Total Eclipse: Dark of the Moon

The December 21, 2010, lunar eclipse dazzled viewers across North America. This image is a two-exposure combination taken in Mead, Colorado, by attaching a digital camera to a 4-inch telescope.
Photo by Richard McCoy

by Michael E. Bakich
Published: June 6, 2011

Don't get scared. This title is not from Hollywood Movies. The first lunar eclipse of 2011 occurs June 15. The timing and the placement of the Moon in its orbit does not favor the Western Hemisphere, however.

Skywatchers can see the entire event from the eastern half of Africa, the Middle East, central Asia, and western Australia. At mid-eclipse, the Moon will lie near the zenith for observers situated in Réunion or Mauritius in the Indian Ocean.

Observers throughout Europe will miss the early stages of the eclipse because they occur before moonrise. But except for northern Scotland and northern Scandinavia, Europeans with clear skies will see totality (when the Moon lies completely within Earth’s umbra).

Likewise, eastern Asia, eastern Australia, and New Zealand will miss the last stages of eclipse because they occur after moonset, but, like those in Europe, most inhabitants will see the total phase. Observers in eastern Brazil, Uruguay, and Argentina also will witness totality. Unfortunately, none of the eclipse will be visible from North America.

A lunar eclipse occurs when the Moon in its orbit passes into Earth’s shadow. Because the Sun isn’t a point of light, the shadow has two parts — the inner, darker umbra and the outer, lighter penumbra. If the entire Moon enters the umbra, the eclipse is total. If the umbra hides only part of our satellite, the eclipse is partial.

This eclipse’s umbral phase begins at 18h22m56s UT (2:22:56 p.m. EDT). As the Moon dips deeper into our planet’s shadow during the next hour or so, darkness gradually overtakes the brilliant orb.

Earth’s shadow takes almost exactly 1 hour to envelop the Moon. Totality begins at 19h22m30s UT (3:22:30 p.m. EDT).

The Moon won’t disappear from view, however. Some sunlight passing through Earth’s atmosphere falls on the lunar surface. The cleaner our atmosphere is, the “lighter” the eclipse will be. “Dark” eclipses generally occur after large volcanic eruptions when the atmosphere contains more dust.

What color will the Moon turn at mideclipse? During previous total eclipses, the Moon has appeared brown, orange, crimson, and brick red. Lunar eclipses exhibit a range of shades because sunlight passing through Earth’s atmosphere during totality becomes scattered and reddened. It’s this dim glow that fills Earth’s shadow and lights the eclipsed Moon. The sky certainly will grow darker, allowing the bright summer stars surrounding our nearest celestial neighbor to spring back to prominence.

Totality lasts 100 minutes, which is rather long. The last eclipse to exceed this duration was in July 2000. During totality, the Moon’s southern edge may appear slightly darker than its northern side. This disparity occurs because the Moon’s southern limb lies a bit closer to the center of Earth’s shadow. After totality ends at 21h02m42s UT (5:02:42 p.m. EDT), it takes the Moon another 60 minutes to leave Earth’s umbra.

Astronomy magazine Contributing Editor Ray Shubinski describes the upcoming eclipse as a missed opportunity: “It’s too bad that nobody in North America will see the eclipse. Luckily, we don’t have all that long to wait until the next one.”

The eclipse to which Shubinski refers will occur December 10. But it won’t be perfect, either. For North Americans, that eclipse will still be in progress as the Moon sets. The farther west you live, however, the larger fraction of the eclipse you will see before moonset. The entire event will be visible for inhabitants of Asia and Australia under clear skies.

NASA probes suggest magnetic bubbles reside at edge of solar system

Old and new views of the heliosheath. Red and blue spirals are the gracefully curving magnetic field lines of orthodox models. New data from Voyager add a magnetic froth (inset) to the mix.
Photo by NASA

By NASA Headquarters, Washington, D.C.
Published: June 9, 2011

While using a new computer model to analyze Voyager data, scientists found the Sun’s distant magnetic field is made up of bubbles approximately 100 million miles wide. The bubbles are created when magnetic field lines reorganize. The new model suggests the field lines are broken up into self-contained structures disconnected from the solar magnetic field. The findings are described in the June 9 edition of the Astrophysical Journal.

Like Earth, the Sun has a magnetic field with a north pole and a south pole. The field lines are stretched outward by the solar wind or a stream of charged particles emanating from the star that interacts with material expelled from others in our corner of the Milky Way galaxy.

The Voyager spacecraft, more than nine billion miles away from Earth, are traveling in a boundary region. In that area, the solar wind and magnetic field are affected by material expelled from other stars in our corner of the Milky Way galaxy.

“The Sun’s magnetic field extends all the way to the edge of the solar system,” said astronomer Merav Opher of Boston University. “Because the Sun spins, its magnetic field becomes twisted and wrinkled, a bit like a ballerina’s skirt. Far, far away from the Sun, where the Voyagers are, the folds of the skirt bunch up.”

Understanding the structure of the Sun’s magnetic field will allow scientists to explain how galactic cosmic rays enter our solar system and help define how our star interacts with the rest of the galaxy.

So far, much of the evidence for the existence of the bubbles originates from an instrument aboard the spacecraft that measures energetic particles. Investigators are studying more information and hoping to find signatures of the bubbles in the Voyager magnetic field data.

“We are still trying to wrap our minds around the implications of the findings,” said University of Maryland physicist Jim Drake, one of Opher’s colleagues.

A new class of stellar explosions

The four supernovae discovered by the Palomar Transient Factory, (from top to bottom) PTF09atu, PTF09cnd, PTF09cwl, and PTF10cwr, are shown both before (left) and after (right) explosion.
Photo by Caltech/Robert Quimby/Nature

By California Institute of Technology, Pasadena
Published: June 10, 2011

They’re bright and blue — and a bit strange. They’re a new type of stellar explosion that was recently discovered by a team of astronomers led by the California Institute of Technology (Caltech). Among the most luminous in the cosmos, these new kinds of supernovae could help researchers better understand star formation, distant galaxies, and what the early universe might have been like.

“We’re learning about a whole new class of supernovae that wasn’t known before,” said Robert Quimby, a Caltech postdoctoral scholar and lead author on the paper. In addition to finding four explosions of this type, the team also discovered that two previously known supernovae, whose identities had baffled astronomers, also belong to this new class.

Quimby first made headlines in 2007 when — as a graduate student at the University of Texas at Austin — he discovered what was then the brightest supernova ever found: 100 billion times brighter than the Sun and 10 times brighter than most other supernovae. Dubbed 2005ap, it was also a little odd. For one thing, its spectrum — the chemical fingerprint that tells astronomers what the supernova is made of, how far away it is, and what happened when it blew up — was unlike any seen before. It also showed no signs of hydrogen, which is commonly found in most supernovae.

At around the same time, astronomers using the Hubble Space Telescope discovered a mysterious supernova called SCP 06F6. This supernova also had an odd spectrum, though there was nothing that indicated this cosmic blast was similar to 2005ap.

Caltech's Shri Kulkarni, a co-author on the paper, recruited Quimby to become a founding member of the Palomar Transient Factory (PTF). The PTF is a project that scans the skies for flashes of light that weren’t there before — flashes that signal objects called transients, many of which are supernovae. As part of the PTF, Quimby and his colleagues used the 1.2-meter Samuel Oschin Telescope at Palomar Observatory to discover four new supernovae. After taking spectra with the 10-meter Keck telescopes in Hawaii, the 5.1-meter telescope at Palomar, and the 4.2-meter William Herschel Telescope in the Canary Islands, the astronomers discovered that all four objects had an unusual spectral signature.

Quimby then realized that if you slightly shifted the spectrum of 2005ap — the supernova he had found a couple of years earlier — it looked a lot like these four new objects. The team then plotted all the spectra together. “Boom — it was a perfect match,” he recalled.

The astronomers soon determined that shifting the spectrum of SCP 06F6 similarly aligned it with the others. In the end, it turned out that all six supernovae are siblings, and that they all have spectra that are very blue — with the brightest wavelengths shining in the ultraviolet.

According to Quimby, the two mysterious supernovae — 2005ap and SCP 06F6 — had looked different from one another because 2005ap was 3 billion light-years away while SCP 06F6 was 8 billion light-years away. More-distant supernovae have a stronger cosmological redshift, a phenomenon in which the expanding universe stretches the wavelength of the emitted light, shifting supernovae spectra toward the red end.

The four new discoveries, which had features similar to 2005ap and SCP 06F6, were at an intermediate distance, providing the missing link that connected the two previously unexplained supernovae. “That’s what was most striking about this — that this was all one unified class,” said Mansi Kasliwal, a Caltech graduate student and co-author on the paper.

Although astronomers now know these supernovae are related, no one knows much else. “We have a whole new class of objects that can’t be explained by any of the models we’ve seen before,” Quimby says. What the researchers do know about them is that they are bright and hot — 10,000 to 20,000 kelvins; that they are expanding rapidly at 6,000 miles per second (10,000 km/s); that they lack hydrogen; and that they take about 50 days to fade away — much longer than most supernovae, whose luminosity is often powered by radioactive decay. So there must be some other mechanism that’s making them so bright.

One possible model that would create an explosion with these properties involves a pulsating star about 90 to 130 times the mass of the Sun. The pulsations blow off hydrogen-free shells, and when the star exhausts its fuel and explodes as a supernova, the blast heats up those shells to the observed temperatures and luminosities.

A second model requires a star that explodes as a supernova but leaves behind what’s called a magnetar, a rapidly spinning dense object with a strong magnetic field. The rotating magnetic field slows the magnetar down as it interacts with the sea of charged particles that fills space, releasing energy. The energy heats the material that was previously blown off during the supernova explosion and can naturally explain the brightness of these events.

The newly discovered supernovae live in dim, small collections of a few billion stars called dwarf galaxies. (Our own Milky Way has 200 to 400 billion stars.) The supernovae, which are almost a hundred times brighter than their host galaxies, illuminate their environments like distant street lamps lighting up dark roads. They work as a kind of backlight, enabling astronomers to measure the spectrum of the interstellar gas that fills the dwarf galaxies in which the supernovae reside, and revealing each galaxy’s composition. Once an observed supernova fades a couple of months later, astronomers can directly study the dwarf galaxy — which would have remained undetected if it weren’t for the supernova.

These supernovae could also reveal what ancient stars might have been like because they most likely originate from stars around a hundred times more massive than the Sun — stars that would have been very similar to the first stars in the universe.

“It is really amazing how rich the night sky continues to be,” Kulkarni said. “In addition to supernovae, the Palomar Transient Factory is making great advances in stellar astronomy as well.”