Black Holes: Gravity's Relentless Pull

Black holes are places where ordinary gravity has become so extreme that it overwhelms all other forces in the Universe. Once inside, nothing can escape a black hole's gravity — not even light.

Yet we know that black holes exist. We know how they are born, where they occur, and why they exist in different sizes. We even know what would happen if you fell into one. Our discoveries have revealed one of the strangest objects in the Universe, and there's still much we don't know.

The nearest black hole is many lightyears away, so we don't have to worry about threats to the Earth. This is as close as you'll ever get to one. So come explore!

For a fully interactive multimedia experience, click Journey to a Black Hole (fast internet connection recommended). For an in-depth study of black holes that presents the site's contents conveniently sorted by topic, click Black Hole Encyclopedia.

Supernova Mystery Remains Just That

White dwarf-red giant binary system
(This artist's rendering depicts the type of system thought to be
responsible for most Type Ia supernovae. A red giant appears on the
left, and a white dwarf (depicted at the moment of explosion) appears
on the right)


Supernova Mystery Remains Just That

Whenever I hear a claimed discovery that overturns conventional wisdom on some important aspect of astronomy, my skepticism meter goes on high alert. Such was the case on Wednesday, when I listened to a NASA press conference in which two astronomers based in Germany presented evidence arguing that the most popular model for Type Ia supernovae is incorrect, at least for elliptical galaxies.

Astronomers are in agreement that Type Ia supernovae (SN Ia) occur when a white dwarf (a collapsed, Earth-size remnant of a star) blows itself to smithereens, producing an extraordinarily powerful explosion that large telescopes can see from billions of light-years away. But despite years of intense study on both observational and theoretical fronts, the details remain shrouded in mystery.

The standard textbook explanation for SN Ia goes something like this: Type Ia supernovae occur in binary systems consisting of a white dwarf and a normal star. As the normal star gives off a wind, some of the gas becomes trapped in a disk around the white dwarf. The gas spirals in, collecting on the white dwarf's surface. If enough gas accumulates on the white dwarf to push its mass close to the Chandrasekhar limit (1.4 solar masses), it ignites a powerful thermonuclear explosion that blows up the entire white dwarf like a giant H-bomb.

But astronomers have also proposed an alternative theory: Some Type Ia supernovae occur in binary systems consisting of two white dwarfs. The white dwarfs slowly spiral inward and merge, pushing the merged object over 1.4 solar masses. This object explodes as a SN Ia.

In the standard picture, the "accreting white dwarf" scenario occurs much more frequently than the merger scenario, because accreting binary systems should be more common. In addition, some theories predict that when two white dwarfs merge, they collapse to form a neutron star, with no supernova explosion whatsoever. As astrophysicist Mario Livio (Space Telescope Science Institute) points out, "Whether or not mergers can indeed produce Type Ia supernovae is not clear from a theoretical standpoint."

But if most SN Ia are triggered by accreting white dwarfs, gas falling onto a white dwarf should give off copious amounts of X-rays. In the NASA press conference, Marat Gilfanov and Akos Bogdan (Max Planck Institute for Astrophysics in Garching, Germany) said they had searched five elliptical galaxies and the central part of the Andromeda Galaxy with NASA's Chandra X-ray Observatory. They found 30 to 50 times less X-ray emission in the elliptical galaxies than they expected to see if accreting white dwarfs make up the majority of SN Ia progenitors.

"Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging," says Bogdan in a Chandra press release. "This is probably not what many astronomers would expect."

To find out if this was a valid claim, I consulted a number of leading experts in the field. Although everyone seemed to agree that Gilfanov and Bogdan took a creative approach toward addressing the problem, the replies were all over the map, and it's clear that the origin of SN Ia remains very much an open question.

Andrew Howell (University of California, Santa Barbara) pointed out that astronomers have known for years that there aren't enough accreting white dwarf binaries to explain the number of observed SN Ia. Speaking of this new study, Howell says, "It's not a leap, but a new step down a long path toward understanding the progenitors of SN Ia. The evidence has been building for years that the accreting-white-dwarf scenario cannot explain all SN Ia."

Alex Filippenko (University of California, Berkeley) adds, "I'm not disagreeing with Gilfanov and Bogdan that some Type Ia supernovae come from white dwarf mergers. People have been saying this for a long time. It's just that we've had trouble finding actual binary white dwarfs that have the potential to merge in the age of the universe."

Brad Schaefer (Louisiana State University) points out that the Gilfanov/Bogdan study assumes that all accreting white dwarf binaries shine in X-rays at a relatively uniform and steady rate, but that doesn't hold true in many known systems that contain a white dwarf and a normal star. "Their conclusion is clearly and easily wrong," he says.

Sumner Starrfield (Arizona State University) notes that astronomers are finding that SN Ia are not as uniform as once thought. "It now looks like there are at least two kinds of SN Ia, and they may occur in different types of galaxies. So, they are probably only addressing one type," he says.

Several astronomers also pointed out that a group led by Rosanne Di Stefano (Harvard-Smithsonian Center for Astrophysics) has already announced similar results, and that these results apply to galaxies of all types. But Di Stefano cautions, "We must be very careful about the implications for the models," because it's possible that accreting white dwarfs reprocess the X-ray radiation they emit, meaning it wouldn't necessarily show up in the Chandra observations of elliptical galaxies.

Despite this new study, the origin of SN Ia remains an unsolved problem; it's quite possible that these cataclysmic explosions have multiple origins. And it's a mystery that has much broader significance. As Schaefer points out, "The Type Ia supernova progenitor problem is one of the biggest and longest-lasting questions in astrophysics. And just 10 years ago the problem became uber-important."

Why? Because cosmologists can calibrate the luminosities of SN Ia, they use them to study the expansion history of the universe. By studying distant SN Ia with both ground- and space-based telescopes, two teams announced in 1998 that cosmic expansion is accelerating, which hints that most of the universe's energy is in some unexplained form known as dark energy.

Since 1998 astronomers have used completely independent methods to confirm the accelerating universe, so this new result casts no doubt that there's a lot of dark energy out there. But lacking a complete understanding of SN Ia makes it more difficult for astronomers to use them for cosmology studies, which will make it harder to pin down the precise details of the universe's expansion history, which in turn could make it more difficult to unravel the profound mystery of dark energy.

Jupiter and Venus to Cross Paths Feb. 16

Sometimes, if you're not paying close attention, the sky can play tricks on you.

For the past couple of months, Jupiter has been a prominent object in the southwestern sky just after sunset, what some would call "the evening star." On Feb. 16 there will be a celestial "bait and switch" as Venus replaces Jupiter as the evening star.

Because Earth moves much faster around the sun than Jupiter does, Jupiter appears to move ever-so-slowly from left to right relative to the sun, as seen in the Northern Hemisphere. One can see this effect over the course of a few nights by noting Jupiter's position at the same time each night. Jupiter has been sinking towards the sun night by night over the past two months.

Venus is closer to the sun than the Earth. It passed behind the sun on Jan. 11, from our point of view, and now is moving from right to left relative to the sun towards greatest eastern elongation on Aug. 20 – that's when it'll be farthest out to the side, forming a triangle with Earth and the sun as seen from above, and that's when Venus will appear highest in our sky.

On the evening of Tuesday, Feb. 16, from our vantage point, Jupiter (moving left to right) will pass Venus (moving right to left). The two planets will appear to be separated from each other by an amount equal to the diameter of our moon in the sky.

Even though Venus is much smaller in diameter than Jupiter, it will be very much brighter to the eye because it is much closer to the sun and to Earth than Jupiter.

On Feb. 16, Venus and Jupiter will easily fit in the field of a small telescope. Jupiter will appear more than three times the size of Venus, a brooding giant far beyond brilliant Venus.

Try to follow the action over the next week as the two planets rapidly approach each other. After Feb. 16, Jupiter will be gone behind the sun for the next month, emerging as a "morning star" late in March. Venus gradually gets farther from the sun and more brilliant, reaching greatest eastern elongation on Aug. 20 and greatest brilliancy on Sept. 23, before passing between Earth and sun on Oct. 29.

New Plan for NASA

New Plan for NASA

NASA officials today unveiled a new budget for the upcoming fiscal year, a plan that would significantly alter the future of human spaceflight. The Obama administration plan, if enacted by Congress, would cancel the Constellation program for returning astronauts to the Moon, but would greatly increase funding to develop new technologies that could enable future human missions to the Moon, near-Earth asteroids, and Mars. The plan would place greater reliance on private industry for ferrying humans to low-Earth orbit, and it would extend U.S. participation in the International Space Station to 2020.

Despite canceling a program in which $9 billion has already been spent, the plan increases NASA's overall funding by $6 billion over the next five years to about $100 billion total. By developing new propulsion technologies, developing capabilities such as on-orbit fuel depots, and new robotic precursor missions to study the environments for future astronauts, the administration and NASA seem to be betting that they can lower costs for deep-space exploration over the long haul. The plan also calls for more international cooperation.

"We will pursue a more sustainable, affordable approach to manned exploration, and facilitate the growth of a new commercial industry," said NASA administrator Charles Bolden during a Monday press conference.

The budget follows closely on the heels of a report issued last year by an independent, nonpartisan committee chaired by former aerospace executive Norman Augustine. The committee, which included former astronauts, engineers, and experts from the aerospace industry, spelled out in clear language what many have been saying for years: the Constellation Moon program has been given woefully inadequate funding to achieve its lofty goals, and it was putting NASA on an unsustainable trajectory toward failure.

Sally Ride, a member of the Augustine committee and America’s first woman in space, strongly endorsed the proposal during the press briefing. Apollo 11 astronaut Buzz Aldrin issued the following statement::

"Today I wish to endorse strongly the President's new direction for NASA. As an Apollo astronaut, I know the importance of always pushing new frontiers as we explore space. The truth is, that we have already been to the Moon — some 40 years ago. A near-term focus on lowering the cost of access to space and on developing key, cutting-edge technologies to take us further, faster, is just what our Nation needs to maintain its position as the leader in space exploration for the rest of this century. We need to be in this for the long haul, and this program will allow us to again be pushing the boundaries to achieve new and challenging things beyond Earth. I hope NASA will embrace this new direction as much as I do, and help us all continue to use space exploration to drive prosperity and innovation right here on Earth."

In general, I liked much of what I heard, and I was encouraged by the fact that the new NASA plan closely follows many of the recommendations set forth by the Augustine committee. Augustine says in a written statement, "The plan released with the President’s FY 2011 budget does appear to respond to the primary concerns highlighted in our committee's report." But I also felt the new plan involves considerable risks, and doesn't yet outline specific mission objectives and timetables.

What I found particularly interesting about the press conference was the Q&A session. Several reporters called in from states such as Florida, Texas, and Alabama — which have major NASA facilities (Kennedy, Johnson, and Marshall) that develop human spaceflight. The questions all centered around what this new plan would entail for jobs in these local areas, and one reporter said that the Florida Congressional delegation has already come out in opposition to the new plan (even though NASA officials claimed the plan would end up modernizing Kennedy and boosting funding for the center).

I need to learn more about this new plan and its ramifications before I can pass judgment. For the future of NASA, the U.S., and human spaceflight, I just hope that the Congressional meat grinder can ultimately evaluate the new NASA plan on its long-term merits and not on its short-term economic impact on local constituencies