Giant star goes supernova and is completely covered by its own dust

Ohio State University, Columbus

While searching the skies for black holes using the Spitzer Space Telescope Deep Wide Field Survey, Ohio State University astronomers discovered a giant supernova that was smothered in its own dust. In this artist’s rendering, an outer shell of gas and dust — which erupted from the star hundreds of years ago — obscures the supernova within. This event in a distant galaxy hints at one possible future for the brightest star system in our own Milky Way.

Published: October 14, 2010

A giant star in a faraway galaxy recently ended its life with a dust-shrouded whimper instead of the more typical bang.

Ohio State University researchers suspect that this odd event — the first one of its kind viewed by astronomers — was more common early in the universe. It also hints at what we would see if the brightest star system in our galaxy became a supernova.

Christopher Kochanek from Ohio State University and his colleagues describe how the supernova appeared in late August 2007 as part of the Spitzer Space Telescope Deep Wide Field Survey.

The astronomers were searching the survey data for active galactic nuclei (AGN) — supermassive black holes at the centers of galaxies. AGN radiate enormous amounts of heat as material is sucked into the black hole. In particular, the astronomers were searching for hot spots that varied in temperature because these could provide evidence of changes in how the material was falling into the black hole.

Normally, astronomers wouldn't expect to find a supernova this way, said Szymon Kozlowski from Ohio State. Supernovae release most of their energy as light, not heat.

But one hot spot, which appeared in a galaxy some 3 billion light-years from Earth, didn't match the typical heat signal of an AGN. The visible spectrum of light emanating from the galaxy didn't show the presence of an AGN either.

Enormous heat flared from the object for a little over 6 months, then faded away in early March 2008 — another clue that the object was a supernova. "Over 6 months, it released more energy than our Sun could produce in its entire lifetime," Kozlowski said.

The astronomers knew that if the source were a supernova, the extreme amount of energy it emitted would qualify it as a big one, or a "hypernova." The temperature of the object was around 1,300° Fahrenheit (700° Celsius) — only a little hotter than the surface of the planet Venus. They wondered what could absorb that much light energy and dissipate it as heat?

The answer: Dust, and a lot of it.

Using what they learned from the Spitzer survey, the astronomers worked backward to determine what kind of star could have spawned the supernova, and how the dust was able to partly muffle the explosion. They calculated that the star was probably a giant, at least 50 times more massive than our Sun. Such massive stars typically belch clouds of dust as they near the end of their existence.

This particular star must have had at least two such ejections, they determined — one about 300 years before the supernova, and one only about 4 years before it. The dust and gas from both ejections remained around the star, each in a slowly expanding shell. The inner shell from 4 years ago would be close to the star, while the outer shell from 300 years ago would be farther away.

"We think the outer shell must be nearly opaque, so it absorbed any light energy that made it through the inner shell and converted it to heat," said Kochanek. That's why the supernova showed up on the Spitzer survey as a hot dust cloud.

Krzysztof Stanek from Ohio State said that stars probably choked on their own dust more often in the distant past.

"These events are much more likely to happen in a small, low-metallicity galaxy," Stanek said — meaning a young galaxy that hadn't been around long enough for its stars to fuse hydrogen and helium into the more complex chemicals that astronomers refer to as metals.

Still, Kozlowski added, NASA's Wide-field Infrared Explorer (WISE) will likely find more such supernovae. "I would expect WISE to see 100 of these events in 2 years, now that we know what to look for," he said.

Because of the alignment of the galaxy with Earth and our Sun, astronomers were not able to see what the event looked like to the naked eye while it was happening. But Kochanek believes that we might see the star brighten a decade or so from now. That's how long it will take for the shock wave from the exploding star to reach the inner dust shell and slam it into the outer shell. Then we'll have something to see here on Earth.

We do have at least one chance to see a similar light show closer to home, though. "If Eta Carinae went supernova right now, this is what it would probably look like," Kochanek said, referring to the brightest star system in our Milky Way Galaxy.

The two stars that make up Eta Carinae are 7,500 light-years away, and they host a distinctive dust shell dubbed the Homunculus Nebula. Astronomers believe that the nebula was created when the larger of the two stars underwent a massive eruption around 1840, and that future eruptions are likely.

10,000 years into the future

The multicolor snapshot at top captures the central region of the giant globular cluster Omega Centauri. All the stars in the image are moving in random directions, like a swarm of bees. From these measurements, they can predict the stars' future movement. The bottom illustration charts the future positions of the stars highlighted by the white box in the top image. Each streak represents the motion of the star over the next 600 years. The motion between dots corresponds to 30 years.
Photo by NASA/ESA/STScI

Looking at the heart of Omega Centauri, a globular cluster in the Milky Way, scientists have calculated how the stars will move over the next 10,000 years.

Published: October 26, 2010 (By STScI/ESA)


Astronomers are used to looking millions of years into the past. Now scientists have used the NASA/ESA Hubble Space Telescope to look thousands of years into the future. Looking at the heart of Omega Centauri, a globular cluster in the Milky Way, they have calculated how the stars will move over the next 10,000 years.

The globular star cluster Omega Centauri has caught the attention of sky watchers ever since the ancient astronomer Ptolemy first cataloged it 2,000 years ago. Ptolemy, however, thought Omega Centauri was a single star. He didn't know that the "star" was actually a beehive swarm of nearly 10 million stars, all orbiting a common center of gravity.

The stars are so tightly crammed together that astronomers had to wait for the powerful vision of NASA's Hubble Space Telescope to peer deep into the core of the "beehive" and resolve individual stars. Hubble's vision is so sharp, it can even measure the motion of many of these stars — and over a relatively short span of time.

A precise measurement of star motions in giant clusters can yield insights into how stellar groupings formed in the early universe and whether an intermediate mass black hole, one roughly 10,000 times as massive as our Sun, might be lurking among the stars.

Analyzing archived images taken over a 4-year period by Hubble's Advanced Camera for Surveys, astronomers have made the most accurate measurements yet of the motions of more than 100,000 cluster inhabitants, the largest survey to date to study the movement of stars in any cluster.

"It takes high-speed, sophisticated computer programs to measure the tiny shifts in the positions of the stars that occur in only 4 years' time," said Jay Anderson from the Space Telescope Science Institute (STScI) in Baltimore, Maryland, who conducted the study with Roeland van der Marel, also from STScI. "Ultimately, though, it is Hubble's razor-sharp vision that is the key to our ability to measure stellar motions in this cluster."

"With Hubble, you can wait 3 or 4 years and detect the motions of the stars more accurately than if you had waited 50 years on a ground-based telescope,” Anderson said.

The astronomers used the Hubble images, which were taken in 2002 and 2006, to make a movie simulation of the frenzied motion of the cluster's stars. The movie shows the stars' projected migration over the next 10,000 years.

Identified as a globular star cluster in 1867, Omega Centauri is one of roughly 150 such clusters in our Milky Way Galaxy. The behemoth stellar grouping is the biggest and brightest globular cluster in the Milky Way, and one of the few that can be seen by the unaided eye. Located in the constellation Centaurus, Omega Centauri is viewable in the southern skies.

Weighing a star using a moon

If a star has a planet, and that planet has a moon, and both of them cross in front of their star, then scientists can measure their sizes and orbits to learn about the star.

Provided by Harvard-Smithsonian Center, Cambridge

Published: October 18, 2010

How do astronomers weigh a star that's trillions of miles away and way too big to fit on a bathroom scale? In most cases, they can't, although they can get a best estimate using computer models of stellar structure.

New work by astrophysicist David Kipping says that in special cases, astronomers can weigh a star directly. If the star has a planet, and that planet has a moon, and both of them cross in front of their star, then scientists can measure their sizes and orbits to learn about the star.

"I often get asked how astronomers weigh stars," said Kipping from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "We've just added a new technique to our toolbox for that purpose," he said.

Astronomers have found more than 90 planets that cross in front of, or transit, their stars. By measuring the amount of starlight that's blocked, they can calculate how big the planet is relative to the star. But they can't know exactly how big the planet is unless they know the actual size of the star. Computer models give a very good estimate, but in science, real measurements are best.

Kipping realized that if a transiting planet has a moon big enough for astronomers to see — by also blocking starlight — then the planet-moon-star system could be measured in a way that lets them calculate exactly how large and massive all three bodies are.

"Basically, we measure the orbits of the planet around the star and the moon around the planet. Then through Kepler's laws of motion, it's possible to calculate the mass of the star," said Kipping.

The process isn't easy and requires several steps. By measuring how the star's light dims when planet and moon transit, astronomers learn three key numbers: 1) the orbital periods of the moon and planet; 2) the size of their orbits relative to the star; and 3) the size of planet and moon relative to the star.

Plugging those numbers into Kepler's third law yields the density of the star and planet. Because density is mass divided by volume, the relative densities and relative sizes gives the relative masses. Finally, scientists measure the star's wobble due to the planet's gravitational tug, known as the radial velocity. Combining the measured velocity with the relative masses, they can calculate the mass of the star directly.

"If there was no moon, this whole exercise would be impossible," said Kipping. "No moon means we can't work out the exact density of the planet, so the whole thing grinds to a halt."

Kipping hasn't put his method into practice yet because no star is known to have both a planet and moon that transit. However, NASA's Kepler spacecraft should discover several such systems.

"When they're found, we'll be ready to weigh them," said Kipping.

Unsteady rocking motion of Saturn's icy moon may keep its oceans liquid

At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus in this dramatically illuminated image.
Photo by NASA/JPL/Space Science Institute

Goddard Space Flight Center, Greenbelt, Maryland

October 7,2010

Saturn's icy moon Enceladus should not be one of the most promising places in our solar system to look for extraterrestrial life. Instead, it should have frozen solid billions of years ago. Located in the frigid outer solar system, it's too far from the sun to have oceans of liquid water, a necessary ingredient for known forms of life on its surface.

Some worlds, like Mars or Jupiter's moon Europa, give hints that they might harbor liquid water beneath their surfaces. Mars is about 4,200 miles (6,800 kilometers) across and Europa almost 2,000 miles (3,200 km) across. However, with a diameter only slightly more than 500 miles (800 km), Enceladus just doesn't have the bulk needed for its interior to stay warm enough to maintain liquid water underground.

With temperatures around -324 degrees Fahrenheit (-198 degrees Celsius), the surface of Enceladus is indeed frozen. However, in 2005, NASA's Cassini spacecraft discovered a giant plume of water gushing from cracks in the surface over the moon's south pole, indicating that there was a reservoir of water beneath the ice. Analysis of the plume by Cassini revealed that the water is salty, indicating the reservoir is large, perhaps even a global subsurface ocean. Scientists estimate from the Cassini data that the south polar heating is equivalent to a continuous release of about 13 billion watts of energy.

To explain this mysterious warmth, some scientists invoke radiation coupled with tidal heating. As it formed, Enceladus, like all solar system objects, incorporated matter from the cloud of gas and dust left over from our sun's formation. In the outer solar system, as Enceladus formed it grew as ice and rock coalesced. If Enceladus was able to gather greater amounts of rock, which contained radioactive elements, enough heat could have been generated by the decay of the radioactive elements in its interior to melt the body.

However, in smaller moons like Enceladus, the cache of radioactive elements usually is not massive enough to produce significant heat for long, and the moon should have soon cooled and solidified. So, unless another process within Enceladus somehow generated heat, any liquid formed by the melting of its interior would have frozen long ago.

This led scientists to consider the role of tidal heating as a way to keep Enceladus warm enough for liquid water to remain under its surface. Enceladus' orbit around Saturn is slightly oval-shaped. As it travels around Saturn, Enceladus moves closer in and then farther away. When Enceladus is closer to Saturn, it feels a stronger gravitational pull from the planet than when it is farther away. Like gently squeezing a rubber ball slightly deforms its shape, the fluctuating gravitational tug on Enceladus causes it to flex slightly. The flexing, called gravitational tidal forcing, generates heat from friction deep within Enceladus.

The gravitational tides also produce stress that cracks the surface ice in certain regions, like the south pole, and may be reworking those cracks daily. Tidal stress can pull these cracks open and closed while also shearing them back and forth. As they open and close, the sides of the south polar cracks move as much as a few feet, and they slide against each other by up to a few feet as well. This movement generates friction, which releases extra heat at the surface at locations that should be predictable with our understanding of tidal stress.

To test the tidal heating theory, scientists with the Cassini team created a map of the gravitational tidal stress on the moon's icy crust and compared it to a map of the warm zones created using Cassini's composite infrared spectrometer instrument (CIRS). Assuming the greatest stress is where the most friction occurs, and therefore where the most heat is released, areas with the most stress should overlap the warmest zones on the CIRS map.

"However, they don't exactly match," said Terry Hurford of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "For example, in the fissure called the Damascus Sulcus, the area experiencing the greatest amount of shearing is about 31 miles (50 kilometers) from the zone of greatest heat."

Hurford and his team believe the discrepancy can be resolved if Enceladus' rotation rate is not uniform — if it wobbles slightly as it rotates. Enceladus' wobble, technically called "libration," is barely noticeable. "Cassini observations have ruled out a wobble greater than about 2 degrees with respect to Enceladus' uniform rotation rate," said Hurford.

The team created a computer simulation that made maps of the surface stress on Enceladus for various wobbles, and found a range where the areas of greatest stress line up better with the observed warmest zones.

"Depending on whether the wobble moves with or against the movement of Saturn in Enceladus' sky, a wobble ranging from 2 degrees down to 0.75 degree produces the best fit to the observed warmest zones," said Hurford.

The wobble also helps with the heating conundrum by generating about five times more heat in Enceladus' interior than tidal stress alone, and the extra heat makes it likely that Enceladus' ocean could be long-lived, according to Hurford. This is significant in the search for life, because life requires a stable environment to develop.

The wobble is probably caused by Enceladus' uneven shape. "Enceladus is not completely spherical, so as it moves in its orbit, the pull of Saturn's gravity generates a net torque that forces the moon to wobble," said Hurford. Also, Enceladus' orbit is kept oval-shaped, maintaining the tidal stress, because of the gravitational tug from Dione, a neighboring larger moon. Dione is farther away from Saturn than Enceladus, so it takes longer to complete its orbit. For every orbit Dione completes, Enceladus finishes two, producing a regular alignment that pulls Enceladus' orbit into an oval shape.

What is a Fuzzball?

Credit: ESA (European Space Agency)

Cygnus X-1, an 8.7‑solar-mass black hole only 6,000 light years away in our own Milky Way galaxy, belongs to a binary system along with a blue supergiant variable star. If Cygnus X-1 is actually a fuzzball, its surface has a diameter of 51 kilometers.

Fuzzballs are theorized by some superstring theory scientists to be the true quantum description of black holes. The theory resolves two intractable problems that classic black holes pose for modern physics:

1. The information paradox wherein the quantum information bound in in‑falling matter and energy entirely disappears into a singularity; that is, the black hole would undergo zero physical change in its composition regardless of the nature of what fell into it.
2. The singularity at the heart of the black hole, where conventional black hole theory says there is infinite spacetime curvature due to an infinitely intense gravitational field from a region of zero volume. Modern physics breaks down when such parameters are infinite and zero.

Fuzzball theory replaces the singularity at the heart of a black hole by positing that the entire region within the black hole’s event horizon is actually a ball of strings, which are advanced as the ultimate building blocks of matter and energy. Strings are thought to be bundles of energy vibrating in complex ways in both the three physical dimensions of space as well as in compact directions—extra dimensions interwoven in the quantum foam (also known as spacetime foam).

Samir Mathur of Ohio State University, with postdoctoral researcher Oleg Lunin, proposed via two papers in 2002 that black holes are actually spheres of strings with a definite volume; they are not a singularity, which the classic view holds to be a zero-dimensional, zero-volume point into which a black hole’s entire mass is concentrated.

String theory holds that the fundamental constituents of subatomic particles, including the force carriers (e.g., quarks, leptons, photons, and gluons), all comprise a one-dimensional string of energy that takes on its identity by vibrating in different modes and/or frequencies. Quite unlike the view of a black hole as a singularity, a small fuzzball can be thought of as an extra-dense neutron star where its neutrons have decomposed, or “melted,” liberating the quarks (strings in string theory) comprising them. Accordingly, fuzzballs can be regarded as the most extreme form of degenerate matter.

Whereas the event horizon of a classic black hole is thought to be very well defined and distinct, Mathur and Lunin further calculated that the event horizon of a fuzzball would, at an extremely small scale (likely on the order of a few Planck lengths),be very much like a mist: fuzzy, hence the name “fuzzball.” They also found that the physical surface of the fuzzball would have a radius equal to that of the event horizon of a classic black hole; for both, the Schwarzschild radius for a median-size stellar-mass black hole of 6.8 solar masses is 20 kilometers—roughly the size of the island of Kauai in Hawaii.

With classical black holes, objects passing through the event horizon on their way to the singularity are thought to enter a realm of curved spacetime where the escape velocity exceeds the speed of light. It is a realm that is devoid of all structure. Further, at the singularity—the heart of a classic black hole—spacetime is thought to have infinite curvature (that is, gravity is thought to have infinite intensity) since its mass is believed to have collapsed to zero (infinitely small) volume where it has infinite density. Such infinite conditions are problematic with known physics because key calculations utterly collapse. With a fuzzball however, the strings comprising an object are believed to simply fall onto and absorb into the surface of the fuzzball, which corresponds to the event horizon—the threshold at which the escape velocity equals the speed of light.

A fuzzball is a black hole; spacetime, photons, and all else that is not exquisitely close to the surface of a fuzzball are thought to be affected in precisely the same fashion as with a classic black hole featuring a singularity at its center. Classic black holes and fuzzballs differ only at the quantum level; that is, they differ only in their internal composition as well as how they affect virtual particles that form close to their event horizons (see Information paradox, below). Fuzzball theory is thought by its proponents to be the true quantum description of black holes.

Since the volume of fuzzballs is a function of the Schwarzschild radius (2,954 meters per solar mass), fuzzballs have a variable density that decreases as the inverse square of their mass (twice the mass is twice the diameter, which is eight times the volume, resulting in one‑quarter the density). A typical 6.8‑solar-mass fuzzball would have a mean density of 4.0×10^17 kg/m3. A bit of such a fuzzball the size of a drop of water would have a mass of twenty million metric tons, which is the mass of a granite ball 240 meters in diameter is tall). Though such densities are almost unimaginably extreme, they are, mathematically speaking, infinitely far from infinite density. Although the densities of typical stellar-mass fuzzballs are quite great—about the same as neutron stars—their densities are many orders of magnitude less than the Planck density (5.155×10^96 kg/m3), which is equivalent to the mass of the universe packed into the volume of a single atomic nucleus.

Fuzzballs become less dense as their mass increases due to fractional tension. When matter or energy (strings) fall onto a fuzzball, more strings aren’t simply added to the fuzzball; strings fuse together, and in doing so, all the quantum information of the in‑falling strings becomes part of larger, more complex strings. Due to fractional tension, string tension exponentially decreases as they become more complex with more modes of vibration, relaxing to considerable lengths. The “mathematical beauty” of the string theory formulas Mathur and Lunin employed lies in how the fractional tension values produce fuzzball radii that precisely equal Schwarzschild radii, which Karl Schwarzschild calculated using an entirely different mathematical technique 87 years earlier.

Due to the mass-density inverse-square rule, all fuzzballs need not have unimaginable densities. There are also supermassive black holes, which are found at the center of virtually all galaxies. Sagittarius A*, the black hole at the center of our Milky Way galaxy, is 4.3 million solar masses. If it is actually a fuzzball, it has a mean density that is “only” 51 times that of gold. At 3.9 billion solar masses, near the upper bounds for supermassive black holes, a fuzzball would have a radius of 77 astronomical units—about the same size as the termination shock of our solar system’s heliosphere—and a mean density equal to that of the Earth's atmosphere at sea level (1.2 kg/m3).

Irrespective of a fuzzball’s mass and resultant density, the determining factor establishing where its surface lies is the threshold at which the fuzzball’s escape velocity precisely equals the speed of light.Escape velocity, as its name suggests, is the velocity a body must achieve to escape from a massive object. For earth, this is 11.2 km/s. In the other direction, a massive object’s escape velocity is equal to the impact velocity achieved by a falling body that has fallen from the edge of a massive object’s sphere of gravitational influence. Thus, event horizons—for both classic black holes and fuzzballs—lie precisely at the point where spacetime has warped to such an extent that falling bodies just achieve the speed of light. According to Albert Einstein, via his special theory of relativity, the speed of light is the maximum permissible velocity in spacetime. At this velocity, infalling matter and energy impacts the surface of the fuzzball and its now-liberated, individual strings contribute to the fuzzball’s makeup.