Innumerable stars reside within 1.6 light-years of the Milky Way’s central black hole. But this same crowded neighborhood has fewer red giants — luminous stars that are large and cool — than expected.
Now astrophysicists have a new theory why: The supermassive black hole, Sagittarius A*, launched a powerful jet of gas that ripped off the red giants’ outer layers. That transformed the stars into smaller red giants or stars that are hotter and bluer, Michal Zajaček, an astrophysicist at the Polish Academy of Sciences in Warsaw, and colleagues suggest in a paper published online November 12 in the Astrophysical Journal.
Today Sagittarius A* is quiet, but two enormous bubbles of gamma-ray-emitting gas rooted at the center of the Milky Way tower far above and below the galaxy’s plane (SN: 12/9/20). These gas bubbles imply the black hole sprang to life some 4 million years ago when something fell into it.
At that time, a disk of gas around the black hole shot a powerful jet of material into its star-studded neighborhood, Zajaček and colleagues propose. “The jet preferentially acts on large red giants,” he says. “They can be effectively ablated by the jet.” The biggest and brightest red giants seem to be missing near the galactic center, Zajaček says.

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Red giants are vulnerable because they are large and their envelopes of gas tenuous. A red giant forms from a smaller star after the star’s center gets so full of helium that it can no longer burn its hydrogen fuel there. Instead, the star starts to burn hydrogen in a layer around the center, which makes the star’s outer layers expand, causing its surface to cool and turn red. As a result, some red giants are more than a hundred times the diameter of the sun, making them easy pickings for the jet.
Still, Zajaček says that as red giants orbit the black hole, they must pass through the jet hundreds or thousands of times before becoming hot, blue stars. The jet is most effective at removing red giants within 0.13 light-years of the black hole, the team calculates.
“The idea is plausible,” says Farhad Yusef-Zadeh, an astronomer at Northwestern University in Evanston, Ill., who was not involved with the study.
Tuan Do, an astronomer at UCLA, adds “it may take a combination of several of these kinds of mechanisms to fully explain the lack of the red giants.” In particular, he says, something other than a jet likely accounts for the paucity of red giants farther away from the black hole.
One candidate, say Zajaček and Do, is a large disk of gas that circled the black hole a few million years ago. This disk spawned stars that now orbit the black hole in a single plane. These young stars exist as far as 1.6 light-years from the black hole, which is also the extent of the red giant gap. As red giants revolved around the black hole and repeatedly plunged through the disk, its gas may have torn off their outer layers, explaining another part of the galactic center’s red star shortage. More

A newly discovered exoplanet is really making astronomers prove their mettle. Planet GJ 367b is smaller than Earth, denser than iron and hot enough to melt, researchers report in the Dec. 3 Science.

“We think the surface of this exoplanet could be molten,” says astronomer Kristine Wei Fun Lam of the Institute of Planetary Research at the German Aerospace Center in Berlin.

Signals of the planet were first spotted in data from NASA’s TESS telescope in 2019. The small world swung around its host star every 7.7 hours.

Using data from TESS and the ground-based HARPS spectrograph at the European Southern Observatory in Chile, Lam and her colleagues measured the planet’s radius and mass. GJ 367b clocked in at about 0.72 times Earth’s radius and 0.55 times its mass. That makes it the first ultrashort-period planet — a class of worlds with years shorter than one Earth day and with mysterious origins — known to be smaller and lighter than Earth.

Using those measurements, the team then calculated the planet’s density: about 8.1 grams per cubic centimeter, or slightly denser than iron. A computer analysis of the planet’s interior structure suggests that 86 percent of it could comprise an iron core, with only a sliver of rock left on top.

Mercury has a similarly large core, Lam notes (SN: 4/22/19). Scientists think that’s a result of a giant impact with another planet that stripped away most of its outer layers. GJ 367b could have formed after a similar collision. It could also have once been a gaseous planet whose atmosphere was blasted off by radiation from its star (SN: 7/1/20).

Whatever its origins, GJ 367b is so close to its star that it’s almost certainly covered in melted metallic lava now. “At 1400° Celsius, I don’t think it would be very nice to stand on it,” Lam says. More

This year marked the end of a decades-long wait for astronomers. The James Webb Space Telescope is finally in action.

The telescope, which launched in December 2021, released its first science data in July (SN: 8/13/22, p. 30) and immediately began surpassing astronomers’ expectations.

“We’ve realized that James Webb is 10 times more sensitive than we predicted” for some kinds of observations, says astronomer Sasha Hinkley of the University of Exeter in England. His team released in September the telescope’s first direct image of an exoplanet (SN: 9/24/22, p. 6). He credits “the people who worked so hard to get this right, to launch something the size of a tennis court into space on a rocket and get this sensitive machinery to work perfectly. And I feel incredibly lucky to be the beneficiary of this.”

The telescope, also known as JWST, was designed to see further back into the history of the cosmos than ever before (SN: 10/9/21 & 10/23/21, p. 26). It’s bigger and more sensitive than its predecessor, the Hubble Space Telescope. And because it looks in much longer wavelengths of light, JWST can observe distant and veiled objects that were previously hidden.

JWST spent its first several months collecting “early-release” science data, observations that test the different ways the telescope can see. “It is a very, very new instrument,” says Lamiya Mowla, an astronomer at the University of Toronto. “It will take some time before we can characterize all the different observation modes of all four instruments that are on board.”

That need for testing plus the excitement has led to some confusion for astronomers in these heady early days. Data from the telescope had been in such high demand that the operators hadn’t yet calibrated all the detectors before releasing data. The JWST team is providing calibration information so researchers can properly analyze the data. “We knew calibration issues were going to happen,” Mowla says.

The raw numbers that scientists have pulled out of some of the initial images may end up being revised slightly. But the pictures themselves are real and reliable, even though it takes some artistry to translate the telescope’s infrared data into colorful visible light (SN: 3/17/18, p. 4).

The stunning photos that follow are a few of the early greatest hits from the shiny new observatory.

Deep space

NASA, ESA, CSA, STScI

JWST has captured the deepest views yet of the universe (above). Galaxy cluster SMACS 0723 (bluer galaxies) is 4.6 billion light-years from Earth. It acts as a giant cosmic lens, letting JWST zoom in on thousands of even more distant galaxies that shone 13 billion years ago (the redder, more stretched galaxies). The far-off galaxies look different in the mid-infrared light (above left) captured by the telescope’s MIRI instrument than they do in the near-infrared light (above right) captured by NIRCam. The first tracks dust; the second, starlight. Early galaxies have stars but very little dust.

Rings around Neptune

NASA, ESA, CSA, STSCI; IMAGE PROCESSING: JOSEPH DEPASQUALE/STSCI, NAOMI ROWE-GURNEY/NASA GODDARD SPACE FLIGHT CENTER

JWST was built to peer over vast cosmic distances, but it also provides new glimpses at our solar system neighbors. This pic of Neptune was the first close look at its delicate-looking rings in over 30 years (SN: 11/5/22, p. 5).

Under pressure

NASA, ESA, CSA, STScI, JPL-Caltech/NASA

The rings in this astonishing image are not an optical illusion. They’re made of dust, and a new ring is added every eight years when the two stars in the center of the image come close to each other. One of the stars is a Wolf-Rayet star, which is in the final stages of its life and puffing out dust. The cyclical dusty eruptions allowed scientists to directly measure for the first time how pressure from starlight pushes dust around (SN: 11/19/22, p. 6).

Galaxy hit-and-run

NASA James Webb Space Telescope/Flickr (CC BY 2.0)

With JWST’s unprecedented sensitivity, astronomers plan to compare the earliest galaxies with more modern galaxies to figure out how galaxies grow and evolve. This galactic smashup, whose main remnant is known as the Cartwheel galaxy, shows a step in that epic process (SN Online: 8/3/22). The large central galaxy (right in the above composite) has been pierced through the middle by a smaller one that fled the scene (not in view). The Hubble Space Telescope previously snapped a visible light image of the scene (top half). But with its infrared eyes, JWST has revealed much more structure and complexity in the galaxy’s interior (bottom half).

Exoplanet portrait

NASA, ESA, CSA, Aarynn Carter/UCSC, The ERS 1386 Team, Alyssa Pagan/STSCI

The gas giant HIP 65426b was the first exoplanet to have its portrait taken by JWST (each inset shows the planet in a different wavelength of light; the star symbol shows the location of the planet’s parent star). This image, released by astronomer Sasha Hinkley and colleagues, doesn’t look like much compared with some of the other spectacular space vistas from JWST. But it will give clues to what the planet’s atmosphere is made of and shows the telescope’s potential for doing more of this sort of work on even smaller, rocky exoplanets (SN: 9/24/22, p. 6).

Shake the dust off

NASA, ESA, CSA, STScI, Hubble Heritage Project/STScI/AURA; Image Processing: Joseph DePasquale, Anton M. Koekemoer and Alyssa Pagan/STScI

Another classic Hubble image updated by JWST is the Pillars of Creation. When Hubble viewed this star-forming region in visible light, it was shrouded by dust (above left). JWST’s infrared vision reveals sparkling newborn stars (above right). More

Source: Space & Astronomy – www.sciencenews.org More

Huge rings of gas surround a large red star named V Hydrae, new images show, signaling its eventual transformation into a much smaller and bluer star.

“It’s definitely going through its metamorphosis,” says Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Such ringlike structures have never been seen in any object like this before.”

Observations of the three concentric rings, all ejected from the star during the last 800 years, could help astronomers understand how giant stars lose mass toward the end of their lives and seed the cosmos with planet- and life-building elements.

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Born roughly twice as massive as the sun and lying about 1,300 light-years from Earth, V Hydrae is what’s known as an asymptotic giant branch star. It once fused hydrogen in its core, as the sun does. But now it is a cool, brilliant, puffed-up star that alternately burns hydrogen and helium in shells around a carbon-oxygen core. Such stars cast lots of material into space.

“The processes by which this happens are not well-understood,” says Sahai, who has studied V Hydrae since the 1980s.

His team used the Atacama Large Millimeter/submillimeter Array of radio telescopes in Chile, also known as ALMA, to detect the three rings of gas. Beyond them lie three additional rings, which are fainter and seen only partially, Sahai and colleagues report in a paper submitted February 18 at arXiv.org.

The outermost complete ring now sits about 260 billion kilometers from the star, or 1,740 times as far as Earth is from the sun — more than 40 times Pluto’s distance from Earth. By measuring the speed at which the three complete rings are moving outward and their current distances from the star, the astronomers calculate that it cast them off about 270, 485 and 780 years ago.

It’s thought that another star orbits the main one every few hundred years on an elliptical orbit. When the companion dives in, it can trigger the giant star to cast more material into space, the team says.

The new image is striking and unusual, and it illustrates how a companion star enhances a giant star’s loss of mass, says Joel Kastner, an astronomer at the Rochester Institute of Technology in New York who was not part of the study. “Mass loss is very important because it’s how the elements of life get distributed from stars into the universe.”

Stars like V Hydrae forged most of the nitrogen in Earth’s air as well as much of our planet’s carbon, the basis for all terrestrial life (SN: 2/12/21; SN: 11/18/21). V Hydrae has so many carbon compounds in its atmosphere that it’s classified as a carbon star. It’s also one of the reddest stars known because those compounds as well as dust particles absorb its blue and violet light.

Sahai expects the star’s ejection of material to continue, but, he says, “it’s anybody’s guess as to how many more rings will be produced.”

When the star loses all of its atmosphere, probably many thousands of years from now, it will expose its hot core, whose ultraviolet light will set the cast-off material aglow, creating a beautiful bubble of gas known as a planetary nebula.

When the nebula dissipates, all that will remain of the magnificent red star will be a tiny blue one — a white dwarf — a little larger than Earth, plus innumerable life-giving elements floating through the Milky Way. More