An Introduction to Astronomy

Part XIII: Black Holes, Quasars and Active Galaxies

Lambert E. Murray, Ph.D.

Professor of Physics

Einstein’s Theory of Special Relativity The Special Theory Deals with Reference Frames

that move at constant velocity. Einstein was able to show that:

– In order to preserve the notion of “cause and effect” nothing can travel faster than the speed of light.

– Because of this fact: Events that are simultaneous in one reference frame are not

simultaneous in another. Moving objects contract in the direction of motion [Lawence

Contraction]. Moving clocks run slower [The Twin Paradox]. Objects that move at the speed of light can get anywhere in the

universe in no time at all.

Einstein’s Theory of General Relativity

The General theory deals with accelerating reference frames – and gravity accelerates objects.

Einstein was able to show that mass “warps” space to create the effect of gravity.

This warping of space causes:– Light to bend (or curve) in the presence of massive

objects.

– Time to slow near move massive objects.

– A gravitational redshift.

An Example of Curved Space

• The flat surface shown in (a) represents two dimensional space in spacetime. You can think of it as a large flat sheet of rubber. In the absence of any matter, straight lines are straight in our intuitive sense (the sheet is flat).

• In the presence of matter, spacetime curves, as shown in (b) by the curvature of the sheet when mass is laid on it (just like the rubber would stretch). Straight lines, defined by the paths that light rays take, are no longer straight in the “usual” sense, but follow the curvature of the warped spacetime, just as a small marble would follow the curvature of the rubber sheet.

• This curvature of space also creates gravity. If you place a large mass on the rubber sheet and then release a much smaller marble near that mass, the stretched rubber sheet will cause the marble to “roll down hill” toward the larger mass.

Curved Spacetime and the Path of Light

• The bending of light by matter was confirmed in 1919 during a total solar eclipse. Photographs of stars in the region of the sky near the sun during this eclipse showed the exact displacement that Einstein’s theory predicted. This is illustrated in the diagram above.

Time Slows Down Near Matter

• If two clocks are synchronized in space (a), and then brought near the Earth and the Moon (b), the clock nearest to the Earth will slow down more that the one nearer the moon.

• This occurs because mass slows down the flow of time, and Earth has more mass (and a higher density, which adds to the effect) than the Moon.

Gravitational Redshift• The color of light from the

same object located at different distances from a mass appears different as seen from far away. The photons that leave the vicinity of the massive object lose energy causing a redshift. The closer the light source is to the mass, the redder the light appears, and hence the name gravitational redshift. The same argument applies to light leaving the surfaces of different stars.

Trapping of Light by a Black Hole• (a) The paths and color of light rays

departing from a main-sequence, giant, or supergiant star are affected very little by the star’s gravitational force.

• (b) Light leaving the vicinity of a white dwarf curves and redshifts slightly, whereas

• (c) near a neutron star, some of the photons actually return to the star’s surface.

• (d) Inside a black hole, all light remains trapped. Most photons curve back in. Those that move straight upward become infinitely redshifted, thereby disappearing.

Characteristics of Black Holes Black Holes retain only three properties that it

possessed before forming:– Its Mass– Its Angular Momentum– Its Electrical Charge

Black Holes lose their internal magnetic field upon collapse.

Most Black Holes are believed to maintain a neutral charge.

Thus, there are only two types of Black Holes to consider:– A Schwarzschild (non-rotating) Black Hole– A Kerr (rotating) Black Hole

Structure of a Schwarzschild (Nonrotating) Black Hole

• A nonrotating black hole has only two notable features: • its singularity, and• its boundary.

• Its mass, called a singularity because it is so dense, collects at its center.

• The spherical boundary between the black hole and the outside universe is called the event horizon.

• The distance from the center to the event horizon is the Schwarzschild radius.

Structure of a Kerr (Rotating) Black Hole

• The singularity of a Kerr black hole is located in an infinitely thin ring around the center of the hole. It appears as an arc in this cutaway drawing.

• The event horizon is again a spherical surface.

• There is a doughnut-shaped region, called ergoregion, just outside the event horizon, in which nothing can remain at rest. Space in the ergoregion is being curved or pulled around by the rotating black hole.

A Model of a Rotating Black

Hole Material in a region

surrounding a rotating black hole would form a swirling “accretion disk” just outside the black hole.

What is the Evidence for Black Holes?

Since solitary black holes are not directly visible, we can infer their existence only when they interact with other matter.– This may occur in binary star systems.– It may also occur when large number of stars

near the center of a galaxy fall into a supermassive black hole.

– It may be evident by “gravitational lensing.”

X Rays Generated by Accretion of Matter Near a Black Hole

• Stellar-remnant black holes, such as Cygnus X-1, LMC X-3, V404 Cygni, and probably A0620-00, are detected in close binary star systems.

• This drawing (of the Cygnus X-1 system) shows how gas from the 30 M companion star, HDE 226868, transfers to the black hole, which has at least 11 Solar Masses.

• This process creates an accretion disk. As the gas spirals inward, friction and compression heat it so much that the gas emits X rays, which astronomers can detect.

Jets Created by a Black Hole in a Binary System

• Some of the matter spiraling inward in the accretion disk around a black hole is superheated and redirected outward to produce two powerful jets of particles that travel at close to the speed of light.

Supermassive Black Holes

• The bright region in the center of galaxy M87 has stars and gas held in tight orbits by a black hole.

• Doppler shift measurements allow us to calculate the central mass of the galaxy.

• M87’s bright nucleus (inset) is only about the size of the solar system and pulls on the nearby stars with so much force that astronomers calculate its mass to be a 3-billion-solar-mass black hole.

Photograph of an Accretion Disk Around a Supermassive Black Hole

Swirling around a 300-million-M black hole in the center of the galaxy NGC 7052, this disk of gas and dust is 3700 ly across. The black hole appears bright because of light emitted by the hot, accreting gas outside its event horizon. NGC 7052 is 191 million ly from Earth in the constellation Vulpecula.

Image of OJ287 – a Supermassive Black Hole Accretion Disk

Einstein rings - Light from distant galaxies is bent into rings around intervening matter

(galaxies or black holes).

The Discovery of Quasars

In 1944, Grote Reber detected 3 strong radio emission sources with a home-made radio telescope in his back yard.

Though two of these were identified as sources within our galaxy (the galactic nucleus and a supernova remnant), the third, Cygnus A, was not clearly identified until 1951.

In 1951 a visible source located at Cygnus A was observed which looked like an odd-shaped elliptical galaxy.

When examined with a spectrometer, the light from this galaxy exhibited unexpected emission lines with a relatively large red-shift.

Cygnus A (3C 405) Today

Today we know that the red-shifted visible spectra comes from the central galaxy and the radio signals emanate from two radio lobes located on either side of the galaxy.

Cygnus A (3C 405) Image

What Sized Red Shift?

A red-shift as large as that measured for Cygnus A would imply (based upon Hubble’s Law) that this galaxy was nearly 220 Mpc from us – farther from us that any previously observed galaxy.

For this radio source to produce a radio signal large enough to be detected by a back-yard radio telescope, and to be that far away it must be emitting a HUGE amount of energy – hundreds of times the output of the Milky Way.

Other Strange Stars Around 1960 several “stars” were observed which

were also strong radio sources – unlike most stars which are poor radio sources. These were 3C 48 and 3C 273.

These stars exhibited strange spectral emission lines that no one could identify.

Initially, scientists believed that these “stars” were in our galaxy. Finally, in 1963, Maarten Schmidt realized that the emission lines were lines that had been red-shifted by large amounts, placing these “stars” at tremendous distances away from our galaxy. Only then did we realize that these stars were in fact the same type animal as Cynus A.

Note: 3C 405 refers to the object number 405 in the Third Cambridge Catalogue of radio sources.

In 1963 Maarten Schmidt at CalTech identified the strange emission lines of 3C 273 as significantly red

shifted lines of ordinary Hydrogen.

3C273

This Quasi-stellar object also exhibited a luminous jet

This object that looks like a star must be

enormously luminous - its redshift indicates it is 4 billion light years

away!!

Star-like Object 3C 48

Quasars Because these strange “stars” had looked so much

like a normal star, while also emitting radio waves, they were originally dubbed “quasi-stellar radio sources”, or “quasars”.

Since these quasars are so distant and still have a large apparent magnitude, we know that no single star could emit that much energy – these objects must be very bright galaxies.

We have now identified a number of very distant very “bright” objects exhibiting large red-shifts – some of which are not radio sources. We associate the name quasar, however, with both types of galaxies.

An Indication of the Size of Quasars

In the mid 1960’s it was discovered that some quasars change in intensity over periods of months, weeks, or even days.

The variation in brightness of an object gives an indication of the size of an object, since an object cannot change in brightness faster than the speed of light can travel across that object. Thus, an object 1 ly in diameter cannot vary in brightness with a period of less than a year.

This means that some quasars must be relatively small in size.

A Quasar Emits a Huge Amount of Energy from a Small Volume

These changes in brightness could indicate that the quasar cannot be larger than a few light years.

The Radio Galaxy M87

Visible Elliptical Galaxy

Radio ImageGalaxy is small bright dot in center.

Hubble Image of Core of Galaxy

The Nature of These Active Galaxies

Many of these active galaxies have double radio lobes associated with them.

These double lobes appear to arise from jets of electrons emitted from the galaxy at relativistic speeds producing synchrotron radiation in the radio part of the spectrum.

The electrons are emitted along the magnetic field lines coming from the poles of the galaxy.

Computer Enhanced Radio Image of Cygnus A

Radio Emitting Lobes are Clearly Visible

(radio lobes are 320,000 lyrs wide)

Centaurus A - radio image superimposed on a visible image...note no light from radio lobes

visible image

Radio lobes

Active galaxies lie at the center of double radio sources

Galaxy (which is actually

quite large)

Intergalactic gas jet

Giant Gas Clouds

(surrounding the galaxy)

Supermassive Black Holes:A Common Solution

Astronomers believe that supermassive black holes at the center of these strange and peculiar galaxies may explain what we see:– A relatively small, but massive energy source– An energy source that radiates energy across

the spectrum– An energy source that emits large quantities gas

in high-speed jets.

The location of the observer makes the difference in what is seen ...

Model of the center of an

Active Galaxy