact object such as a white dwarf, neutron star, or black hole. Matter falling onto the White Dwarf from the accretion disk can cause a nova to occur Gas from a star is flowing to a close compact companion, a white dwarf, surrounded by an accretion disk

Mechanism of an X-Ray Burster

(A) In an X-ray burster, the mechanism is thought to be similar to the nova, except that the star onto which the matter accretes is a neutron star. (B) Because the gravitational field of a neutron star is much stronger than that of a white dwarf, the accretion under degenerate conditions leads to much higher temperatures than in the nova outburst. This in turn tends to produce X-rays rather than visible light in the thermonuclear runaway on the surface of the neutron star. (slide 84)

First Test of General Relativity

(A) One of Arthur Eddington's photos of the totally-eclipsed sun taken on May 29, 1919. Stars used to prove Einstein's prediction are shown between pairs of tick marks (circled). (Philosophical Transactions of the Royal Society of London) (B) The New York Times, November 10, 1919, that announced Einstein's triumph to the world.

Supermassive black hole

(A) The supermassive black hole Sgr A* at our galaxy's center is thought to be surrounded by (B) a disc of gas (yellow and red) in which massive stars (blue) have formed. The Chandra X-ray Observatory results show that stars have formed locally, rather than being deposited there

First cosmic event observed in both gravitational waves & light

(A,B) For the first time, scientists have directly detected gravitational waves in addition to light from the collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light. The discovery was made using the U.S.-based LIGO; the Europe-based Virgo detector; and some 70 ground- and space-based observatories. Approximately 130 million years ago, two neutron stars were in their final moments of orbiting each other, separated only by about 300 km (200 mi), and gathering speed while closing the distance between them. As the stars spiraled faster and closer together, they stretched and distorted the surrounding spacetime, giving off energy in the form of powerful gravitational waves, before smashing into each other. The inspiraling objects were estimated to be in a range from 1.1 to 1.6 times the mass of the Sun. While binary black holes produce "chirps" lasting a fraction of a second in the LIGO detector's sensitive band, the August 17, 2017 chirp lasted approximately 100 seconds. The new light-ABbased observations show that heavy elements, such as lead and gold, are created in these neutron star collisions and ultimately distributed throughout the universe.

White Dwarf (Type 1a) Supernovae

1. Two normal stars are in a binary pair 2. The more massive star becomes a giant 3. Which spills gas onto the second star, causing it to expand and become engulfed 4. The secondary, lighter star and the core of the giant star spiral inward within a common envelope 5. The common envelope is ejected, while the separation between the core and the secondary star decreases. 6. The remaining core of the giant collapses and becomes a white dwarf. 7. The aging companion star starts swelling, spilling gas onto the white dwarf. 8. The white dwarf's mass increases until it reaches a critical mass and explodes 9. ..causing the companion star to be ejected away.

(1) A white dwarf in a close binary system can form what structure? (2) This structure can then cause the white dwarf to do what?

1.An accretion disk (matter is transferred from the companion star). 2.Matter that falls onto the white dwarf via the accretion disk can ignite causing the white dwarf to become a nova or a white dwarf supernova.

(1) Describe the environment around a black hole at a point where your spaceship experiences the force of 1 g. (2) Describe the environment around a black hole at the event horizon.

1.Your spaceship will orbit the black hole like any other object of the same mass—black holes don't suck! 2.At the event horizon of the black hole, time slows down and tidal forces are very strong (probably strong enough to destroy your spaceship).

Black holes warp space and time

A black hole's mass strongly warps space and time in the vicinity of the event horizon. The event horizon is larger for black holes of larger mass. If the Sun shrank into a black hole, its gravity would be different only near the event horizon. Black holes don't suck! Mass warps spacetime. The greater the mass, the more warping of spacetime there is. If an enormous amount of mass is concentrated in a small enough volume, a black hole will form.

Making a Magnetar

A cross-section diagram shows a neutron star in its first seconds of life. It is still a superhot liquid with two or three layers of convection carrying heat to the surface. If the neutron star is spinning at more than 200 rotations/second, it sets up a dynamo effect that forms an intense magnetic field, and a magnetar is born. Hot, newborn neutron star churns and mixes Internal convection carries off heat If spinning faster than 200 revolutions/second, the dynamo action quickly builds up the magnetic field

Size of a neutron star

A neutron star is about the same size as a small city: ~20 kilometers or 12 miles.

Neutron Stars

A neutron star is the ball of neutrons left behind by a massive-star supernova. Degeneracy pressure of neutrons supports a neutron star against gravity. Neutron stars pack the mass of the sun into a diameter of about 10 km (6 miles). Precise neutron star limit is not known, but it lies below 3 M(sun symbol). Supernova remnant Puppis A, one of the sky's brightest X-ray sources. It is about 100 l-y in diameter and 6,500-7,000 l-y (1,991-2,145 pc) distant. (Inset) This close-up view, a pinpoint source of X-rays, is most likely a neutron star ejected by the asymmetric explosion and moving away from the site of the original supernova at about 600 miles/second. This hypervelocity neutron star is known as the Cosmic Cannonball.

Rotation about a black hole

A schematic diagram of velocity measurements of a rotating disk of hot gas in the core of active galaxy M87. The measurement was made by studying how the light from the disk is redshifted and blueshifted —as part of the swirling disk spins in Earth's direction and the other side spins away from Earth. The gas on one side of the disk is speeding away from Earth, at a speed of about 550 km/s (1.2 million mph). The gas on the other side of the disk is orbiting around at the same speed, but in the opposite direction, as it approaches viewers on Earth. This high velocity is the signature of the tremendous gravitational field at the center of M87. This is clear evidence that the region harbors a supermassive black hole, since it contains only a fraction of the number of stars that would be necessary to create such a powerful attraction. The object at the center of M87 weights as much as three billion suns but is concentrated into a space no larger than our solar system.

X-Ray jets in a binary system

A series of Chandra images has allowed scientists to trace the evolution of large-scale X-ray jets produced by a black hole in a binary star system. As the schematic shows, gaseous matter pulled from a normal star forms a disk around the black hole. The gas is heated to temperatures of millions of degrees, and intense electromagnetic forces in the disk can expel jets of high-energy particles.

Lighthouse model of Pulsars

As the neutron star rotates, the radiation beams sweep through space like lighthouse beams. (A) A rotating neutron star is called a pulsar. (B) A pulsar beams its radiation similar to that of a lighthouse.

Theroretical interperation

Astronomers can deduce the presences of a central supermassive black hole and measure its mass from the motions of stars near the nucleus of large star systems. Intermediate sized black holes have been found at the centers of globular clusters, such as M15.

Black hole

A star born with 8 M(sun symbol) or more will die in a supernova. Most supernovas end as neutron stars. If enough mass falls back on the core, a black hole can result. A black hole is an object whose gravity is so powerful that not even light can escape it. GRS 1915+105 or V1487 Aquilae is an X-ray binary star system which features a regular star and a black hole. The near-infrared counterpart was confirmed by spectroscopic observations. The binary system lies 40,000 l-y (11,000 parsecs) away in the constellation Aquila. GRS 1915+105 is the one of the heaviest of the stellar black holes so far known in the Milky Way Galaxy, with 10-18 times the mass of the Sun. It is also a microquasar, and it appears that the black hole may rotate at 1,150 times per second. (Inset) X-rays shine from heated material falling into black hole GRS 1915+105.

White Drawfs-1

A white dwarf is an inert core left over after a star has ceased nuclear fusion. Electron degeneracy pressure supports these stars against gravity. 1 M(sun symbol) white dwarf will be made mostly of carbon. Very low-mass stars end up as helium white dwarfs. (A) White dwarfs (circled) appear as tiny points in our galaxy as imaged by the Hubble Space Telescope. (NASA) (B) Animation of a white dwarf as it ages from a high surface temperature collapsed star ("white hot" just after formation) to a slowly cooling and dimming stellar core the size of the Earth. White dwarfs will eventually become black dwarfs, but this will take longer than the universe's current age.

As stars enter the final phase of their lives, they either eject a good portion of their gaseous envelopes or outright explode. The mass of the stellar core that remains will determine the ultimate fate of the star. For the stars listed below (initial mass at birth), give the name of the final object that forms and its mass.

A.<2 M:white dwarf with core mass <1.44 M B.2-8 M:white dwarf with core mass <1.44 M C.>8 M:neutron star with core mass <3 Mor black hole with core mass >3 M box means sun symbol

What can happen to a neutron star in a close binary system? What type of radiation is produced? Fusion events on the neutron star's surface can produce what result? (Hint: This is similar to what happens to a white dwarf in a close binary system.)

A.The accretion disk around a neutron star gets hot enough to produce X-rays, making the system an X-ray binary. B.Sudden fusion events periodically occur on a the surface of an accreting neutron star, producing X-ray bursts.

Mass Curves Spacetime

According to Einstein's General Relativity Theory, large masses warp the fabric of spacetime. In our solar system, planetary orbits are caused by the Sun's effect on our local spacetime. Planets will follow the straightest (most efficient) possible paths allowed by the curvature of spacetime. Spacetime without mass is flat. A large mass will deform or warp spacetime (shown here as the green mesh). According to Einstein, gravity is the result of the presence of mass in spacetime. Simplified, matter tells spacetime how to curve and curved spacetime, then, tells matter how to move. The idea of acceleration and how it relates to gravity comes from Einstein's Equivalence Principle, which states that you cannot tell the difference between a gravitational field and an equivalent uniform acceleration. So that objects in free fall under gravity all accelerate by the same amount, or in other words, they move the same way as if there was no gravity (i.e., weightlessness). Thus, what seems to be the result of gravity is really the result of curved spacetime. Since spacetime is curved around massive objects, the Earth is merely traveling along the shortest path in curved spacetime, which has the same appearance as if the Sun were pulling on the Earth due to gravity. Earth does not fall into the Sun because it also possesses forward momentum.

X-Ray burster

Accreting matter adds angular momentum to a neutron star, increasing its spin. Episodes of fusion on the surface of a neutron star lead to X-ray bursts. (A) An X-ray binary. Gas falling on the neutron star from its binary companion will periodically flare and produce X-rays. (NASA /ESA) (B) Circinus X-1, an X-ray binary, is about 26,000 l-y (8.0 kpc) distant.

Double pulsar & general realativity

After three years of observation, the double pulsar system, PSR J0737-3039A and B, has been used to demonstrate that Einstein's theory of general relativity is correct to within 0.05%. A key result of these observations is that the pulsar's separation is seen to be shrinking by 7 mm/day. Einstein's theory predicts that the double pulsar system should be emitting gravitational waves. The double pulsar system should lose energy causing the two neutron stars to spiral in towards each other by precisely the amount that has been observed. Thus, the observations give an indirect proof of the existence of gravitational waves. The double pulsar is 2000 light-years away in the direction of the constellation Puppis. The double pulsar is shown within the fabric of spacetime (depicted here as a green grid). Note how spacetime around each pulsar is warped by the pulsar's intense gravity

Sirius System in X-Rays

An X-ray image of the Sirius star system located 8.6 light-years from Earth. This image shows two sources and a spike-like pattern due to the support structure for the transmission grating. The bright source is Sirius B, a white dwarf star that has a surface temperature of about 25,000 K, which produces very low-energy X-rays. The dim source at the position of Sirius A—a normal star more than twice as massive as the Sun—may be due to ultraviolet radiation from Sirius A leaking through the filter on the detector. In contrast, Sirius A is the brightest star in the northern sky when viewed with an optical telescope, while Sirius B is 10,000 times dimmer. Because the two stars are so close together, Sirius B escaped detection until 1862 when Alvan Clark discovered it while testing one of the best optical telescopes in the world at that time.

Gamma Ray Bursts

An artist's impression of a gamma-ray burst. Gamma ray bursts are events that tap extraordinary energies (10<45 to 10<47 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they (or, at least the majority of the sample) can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high-energy radiation, so the two subsets are known as long/soft and short/hard bursts. The short/hard bursts appear to arise from coalescing binary systems (probably pairs of neutron stars or black holes), but the long/soft bursts appear to originate in the collapse of very massive stars. The latter sources are therefore almost certainly associated with star formation, so they act as signposts to active star-forming regions in the high-redshift universe. However, it should be noted that not all GRBs fall into these two categories. Gamma-ray bursts come from very distant explosions in other galaxies and are the most powerful bursts of energy that occur in the universe. A GRB is generated during the collapse of a massive star into a black hole or neutron star. The physics behind a GRB is highly complex, but the most accepted model is that as a massive star collapses to form a black hole, the in-falling material is energetically converted into a blast of high-energy radiation. It is thought the burst is highly collimated from the poles of the collapsing star. Any local matter downstream of the burst will be vaporized. Stills from a computer animation of a gamma-ray burst destroying a star. This blue Wolf-Rayet star—containing about 10 solar masses worth of helium, oxygen, and heavier elements—has depleted its nuclear fuel. This has triggered a Type Ic supernova/gamma-ray burst event. The core of the star has collapsed, without the star's outer part knowing. A black hole forms inside surrounded by a disk of accreting matter, and, within a few seconds, launched a jet of matter away from the black hole that ultimately made the gamma-ray burst. Here we see the jet (white plume) breaking through the outer shell of the star, about nine seconds after its creation. The jet of matter, in conjunction with vigorous winds of newly forged radioactive nickel-56 blowing off the disk inside, shatters the star within seconds. This shattering represents the supernova event.

Event horizon and time

As a consequence of general relativity, occupants of a spaceship near a black hole's event horizon will see time pass more slowly.

Black hole types

As one approaches a stellar black hole, the tidal force increases dramatically. Tidal forces near a supermassive black hole—known to be at the center of many galaxies—will be far less intense (and lethal) because supermassive black holes are more spread out than stellar black holes. Singularity is the point at which all the mass that created the black hole resides. Full understanding of a singularity awaits a quantum theory of gravity. Super-massive black hole, which is surrounded by an accretion disk of very hot, infalling material and, further out, a dusty torus. Also shown are high-speed jets of material ejected at the black hole's poles. (ESO) (B) A conceptual model of a stellar black hole in spacetime

Mass Radius for White Dwarfs

As the mass of the white dwarf increases (given in solar masses), the radius of the white dwarf decreases (given in Earth radii). As the white dwarf approaches the Chandrasekhar limit of 1.44 solar masses, the radius can decrease no further, and the white dwarf explodes.

QPOs and Black Hole Mass

Astronomers have discovered a relation-ship between black holes and the inner part of their surrounding disks, where gas spirals inward before making the fatal plunge. When the infall of gas reaches a moderate rate, hot gas piles up near the black hole and radiates a torrent of X-rays. The X-ray intensity varies in a pattern that repeats itself over a nearly regular interval. This signal is called a quasi-periodic oscillation, or QPO. In 1998, it was determined that a QPO's frequency depends on the black hole's mass. The congestion zone lies close in for small black holes, so the QPO clock ticks quickly. As black holes increase in mass, the congestion zone is pushed farther out, so the QPO clock ticks slower and slower. black hole and its surrounding disk, gas spiraling toward the black hole piles up just outside it, creating a traffic jam. The traffic jam is closer in for smaller black holes, so X-rays are emitted on a shorter timescale. The timing of the X-rays can be used to determine the black hole's mass.

The ultimate fate of a star

At the end of a star's life, either degeneracy pressure has come into permanent balance with gravity, or the star has become a black hole. The nature of the end state depends on the mass of the remaining core (or stellar corpse). The end state depends solely on the mass of the stellar corpse.

Singularity

Beyond the neutron star limit, no known force can resist the crush of gravity. As far as we know, gravity crushes all the matter into a single point known as a singularity. A black hole in spacetime. The singularity is the theoretical point at which gravity crushes all matter. The singularity represents infinite density and infinite curvature. Currently, our physics cannot explain what occurs inside of a black hole.

Black holes can form accretion disks

Black holes can form accretion disks similar to those accompanying neutron stars.

Observing Gamma Ray Bursts

Brief bursts of gamma-rays coming from space were first detected in the 1960s. Observations in the 1990s showed that many gamma-ray bursts were coming from very distant galaxies. They are among the most powerful explosions in the universe and could result from the formation of a black hole. The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory recorded this burst, known as GRB971212, in December 1997. This was the 2000th gamma-ray burst recorded by the now de-orbited space observatory.

Why Pulsars Must Be Neutron Stars

Circumference of neutron star = 2π (radius) ~ 60 km Spin Rate of Fast Pulsars ~ 1,000 cycles per second Surface Rotation Velocity ~ 60,000 km/s Escape velocity from neutron star ~ 20% speed of light Anything else would be torn to pieces! Pulsars spin fast because the core's spin speeds up as it collapses into neutron star—a consequence of the conservation of angular momentum.

Gamma Ray Bursts could pose a threat to life on Earth

Drawing of a massive star collapsing to form a black hole. Energy released as jets along the axis of rotation forms a gamma ray burst that lasts from a few milliseconds to minutes. Such an event within several thousand light-years of Earth could disrupt the biosphere by wiping out half of the ozone layer, creating nitrogen dioxide, and potentially cause a mass extinction.

Neutron stars

Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star. (A) As the iron core of a massive star collapses, electrons and protons combine to form neutrons and neutrinos. (B) Neutron star 9732 taken by the Hubble Space Telescope. Its surface temperature is 667,000 K (1.2 million degrees F). Its diameter is 28 km (16.8 mi). This neutron star is about 400 light-years away in the southern constellation Coronae Australis.

Accretion Disks

Friction between orbiting rings of matter in the disk transfers angular momentum outward and causes the disk to heat up and glow. The mass falling toward a white dwarf from its close binary companion has some angular momentum. The matter therefore, orbits the white dwarf in an accretion disk. In this scenario, the mass transfer occurs between a main sequence companion star and a white dwarf.

Black holes in globular clusters

In 1998, Hubble Space Telescope discovered intermediate-sized black holes in the cores of two globular star clusters, M15 and G1. Using spectral observations, astronomers discovered that the stars orbiting the cores of M15 and G1 moved at a much faster rate than expected, which suggested the presence of unseen massive bodies. The black hole in M15 (left) is 4,000 times more massive than our Sun. G1 (right), a much larger globular cluster, has a heftier black hole, about 20,000 M(sun symbol). M15 resides 32,000 l-y distant in the constellation Pegasus. G1, 2.2 million l-y distant in M31, the Andromeda galaxy, has a total mass of 10 million suns, making it one of the most massive globular clusters known.

Least massive black hole

In 2008, astronomers announced the least massive black hole ever seen, with a mass only 3.8 times that of the Sun, and a diameter of only 25 km (15 mi) across. The "tiny" black hole, known as XTE J1650-500, was discovered in 2001 in a binary system with a normal star. Astronomers had known about the binary system for several years, but they were finally able to make accurate measurements using NASA's Rossi X-ray Timing Explorer (RXTE) to pin down the mass. The smallest possible black hole is thought to be between 1.7 and 2.7 M(sun symbol). Astronomers had known about the binary system for several years, but they were finally able to make accurate measurements using NASA's Rossi X-ray Timing Explorer (RXTE) to pin down the mass. The smallest possible black hole is thought to be between 1.7 and 2.7 M(sun symbol).

2017 Noble Prizes in physics

In 2017, (A) Rainer Weiss (Photo by Michael Hauser, December 2006), (B) Kip Thorne (Photo by Keenan Pepper, August 2007), and (C) Barry C. Barish (Photo by R. Hahn, August 2005) were awarded the Nobel Prize in Physics "for decisive contributions to the LIGO detector and the observation of gravitational waves." (D) Gravitational waves generated by colliding black holes.

Close Binary Systems-1

In a close binary system, mass from the larger star can be attracted to the smaller but denser white dwarf. The infalling matter will form a whirlpool-like disk around the white dwarf called an accretion disk. Accretion provides the white dwarf with a new energy source that will cause the accretion disk to radiate UV or X-ray radiation. A white dwarf star is depicted in a close binary system with a companion star. The white dwarf can pull mass from the companion star into an accretion disk.

Mass warps spacetime, causing light rays to deflect toward the massive body

In general relativity, light travels along the curved space taking the shortest path between two points. Thus, light is deflected toward a massive object as it warps spacetime. The stronger the local gravity is, the greater the light path is bent. A massive object, like the Sun, warps the fabric of spacetime. Light is deflected from its original straight path During a total solar eclipse in 1919, astronomers were able to measure the angle of deflection of light coming from a star whose light rays passed near to the Sun. The angle measured was in agreement with that predicted by Einstein's Theory of General Relativity.

Magnetars

In ordinary neutron stars, the crust is stable. In magnetars, the crust is stressed by unbearable forces as the colossal magnetic field drifts through it, deforming the crust and sometimes cracking it. Violent seismic waves then shake the star's surface, which energize clouds of particles above the surface of the star. Atmosphere composed of super hot plasma Outer crust- a crystal lattice 200m deep- is composed of nuclei and electrons and experiences starquakes. Inner crust- a crystal lattice 1 km deep- is composed of nuclei, electrons, and a neutron drip and experiences starquakes. Magnetar diameter is 20 km (12 mi) Outer core is composed of atomic particle fluid Inner core is composed of a solid block of subatomic particles (very speculative)

Mass Transfer

In this scenario, a star that started with less mass gains mass from its larger companion. Now a red giant, the mass-losing star eventually will become a white dwarf.

Close Binary Systems-2

Instability in the accretion disk can lead to a sudden infall of matter onto the white dwarf that can result in the release of gravitational potential energy in the form of brightening (called a dwarf nova). Even more dramatic is a nova, where infalling matter can ignite suddenly in a thermonuclear flash of hydrogen shell burning. Note that novas and supernovas are quite different events. Z Camelopardalis, a binary and a recurrent dwarf nova, shows evidence of a shell of ionized gas that can only be explained as the remnant of a full-blown classical nova explosion. The image combines data gathered from the far-ultraviolet and near-ultraviolet detectors on NASA's Galaxy Evolution Explorer. Z Cam has an apparent visual magnitude that varies between 10.0 and 14.5.

White Dwarfs-2

Intermediate-mass stars progress to carbon burning (600 million K) but do not create any iron, leaving white dwarfs containing large amounts of oxygen and even heavier elements. 1 M(sun symbol) white dwarf will shrink to the size (diameter) of the Earth. More massive white dwarfs are smaller in size than less massive ones: 1.3 M(sun symbol) white dwarf is ½ the diameter of a 1.0 M(sun symbol) white dwarf. The white dwarf has a surface temperature of 35,500 K. (slide 9)

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

It increases

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

It increases

Can I safely orbit a black hole?

It is possible to be near a black hole without falling into it, provided you move rapidly. This is similar to what happens in the solar system: Earth does not fall into the Sun because we move around it at a speed of some 67,000 mph. But the orbits near a black hole can have various interesting shapes, whereas those in the solar system are always elliptical (and almost circular). If you launch your spaceship with low speed, you will spiral into the black hole If you launch your spaceship with high speed, you will fly into the far off distance If you launch your spaceship with intermediate speed, you will orbit the black hole in a complicated pattern known as rosetta orbit There is exactly one launch speed that will put you on a circular orbit around the black hole

Karl Schwarzschild

Karl Schwarzschild (1873-1916) was a noted German Jewish physicist and astronomer. During World War I while serving in Russia in 1915, he wrote three main papers, two on relativity theory and one on quantum theory. His work on relativity produced the first exact solutions to the general gravitational equations—one for non-rotating spherically symmetric bodies and one for static isotropic empty space surrounding any massive body. From the second solution, he undertook some pioneering work on classical black holes. Properties of black holes have been given his name—the Schwarzschild metric and the Schwarzschild radius. The latter is the radius of the event horizon of a non-rotating black hole. Karl Schwarzschild (1873-1916) worked out the mathematics describing a black hole.

Supernova Type: Massive Star (Type II)or White Dwarf (Type Ia)?

Light curves differ between the two types of supernova. Spectra differ between the two supernova types. For example, exploding white dwarfs do not have hydrogen absorption lines. The spectra of Type Ia supernovae are distinguished by the lack of hydrogen emission lines and the presence of a silicon absorption feature. In a normal Type Ia spectrum, absorption lines of many elements and their ionic states appear.

Gravitational redshift and black holes

Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift. (A) In a deep gravity well, light takes more time to escape leading to what is called a gravitational redshift. (B) Because of the intense gravity, spectral lines are redshifted.

Summary of Stellar Evolution

Low-mass stars (< 8 Msun symbol)evolve into white dwarfs. High-mass stars (> 8 Msun symbol) become supernovas before evolving into either neutron stars or black holes. A flow chart for stellar evolution illustrates the two paths for stars: Low-mass stars evolve into white dwarfs. High-mass stars evolve into either neutron stars or black holes. (slide 6, for flow chart)

Magnetars: Neutron Stars & Magnetic Fields

Magnetars are neutron stars with extremely powerful magnetic fields, hundreds of trillions of times more powerful than the Earth's. The decay of these powerful magnetic fields powers the emission of very energetic radiation, usually in the form of X-rays or gamma gays.

Why Magnetars Shine in X-Rays

Magnetars form a class of neutron stars with magnetic fields 1,000 times stronger than that of ordinary neutron stars. But astronomers have been unsure exactly why magnetars shine in X-rays. Data from ESA's XMM-Newton and Integral orbiting observatories are being used to test the X-ray properties of magnetars. Magnetars are different from "ordinary" neutron stars because their internal magnetic field is thought to be strong enough to twist the stellar crust. Like in a circuit fed by a gigantic battery, this twist produces currents in the form of electron clouds which flow around the star. These currents interact with the radiation coming from the stellar surface, producing the X-rays. Using the satellite data, researchers found evidence that large electron currents do actually exist, and were able to measure the electron density which is a thousand times stronger than in a "normal" pulsar.

X-Ray bursts

Matter accreting onto a neutron star can eventually become hot enough for helium fusion. The sudden onset of fusion produces a burst of X-rays.

Pulsar in a binary system

Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary. X-ray bright pulsar in a binary system with a low-mass star as a companion. When the gravitational pull of the pulsar—which is a very dense object —starts drawing matter from the companion star, the pulsar starts accreting matter via an accretion disk and emitting X-rays. This emission, supported by the accretion process, is shown as wide, white beams. As the pulsar accretes matter, it also gains angular momentum, and its rotation becomes faster. Astronomers believe that accretion in a binary system is the mechanism responsible for speeding up millisecond pulsars, which are known to spin much faster than expected for their old age.

Chandra Discovers A Massive Stellar Black Hole

Most stellar black holes contain a few solar masses. Using the orbiting Chandra X-Ray Observatory, astronomers have discovered black hole X-7 in M33, a galaxy 3 million l-y from Earth. M33 X-7 contains 15.7 solar masses and orbits a huge star with 70 solar masses. Eventually, this massive star will also become a black hole. Like X-7, a black hole in a close binary system is easier to "see" because the hot gas falling into the black hole from its accretion disk produces strong X-rays that can be detected.

Accretion Disk in a Close Binary System

Note that accretion disks can occur in close binary systems that have a white dwarf, neutron star, or black hole as one of the objects. Star in a close binary system infalling gas compact object (white dwarf, neutron star, or black hole) Accretion disk X-ray radiation from accretion disk Orbital plane of binary system

Pulsars-2

Note that all pulsars are neutron stars, but not all neutron stars are pulsars. Neutron stars in close binary systems can also form accretion disks that lead to X-ray emissions; often called X-ray binaries. Like accreting white dwarfs that can erupt into novas, accreting neutron stars can erupt also and are called X-ray bursters. Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky. The pulses were coming from a spinning neutron star—a pulsar. Jocelyn Bell and Antony Hewish accidentally discovered the extraordinary celestial objects know as pulsars in 1967 while they were searching for twinkling sources of radio radiation. The explanation for the rapid radio pulses Bell and Hewish observed proved the existence of neutron stars, incredibly dense remains of massive collapsed stars. Neutron stars were predicted in the 1930s.

No escape from a black hole

Nothing can escape from within a black hole's event horizon because nothing can go faster than light. No escape means there is no more contact with something that falls in. The matter that falls in increases the black hole's mass, changes the spin or charge, but otherwise loses its identity. A.) Space debris falls into a black hole. B.) The escape velocity, ve, is derived from energy relationships for kinetic and potential energy. The escape velocity for a black hole is (Squareroot)(2GM/r)>c<2, where c is the speed of light. Using the Schwarzchild radius, Rs=2GMlc<2, one can calculate the size of a black hole initial kinetic energy(slide 94)

Merging black holes creates powerful gravitational waves

Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the blue lines represent the orbits of the black holes, and the green arrows represent their spins. Einstein predicted gravitational waves in 1915.

OJ 287: Binary Galactic SMBH

OJ 287, a BL Lac object (a type of active galaxy with an active galactic nucleus or AGN), is 3.5 billion l-y distant in Cancer. It has produced quasi-periodic optical outbursts going back approximately 120 years (appearing first on photographic plates from 1891). Its central supermassive black hole is among the largest known, with a mass of 18 billion solar masses. The optical light curve shows that OJ 287 has a periodic variation of 11-12 years with a narrow double peak at maximum brightness. This kind of variation suggests the existence of a binary supermassive black hole where a smaller black hole with a mass of only 100 million M(sun symbol) orbits the larger one with an observed 11-12 year orbital period. The maximum brightness is obtained when the minor component moves through the accretion disk of the supermassive component at closest approach.

Supernovae and GRB

Observations show that at least some gamma-ray bursts are produced by supernova explosions. Some others may come from collisions between neutron stars. From this supernova a burst of gamma rays (GRB 0111210) was detected on November 21, 2001. The series of Hubble Space Telescope images show the fading transient lies to the right of a fuzzy, distant galaxy, likely home to the gamma-ray burster. The transient did not simply fade away though. Instead, it brightened up again days after the burst. This is evidence that at least some gamma-ray bursts are produced when a star dies.

LIGO Confirms Gravity Waves LIGO Confirms Gravity Waves

On 11 February 2016, the LIGO (Laser Interferometer Gravitational-Wave Observatory) collaboration announced the detection of gravitational waves, from a signal detected at 10:51 GMT on 14 September 2015 of two black holes with masses of 29 and 36 solar masses merging together around 1.3 billion light-years away. The mass of the new black hole obtained from merging the two was 62 solar masses. Energy equivalent to three solar masses was emitted as gravitational waves. The signal was seen by both LIGO detectors, in Livingston, LA, and Hanford, WA, with a time difference of 7 milliseconds due to the angle between the two detectors and the source. The signal came from the southern celestial hemisphere, in the rough direction of (but much further away than) the Magellanic Clouds. The confidence level of this being an observation of gravitational waves was 99.99994%. LIGO measurement of the gravitational waves at the Livingston (right) and Hanford (left) detectors, compared with the theoretical predicted values. This observation confirmed the existence of gravitational waves. The gravitational wave spectrum with sources and detectors. The LIGO detector confirmed the existence of gravitational waves in September 2015.

Black hole vertification

One can observe the complete fading of light emission near the center in the case of a black hole. Conversely, a neutron star has a solid surface, and there is a peak in the brightness at the center, where the gas collides with it. Technique used by NASA's Chandra satellite to distinguish between a black hole and a neutron star. Black hole center is dark Neutron Star the disk radiates up to the center

A Schwarzschild Black Hole

Photon sphere: 1.5 times bigger than the Schwarzschild radius, RS. Event horizon: the "border" of the black hole. Its distance from the singularity is called the Schwarzschild radius. Singularity: a point where space and time have an infinite curvature. Physically, we must say that the meaning of this curvature is less than obvious. Basic components of a black hole: Singularity Event horizon Photon Sphere

Binary black holes

Powerful gravitational waves should emanate from a pair of orbiting black holes. Gravity waves from such a pair of merged black holes was detected in late 2015. (A) Like waves rippling through water when a pebble is dropped into a pond, gravity waves from a binary black hole system emanate through spacetime. (B) Simulation showing gravitational waves produced during the final moments before the collision of two black holes. In the video, the waves could be seen to propagate outwards as the black holes spin past each other.

Gravitational waves

Prediction:Objects with mass should create ripples in the surrounding spacetime as they move, called gravitational waves. These waves do not travel through spacetime but are the oscillations of spacetime itself. The spacetime ripples move at the speed of light. However, the waves are very small and extremely hard to detect. Observation: The LIGO experiment first detected gravitational waves—the tiny stretching-shrinking of spacetime caused by the merger of two black holes—in 2015. However, the decaying orbits of a binary pulsar system discovered in 1974 by Russell Hulse and Joseph Taylor can only be explained by gravity waves carrying away energy from the pulsars as they orbit each other. This earlier observation provided a very strong gravity field test of General Relativity Theory. A binary system of compact massive objects (like binary pulsar PSR1913+16) rapidly orbiting each other produces ripples in spacetime. (Nick Strobel)

The statement "All pulsars are neutron stars, but not all neutron stars are pulsars" is true. Why are some neutron stars not observed as pulsars?

Pulsars are detected by their beams of radiation. If the pulsar's beams do not cross our line of sight, then we will not perceived the neutron star as a pulsar.

Pulsars-1

Pulsars, or rotating neutron stars, rotate very rapidly because of conservation of angular momentum—as the iron core collapses, it must rotate faster. Intense magnetic fields of the collapsed core direct beams of radiation out along the magnetic poles. As the neutron star rotates, the beams sweep around like the light on a lighthouse. All pulsars are gradually slowing down as electromagnetic radiation carries away energy and angular momentum. Pulsars are rotating neutron star that sent out radio pulses much like a lighthouse sends out visible light. The first pulsar was discovered in 1967.

White Dwarf Limit

Quantum mechanics says that electrons must move faster as they are squeezed into a very small space. As a white dwarf's mass approaches 1.44 M(sun symbol), its electrons must move at nearly the speed of light. Because nothing can move faster than light, a white dwarf cannot be more massive than 1.44 M(sun symbol), the white dwarf limit, or Chandrasekhar limit.

Neutron star limit

Quantum mechanics says that neutrons in the same place cannot be in the same state. Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds 3 M(sun symbol). (This limit is not well established.) Some massive star supernovae can make a black hole if enough mass falls onto their respective cores. A massive star core implosion produces either a neutron star or black hole.

Do Quark Stars exisit?

Recent observations of ultra-luminous supernovae suggest that these explosions may create an even more exotic remnant—quarkstars. Neutron stars can form after a star ends its life; measuring only 16 km across, these small but massive objects (1½ times the Sun's mass) may become too big for the structure of neutrons to hold it together. If the structures of the neutrons inside a neutron star collapse, quark stars (a.k.a. "strange" stars) may be the result, smaller and denser than neutron stars. Currently, their existence is only hypothetical.

Debris Rings around Pulsars

Rings of debris formed in the aftermath of stellar explosions could fuel the birth of new, rocky planets around dead stars. Using NASA's Spitzer Space Telescope, researchers detected a cool disk of material glowing in infrared light around a young X-ray pulsar. If material cast off from the explosion does not have enough velocity to escape the star's gravitational grasp, it will stall and fall back.This so-called "fallback" material can land back on the neutron star's surface or coalesce into a spinning debris disk around the star. If the fallback material lands back onto the neutron star (> 3 Msun symbol), it can cause the star to become a black hole. triple planet system discovered around pulsar PSR B1257+12 ("Lich") by Alek-sander Wolszczan in 1992.The planets are named Draugr, Poltergeist, and Phobetor. The pulsar is located 2,300 l-y (705 pc) distant in the constellation Virgo. Its mass is estimated at 1.4 M☉, its radius at 10 km, and its rotation period at 6.22 milliseconds (9,650 rpm). dusty debris disk formed around pulsar 4U 0142+61 after a supernova explosion. The disk orbits about 1.6 million km away from the pulsar and probably contains about 10 Earth-masses of material. The pulsar is 13,000 l-y (4.0 kpc) distant in Cassiopeia.

Type Ia Supernova Theories

Single Degenerate Theory for Type Ia Supernova Production: Companion Star-White dwarf in close binary system--> Companion star under going mass transfer- White dwarf with accretion disk--> White dwarf exceeds Chandrasekhar limit and explodes as a supernova type 1a. Double Degenerate Theory for Type Ia Supernova Production: Two white dwarfs in close binary system--> The white dwarfs spin inward and eventually merge--> White dwarf exceeds Chandrasekhar limit and explodes as a supernova type 1a. A Type Ia supernova can result from two different paths. Discoveries in 2012 point to the double degenerate theory as explaining many of the Type Ia's observed. However, this second method may not produce consistent energy output, which could undermine the use of Type Ia's as standard candles in determining the universe's rate of expansion.

Sirius B: The First White Dwarf

Sirius B was the first white dwarf star discovered. (A) This Hubble Space Telescope image shows Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B. Astronomers overexposed the image of Sirius A so that the dim Sirius B could be seen. The cross-shaped diffraction spikes and concentric rings around Sirius A, and the small ring around Sirius B, are artifacts produced within the telescope's imaging system. The two stars revolve around each other every 50 years. Sirius A, only 8.6 l-y from Earth, is the fifth closest star system known. (NASA, ESA, H. Bond/STScI, and M. Barstow/U. Leicester) (B) The orbit of Sirius B around Sirius A as viewed from Earth. The angular separation between the two stars is continuing to widen.

White Dwarf Supernova

Some white dwarfs can gain enough mass to approach the 1.44 M(sun symbol) white dwarf limit. The electron degeneracy pressure is not overcome by gravity. The fusion point of carbon happens to occur just before gravity would have overcome the pressure. Because the star is supported by the electron degeneracy pressure, which is unchanged by increased temperature, the white dwarf will not expand and cool when the fusion runs too quickly. Thus, carbon fusion ignites almost instantly, and this runaway reaction detonates the star. The white dwarf explodes completely in a white dwarf supernova (also called a Type Ia supernova). Nothing remains after a detonation of a white dwarf supernova. Supernova 2002bo (circled) is the explosion of a white dwarf in the galaxy NGC 3190 in the constellation Leo, which is 80 million l-y (24.5 Mpc) distant. NGC 3190 is a primary member of galaxy cluster Hickson 44.

Spacetime

Spacetime is a mathematical model that combines the three spatial dimensions with the single dimension of time into a 4-D continuum. The concept of spacetime can be traced back to the mid-18th century. Our modern view of spacetime derived as a consequence of Albert Einstein's 1905 theory of special relativity. Spacetime was first proposed mathematically by the mathematician Hermann Minkowski (one of Einstein's teachers) in 1908. At the Planck scale (10−35 m), spacetime is predicted as quantized (i.e., not continuous). Two-dimensional analogy of spacetime distortion. Matter changes the geometry of spacetime, this (curved) geometry being interpreted as gravity. White lines do not represent the curvature of space but instead represent the coordinate system imposed on the curved spacetime, which would be rectilinear in a flat spacetime.

X-Ray Bursts and Neutron Stars

Spinning neutron star (pulsar) approximately 10 km (6 mi) in diameter. When a neutron star orbits another star, its strong gravitational field can pull gas from the other star. This coats the surface of the neutron star. When the coating reaches a height of between 5-10 meters, the gas ignites in a thermonuclear explosion. This massive release of energy generally lasts from between several seconds to several minutes, and a burst of X-rays is released.

Making a neutron star

Step 1: A massive star (>8 Msun symbol) with a dense, hot core is within its final stage fusing silicon into iron. Step 2: The iron core collapses, and the resulting shockwave creates a supernova explosion that blows off the star's outer layers Step 3: The ejected outer layers of gas expand as a supernova remnant (SNR); the neutron star is left at the position of the star's original core. Steps in making a neutron star: (1) Through multiple shell nuclear fusion, a massive star creates elements up to iron. (2) Iron builds up in the core as it cannot fuse. The core collapses as gravity overcomes the nuclear structure of iron. The core bounces and sends out an enormous shockwave that blows the star apart. (3) The ejected layers of the star travel outward, leaving behind a neutron star.

Relative Sizes of Stars During Their Stages of Evolution

Sun: Radius= 695,500 km 109R Earth White Dwarf: Radius= 6,400 km 1 R Earth Red Giant: Radius= 100 R(sun symbol)= 69,550 km= 0.5 AU Mercury's orbit= 0.39 AU; Venus's orbit=0.72 AU The relative size of the Sun at three stages in its evolution. (A) The Sun at present (a main sequence hydrogen burning star). (B) The Sun as a red giant (having left the main sequence. (C) The Sun at the end of its life as a white dwarf.

Neutron Star Collisions Create Largest Magnetic Fields Known in Universe

Supercomputer simulation sequence of the first 11 milliseconds of a neutron star merger. The colors indicate the strength of the magnetic fields with blue representing the lowest (10<9 gauss) and yellow representing the highest strength (1015gauss). Earth's magnetic field at the surface ranges from 0.3 to 0.6 gauss.

Nova or Supernova?

Supernovae are about 10 million times (107) more luminous than novae! Nova: hydrogen to helium fusion occurs in a layer of accreted matter on a white dwarf in a close binary system; the white dwarf is left intact. Supernova: complete explosion of a white dwarf that exceeds the 1.44 M(sun symbol) limit (in a close binary system) with nothing left behind. Note that supernova also occur in massive stars whose initial mass is greater than 8 solar masses.

Supernova Remnant

The Crab Nebula is the remnant of the supernova seen in A.D. 1054 by the Chinese. It is 6,500 l-y (1,991 pc) distant. Tycho Brahe witnessed a supernova in 1572, and Johannes Kepler saw one in 1604. G11.2-0.3 is a circularly symmetric supernova remnant in Sagittarius that contains a pulsar at its center, representing a textbook case of what the remnant of an exploding star should look like after a couple thousand years. Combined X-ray and radio image of supernova remnant G11.2-03, the remains of a supernova observed by Chinese astronomers in A.D. 386. The white dot at the center represents X-rays emitted from the neutron star left behind.

Parts of a Black Hole

The boundary between black hole and the outside universe is called the event horizon. Within the event horizon, escape velocity exceeds the speed of light (which is why black holes are "black"). Radius of the event horizon is the Schwarzchild radius. All we can know about a black hole is mass, angular momentum, and electrical charge.

Nova Light Curve

The change in brightness of a nova can be as much as a factor of 10<6, or 15 magnitudes. (slide 28)

Double pulsar system

The double pulsar PSR J0737-3039A/B consists of a binary system made up of two pulsars in a 2.4-hour orbit. Each pulsar emits radio waves along its magnetic poles that illuminate Earth-based radio-telescopes like rotating lighthouse beacons as they spin; one every 23 milliseconds and the other every 2.8 seconds. The fortunate almost-perfect alignment of our line of sight with the orbital plane of the system gives rise to an eclipse of the 23-ms pulsar, once per orbit, as it moves behind its 2.8-s pulsar companion. The eclipse is created by the magnetosphere of the 2.8-s pulsar, a region in which a dense cloud of plasma is trapped by the magnetic field of the pulsar. These eclipses allow us to infer the orientation the 2.8-s pulsar since changes in the geometry would affect the way that light emitted by the other pulsar is transmitted to us during the eclipse. According to classical Newtonian physics, the spin axis about which a star rotates should remain fixed with respect to the background stars as it orbits another star. Einstein's general relativity predicts, however, that the spin axis should slowly precess, like the gentle wobble of a tilted spinning top.

Size of a Black Hole

The event horizon of a 3 M(sun symbol) black hole is also about as big as a small city: 18 kilometers or 11 miles.

Degenerate Matter

The extreme density of matter in a degenerate state illustrates that the atoms that make up ordinary matter are composed mostly of "empty" space. (slide 5)

What would gas in an accretion disk do if there were no friction? Which one of Newton's laws describes this situation?

The gas would orbit indefinitely. Newton's First Law

Normal Pulsar's life journey

The life journey of normal pulsars (excluding millisecond pulsars) is best visualized through a magnetic field-spin period diagram. Pulsars are born in the top left- hand side of the diagram and spin down to longer periods within a few million years. They eventually cease radiating near the "extinction line" and are classified as extinct or dead.

Most massive black hole

The record for the mass of a black hole has been broken with the discovery of a black hole containing 24 to 33 M(sun symbol). (A) The new black hole is located in the nearby irregular dwarf galaxy IC 10, 1.8 million l-y distant in the constellation Cassiopeia. (Adam Block/NOAO/ AURA/ NSF) (B) In this artist's portrayal of the IC 10 X-1 system, the black hole lies at the upper left and its companion star is on the right. The two objects orbit around a center of gravity once every 34.4 hours. The stellar companion is a type known as a Wolf-Rayet star. Such stars are highly evolved and destined to explode as supernovae. The companion star is shedding its outer envelope in a powerful wind, and some of this gas is captured by the black hole's powerful gravity.

What happens to a white dwarf when it accretes enough matter to reach the 1.44 Mlimit?

The star explodes.

Nova

The temperature of accreted matter eventually becomes hot enough for hydrogen fusion. Fusion begins suddenly and explosively, causing a nova. Classical nova eruptions are likely created in a close binary star system consisting of a white dwarf and either a main sequence, sub-giant, or red giant star. When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto its surface, which creates a dense, but thin, atmosphere. This mostly hydrogen atmosphere is thermally heated by the hot white dwarf and eventually reaches a critical temperature that causes a rapid runaway ignition by fusion. The explosion drives accreted matter out into space. The nova star system (circled in the image) temporarily appears much brighter. Process by which a white dwarf star in a close binary system can flare up as a nova. (B) Spraying starter fluid on hot charcoal can cause a flare-up. Similarly, when hydrogen gas falls onto a white dwarf from a binary companion, a nuclear fusion flare-up can occur. We see this flare-up as the brightening of a nova.

Neutron Star Merger and γ-Rays

These images show the merger of two neutron stars recently simulated using a new supercomputer model. Redder colors indicate lower densities. Green and white ribbons and lines represent magnetic fields. The orbiting neutron stars rapidly lose energy by emitting gravitational waves and merge after about three orbits, or in less than 8 milliseconds. The merger amplifies and scrambles the merged magnetic field. A black hole forms and the magnetic field becomes more organized, eventually producing structures capable of supporting the jets that power short gamma-ray bursts.

Brightest Ever γ-Ray Burst

This brightest ever γ-ray burst decayed from the brightness of a 5th magnitude star to 11th magnitude over four minutes, allowing it to be seen by the naked eye when it was at its brightest. GRBs lasting for longer than two seconds are attributed to massive stars collapsing and forming black holes. (A) On March 18/19, 2008, the "Pi of the Sky" apparatus located at Las Campanas Observatory was observing the Swift satellite field of view with 10-second exposures and observed an exceptionally bright optical flash reaching 5.8 magnitude. The GRB occurred in the constellation Boötes. (Pi of the Sky) (B) Gamma ray burst (GRB) 080319B as observed by NASA's orbiting Swift X-ray telescope. The explosion occurred some 7.5 billion light years from Earth, making it the most distant object visible to the naked eye.

Crab Pulsar

This composite picture shows a time sequence for the pulsar in the Crab nebula, as well as an image of the supernova remnant. Both were taken with the Kitt Peak 4-m Mayall telescope. The enlarged region is a mosaic of 33 time slices, ordered from top to bottom and from left to right. Each slice represents approximately one millisecond in the period of the pulsar. The brighter, primary pulse is visible in the first column: the weaker, broader inter-pulse can be seen in the second column. The observed period at the telescope was 33.36702 milliseconds (0.033 s). The Crab pulsar is both a radio and rare optical pulsar.

3-D Simulation of Gravitational Waves Produced by Merging Black Holes

This visualization shows what Einstein envisioned. Researchers crunched Einstein's theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. The honeycomb structures are the contours of the strong gravitational field near the black holes.

Degeneracy Pressure-1

Throughout most of a star's life, it is thermal pressure that holds back the inward crush of gravity. After the star dies—in a planetary nebula or a supenova —the fate of the star lies in the outcome of this final "battle" between gravity and degeneracy pressure. White dwarfs, neutron stars, brown dwarfs, and inert stellar cores are examples of degenerate objects composed of degenerate matter. brown dwarf, which is an example of an object composed of degenerate matter. (B) WISEPC J045853.90 +643451.9 is the first ultra-cool brown dwarf discovered by NASA's Wide-field Infrared Survey Explorer (WISE). Methane in the atmospheres of brown dwarfs absorbs color-coded blue light, and because the objects are too faint to give off much red light, that leaves green. The green dot of a brown dwarf stands out against the hotter blue stars in this infrared image. This brown dwarf is located 18-30 l-y away in the constellation Camelopardalis (Giraffe) and is one of the coolest brown dwarfs known at roughly 600 K.

Tidal Forces

Tidal forces near the event horizon of a 3 M(sun symbol) stellar black hole would be instantly lethal to humans. Tidal forces would be gentler near a supermassive black hole because its radius is much, much bigger. Gravity is so enormous near a black hole that over a span of just a meter or so, the difference in gravitational force (i.e., the tidal force) would be enough to tear a human being apart.

Supernova Explosions: Type Ia and Type II

Type 1a: Companion star adds mass to white dwarf (mass< 1.4 M sun symbol)--> As mass becomes > 1.4M(sun symbol), white dwarf collapses amid runaway thermonucluear fusion.--> White dwarf explodes as a Type 1a supernova and probably leaves no remenant.--> Type II: Massive star (>8M sun symbol) at the end of supergiant stage fuses silicon into iron in its core.--> Because iron can't fuse, iron core collapses violently, with core bounce producing a massive shockwave.--> Shockwave blasts star apart creating a Type II supernova and leaving either a neutron star or black hole.

Two-Stage Explosion Process in Type Ia Supernovae

Type Ia supernovae are used to measure distance in the universe because they explode with the same brightness, detonating when a white dwarf star consumes a specific amount of material from a binary companion. The accuracy of these distance measurements depends on the shape of the blast. New research indicates that Type Ia supernovae explosions start out clumpy and uneven, but a second, supersonic, spherical blast overwhelms the first creating a smooth residue. This sets the limits of uncertainty on distance measurements that use Type Ia supernovae. The radioactive decays of nickel-56 and cobalt-56 that produce a Type IA's supernova visible light curve.

Space falls into a black hole

Viewed most simply, space falls into a black hole. Outside the horizon, space is falling into the black hole at less than the speed of light, and photons can make way against the flow. At the horizon, space is falling into the black hole at the speed of light. At the horizon, a photon will just stay there, trying to move outward, but not going anywhere, the inward flow of space exactly canceling the photon's motion. Inside the horizon, space falls faster than the speed of light, carrying everything with it.

Determining a black hole

We need to measure the mass of the compact object to verify if it is a black hole: Use orbital properties of companion. Measure velocity and distance of orbiting gas. It is a black hole if it is not a star, and its mass exceeds the neutron star limit (~3 M(sun symbol). Some X-ray binaries (like the depiction of Cygnus X-1 shown here) contain compact objects of mass exceeding 3 M(sun symbol) that are likely to be black holes.

Radiation pressure and the mass of black holes

When matter is pulled toward a black hole, it is heated and produces X-rays. These X-rays create a radiation pressure which pushes out on the matter. If the matter continues to fall in, the radiation pressure of the X-rays must be less than the pull of the black hole's gravity. This effect, called the Eddington limit, enables astronomers to estimate the mass of a black hole.

Two types of Supernova

White dwarf supernova (Type Ia): Carbon fusion suddenly begins as a white dwarf in a close binary system reaches the white dwarf mass limit, causing a total explosion. Massive star supernova (Type II): Iron core of a massive star reaches the white dwarf limit and collapses into a neutron star, causing a massive explosion. Material falling on the core from above hits the core and bounces. This bounce creates a pressure wave that travels out through the envelope and blows the star apart. Image shows computer simulation of core bounce and shockwave.

Degeneracy Pressure-2

White dwarfs are supported against gravity by electron degeneracy pressure—densely packed electrons. Neutron stars are supported against gravity by neutron degeneracy pressure—densely packed neutrons. When gravity wins the battle against degeneracy pressure, we have a black hole.Fig. 12-2: HST image of the first direct observation in visible light of a neutron star Milky Way Galaxy's center where astronomers have detected a supermassive black hole

White Dwarfs

White dwarfs cool off and grow dimmer with time and are the end-stage of the life cycle of low-mass stars like the Sun. The H-R diagram can be used to track the life stages of a star. Note that white dwarfs typically are hot but have low luminosities because their radii are so small.

Degeneracy Pressure

White dwarfs exhibit electron degeneracy pressure. Neutron stars exhibit neutron degeneracy pressure. In black holes, gravity overpowers atomic structure. Gravity to: White Dwarf: Atoms packed closely together. Outer shell electrons have no room to move around and prevent further collapse. Radius=5,000-10,000 km Mass< 1.4 solar masses Neutron Star: Electrons+Protons combine to form neutrons. Closely packed neutrons have no room to move around and prevent further collapse. Radius=10 km. Mass> 1.4 solar masses but mass <3 solar masses Black Hole: Gravity wins as no atomic structure can prevent collapse. Radius=8 km. Mass>3 Solar Masses

Size of a White Dwarf

White dwarfs with the same mass as the Sun are about same size as the Earth. Higher mass white dwarfs are smaller. 1.0 M(sun symbol) White Dwarf 1.3 M (sun symbol) white dwarf Relative sizes of the Earth, a 1 solar mass white dwarf, and a 1.3 solar mass white dwarf. Note that the more massive the white dwarf, the smaller the radius.

NASA's Fermi Gamma-ray Space Telescope

discovered the first pulsar that beams only in gamma rays. The gamma-ray-only pulsar lies within a supernova remnant known as CTA 1, which is located about 4,600 l-y away in the constellation Cepheus. Its lighthouse-like beam sweeps Earth's way every 316.86 milliseconds. The pulsar, which formed about 10,000 years ago, emits 1,000 times the energy of our Sun. (NASA/S. Pineault, DRAO) (Inset) Clouds of charged particles move along the pulsar's magnetic field lines (blue) and create a lighthouse-like beam of gamma rays (purple) in this illustration.

Microlensing Reveals Isolated Black Hole

n 2000, astronomers using HST and ground-based telescopes discovered the first examples of isolated stellar-mass black holes in our galaxy. The two isolated 6 solar mass black holes were detected indirectly by the way their extreme gravity bends the light of a more distant star behind them. The black hole's gravity acts like a powerful lens, bending the light of a background star so that it appears as two separate images when the black hole slowly drifts in front of it. The bending angle is ~100 times smaller than the angular resolution of Hubble, so the two distorted images of the background star cannot be separated, even in high-resolution Hubble images. However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. These passages are called gravitational microlensing events. Isolated stellar-mass black holes have been detected indirectly by the way their extreme gravity bends the light from a more distant star.