Roman Brajsa Hvar Observatory, University of Zagreb
Ivica Skokic Astronomical Institute of the Czech Academy of Sciences Ondrejov, Czech Republic
Astronomers have harnessed the Atacama Large Millimeter/submillimeter Array (ALMA)‘s capabilities to image the millimetre-wavelength light emitted by the Sun’s chromosphere — the region that lies just above the photosphere, which forms the visible surface of the Sun. The solar campaign team, an international group of astronomers with members from Europe, North America and East Asia, produced the images as a demonstration of ALMA’s ability to study solar activity at longer wavelengths of light than are typically available to solar observatories on Earth.
Astronomers have studied the Sun and probed its dynamic surface and energetic atmosphere in many ways through the centuries. But, to achieve a fuller understanding, astronomers need to study it across the entire electromagnetic spectrum, including the millimetre and submillimetre portion that ALMA can observe.
Peering into the heart of the Milky Way galaxy, NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) has spotted a mysterious glow of high-energy X-rays that, according to scientists, could be the “howls” of dead stars as they feed on stellar companions.
“We can see a completely new component of the center of our galaxy with NuSTAR’s images,” said Kerstin Perez of Columbia University in New York, lead author of a new report on the findings in the journal Nature. “We can’t definitively explain the X-ray signal yet — it’s a mystery. More work needs to be done.”
The center of our Milky Way galaxy is bustling with young and old stars, smaller black holes and other varieties of stellar corpses — all swarming around a supermassive black hole called Sagittarius A*.
NuSTAR, launched into space in 2012, is the first telescope capable of capturing crisp images of this frenzied region in high-energy X-rays. The new images show a region around the supermassive black hole about 40 light-years across. Astronomers were surprised by the pictures, which reveal an unexpected haze of high-energy X-rays dominating the usual stellar activity.
“Almost anything that can emit X-rays is in the galactic center,” said Perez. “The area is crowded with low-energy X-ray sources, but their emission is very faint when you examine it at the energies that NuSTAR observes, so the new signal stands out.”
Astronomers have four theories to explain the baffling X-ray glow, three of which involve different classes of stellar corpses. When stars die, they don’t always go quietly into the night. Unlike stars like our sun, collapsed dead stars that belong to stellar pairs, or binaries, can siphon matter from their companions. This zombie-like “feeding” process differs depending on the nature of the normal star, but the result may be an eruption of X-rays.
According to one theory, a type of stellar zombie called a pulsar could be at work. Pulsars are the collapsed remains of stars that exploded in supernova blasts. They can spin extremely fast and send out intense beams of radiation. As the pulsars spin, the beams sweep across the sky, sometimes intercepting Earth, like lighthouse beacons.
“We may be witnessing the beacons of a hitherto hidden population of pulsars in the galactic center,” said co-author Fiona Harrison of the California Institute of Technology in Pasadena, principal investigator of NuSTAR. “This would mean there is something special about the environment in the very center of our galaxy.”
Other possible culprits include heavy-set stellar corpses called white dwarfs, which are the collapsed, burned-out remains of stars not massive enough to explode in supernovae. Our sun is such a star, and is destined to become a white dwarf in about five billion years. Because these white dwarfs are much denser than they were in their youth, they have stronger gravity and can produce higher-energy X-rays than normal. Another theory points to small black holes that slowly feed off their companion stars, radiating X-rays as material plummets down into their bottomless pits.
Alternatively, the source of the high-energy X-rays might not be stellar corpses at all, astronomers say, but rather a diffuse haze of charged particles called cosmic rays. The cosmic rays might originate from the supermassive black hole at the center of the galaxy as it devours material. When the cosmic rays interact with surrounding, dense gas, they emit X-rays.
However, none of these theories match what is known from previous research, leaving the astronomers largely stumped.
“This new result just reminds us that the galactic center is a bizarre place,” said co-author Chuck Hailey of Columbia University. “In the same way people behave differently walking on the street instead of jammed on a crowded rush-hour subway, stellar objects exhibit weird behavior when crammed in close quarters near the supermassive black hole.”
The team says more observations are planned. Until then, theorists will be busy exploring the above scenarios or coming up with new models to explain what could be giving off the puzzling high-energy X-ray glow.
“Every time that we build small telescopes like NuSTAR, which improve our view of the cosmos in a particular wavelength band, we can expect surprises like this,” said Paul Hertz, the astrophysics division director at NASA Headquarters in Washington.
NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory in Pasadena, California, for NASA’s Science Mission Directorate in Washington.
(PRESS RELEASE – NASA): NASA Television will air an event from 1 – 2 p.m. EDT on Tuesday, April 7, featuring leading science and engineering experts discussing the recent discoveries of water and organics in our solar system, the role our sun plays in water-loss in neighboring planets, and our search for habitable worlds among the stars.
The event, which is open to the public, will take place in the Webb Auditorium at NASA Headquarters, 300 E Street SW in Washington.
The panel also will highlight the fundamental questions NASA is working to answer through its cutting-edge science research: Where do we come from? Where are we going? Are we alone?
Panel participants include:
John Grunsfeld, astronaut and Science Mission Directorate associate administrator, NASA Headquarters, Washington
Ellen Stofan, chief scientist, NASA Headquarters
James Green, director of Planetary Science, NASA Headquarters
Jeffery Newmark, interim director of Heliophysics, NASA Headquarters
Paul Hertz, director of Astrophysics, NASA Headquarters
While members of the press can participate by phone, both the media and the public also may ask questions during the event via Twitter using the hashtag #askNASA.
For NASA TV streaming video, schedules and downlink information, visit: