Black Holes at Yale Exhibit Panels

Stellar Black Holes

A dying star that has a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life. Once all the fuel in a star is exhausted, the thermonuclear forces that have held the star together
disappear and the core contracts due to the strong gravity. In the most massive stars this collapse culminates in a supernova, an explosion that sends the outer part of the star streaming into space. At the same time the
gravitational force in the star’s inner regions causes the core to collapse into a single point of infinite density — a black hole.

 

Stellar-mass black holes are often paired with a “companion star” in a binary system. Such a system is made up of two objects: an ordinary Sun-like star and a dense “compact object” that is either a neutron star or a black hole. The black hole’s gravity pulls matter off the companion star Charles Bailyn and into itself, generating heat and light that are detectable in the X-ray spectrum.

 

Searching for Black Holes

Yale astronomer Charles Bailyn was among the first scientists to positively identify black holes. Although the presence of black holes was predicted in 1939 by Einstein’s general theory of relativity, it was not until 1986 that the first black hole was confirmed. Bailyn was part of the team that detected a second black hole in 1992. Today there are about two dozen confirmed stellar black holes, around half of which were discovered by Bailyn’s team. It is very likely that there are thousands more throughout the Milky Way Galaxy.

 

The presence of a black hole is confirmed when an object’s mass is too great for it to be a neutron star, or anything else. The orbit of a companion star can tell us the mass of a compact object — the more massive the compact object, the faster its companion star must move to maintain its orbit. We measure this motion by observing the star’s Doppler shift, an effect that makes the light of an approaching object look bluer and a receding object look redder. The Doppler shift of a companion star orbiting a black hole changes from red to blue and back again.

 

Once we know the mass of the black hole, we can understand the details of the X-ray emissions from the material it is pulling in. Bailyn and his collaborators at the Massachusetts Institute of Technology and the Smithsonian Astrophysical Observatory have been studying these X-rays to measure the spin of a black hole. A spinning black hole twists the spacetime in its vicinity, changing the trajectory of the matter falling into it. The X-rays can tell us about the behavior of this infalling matter, enabling astronomers to determine how fast the black hole is spinning.

 

Image Middle Far Left

Charles Bailyn

 

Image Bottom Far Left

This Chandra X-ray Observatory image is a spectrum of the black hole in the binary system XTE J1118+480. It is similar to the colorful spectrum of sunlight produced by a prism.

 

NASA/CfA/J.McClintock & M.Garcia

 

Image Bottom Center Left

An illustration of the binary system GRO J1655-40. This black hole, with a mass seven times that of the sun, is pulling matter from its companion star that is about twice as massive as the sun. The X-ray spectrum (see inset) indicates that turbulent winds of multimillion degree gas are swirling around the black hole. While much of the hot gas is spiraling inward toward the black hole, about 30% is blowing away. Powerful magnetic fields, likely carried by the gas flowing from the companion start, drive winds from the gaseous disk that carry momentum outward away from the black hole.

NASA/CXC/M.Weiss; X-ray Spectrum: NASA/CXC/U.Michigan/J.Miller et al.

 

Image Bottom Center Right

The Doppler Shift


The light from the companion star looks redder as it moves away from the viewer and bluer as it approaches the viewer. Astronomers can calculate how fast the star is orbiting the black hole by observing this shift in color.

 

Image Bottom Far Right

This illustration depicts a non-spinning (left) and spinning (right) black hole. Spinning black holes drag space with them as they spin, making it possible for particles to orbit nearer to the black hole. A possible explanation for the differences in spin among stellar black holes is that they are born spinning at different rates. Another is that the gas flowing into the black hole speeds it up.


NASA/CXC/SAO/J.Miller et al.

Supermassive Black Holes

Supermassive black holes undoubtedly play a key role in the formation and evolution of galaxies. But how is the development of a galaxy tied to the making of stars and to the growth of the black hole at its center? Urry and her team use radio, infrared, optical and X-ray wavelength observations to study how black holes and their host galaxies evolve over time.

 

Unlike the Milky Way, about a tenth of all galaxies have unusually bright centers powered by matter falling into their supermassive black holes. Known as “active galactic nuclei,” the centers of these galaxies can be seen from vast distances. Yale astrophysicist Meg Urry and her colleagues study the energetics, structure and evolution of active galaxies, especially the effect black holes have on the evolution of the galaxies around them. Black holes grow in essentially every galaxy, so having a bright, active galactic nucleus must be a common phase in galaxy evolution.

 

Co-evolution of Galaxies and their Supermassive Black Holes

Supermassive black holes undoubtedly play a key role in the formation and evolution of galaxies. But how is the development of a galaxy tied to the making of stars and to the growth of the black hole at its center? Urry and her team use radio, infrared, optical and X-ray wavelength observations to study how black holes and their host galaxies co-evolve over time.

Observational data suggest that there is a very close connection between the formation of a galaxy and the growth of a black hole. As matter from the inner galaxy nears the event horizon, it is captured by gravitational attraction. The black hole grows more and more massive, feeding on its host galaxy this way. The outflows and light released as matter falls toward the black hole should suppress the formation of new stars. Instead Urry's team, led by postdoctoral researcher Kevin Schawinski, discovered in 2009 that nearby galaxies stop forming stars long before their central supermassive black holes reach their most powerful stage. In these galaxies, at least, it appears the black holes do not shut down star formation. But subsequent work by the Urry team, led by graduate student Carolin Cardamone, showed that black hole growth in more distant galaxies probably does stop star formation. Although the "feedback" between a galaxy and a black hole is not completely understood, this research at Yale implies it is more important in the distant Universe, where star formation and black hole growth were most vigorous

 

Image Middle Far Left

Meg Urry

 

Image Bottom Far Left

Artist’s conception of the innermost region of an active galactic nucleus. A hot accretion disk feeds the central supermassive black hole and jets of highly energetic particles are launched from near the black hole’s event horizon. Clouds of dust and gas only light-days to light-months away are illuminated by the intense radiation given off by the accretion disk.


NASA/CXC/M.Weiss

 

Image Bottom Center Left

The supermassive black hole at the center of the Milky Way is known as Sagittarius A* (or Sgr A* for short). It is about 26,000 light years away from Earth.


NASA/CXC/MIT/F.K. Baganoff et al./E. Slawik.K. Baganoff et al.

 

Image Bottom Center Right

An active galaxy in turmoil. This composite image shows the galaxy Centaurus A in X-ray (blue), radio (pink and green), and optical (orange and yellow) wavelengths. A broad band of
dust and cold gas across the Centaurus A galaxy is bisected at an angle by opposing jets of high-energy particles blasting away from the supermassive black hole in the nucleus. Two large arcs of X-ray emitting hot gas, ultimately heated by this outflow, fill the outskirts of the galaxy. The active accretion onto the black hole may be due to the merger of a small spiral galaxy and Centaurus A about 100 million years ago.

 

NASA

 

Image Bottom Far Right

This Hubble Ultra Deep Field picture is a million-second-long exposure with the Hubble Space Telescope's most sensitive camera. It is the deepest portrait of the visible universe ever achieved, and it reveals the  youngest galaxies ever imaged. These formed a few hundred million years after the Universe emerged from the so-called “dark ages,” the time lasting from a few hundred thousand years after the big bang, when the Universe became very cold and inert, to a few hundred million years later, when the first stars reheated the cold, dark universe.


NASA/ESA/S. Beckwith(STScI) and The HUDF Team.

SMARTS Research Telescopes

SMARTS (Small and Moderate Aperture Research Telescope System) is a consortium of institutions, led by Yale’s Charles Bailyn that operates the small and medium telescopes at Cerro Tololo in Chile. The high desert mountains of the Chilean Andes provide the best astronomical observation sites in the Southern Hemisphere. With over 300 clear nights each year, telescopes in Chile provide unparalleled views of the southern skies.

 

The SMARTS telescopes are used to observe variable objects. Chilean astronomers record observations for dozens of different projects each night that keep track of changes in the sky. Among these is a project to monitor the orbits of stars around stellar-mass black holes and another to observe the optical light from blazar jets. Other projects include tracking asteroids and dwarf planets in our own solar system, identifying planets around other stars, and measuring the gravitational deflection of light by distant clusters of galaxies.

 

A Unique Blazar

Yale astronomers are currently using SMARTS and the space-based Fermi telescope simultaneously to monitor the highly variable light emission from blazars. In one of the first published reports using data from Fermi, Yale post-doctoral associate Erin Bonning, together with Charles Bailyn, Meg Urry and other Yale astronomers, showed that the gamma-ray and optical emission from the blazar 3C454.3 varied together. When the blazar shines brightly in gamma-rays, the optical light is also at its brightest. This confirmed the basic picture scientists had worked out about how blazar jets shine, and refuted a number of competing theories. Without the  sensitivity of the Fermi satellite and the frequent monitoring provided by SMARTS, such simultaneous observations would not have been possible.

 

Image Left Middle

The two telescopes with their domes open are the SMARTS 1.5 meter (in middle background) and 0.9 meter (at right).

 

Image Left Bottom

SMARTS is part of the Cerro Tololo Inter-American Observatory which is located in the foothills of the Andes Mountains, approximately 80 kilometers east of La Serena, Chile.

 

Image Middle

The CerroTololo Inter-American Observatory summit showing the Milky Way, Magellanic Clouds and the Southern Cross, seen to the right of the main dome. The SMARTS 1.0m telescope is in the right foreground.

 

Credit?

 

Image Middle Right

Erin Bonning

 

Image Bottom Right

This figure shows the brightness of the blazar 3C 454.3 over different regions of the electromagnetic spectrum. At top, the gamma-ray intensity (measured by the Fermi telescope) varies significantly over periods of days to weeks. The blue points were measured by the Swift satellite in the ultraviolet. The optical and infrared light was measured by the Yale team with the SMARTS telescopes. All of these wavelengths of light move up and down together indicating that they are produced from the same energetic gas in the jet. At the bottom of the figure, X-rays measured by the Swift satellite do not change much—they come from some other region or population of particles in the jet.

Fermi Gamma Ray Space Telescope

The Fermi Gamma-ray Space Telescope, launched on June 11, 2008, detects extremely energetic radiation called gamma rays. Ground-based telescopes can only see optical and radio wavelengths. Most infrared radiation
and all ultraviolet, X-ray and gamma-ray radiation are absorbed by Earth’s atmosphere. A series of NASA orbiting observatories study the Universe by observing radiation that never reaches the ground. Hundreds of times more sensitive than previous gamma-ray observatories, the Fermi telescope maps the entire sky several times a day. This means that sudden events, like the accretion of a chunk of matter by a black hole, can be identified and studied.

 

What are Gamma Rays?

Visible light makes up only a tiny fraction of the broad range of the electromagnetic radiation spectrum. This spectrum stretches from very low-energy radiowaves through infrared radiation, visible light, ultraviolet light and X-rays, and finally to very high-energy gamma rays. The processes that produce the photons (packets of electromagnetic radiation) of each type of radiation differ, as does their energy, but all forms of radiation are part of the electromagnetic spectrum family. The only real difference between a gamma-ray photon and a visible light photon is its energy. Gamma rays can have more than a billion times the energy of visible light. Gamma rays are so energetic that they are harmful to life on Earth. Luckily, Earth’s atmosphere absorbs gamma rays.

 

Blazars

Gamma rays are emitted by very hot, energetic cosmic sources, especially a special class of active galaxies called blazars. Blazars have relativistic jets — outflows of matter moving at nearly the speed of light — aimed directly toward us. Gamma rays from blazars, along with emissions at other wavelengths, reveal the speed and energy content of a jet, which is often much greater than the light of the blazar. Yale astronomers are using the SMARTS ground-based telescopes and the Fermi telescope simultaneously to monitor the highly variable light emission from blazars. In one of the first published reports using data from Fermi, Yale postdoctoral associate Erin Bonning, together with Charles Bailyn, Meg Urry and other Yale astronomers, showed that the gamma-ray and optical emissions from blazar 3C454.3 varied together: when the blazar shines brightly with gamma rays, the optical light is also at its brightest. This confirmed the basic picture scientists had worked out about how blazar jets shine and refuted several competing theories. Without the sensitivity of
the Fermi space telescope and the frequent monitoring provided by the SMARTS telescopes, such simultaneous observations would not have been possible.

 

Image Middle Left

The Fermi Gamma-ray Space Telescope.

 

Image Bottom Left

The Fermi Gamma-ray Space Telescope was named in honor of Enrico Fermi, a Nobel Prize-winning physicist and one of the leading scientists of the 20th century.

 

Image Middle Right

An artist's conception of the blazar PKS 2155-304, which is located 1.5 billion light-years away in the southern constellation of Piscis Austrinus.

 

NASA/Goddard Space Flight Center Conceptual Image Lab

 

Image Middle Far Right

Blazar 3C 454.3’s Record Flare

Unprecedented flares from the blazar 3C 454.3 (in the constellation Pegasus) make it the brightest persistent gamma-ray source in the sky. That title usually goes to the Vela pulsar in our galaxy, which is millions of times closer. These all-sky images, which show the numbers of high-energy gamma-rays captured by Fermi on November 3 and December 2, 2009, clearly show the change.

 

NASA/DOE/Fermi LAT Collaboration

 

Image Bottom Right

The blazar 3C 454.3 as seen by the SMARTS 1.3 meter telescope.

Alberto Miranda