Galaxies - Introduction

A galaxy is an organized system of hundreds of millions to thousands of billions of stars, sometimes mixed with interstellar gas and dust.

Our sun and solar system are part of the Milky Way galaxy.

Galaxies can be seen in every direction in space, each with billions of stars. Galaxies often appear to be distinct but fuzzy patches of light.

Charles Messier (1730-1817) cataloged more than 100 fuzzy celestial objects, sometimes called Messier objects, and named M1 to M110.

Dreyer compiled the New General Catalog of nearly 8000 objects around 1900. Most of these fuzzy objects are planetary nebulae and star clusters that are part of our galaxy, but extragalactic objects (or galaxies) were also included.

Nearest neighboring large galaxy = Andromeda Galaxy (M31). The relatively "nearby" Andromeda Galaxy (M31) is about 2.2 million light years away.

The Local Group is a group of our nearest galaxy neighbors, held together by their mutual gravitational attraction. About 20 galaxies are in this area.

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Classification of Galaxies

In the 1920's, Hubble devised a classification of galaxies:

1. Spiral galaxies (30%)
2. Elliptical galaxies (most common - 60%)
3. Lenticular galaxies (transitional orms between sprial and elliptical galaxies)
4. Irregular galaxies (10%)

Spiral galaxies are flat disks with a nuclear bulge, a halo of old stars, and spiral arms with young stars. Some have a bar-shaped concentration of stars in the center (barred spirals). Arms emerge fromt he ends of the bar. Dust is readily visible as dark streaks. The Milky Way galaxy is a spiral galaxy.

Globular clusters encircle spiral galaxies. Elliptical galaxies are spheroidal in shape (elliptical in two dimensions). Old stars are dominant. There is no prominent internal structure. They are circled by a halo of globular clusters. Little or no gas and dust are present. Almost all has been converted into stars.

Irregular galaxies are ot disk-like or spheroidal and have no nucleus. They have a chaotic, irregular appearance. Some have bars, but no arms. Sites of active star formation with young stars and luminous gas clouds. Some very old stars are present in globular clusters.

Examples of irregular galaxies are the Small Magellanic Cloud and the Large Magellanic Cloud.

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universe

Solar Activity Heats Up

August 28 saw a major solar flare erupting from a complex sunspot group crossing the Sun's southern hemisphere. The intensity of the eruption placed it in the most powerful "X" category. The flare was accompanied by a coronal mass ejection (CME) which was clocked at 600 km/hr as it headed past the Earth. This CME passed south of Earth's orbital plane and did not cause any major effects here. However, CMEs that collide directly with the Earth can excite geomagnetic storms, which have been linked to satellite communication failures. In extreme cases, such storms can induce electric currents in the earth and oceans that can damage electric power transmission equipment. Scientists expect to see solar flare-ups daily during the solar max, which is expected in mid-2000.

Leaking Earth Could run dry

Researchers from the Tokyo Institute of Technology say that Earth could be dry and barren within a billion years because the oceans are draining into the planet's interior. They have calculated that about 1.12 billion tonnes of water leaks into the Earth each year. Although a lot of water also moves in the other direction, not enough comes to the surface to balance what is lost. The scientists believe that eventually all of it will disappear. They predict that the Earth's surface will look a lot like the surface of the planet Mars where a similar process seems to have taken place. This research was published in the New Scientist magazine. Mir drifts free

Russian space station Mir has gone into "free drift" mode as control passes to a new onboard computer. The free flight is necessary to reduce power consumption so that Mir can survive for months in space before a possible return by cosmonauts. The last crew left on August 27, just 10 days before Mir would have celebrated 10 years of continuous crewing. The new computer keeps the solar panels pointed at the Sun so Mir's batteries remain charged. Russia is expected to make a decision late this year or early next on whether Mir will remain in orbit. The station costs about $250 million a year to operate and while this is not too high in space terms, Russia just doesn't have the money.

Shaking Earth

The centuries-old mystery of why the Earth appears to wobble has been solved. Every 1.2 years, the planet appears to move about its axis by about 20 ft at the North Pole, but since discovering the so-called Chandler Wobble in 1891, scientists had been unable to explain it. Now NASA believes that the cause lies in the Earth's oceans. Fluctuating pressure of water on the ocean bed caused by temperature, salinity and current changes - forces the Earth to move slightly on its axis. Atmospheric fluctuations add to the wobble. The findings were made by analyzing data from the International Earth Rotation Service, set up in Paris in 1988.

Moon Magic

The Moon has always been an object of fantasy as well as research but we still don't know exactly how the Earth got its moon.

According to the 'giant impact theory', proposed in the 1970s, the moon was formed after the Earth was hit by a huge object, as big as Mars.

Using the new model, researchers at the Southwest Research Institute and the University of California at Santa Cruz, created high-resolution simulations to show that an oblique impact by an object with 10 per cent of the mass of Earth could have ejected sufficient iron free material into Earth's orbit to eventually coalesce into the moon, while also leaving the Earth with its present mass and correct initial rotation rate.

The simulation also implies that the moon formed near the very end of Earth's formation, some 4.5 billion years ago. The moon is believed to have played an important role in making the Earth habitable because of the stabilizing effect it had on the tilt of Earth's rotation.

New Solar System Is Like Ours

After 15 years of searching, astronomers say they have found an alien planetary system that reminds them a lot of home. This is the first time planet hunters have detected what they believe is a Jupiter-like gas ball orbiting a star much like our Sun, at a distance that allows for the possibility of an unseen Earth-type planet orbiting in between.

In the last decade and a half, scientists have found more than 90 so-called extra-solar planets around stars outside our solar system. But none of these earlier discoveries has held the same potential to answer an essential question: Might there be other Earths in the universe?

"We have a (planetary) system that is maybe not a sibling of the solar system… it might be more accurately classified as a first cousin," Paul Butler of the Carnegie Institution said on Thursday.

Butler and fellow planet-hunter Geoffrey Marcy of the University of California-Berkeley noted that the newly discovered Jupiter-type planet is the third thought to orbit 55 Cancri, a star in the constellation Cancer that can be seen without telescopes or even binoculars. It is about as old – five billion years or so – and about the same size as our Sun. Aside from it known planets, the new planetary system has a tantalizing gap between the new Jovian discovery and two other big gas planets orbiting very close to the star, Marcy said.

There’s a huge region centered at about Earth-Sun distance, and in that gap… an Earth-mass planet could exist … and such a planet would be stable, " he said. "It could persist there for billions of years, so it’s conceivable that this system has rocky planets like Mars, Venus or Earth and we simply can’t detect them," Marcy said.

Heavy Traffic Heads for Mars

American space agency NASA has outlined ambitious, long-term plans to explore the planet Mars. It says six major missions will take place in little more than 10 years, with Italy and France also participating. At an annual cost estimated at $400 million to $450 million a year for the next five years, the agency will dispatch a combination of orbiting spacecraft and landers to the Red Planet. Then, after 2010, the agency will undertake a mission to bring back samples from Mars. In 1999, NASA lost two Martian missions : the Mars Polar Lander and the Mars Climate Orbiter. The failures were a huge blow and prompted a major review of the way NASA carries through its space operations. The campaign to explore Mars is unparalleled in the history of space exploration. It’s meant to be a robust, flexible, long-term programme that will give the highest chances for success. The new strategy is aimed to answer questions about Mars’ mineralogy, geology and climate history. The idea is to ‘follow the water’ so we may know the answers to far-reaching questions about the red planet humans have asked over the generations: Did life ever arise there, and does life exist there now ?"

Astronomers spot winking baby star

A Sun-like star just out of infancy has winked at astronomers, indicating its eclipse by cosmic dust and rocks, the stuff of which planets like Earth could possibly form, scientists reported on Wednesday.

The star, located in the Unicorn constellation about 2,400 light years from Earth, disappeared from view for regular periods of about 48 days over the past six years. Its disappearance suggested an eclipse, but not a typical one caused by an intervening planet, star or moon.

Only a collection of smaller objects, like dust and rocks, could cause the long eclipse the astronomers saw. Known as KH 15D, the star is only about 3 million years old, a prime age for monitoring by astronomers interested in our solar system's planet-forming past.

"We've monitored thousands of these stars over the years and this is the only that behaves this way," said astronomer William Herbst of Wesleyan University in Connecticut. "Essentially the star winks at us."

The dust that caused the wink is different from the fine interstellar dust that is distributed throughout the cosmos. Herbst said. Its particles are bigger, indicating that it is clumping into what astronomers call a protoplanetary disk - the disk from which planets can form.

"Is there a mass in here that is somehow sculpting the obscuring clouds so that it's producing these rings of material which then circle around the star and alternately block the object? We think that's very possible," Herbst said. There could be two blobs circling the star, or just one, but there is no confirmation as yet of exactly what could be causing this kind of disk to form, said Herbst's colleague Catrina Hamilton.

At just 3 million years old, KH 15D is a cosmic toddler barely out of infancy. By contrast, our solar system is thought to be about 4.5 billion years old. However, some astronomers believe the planets may have begun forming when the Sun was a few million years old.

The disk is forming quite close to the star, closer than the planet Mercury is to the Sun. "The star is ... like the Sun was when it was 3 million years old, so the processes that are going on in this inner disk region, where terrestrial planets would be forming - could be analogous to what was going on with the formation of Earth," Herbst said.

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Star crust 10 billion times stronger than steel

BLOOMINGTON, Ind. -- Research by a theoretical physicist at Indiana University shows that the crusts of neutron stars are 10 billion times stronger than steel or any other of the earth's strongest metal alloys.

Charles Horowitz, a professor in the IU College of Arts and Sciences' Department of Physics, came to the conclusion after large-scale molecular dynamics computer simulations were conducted at Indiana University and Los Alamos National Laboratory in New Mexico. The research will appear Friday (May 8) in Physical Review Letters.

Exhibiting extreme gravity while rotating as fast as 700 times per second, neutron stars are massive stars that collapsed once their cores ceased nuclear fusion and energy production. The only things more dense are black holes, as a teaspoonful of neutron star matter would weigh about 100 million tons.

Scientists want to understand the structure of neutron stars, in part, because surface irregularities, or mountains, in the crust could radiate gravitational waves and in turn may create ripples in space-time. Understanding how high a mountain might become before collapsing from the neutron star's gravity, or estimating the crust's breaking strain, also has implications for better understanding star quakes or magnetar giant flares.

"We modeled a small region of the neutron star crust by following the individual motions of up to 12 million particles," Horowitz said of the work conducted through IU's Nuclear Theory Center in the Office of the Vice Provost for Research. "We then calculated how the crust deforms and eventually breaks under the extreme weight of a neutron star mountain."

Performed on a large computer cluster at Los Alamos National Laboratory and built upon smaller versions created on special-purpose molecular dynamics computer hardware at IU, the simulations identified a neutron star crust that far exceeded the strength of any material known on earth.

The crust could be so strong as to be able to elicit gravitational waves that could not only limit the spin periods of some stars, but that could also be detected by high-resolution telescopes called interferometers, the modeling found. An online version of the research paper, "The breaking strain of neutron star crust and gravitational waves," can be found at http://arxiv.org/PS_cache/arxiv/pdf/0904/0904.1986v1.pdf.

"The maximum possible size of these mountains depends on the breaking strain of the neutron star crust," Horowitz said. "The large breaking strain that we find should support mountains on rapidly rotating neutron stars large enough to efficiently radiate gravitational waves."

Because of the intense pressure found on neutron stars, structural flaws and impurities that weaken things like rocks and steel are less likely to strain the crystals that form during the nucleosynthesis that occurs to form neutron star crust. Squeezed together by gravitational force, the crust can withstand a breaking strain 10 billion times the pressure it would take to snap steel.

Earlier this year, Horowitz was elected a fellow of the American Physical Society, the preeminent organization of physicists in the United States, for his contribution to research in dense nuclear matter. His most recent work on neutron stars was supported by a grant from the U.S. Department of Energy and through Shared University Research Grants from IBM to IU. Working with Horowitz were Don Berry, a principal systems analyst with the High Performance Applications Group in University Information Technology Services at Indiana University, and Kai Kadau at Los Alamos National Laboratory.

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Hubble repair mission

A $70 million instrument designed by the University of Colorado at Boulder to probe the evolution of galaxies, stars and intergalactic matter from its perch on the orbiting Hubble Space Telescope is on schedule for its slated May 11 launch from Kennedy Space Center in Florida aboard NASA's space shuttle Atlantis.

Originally scheduled for launch in 2004, NASA's Hubble Servicing mission has been beset by delays over the years by causes ranging from the Columbia space shuttle accident to mechanical glitches. But CU-Boulder Professor James Green of the Center for Astrophysics and Space Astronomy, principal investigator for $70 million Cosmic Origin Spectrograph, or COS, said from the Kennedy Space Center today things look very good for the launch of Atlantis next Monday at 2:01 p.m. EDT.

""There have been no hiccups this time around and everything is going very smoothly," said Green. We are right on schedule and the team is optimistic about the launch."

The telephone-booth-sized COS, built primarily by CU-Boulder's industrial partner, Ball Aerospace & Technology Corp. of Boulder, should help scientists better understand the "cosmic web" of material believed to permeate the universe, said Green. COS will gather information from ultraviolet light emanating from distant objects, allowing scientists to look back several billion years and reconstruct the physical conditions and evolution of the early universe.

Distant quasars will be used as "flashlights" to track light as it passes through the cosmic web of long, narrow filaments of galaxies and intergalactic gas separated by enormous voids, said Green. Astrophysicists have theorized that a single cosmic web filament may stretch for hundreds of millions of light-years, an astonishing length considering a single light-year is about 5.9 trillion miles.

Light absorbed by material in the web should reveal "fingerprints" of matter like hydrogen, helium and heavier elements, allowing scientists to build up a picture of how the gases are distributed and how matter has changed over time as the universe has aged, Green said.

The spectrograph will break light into its individual components much like a prism, revealing the temperature, density, velocity, distance and chemical composition of galaxies, stars and gas clouds, said Professor Michael Shull of CASA, a co-investigator on COS. The team has chosen hundreds of astronomical targets in all directions of space, which will allow them to build a picture of the way matter is organized in the universe on a grand scale, Shull said.

Shull said one of the earliest COS targets will be a quasar previously looked at by Hubble that is believed to have formed about 5 billion years ago – more than one-third of the way back in time and space to the Big Bang. "This instrument is ten times more sensitive than any previous Hubble ultraviolet instruments, so we are looking forward to studying intergalactic space at this distant epoch in detail."

While matter is thought to have been distributed uniformly throughout space just after the Big Bang, gravity has shaped it into its present filamentary structure known as the cosmic web, said Shull. "Pointing our instrument at hundreds of targets over time will allow us to take a CAT scan of the universe."

COS also will be used to detect young hot stars shrouded in the thick dust clouds they formed in, providing new information on star birth, said CASA Senior Research Associate Cynthia Froning, COS project scientist. Scientists also will point COS at gas surrounding the outer planets of the solar system to glean new clues about planetary evolution.

Green and his COS science team, which is made up of 14 CU-Boulder scientists and engineers and 10 scientists from other institutions, have been allotted 552 orbits of observation time on Hubble. CU-Boulder's CASA is in the process of hiring several dozen postdoctoral researchers, graduate students and undergraduates to work on the project in the coming years, Green said.

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The cosmos is green

For the first time, researchers have observed a singular cosmic act of rebirth: the transformation of an ordinary, slow-rotating pulsar into a superfast millisecond pulsar with an almost infinitely extended lifespan.

The discovery was made during a large radio sky survey by an international team of astrophysicists at McGill University, the University of British Columbia (UBC), West Virginia University, the U.S. National Radio Astronomy Observatory (NRAO) and several other institutions in the United States, the Netherlands and Australia.

The sky survey used the Robert C. Byrd radio telescope at Green Bank, West Virginia to observe nearly a third of the celestial sphere. The team's results will be published online by the journal Science on May 21.

The discovery was made by astrophysics PhD candidate Anne Archibald and her supervisor, Prof. Victoria Kaspi of the McGill Pulsar Group. "This survey has found many new pulsars, but this one is truly special -- it is a very freshly 'recycled' pulsar that is emerging straight from the recycling plant." said Archibald. The McGill researchers worked with Asst. Prof. Ingrid Stairs of UBC and Scott Ransom of NRAO as well as others from the collaboration to carry out more observations of this unusual pulsar.

Pulsars are rapidly rotating, highly magnetized neutron stars, the remnants left after massive stars have exploded as supernovae. Pulsars emit lighthouse-like beams of radio waves that sweep around as the star rotates. Most rotate relatively slowly, ten times a second or less, and their magnetic fields ordinarily slow them down even further over the course of millennia. Millisecond pulsars, however, rotate hundreds of times a second.

"We know normal pulsars typically pulsate in the radio spectrum for one million to ten million years, but eventually they slow down enough to die out," explained Kaspi. "But a few of these old pulsars get 'recycled' into millisecond pulsars. They end up spinning extremely fast, and then they can pulsate forever. How does nature manage to be so green?"

It has long been theorized that millisecond pulsars are created in double-star systems when matter from the companion star falls into the pulsar's gravity well and increases the rotation speed, but until now the process has never been observed directly.

"Imagine a ping-pong ball in the bathtub, and then you take the plug out of the drain," explained Archibald. "All the water swirling around the ping-pong ball suddenly makes it spin a lot faster than when it was just bobbing on the surface.

"We've seen systems that are undergoing spin-up, because when the matter is falling in, the stars get really bright in X-rays and they're easy to see," she added. "But we've never seen radio pulsations from these stars during the process of spin-up. At last we've found a true radio pulsar that shows direct evidence for having just been recycled."

The pulsar found by the survey team was fortuitously observed by an independent, optical research group to have had swirling matter surrounding it roughly a decade ago -- the blink of an eye in astronomical time. That group recorded the observation as puzzling, never dreaming that a full-fledged radio pulsar would emerge.

"In other words, for the first time, we have caught a glimpse at an actual cosmic recycling factory in action," said Ingrid Stairs of UBC, who has been visiting the Australia Telescope National Facility and Swinburne University of Technology this year. "This system gives us an unparalleled cosmic laboratory for studying how millisecond pulsars evolve and get reborn."

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Are black holes at the center of galaxies?

Bubbles of dark matter could be masquerading as supermassive black holes at the centres of galaxies. If so, they could explain the puzzling pattern of X-ray emissions from the heart of the Milky Way.

Cosmologists know that most galaxies host a compact, supermassive object at their centre and they believe these must be black holes. Such a black hole is thought to be responsible for the X-ray flares coming from the middle of our galaxy, which would be caused by the black hole devouring surrounding matter. But recent observations show that these flares fire roughly every 20 minutes – a regularity that is hard to explain in terms of the behaviour of a black hole.

Now Anatoly Svidzinsky, a physicist at Texas A&M University in College Station, Texas, thinks that hypothetical particles called axions could solve the mystery. Axions have very little mass and no electric charge, and they barely interact with other particles. They were originally proposed to fix a problem with the strong force in particle physics, but have more recently been considered as possible candidates for dark matter, the unseen stuff thought to make up nearly 90 per cent of a galaxy's mass.

In the 1990s, computer simulations of clouds of dark matter made of axions showed that giant bubbles of these particles would burst out from the clouds. Svidzinsky thinks that such bubbles exist at the centre of galaxies. His model shows that the axion bubbles would expand and contract with a period of 20 minutes – matching the period of infrared and X-ray flares from Sagittarius A*, the location of the supermassive compact object at the centre of our galaxy. The model predicts that stable axion bubbles would weigh between about 1 million and 2.5 billion times the mass of the sun – exactly the mass range observed for compact objects at the centres of galaxies (www.arxiv.org/ astro-ph/0607179).

"The proposal looks quite intriguing," says Tim Sumner, who is leading the search for galactic dark matter, including axions, at Imperial College London in the UK. "But it obviously needs a lot more evidence and assessment before it can really displace the more established scenarios."

One big assumption in Svidzinsky's model is that gravity starts to repel as the gravitational field gets stronger – a tweak to general relativity proposed by physicist Huseyin Yilmaz in the 1990s. And this is what causes the bubbles to oscillate. As the bubble grows, its surface tension pulls it back. As it collapses, its gravity eventually becomes repulsive and the bubble expands again.

The fact that Svidzinsky's model relies on this controversial version of gravity doesn't necessarily count against it, says Konstantin Zioutas of the particle physics laboratory CERN in Geneva, Switzerland. "There are various studies in progress around the world which suggest that Einstein did not speak the last word on gravity," says Zioutas. For example, extra dimensions can change the way that gravity behaves in extreme cases.

The other important question is whether axions really exist. There have been attempts to create them in the lab (New Scientist, 14 July, p 35) and even a possible indirect sighting in the sun's halo by Zioutas (New Scientist, 17 April 2004, p 8). "Until their existence is confirmed, axions will appear to be a deus ex machina," says Zioutas.

Still, Zioutas adds, if Svidzinsky is correct, his idea could solve another mystery perplexing astronomers. As well as X-ray flares, astronomers can see diffuse X-rays emanating from our galactic centre. "We don't know what can be causing these," says Zioutas. "Any gas that would be hot enough to emit this radiation would be moving too fast to be held in our galaxy." Dark matter axions, however, could be releasing these X-rays as they decay, he says.

Evidence one way or the other may be just around the corner. "Within a few years astronomers will be able to resolve compact objects at the centre of galaxies with radio interferometers," says Svidzinsky. Black holes will have a constant size, whereas an axion bubble's radius will oscillate, he says. Zioutas is looking forward to the answer. "There is so much at stake here – rewriting both Einstein and dark matter," he says.

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Dark Stars, Black Holes, Bright Galaxies

"Hearts of Darkness"

Galaxies

We live in a spiral galaxy. Our Solar System resides about three quarters of the way out from the centre of our Galaxy, or "Milky Way", in a spiral arm consisting of gas and young stars. However, galaxies exist in several different forms. Elliptical galaxies are large, round, aggregates of predominantly old stars. Spirals, like our Galaxy, possess disks with catherine wheel-like arms that are the sites of ongoing star formation.

An infrared image of our Galaxy taken by the Diffuse Infrared Background Experiment (DIRBE) instrument on the NASA Cosmic Background Explorer (COBE) satellite. The galactic plane runs horizontally along the middle of the image. Absorption by interstellar dust is minimized at infrared wavelengths allowing a clearer view of the plane and centre of our Galaxy.

Irregular galaxies, as their name implies, lack a well defined structure, but usually possess numerous star formation regions and large amounts of gas and interstellar dust (micron sized particles made up of carbon and silicon). Galaxies inhabit variously populated regions of space. The low density regions are well populated by spiral and irregular galaxies, whilst the denser, rich clusters are dominated by elliptical galaxies.

An image of Messier 87, a giant elliptical galaxy in the Virgo cluster.

It has become clear over the last 30 years that extremely dense objects exist both in our Galaxy and in the centres of many nearby galaxies. In our Galaxy (and most likely others) small regions of space weighing more than about 5 of our Suns exist. They consume nearby gas and stars and nothing ever escapes their grasp. In the centres of large galaxies similar regions of space exist that also consume stars and gas. However these regions can weigh as much as several billion (1 billion = 1,000,000,000 or 10⁹) Suns.

This web site will describe the theory and observations of these black holes and recent observations of the centres of galaxies that are providing new ideas about galaxy structure and evolution. The galaxies with these exotic, extremely massive objects at their centres may well be called "Hearts of Darkness".

Dark Stars, Black Holes

Shine a torch upwards in the night sky. The light travels along a straight line then eventually fades, scattered by dust particles in the air. Travelling at 300,000 kilometres per second light is not hindered by the gravitational field of the Earth that requires an object to travel at least 11 kilometres per second to escape its influence. What mass would Earth need to be to stop the torch light from escaping? Based on Newton's gravitational laws the Earth would need a mass equivalent to 2100 times that of our Sun. Such a massive Earth would not be a very hospitable place to live! The intense gravitational field would crush pre-existing structures. If however we used the existing mass of Earth and could squeeze Earth into a sphere slightly smaller than a golf ball, again, light would not escape from its surface.

Theorists from the late 1930s onward predicted that small sized stellar objects could exist as the final products of stellar evolution. A "star" with a radius of 5 kilometres would need to weigh about 1.7 times the mass of the Sun to stop light escaping from its surface. Did such "dark" stars exist?

A schematic view of the formation of a neutron star. A supernova explosion leaves a massive core of neutrons behind.

The partial answer to this question was the discovery in 1967 of radio pulses that came from rotating neutron stars, or pulsars. Pulsars are extremely small, massive stars made of tightly packed neutrons. They are formed during a supernova explosion which occurs to high mass stars. Since their discovery, over one thousand pulsars in the Galaxy have been discovered. A New Zealand astronomer, Richard Manchester, who works at the Australian Telescope National Facility, is one of the worlds leading researchers of pulsars. Whilst neutron stars or pulsars are extremely massive and small, their largest escape velocity is still only about 80% of the speed of light. So they are close to being dark stars, but not quite!

People have been thinking about "dark stars" for over two centuries! In 1783 the Reverend John Michell delivered a paper to the Royal Society in London announcing that invisible stars may exist if they were massive enough. The Frenchman Pierre Laplace discussed a similar phenomenon several years later. Early this century the German astronomer Karl Schwarzschild succeeded in finding solutions to some outstanding problems in Einstein's theory of General Relativity, which describes gravity. Some solutions of Einstein's equations become infinite (called a singularity) at zero radius. Schwarzschild calculated that a singularity, could exist at a small radius for a very dense object. For the Sun this radius would be 3 kilometres. We know this radius nowadays as the Schwarzschild (or gravitational) radius, and it is that required by an object so that radiation cannot escape from it. In 1933 astronomers Walter Baade and Fritz Zwicky suggested that the remnant of a supernova explosion could be a very dense star composed of neutrons.

An artist's impression of a supernova, the explosion of a star.

In 1939 Robert Oppenheimer and colleagues used quantum theory to determine that stable neutron stars could exist, and then went further, publishing a paper that would become a classic. It described massive stars that, once finished thermonuclear burning, would collapse forever. A physical model for a "dark star" had been found!

A photograph of Supernova 1987A (the bright star lower, right) next to the Tarantula Nebula in the Large Magellanic Cloud. This was taken by Alan Gilmore on the 8th of March 1987 using the 60cm reflector at Mount John University Observatory, Lake Tekapo. The image has been inverted so that bright features appear dark.

Let's stop for a moment. A problem is looming! How would you detect an object whose gravitational field is so great that all radiation (light emitted from a torch is just one type of radiation) cannot escape from it? The answer is that you cannot observe it directly, but possibly indirectly, by observing its effect on surrounding objects.

As it turns out, any star greater than 3 solar masses must eventually form such a "dark star" after thermonuclear reactions have ceased, since no known source of pressure can support it. These objects are called "black holes" and this term was first coined by the physicist John Wheeler.

In 1963 a New Zealand mathematician, Roy Kerr, then working at the University of Texas, found solutions to the general relativistic field equations for the case of a rotating star. Since stars rotate, black holes should rotate, and these solutions were critical in understanding the space-time effects of spinning black holes. A major breakthrough had been made. Kerrs solutions showed that as well as having an event horizon (at the gravitational radius) a spinning black hole had another important horizon, at a greater radius than the event horizon, called the static limit. The region between the event horizon and the static limit is called the ergosphere. Later studies by Penrose, Wheeler, Bekenstein and Hawking amazingly showed that black holes could emit radiation from the ergosphere. In general, the smaller a black hole, the larger the amount of radiation could be emitted. However, even for stellar mass black holes the rate of radiation is very small, so that they exist for hundreds of billions of years.

So, are there any black hole candidates? Yes, there exists strong, indirect, evidence for many. One observational signature is the rapid variation of high energy X-rays from an object. This variation can be caused by a binary star system that consists of a black hole orbiting a very large (supergiant) star. Gas from the supergiant is gravitationally attracted to the black hole and as the gas approaches it heats up to 1 million degrees and emits high energy X-rays. A decrease in the strength of X-rays from the binary system is explained when the black hole goes behind the supergiant during its orbit. Many such binary systems are known. One system, Cygnus X-1, in the northern sky constellation Cygnus, is one of the best candidates for a black hole.

Artist's impression of the Cygnus X-1 binary system, with the supergiant star on the left, and the black hole surrounded by an accretion disk of gas, on the right.

Another strong candidate for a black hole is LMC X-1. LMC stands for Large Magellanic Cloud, a close neighbour galaxy to our Galaxy. LMC X-1 is the strongest source of X-rays in the LMC and it originates from an unusually energetic binary star system. This source is thought to be a normal and compact star orbiting each other, similar to the Cygnus X-1 system. The X-rays shining from the system knock electrons off atoms, causing some atoms to glow noticeably in X-rays. Motion in the binary system indicates the compact star is probably a black hole, since its high mass - roughly five times that of our Sun - should be massive enough to cause even a neutron star to collapse.

An X-ray image of LMC X-1 taken with Röntgensatellit (ROSAT).

Active Galaxies and Central Energy Sources

Many galaxies possess nuclei that emit vast amounts of radiation. The amounts can vary from a small fraction to several thousand times greater than the radiation output of an entire normal host galaxy. In the 1950s and 1960s radio astronomy provided important clues to the nature of such galaxies. Powerful radio sources in the sky were found to be associated with faint elliptical galaxies. Many showed dual lobes of radio emission on opposite sides of the optical galaxy. The radio emission was caused by radiation from high velocity, spiralling electrons in strong magnetic fields. This radiation is called synchrotron radiation. It was quickly realised that the majority of the radiation from such galaxies (called active) was not from stellar sources, but due to this type of high velocity particle emission.

A schematic illustration of synchrotron radiation. Electrons spiral around magnetic field lines emitting photons of radiation.

Some clues indicated the probable extreme power source of activity in galaxies. The radio lobes observed on either side of the optical galaxy were sometimes connected to a small, emission region in the nucleus of the galaxy via narrow, straight jets. Energy arguments suggested that the lobes of emission had to be continually replenished by fast moving electrons. The presence of jets joining the nucleus to the lobes suggested that something in the small nucleus was the energy source. Variability in the optical and radio emission of the nucleus on time scales of hours also suggested a very small energy producing region (of light hours diameter, similar in size to the Solar System).

Cygnus A: An image obtained with the Very Large Array (VLA) radio telescope in New Mexico at a wavelength of 6 centimetres. Note the bright lobes, and narrow jets that point back to the nucleus. The optical galaxy lies well within the radio lobes, centred on the radio nucleus.

It is now generally believed that such activity in galaxies is powered by supermassive objects in their nuclei.

Supermassive objects or black holes?

The presence of supermassive objects in galaxy centres was first inferred in the late 1970s. Imaging and spectral observations of the nucleus of the large elliptical galaxy in the Virgo cluster of galaxies, Messier 87 (or M87, see image above), by Peter Young and Wallace Sargent and collaborators, suggested the existence of a compact object of 5 billion solar masses within 300 light years of the nucleus. This amount of mass is difficult to explain by normal populations of stars, and many astronomers were convinced that supermassive black holes (SBHs) easily explained the observations.

Further, the very small size and enormous energy outputs of these nuclear regions strongly suggest black hole accretion (mass converted to energy by the extreme gravitational field of the black hole) as the energy source. Rapid progress has been made recently in the study of central regions of galaxies by using the high resolution capabilities of the Hubble Space Telescope (HST) and radio telescopes on Earth. HST is in orbit around the Earth, and is above the atmosphere that blurs ground-based optical telescope images.

HST above the Space Shuttle. The gold panels are solar arrays used to power the telescope. The central white rectangle is the cover of the Wide Field Planetary Camera 2 instrument that has taken many high resolution images of galaxy nuclei.

A matter of perspective? The Unified Model

It is now apparent that many features of active galaxies are common. A model has been put forward that tries to reconcile the differing properties of activity by assuming that the physical structure in the nucleus of all active galaxies is similar. The "unified model" assumes that all active galaxies possess a SBH surrounded by dust in the shape of a torus (doughnut-like). Relativistic jets (ie. radio jets) if detected will appear at right angles to the major axis of the torus.

Variations to the model include the evolutionary status of the SBH (eg. its mass, possible spin), the type of host galaxy (ie. spiral or elliptical), the accretion rate of fuel (ie. gas, stars) into the nuclear (accretion disk + SBH) region, and importantly, the aspect or orientation of the torus to our line of sight. Such model variations go a long way to explain the variety of physical properties seen in active galaxies.

Schematic diagram, not to scale, of the central region of a Seyfert galaxy illustrating the effect of viewing angle. HBLR/BLR stands for Hidden/Broad Line Region (high velocity gas) close to the nucleus, NLR is Narrow Line Region (low velocity gas). Broad spectral lines are produced by gas clouds with large internal velocities.

By looking along a line of sight into the hole of the torus, we see the highest velocity gas clouds, nearest to the SBH. Such galaxies are classified as Seyfert 1, Quasar and Blazar. If the torus obstructs our direct view, we can only observe lower velocity gas clouds, further from the SBH, and possibly scattered light from the nuclear region, and we then detect active galaxies of the Seyfert 2 and radio galaxy types. In rough order of increasing luminosity the active galaxies are Seyferts, Radio Galaxies, Blazars and Quasars. It is now thought that the host galaxies of Seyferts are spirals, and elliptical galaxies host radio galaxies and quasars although there could be some overlap. Also, many distant quasars imaged by HST show peculiar structures that are indicative of interacting or merging galaxies, suggesting that collisions between galaxies may help to produce the high luminosity quasars.

An artist's impression, based on HST observations, of a warped, dusty disk around a suspected SBH in NGC 6251. Perpendicular to the disk is a jet of relativistic particles ejected along the SBH spin axis.

Nearby Monsters

NGC 4261 - A large, dusty disk

NGC 4261 is a bright elliptical galaxy. It has radio jets extending well outside the optical galaxy. The HST image shows a large, about 400 light years in diameter, dusty disk slightly inclined to our line of sight. Note that the radio jets are aligned perpendicularly to the major axis of the dusty disk (ie. the extended cool region of a torus) consistent with the unified model. HST spectral observations of gas in the nucleus suggest a 5 x 10⁸ solar mass SBH.

NGC 4261, Left: A ground based composite optical (white) and radio (yellow/orange) image. Right: HST image of the galaxy centre showing the disk of dust. Interestingly, the suspected SBH is some 20 light years from the geometrical centre of the galaxy. The reason for this misalignment is unknown.

A word of warning. Even though HST allows us the clearest optical view of galaxy centres, we do not directly resolve the SBHs or their gaseous accretion disks. For example, NGC 4261 is approximately 82 million light years distant, and at that distance, an SBH accretion disk of 1 light week diameter would span about 1/1000 the size of a HST imaging pixel element. What we do see in HST images however are the cooler, dusty disks surrounding the SBH and hot accretion disk. However, the resolving power of HST does allow important velocity measurements at small distances from the nucleus, which constrains the mass contained within that distance.

Messier 87 - revisited

M87 is one of the nearest ellipticals that shows signs of activity. As long ago as 1918 H. D. Curtis discovered an optical "jet" originating from the nucleus. The optical emission from the jet is also synchrotron radiation, seen usually as radio emission. The synchrotron jet occurs at optical wavelengths when the fast moving electrons are very energetic. M87 is a powerful radio source (known as 3C 274 and Virgo A) and the radio source at the nucleus is compact, spanning a diameter of less than 3 light-months.

M87 as observed by HST showing the nuclear gas disk (lower left) and jet.

HST detects a small disk of gas in the nucleus. The disk is approximately elliptical in shape, and its minor axis is close to the direction of the optical synchrotron jet. Radial velocity measurements along the gas disk shows high recession and approach velocities of 500 kilometres per second. A central mass of 2 billion solar masses is deduced. The authors conclude that the disk of gas is feeding a SBH in the nucleus, consistent with (but smaller than, by a factor of about two) the mass inferred from the measurements in the 1970s mentioned previously.

HST optical observations of M87, showing the nuclear gas disk, and the spectral signature of rotation. A gas emission line from two regions of the disk shows a shift in wavelength indicative of very high relative velocities.

A Mini-Monster in our backyard!

For a number of years evidence has been growing that the centre of our Galaxy may harbour a SBH. The motions of stars around our Galaxy centre indicate increased velocities down to very small distances, about 10 light days. The density of matter needed to explain such motions rules out most alternatives to a SBH.

Left: A near-infrared image of the central 3 light years of the Galaxy centre. The observation was made with the SHARP I camera on the NTT telescope at ESO, La Silla, Chile. Right: A contour plot of the image. The compact radio source Sgr A^*, which is associated with a 3 million solar mass black hole, is just above the central label "SW".

A radio image of the Galactic Centre at a wavelength of 6 cm, taken with the VLA. The region is known as Sgr A West (encompassing Sgr A^*) and the emission is due to gas being heated by nearby hot, young stars.

As in the case of stellar mass black hole systems, we may expect to detect large amounts of X-rays from an accretion disk around a Galactic Centre SBH. However, observations have resolved most of the X-ray emission in the region to a handful of unrelated X-ray binary systems. The X-ray luminosity of the Galactic Centre is some 7 orders of magnitude lower than expected for an accretion disk around a 3 million solar mass SBH. It is therefore possible that if a SBH does reside in the centre of our Galaxy, it is dormant.

Where to now?

The picture that has emerged is as follows. SBHs are probably a normal feature of the central regions of bright galaxies that have spheroidal components (eg. elliptical galaxies, spiral galaxies with a bulges). SBHs have not been detected in irregular galaxies. The SBH masses scale roughly with the mass of the host galaxy, implying a strong link between the growth of the galaxy as a whole, and the growth of the SBH.

Some fundamental questions remain however. What is the link between SBHs seen today in relatively nearby and lower luminosity galaxies to distant, very luminous quasars? Quasars were more populous in the early universe, and so it is possible that many nearby galaxies were quasars in their youth, and now harbour relic SBHs that earlier emitted high (quasar) luminosities. How do SBHs evolve? We also believe that galaxy mergers were more prevalent at earlier times in the Universe. What part then do galaxy mergers play in SBH evolution? How would two pre-existing SBHs behave if their host galaxies merged? Such events may not be observable by the usual optical, radio or X-ray telescopes, but by the detection of gravitational waves. A merger of two 10⁷ solar mass SBHs would radiate energy at a frequency of about 10^-4 Hz.

The European Space Agency (ESA) is planning a space-based gravity wave detector, called Laser Interferometric Space Array (LISA). The primary objective of the LISA mission is to detect and observe gravitational waves from massive black holes and galactic binary stars in the frequency range 10^-4 to 10^-1 Hz. Useful measurements in this frequency range cannot be made on the ground because of the unshieldable background of local gravitational noise. From recent research and upcoming missions like LISA we are finally shining some light on these enigmatic hearts of darkness.

An artist's impression of LISA. It consists of six identical spacecraft forming an equilateral triangle in space with two closely spaced (200 kilometres) "near" spacecraft at each vertex. When a gravity wave passes through the system it causes a strain distortion of space which will be detected by measuring the fluctuations in separation between proof masses inside the spacecraft.

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Star Birth Gone Wild in 'Cosmic Hurricane'

	Star Birth Gone Wild in 'Cosmic Hurricane'

A shower of hot gas spewed from a galaxy loaded with pockets of intense star formation offers a window to the more violent early universe.

The rapid-fire star birth in M82 was triggered by a collision with another galaxy, and the tremendous activity fuels a "cosmic hurricane is travelling at more than a million miles an hour [447 kilometers per second] into intergalactic space," said Linda Smith of the University College London.

The gas travels in two opposite directions and extends thousands of light-years. Traced back to their sources, the two plumes are revealed to originate in the many separate clumps of star formation and the quick, explosive deaths of massive stars that generate new elements.

"Our goal here is to understand the structure of the wind's plumes, which are key factors in the evolution of this galaxy and the eventual pollution of nearby intergalactic space with new chemical elements," Smith said.

An image of the scene was released Friday. It was created by combining Hubble Space Telescope observations that detail the inner part of the galaxy with a view from the WIYN Telescope on Kitt Peak in Arizona, which showed the extended winds, explained Mark Westmoquette, also of the University College London.

It is not unusual to see jets or plumes of material escaping along the rotation axis of stars, a black hole or an entire galaxy. But M82 is noted for its "superwinds," as astronomers call the bipolar outflows.

"The M82 wind is made up of gas jets from multiple chimneys, each of which is relatively distinct," said Jay Gallagher of the University of Wisconsin-Madison, another member of the study team. "We hypothesize that these originate from individual star-forming clumps within M82."

Some of the clusters contain as much mass as a million Suns packed within 30 light-years of space, Gallagher said earlier this month in discussing his group's work at an astronomy meeting at the Space Telescope Science Institute.

M82 is about 10 million light-years away, which is relatively close in space and time. Gallagher said the scene can help astronomers understand what occurred in the early universe, when star birth was rampant. Because primordial galaxies are incredibly far away -- billions of light-years -- detailed examination of their structures is not practical with current telescopes.

Yet astronomers have seen enough to know that there are big differences between early galaxies and most of the mature galaxies closer by.

"Observations of the distant universe have really shown us now -- and we have to confront this -- that star formation in early epochs was really intense," Gallagher said. "The universe has gone from an intense mode of star formation in galaxies to a lazier mode nowadays."

So it is imperative, he said, to understand the mechanics of so-called starburst galaxies like M82.

In particular, Gallagher told SPACE.com, the distinct clumping of star formation in M82 is thought to be similar to how it worked when some of the earliest galaxies were under construction.

The impetus for star formation in M82 came from a collision with another galaxy, M81, about 300 million years ago, astronomers say. Collisions were common when the universe was younger and smaller, and are thought to have played an important roll in star birth. Here's what happens in a typical collision:

"Huge amounts of gas are funneled into dense regions faster than the galaxy can get rid of it," Gallagher explained. "The galaxy overheats and explodes into stars."

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Universe Measured: We're 156 Billion Light-years Wide!

f you've ever wondered how big the universe is, you're not alone. Astronomers have long pondered this, too, and they've had a hard time figuring it out. Now an estimate has been made, and its a whopper.

The universe is at least 156 billion light-years wide.

In the new study, researchers examined primordial radiation imprinted on the cosmos. Among their conclusions is that it is less likely that there is some crazy cosmic "hall of mirrors" that would cause one object to be visible in two locations. And they've ruled out the idea that we could peer deep into space and time and see our own planet in its youth.

First, let's see why the size is a number you've never heard of before.

Stretching reality

The universe is about 13.7 billion years old. Light reaching us from the earliest known galaxies has been travelling, therefore, for more than 13 billion years. So one might assume that the radius of the universe is 13.7 billion light-years and that the whole shebang is double that, or 27.4 billion light-years wide.

But the universe has been expanding ever since the beginning of time, when theorists believe it all sprang forth from an infinitely dense point in a Big Bang.

"All the distance covered by the light in the early universe gets increased by the expansion of the universe," explains Neil Cornish, an astrophysicist at Montana State University. "Think of it like compound interest."

Need a visual? Imagine the universe just a million years after it was born, Cornish suggests. A batch of light travels for a year, covering one light-year. "At that time, the universe was about 1,000 times smaller than it is today," he said. "Thus, that one light-year has now stretched to become 1,000 light-years."

All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger.

"It can be thought of as a spherical diameter is the usual sense," Cornish added comfortingly.

(You might have heard the universe is almost surely flat, not spherical. The flatness refers to its geometry being "normal," like what is taught in school; two parallel lines can never cross.)

Hall of mirrors

The scientists studied the cosmic microwave background (CMB), radiation unleashed about 380,000 years after the Big Bang, when the universe had first expanded enough to cool and allow atoms to form. Temperature differences in the CMB left an imprint on the sky that was used last year to reveal the age of the universe and confirm other important cosmological measurements.

The CMB is like a baby picture of the cosmos, before any stars were born.

The focus of the new work, which was published last week in the journal Physical Review Letters, was a search of CMB data for paired circles that would have indicated the universe is like a hall of mirrors, in which multiple images of the same object could show up in different locations in space-time. A hall of mirrors could mean the universe is finite but tricks us into thinking it is infinite.

Think of it as a video game in which an object disappearing on the right side of the screen reappears on the left.

"Several years ago we showed that any finite universe in which light had time to 'wrap around' since the Big Bang would have the same pattern of cosmic microwave background temperature fluctuations around pairs of circles," Cornish explained. They looked for the most likely patterns that would be evident in a CMB map generated by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).

They didn't find those patterns.

Don't look back

"Our results don't rule out a hall-of-mirrors effect, but they make the possibility far less likely," Cornish told SPACE.com, adding that the findings have shown "no sign that the universe is finite, but that doesn't prove that it is infinite."

The results do render impossible a "soccer ball" shape for the universe, proposed late last year by another team. "However, if they were to 'pump up' their soccer ball to make it larger, they could evade our bounds" and still be in the realm of possibility, Cornish said. Other complex shapes haven't been ruled out.

The findings eliminate any chance of seeing our ancient selves, however, unless we can master time travel.

"If the universe was finite, and had a size of about 4 billion to 5 billion light-years, then light would be able to wrap around the universe, and with a big enough telescope we could view the Earth just after it solidified and when the first life formed," Cornish said. "Unfortunately, our results rule out this tantalizing possibility."

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Impossible? Cornish Explains Further

Update, 8:25 a.m. Tuesday, May 25

This article generated quite a few e-mails from readers who were perplexed or flat out could not believe the universe was just 13.7 billion years old yet 158 billion light-years wide. That suggests the speed of light has been exceeded, they argue. So SPACE.com asked Neil Cornish to explain further. Here is his response:

"The problem is that funny things happen in general relativity which appear to violate special relativity (nothing traveling faster than the speed of light and all that).

"Let's go back to Hubble's observation that distant galaxies appear to be moving away from us, and the more distant the galaxy, the faster it appears to move away. The constant of proportionality in that relationship is known as Hubble's constant.

"One seemingly paradoxical consequence of Hubble's observation is that galaxies sufficiently far away will be receding from us at a velocity faster than the speed of light. This distance is called the Hubble radius, and is commonly referred to as the horizon in analogy with a black hole horizon.

"In terms of special relativity, Hubble's law appears to be a paradox. But in general relativity we interpret the apparent recession as being due to space expanding (the old raisins in a rising fruit loaf analogy). The galaxies themselves are not moving through space (at least not very much), but the space itself is growing so they appear to be moving apart. There is nothing in special or general relativity to prevent this apparent velocity from exceeding the speed of light. No faster-than-light signals can be sent via this mechanism, and it does not lead to any paradoxes.

"Indeed, the WMAP data [on cosmic microwave background radiation] contain strong evidence that the very early universe underwent a period of accelerated expansion in which the distance been two points increased so quickly that light could not outrace the expansion so there was a true horizon -- in precise analogy with a black hole horizon. Indeed, the fluctuations we see in the CMB are thought to be generated by a process that is closely analogous to Hawking radiation from black holes.

"Even more amazing is the picture that emerges when you combine the WMAP data with [supernova] observations, which imply that the universe has started inflating again. If this is true, we have started to move away from the distant galaxies at a rate that is increasing, and in the future we will not be able to see as many galaxies as they will appear to be moving away from us faster than the speed of light (due to the expansion of space), so their light will not be able to reach us."

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Black Hole Gives Up Some Secrets

ATLANTA - Three

separate teams of researchers have unlocked some longstanding secrets of two stars that have puzzled astronomers for more than 20 years.

The studies focus on the pair of objects known as binary system SS 433, some 16,000 light-years from Earth. The pair consists of an old, faint star locked in a tight orbit with a stellar corpse -- either a black hole or dense neutron star. The presumed black hole constantly strips gas from its companion, channels it into a flat "accretion disk" and then later spits the material out in opposing, polar jets that shoot out at 90-degree angles to the disk.

Astronomers have seen SS 433 do some strange things since the system was first discovered in the 1960s. Not only are its jets detectable in infrared and X-rays, but also visibly too. And the light spectrum seen by astronomers seems to change over time.

The system "remains unique and hard to understand 25 years after its discovery," said Bruce Margon, associate director for the Space Telescope Science Institute, which runs the Hubble Space Telescope. "For example, why is this the only star system we see with these relativistic jets?"

Relativistic is a term used to describe material moving at a significant fraction of the speed of light.

The jets were the target of researchers at Massachusetts Institute of Technology (MIT), who used the Chandra X-ray Observatory to study SS 433s emissions, which stream from the object at a roughly one-fourth of light's speed. Their results may help develop new ways to measure masses of black holes.

The system resembles the output of giant galaxies in the making, objects called quasars that exist mostly far away and in the early universe.

"We believe its very much like a quasar jet," MITs Herman Marshall, lead investigator for his team, said of SS 433. "Its a jet laboratory thats closer to home."

Another effort by astronomers with the National Radio Astronomy Observatory (NRAO) made daily observations of SS 433s apparent black hole, compiling the images into a movie that allowed them to track individual ejections of jet material as they spewed forth. The movie helped researchers determine the source of brightness variations in the jets.

A separate team led by astronomer Todd Hillwig of Georgia State University made the first observations the companion star in SS 433.

The results of all three studies were presented here Monday during the 203^rd meeting of the American Astronomical Society.

Honing in on a microquasar

SS 433 is what astronomers call a microquasar, a system where the central massive object -- a neutron star or black hole -- mimics the behavior of quasars, which are thought to consist of gigantic black holes at the center of distant, developing galaxies and which consistently spit bright jets of materials from their poles.

But since microquasars are closer to Earth than their quasar cousins -- which can sit 10 billion light-years in the distance -- they are easier to observe and changes occur more noticeably over time.

"If you were trying to [see] this with an extragalactic jet, it would take years, even decades" said Amy Mioduszeweski, who led one study for the NRAO.

NRAO astronomers used the Very Large Baseline Array, a network of 10 radio telescopes arranged across 5,000 miles (8,046 kilometers) from Hawaiis Mauna Kea to St. Croix in the U.S. Virgin Islands to track SS 433s jets.

Observations of past microquasars have shown that their jets typically get fainter as they taper out into space. But SS 433 doesnt. As the jets shoot out, they sometimes flare up to become even brighter.

The NRAO study also detected other, non-jet related radio waves emanating from SS 433s microquasar core, leading researchers to believe the objects accretion disk forms a wind that interacts with a denser wind from the black holes companion star, causing the added emissions. The disk wind may also interact with jet material to cause the belated jet flare-ups.

"The most obvious place for it to come from is from the center of this system," Mioduszeweski said. "But we dont know for sure where this outflow is coming from."

The system is in the constellation Aquila, the Eagle.

A matter of timing

Hillwigs team had a hard time catching a glimpse of the SS 433s companion star. The accretion disk and bright jets of the black hole are so bright they blot out any chance of seeing the companion star, except during a time of eclipse, when the star crosses in front of the black hole as seen by observers on Earth.

"We think its a matter of timing," explained Douglas Gies, a Georgia State astrophysicist and team member who presented the study. "The best time to see to see the mass-donor star is when the accretion disk wobbles in precession during an eclipse. This set of requirements exists just twice each year."

Although the star passes in front of the black hole every 13 days, only twice a year does it do so at an angle high enough above the black hole to be noted by ground-based instruments.

After a failed first attempt, Hillwigs team successfully identified the star from its companion using the 13-foot (four-meter) telescope at Arizonas Kitt Peak National Observatory. What the team found was a light pattern suggesting SS 433s star is an old, swollen supergiant with a surface temperature of about 13,000 Fahrenheit. It has a mass 11 times greater than that of Earths Sun. The large mass provides a feast for SS 433s much more compact black hole.

"Apparently, the black hole cant digest the overwhelming amount of gas its companion star is giving," Gies said. "So it may be funneling the excess into these powerful jets."

How much less massive the black hole is than its neighbor is up for debate. Hillwigs team claimed the black hole was roughly three solar masses while the MIT study originally found it significantly larger, some 16 solar masses. MIT researchers later reduced that estimate to about eight solar masses. The two teams plan to work together in the future to hone in on a more definitive mass for the black hole.

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