NASA Astronomy Picture of the Day

Apr 10 2023

IC 2944: The Running Chicken Nebula

To some, it looks like a giant chicken running across the sky. To others, it looks like a gaseous nebula where star formation takes place. Cataloged as IC 2944, the Running Chicken Nebula spans about 100 light years and lies about 6,000 light years away toward the constellation of the Centaur (Centaurus). The featured image, shown in scientifically assigned colors, was captured recently in a 16-hour exposure over three nights. The star cluster Collinder 249 is visible embedded in the nebula's glowing gas. Although difficult to discern here, several dark molecular clouds with distinct shapes can be found inside the nebula.

https://apod.nasa.gov/apod/astropix.html?

Historic Nebula Seen Like Never Before With NASA's IXPE

Apr 7, 2023

On Feb. 22, 1971, a sounding rocket lifted off from Wallops Island, Virginia, with specialized sensors aimed at the Crab Nebula, a bright cosmic object 6,500 light-years away. In those days, before recovering physical tapes from the experiment, scientists first received scientific data on a strip chart recorder, a device that printed signals on paper. Astronomer Martin Weisskopf and his colleagues began their analysis on launch day by measuring the distance between signals using a ruler and pencil.

“What makes science so beautiful and exciting is that for those few moments, you're seeing something that no one has ever seen before,” said Weisskopf, now an emeritus astronomer at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

Decades later, Weisskopf proposed the development of an Earth-orbiting satellite with powerful instruments that could gather much more detailed measurements of the same kind about the Crab Nebula and other mysterious cosmic objects. That satellite became NASA’s Imaging X-ray Polarimetry Explorer (IXPE), which launched on December 9, 2021.

Now, more than 50 years after the sounding rocket experiment, scientists have used IXPE to create a detailed, nuanced map of the Crab Nebula’s magnetic field, revealing more of its inner workings than ever before. The new results, published in the journal Nature Astronomy (preprint available), help resolve longstanding mysteries about the well-studied Crab Nebula and open new questions for future study.

IXPE data show that the Crab Nebula’s magnetic field resembles that of the Vela Pulsar Wind Nebula, which is also donut-shaped. But at the Crab, scientists were surprised that areas of magnetic field turbulence were more patchy and asymmetrical than expected.

“This is a clear indication that even the more complex models developed in the past, with the use of advanced numerical techniques, do not fully capture the complexity of this object,” said Niccolò Bucciantini, lead author of the study and astronomer at the INAF Arcetri Observatory in Florence, Italy.

A favorite object of study among astronomers, the Crab Nebula resulted from a supernova documented in the year 1054. The explosion left behind a dense object called the Crab Pulsar, about the diameter of Huntsville, Alabama or the length of Manhattan, but with as much mass as about two Suns. The chaotic mess of gases, shock waves, magnetic fields and high-energy light and particles coming from the rotating pulsar is collectively called a “pulsar wind nebula.” These extreme conditions make for a bizarre environment that is not yet thoroughly understood.

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Weisskopf and colleagues were hoping to understand this extreme environment in a new way by measuring the polarization of X-rays from the Crab Nebula, which shines brightly in X-rays. X-ray polarization gives scientists clues to the direction where the magnetic field points in different parts of a cosmic object, as well as how well ordered the magnetic field is. The magnetic field’s geometry and turbulence determines how particles get catapulted toward the speed of light.

In the five minutes that the 1971 sounding rocket experiment spent above Earth’s atmosphere, it produced the world’s first X-ray polarization measurements.

Scientists followed up with a satellite called OSO-8 in 1975, which also measured the X-ray polarization of the Crab Nebula. The rocket and the satellite produced generally the same result: That the Crab Nebula has an average polarization of about 20%.

As project scientist of NASA’s Chandra X-Ray Observatory, which launched in 1999, Weisskopf continued his exploration of the Crab Nebula in new ways. With Chandra, “we took beautiful images of the nebula and pulsar, and we could see the jets and the various structures,” he said. Chandra’s X-ray imaging revealed wisp-like structures that move in the nebula, and helped scientists to further understand the relationship between the pulsar’s energy and X-ray emissions.

Nearly every recent large telescope has pointed to the Crab Nebula to better understand this mysterious supernova remnant. But only IXPE can study X-rays from Crab in terms of polarization, a measure of the organization of electromagnetic fields.

“The Crab is one of the most-studied high-energy astrophysical objects in the sky. So it is extremely exciting that we could learn something new about this system by looking through IXPE's ‘polarized lenses,’” said Michela Negro, a research scientist at NASA Goddard Space Flight Center affiliated with the University of Maryland, Baltimore, and a co-author of the study.

Across the entire nebula, IXPE found about the same average polarization as Weisskopf and colleagues did in the 1970s. But with more sophisticated instruments, IXPE was able to refine the angle of polarization and examine the differences in polarization across the entire object. Scientists see areas of much polarization in the outer regions of the nebula, light-years away from the pulsar, where polarization is lower.

This enabled scientists to investigate not just X-rays from the Crab Nebula but also those coming from the pulsar itself, or the sphere of magnetic fields around it. The findings suggest that those X-rays originate in the outer magnetic field region, called the “wind” region, although exactly where and how is still unknown. Within the magnetic field, shocks generated by the pulsar’s “wind” are propelling particles close to the speed of light.

“I'm very proud of everybody associated with IXPE,” said Weisskopf, who was the mission’s first principal investigator. “Everybody has worked so hard, and it works as advertised.” Reflecting on his work on the 1971 experiment that laid the groundwork for the new results, Weisskopf says, “It's like somebody said to me, ‘Martin, you did good.’”

https://www.nasa.gov/mission_pages/ixpe/feature/nasa-s-ixpe-unveils-crab-nebula-s-magnetic-field-structure

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Webb Reveals Never-Before-Seen Details in Cassiopeia A

Apr 7, 2023

The explosion of a star is a dramatic event, but the remains the star leaves behind can be even more dramatic. A new mid-infrared image from NASA’s James Webb Space Telescope provides one stunning example. It shows the supernova remnant Cassiopeia A (Cas A), created by a stellar explosion 340 years ago from Earth’s perspective.. Cas A is the youngest known remnant from an exploding, massive star in our galaxy, which makes it a unique opportunity to learn more about how such supernovae occur.

“Cas A represents our best opportunity to look at the debris field of an exploded star and run a kind of stellar autopsy to understand what type of star was there beforehand and how that star exploded,” said Danny Milisavljevic of Purdue University in West Lafayette, Indiana, principal investigator of the Webb program that captured these observations.

“Compared to previous infrared images, we see incredible detail that we haven't been able to access before,” added Tea Temim of Princeton University in Princeton, New Jersey, a co-investigator on the program.

Cassiopeia A is a prototypical supernova remnant that has been widely studied by a number of ground-based and space-based observatories, including NASA’s Chandra X-ray Observatory. The multi-wavelength observations can be combined to provide scientists with a more comprehensive understanding of the remnant.

Dissecting the Image

The striking colors of the new Cas A image, in which infrared light is translated into visible-light wavelengths, hold a wealth of scientific information the team is just beginning to tease out. On the bubble’s exterior, particularly at the top and left, lie curtains of material appearing orange and red due to emission from warm dust. This marks where ejected material from the exploded star is ramming into surrounding circumstellar gas and dust.

Interior to this outer shell lie mottled filaments of bright pink studded with clumps and knots. This represents material from the star itself, which is shining due to a mix of various heavy elements, such as oxygen, argon, and neon, as well as dust emission.

“We’re still trying to disentangle all these sources of emission,” said Ilse De Looze of Ghent University in Belgium, another co-investigator on the program.

The stellar material can also be seen as fainter wisps near the cavity’s interior.

Perhaps most prominently, a loop represented in green extends across the right side of the central cavity. “We’ve nicknamed it the Green Monster in honor of Fenway Park in Boston. If you look closely, you’ll notice that it’s pockmarked with what look like mini-bubbles,” said Milisavljevic. “The shape and complexity are unexpected and challenging to understand.”

Origins of Cosmic Dust – and Us

Among the science questions that Cas A may help answer is: Where does cosmic dust come from? Observations have found that even very young galaxies in the early universe are suffused with massive quantities of dust. It’s difficult to explain the origins of this dust without invoking supernovae, which spew large quantities of heavy elements (the building blocks of dust) across space.

However, existing observations of supernovae have been unable to conclusively explain the amount of dust we see in those early galaxies. By studying Cas A with Webb, astronomers hope to gain a better understanding of its dust content, which can help inform our understanding of where the building blocks of planets and ourselves are created.

“In Cas A, we can spatially resolve regions that have different gas compositions and look at what types of dust were formed in those regions,” explained Temim.

Supernovae like the one that formed Cas A are crucial for life as we know it. They spread elements like the calcium we find in our bones and the iron in our blood across interstellar space, seeding new generations of stars and planets.

“By understanding the process of exploding stars, we’re reading our own origin story,” said Milisavljevic. “I’m going to spend the rest of my career trying to understand what’s in this data set.”

The Cas A remnant spans about 10 light-years and is located 11,000 light-years away in the constellation Cassiopeia.

https://www.nasa.gov/feature/goddard/2023/webb-reveals-never-before-seen-details-in-cassiopeia-a