Science News

The Universe in Black and White

The Universe in Black and White

Contained within the most massive and active star-forming region in the Small Magellanic Cloud, star cluster NGC 346 delivers energetic radiation that excites nearby gas, causing it to glow. The result is one of the most dynamic and intricately detailed images of a nearby star-forming region that has ever been taken by the Hubble Space Telescope.

The complexity of the gas and dust that surrounds NGC 346 shows a series of arched, ragged filaments and a distinct ridge created when high-energy radiation from the young, hot stars eats into the denser areas of the nearby interstellar medium. The intricate, dark beaded edge of the ridge, seen from Hubble’s perspective in silhouette, contains several small dust globules that point back towards the central cluster.

Energetic outflows and radiation from hot young stars are eroding the dense outer portions of the star-forming region, formally known as N66, exposing new stellar nurseries. The diffuse natal parts of the nebula block the energetic outflows from streaming away from the cluster, leaving a series of filaments marking the path of the outflows.

Image Credit: NASA, ESA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: A. Nota (STScI/ESA)

A Vivid View

Supernova Tycho Brahe

More than four centuries after the brilliant star explosion witnessed by Tycho Brahe and other astronomers of the era, NASA’s Spitzer and Chandra space observatories and the Calar Alto observatory in Spain captured this image of the supernova remnant. This composite image combines infrared and X-ray observations.

The explosion left a blazing hot cloud of expanding debris (green and yellow). The location of the blast’s outer shock wave can be seen as a blue sphere of ultra-energetic electrons. Newly synthesized dust in the ejected material and heated pre-existing dust from the area around the supernova radiate at infrared wavelengths of 24 microns (red). Foreground and background stars in the image are white.

Image Credit: MPIA/NASA/Calar Alto Observatory

Witch Head Nebula

Witch Head Nebula

As the name implies, this reflection nebula associated with the star Rigel looks suspiciously like a fairytale crone. Formally known as IC 2118 in the constellation Orion, the Witch Head Nebula glows primarily by light reflected from the star. The color of this very blue nebula is caused not only by blue color of its star, but also because the dust grains reflect blue light more efficiently than red. A similar physical process causes Earth’s daytime sky to appear blue.

Image Credit: NASA/STScI Digitized Sky Survey/Noel Carboni

Enchanting Saturn

Enchanting Saturn

With giant Saturn hanging in the blackness and sheltering Cassini from the sun’s blinding glare, the spacecraft viewed the rings as never before.

This marvelous panoramic view was created by combining a 165 images taken by the Cassini wide-angle camera over nearly three hours on Sept. 15, 2006. The mosaic images were acquired as the spacecraft drifted in the darkness of Saturn’s shadow for about 12 hours, allowing a multitude of unique observations of the microscopic particles that comprise Saturn’s faint rings.

The narrowly confined G ring is easily seen here, outside the bright main rings. Encircling the entire system is the much more extended E ring. The icy plumes of Enceladus, whose eruptions supply the E ring particles, betray the moon’s position in the E ring’s left side edge.

Interior to the G ring and above the brighter main rings is the pale dot of Earth. Cassini views its point of origin from close to a billion miles away in the icy depths of the outer solar system.

Image Credit: NASA/JPL/Space Science Institute

Genome sequence deepens mystery of inconspicuous sea creature

Trichoplax adhaerens. Ana Signorovitch/Yale UniversityResembling a smudge more than an animal, a mysterious life form known as a placozoan has now joined other obscure and primitive creatures whose genomes are providing insight into how animals first arose more than 650 million years ago.

Called Trichoplax adhaerens, the creature’s genome sequence, reported in the Aug. 21 issue of Nature, has surprised some scientists, who thought the organism was so rudimentary - possibly the simplest free-living animal known - that it had to be the most ancient animal ancestor.

“Because of how simple they look, people hypothesized that Trichoplax represented an early form of animal life. Other people thought they were just simplified cnidarians - jellyfish and hydras,” said first author Mansi Srivastava, a University of California, Berkeley graduate student in the Department of Molecular and Cell Biology. “We wanted to figure out where they fit with other animals, in particular the cnidarians, the sponges and more complex animals.”

A microscopic pancake only three cells thick and a few millimeters across, Trichoplax looks like a multicellular amoeba. It has a top, a bottom and a one-cell thick interior, and it appears to have no nerve, sensory or muscle cells. It has never been observed reproducing sexually; it usually just splits into two nondescript splotches.

Based on the genome analysis, Trichoplax is not our earliest ancestor, but seems to have separated from the animal lineage after the sponges and before the jellyfish and later “bilaterians” - animals, like humans, whose right and left sides are mirror images of one another.

Though Trichoplax is not at the base of the animal family tree, “comparing its genome with other genomes can tell us what the basic molecular toolkit was in the common ancestor of all animals,” Srivatava said.

The Trichoplax genome was sequenced by the U.S. Department of Energy’s Joint Genome Institute (JGI) in Walnut Creek, Calif. Over the past few years, JGI researchers have sequenced many basal animal genomes, including those of a jellyfish, sea squirt, choanoflagellate and sea anemone.

“If you want to understand the early origin and diversification of animals, you have to infer this from the fossil record or the genome sequences of animals that lived and diverged early, most of which didn’t leave fossils,” said senior author Daniel S. Rokhsar, a faculty member at UC Berkeley’s Center for Integrative Genomics and program head for computational genomics at JGI.

Placozoans were discovered in 1880 on the glass of a saltwater aquarium and then promptly forgotten until the 1970s, when the late German biologist Karl Grell discovered how to keep them alive and first studied their biology. The strain of Trichoplax sequenced is a direct descendent of Grell’s saltwater collection provided by Grell’s successor and study co-author, Bernd Schierwater, director of the Institute of Animal Ecology & Cell Biology and head of the Center for Biodiversity at the University of Veterinary Medicine in Hannover, Germany.

Since then, placozoans have been discovered throughout the world’s subtropical oceans, mostly near shore and particularly among mangroves. Though various strains have been identified, Trichoplax is the only placozoan species that has been named.

Based on the new sequencing data, the genome of Trichoplax contains 11,514 genes, compared with 20,000 to 25,000 genes in humans. By comparing about 100 of these genes with their homologs in other creatures, the research team was able to construct a tentative family tree for the animals.

Despite the hundreds of millions of years separating placozoans and humans, both still have many of the same genes clustered together in their genomes.

The fact that placozoans have the genes for more complex behavior makes their simple lifestyle all the more surprising. Rokhsar suspects there is much to be discovered about the behavior and lifestyle of these creatures, perhaps even a secret sex life.

“It’s remarkable that we have the whole genome sequence but we still know so little about this animal in the wild,” he said. “Hopefully, the genome sequence will stimulate more studies of this enigmatic creature.”

“Some of our new placozoan species show frequent sexual reproduction, while others never show any signs of sex,” added Schierwater. “The genome data allow us to search for the genes responsible for sex and life cycle complexity.”

One intriguing aspect of the animal is its feeding style. It eats by sliding atop its food and arching its “back” to create a cavity underneath into which it secretes digestive enzymes. Placozoan eating may be a precursor to the one-way, sac-like gut of jellyfish and other cnidarians, which later evolved into the through-gut of bilaterians, including humans.

“It can make a gut as needed, so it could be the original proto-animal,” Rokhsar said.

By Robert Sanders. UC Berkeley