A predawn Moon-and-planets meetup, a returning comet, a great chance to see the Milky Way, and Saturn’s rings at a new angle.
Skywatching Highlights
July 7: Last Quarter Moon
July 11 + 12: Dawn alignment of the Moon, Mars, Saturn, and Uranus
July 14: New Moon; best dark-sky window for Comet 10P/Tempel 2 and the Milky Way
Later in July: Saturn’s unusually thin rings are a rewarding telescope target
July 21: First Quarter Moon
July 29: Full Moon
Transcript
An early morning hangout with the Moon and planets, a comet swings by, prime time for the Milky Way, and Saturn’s rings shine at a new angle. That’s What’s Up for July.
Before sunrise on July 11 and 12, look toward the eastern sky for a lineup of the Moon and planets. On these mornings, the waning crescent Moon helps point the way to Mars, with Saturn shining nearby in the morning sky.
Uranus is in the same general part of the sky, too, but it is much fainter, so you will need binoculars or a telescope to see it.
Mars will look like a small reddish point of light, Saturn is brighter and easier to spot, and the Moon makes the whole scene easy to locate.
Before sunrise on July 11 and 12, the Moon, Mars, Saturn, and Uranus will parade in the eastern sky.
NASA/JPL-Caltech
Around the New Moon on July 14, Comet 10P/Tempel 2 swings by.
This is a short-period comet, meaning it returns to the inner solar system on a regular orbit. In this case, it comes back about every 5½ years. It is not a dramatic comet that you see just by looking up at the sky, though.
Through binoculars or a telescope, find the constellation Capricornus and look for a small fuzzy glow nearby, possibly with a brighter central knot and a short, broad, fan-shaped tail.
For the best chance to view the comet, head somewhere dark, away from city lights. Start looking once the sky is fully dark, ideally about 45 to 60 minutes after sunset.
NASA/JPL-Caltech
Those same dark nights around the July 14 New Moon are also the best time this month to look for the Milky Way.
From a dark location, away from city lights, the Milky Way appears as a pale, cloudy band across the summer sky. The bright, cloudy region of the Milky Way marks the direction of the galactic center. It looks so dense because we’re looking toward one of the most crowded parts of our galaxy, where countless stars glow behind dark clouds of cosmic dust.
Late in the evening, look low in the southern sky for a group of stars shaped like a big hook or scorpion tail. That’s Scorpius. The bright, cloudy part of the Milky Way is nearby, close to another group of stars called Sagittarius.
For the best chance to see the Milky Way, go somewhere dark, give your eyes time to adjust, and try not to look at your phone.
NASA/JPL-Caltech
Later in July, Saturn is a rewarding target for telescope users.
Saturn’s rings are still tilted at a very shallow angle from our point of view, making them look unusually thin. The rings aren’t disappearing, but how they appear from Earth is changing. It’s a great reminder that our view of the solar system is always in motion.
Saturn is famous for the intriguing rings that encircle it. As Saturn orbits the Sun, though, our view of its rings changes. Roughly every 15 years (halfway through Saturn’s almost-30-year orbit), Saturn’s rings appear edge-on, sometimes seeming to disappear altogether. On Feb. 24, 2009, when Saturn’s rings were nearly edge-on, Hubble tracked four of Saturn’s moons as they passed across the face of the giant ringed planet.
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Here are the phases of the Moon for July.
NASA/JPL-Caltech
You can stay up to date on all of NASA’s missions exploring the solar system and beyond at science.nasa.gov. I’m Raquel Villanueva from NASA’s Jet Propulsion Laboratory, and that’s What’s Up for this month.
This sequence begins with an artist’s concept showing the Milky Way galaxy as seen from above, with the estimated positions of spiral arms based on previous data. Next is an updated artist’s concept of the Milky Way, where the positions of the two spiral arms most distant from the center of the galaxy have been adjusted based on newly processed X-ray data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. Both arms may be more distant than previously thought.
NASA/CXC/A. Hobart
A new result using NASA’s Chandra X-ray Observatory shows that the outer spiral arms in the Milky Way galaxy may reach wider than previously thought. This finding may lead astronomers to adjust their understanding of our home galaxy’s structure.
A team of astronomers made this discovery by making precise measurements of distances to dust clouds in the Milky Way’s spiral arms using data from both NASA’s Chandra and XMM-Newton, an ESA (European Space Agency) mission with NASA contributions. The results are described in a new paper published Wednesday in the Astronomy & Astrophysics journal.
The researchers determined the distances by studying rings around gamma-ray bursts, some of the brightest bursts of light in the universe, which arise from the collapse of massive stars or the merger of neutron stars. They are located at enormous distances, well beyond the confines of our galaxy.
An artist’s concept showing the Milky Way galaxy as seen from above, with the estimated positions of spiral arms based on previous data, in blue. Overlaid on this is an updated view of the Milky Way showing different positions for the two outermost spiral arms, shown in red and bordered by dashed lines. Both arms may be more distant than previously thought, based on newly processed X-ray data from Chandra and XMM.
NASA/CXC/SAO/M.Weiss
This distance measurement technique capitalized on the phenomenon of light echoes, where the light from the gamma-ray burst bounced off dust clouds in the spiral arms. The diameters of the rings in X-rays give the distances to Earth, with larger rings being generated by dust clouds closer to us.
“This is a very direct way – relying only on geometry – to precisely measure distances to the Milky Way’s spiral arms,” said Beatrice Vaia, who led the study while a PhD student in a joint program between Scuola Universitaria Superiore IUSS Pavia and University of Trento in Italy. “Most other methods rely on assumptions about how the Milky Way rotates, which become increasingly uncertain in the outer regions of our galaxy.”
Despite a century of awareness of the Milky Way’s spiral arms, astronomers are still working toward precise characterization of its arms because of Earth’s position within one. Dust and gas also block the view to other arms.
The researchers used three different gamma-ray bursts to determine the distances to three spiral arms in the Milky Way. In order of increasing distances from the Galactic Center, they are the Perseus, the Outer, and the Outer Scutum-Centaurus arms. Along the direction of one of the bursts, they found that both the Outer and Outer Scutum-Centaurus arms are about 10% more distant than astronomers previously thought.
“The differences are small, but any revision of these distances is important because they are so fundamental for understanding our galaxy,” said co-author Ilaria Fornasiero, who was a PhD student in the same program as the leading author. “For example, this could mean that astronomers have to revise estimates of the mass of the galaxy, because that affects how wide the arms stretch.”
The images include X-ray data from Chandra and optical data from Pan-STARRS. The composite image shows X-ray rings generated by a gamma-ray burst (GRB), a bright X-ray source located outside our galaxy. In a phenomenon called light echoes, the X-rays from the GRB bounced off dust clouds in the spiral arms of our galaxy. The diameters of the rings in the Chandra data give the distances of the dust clouds to Earth, with larger rings being generated by dust clouds closer to us. The GRB is located at the center of the circles defining the rings, to the left of the X-ray data outlined by the white square.
The team also used their data to estimate that the dust cloud in the most distant arm is about 3,500 light-years wide. These findings show that their measurements apply to the full thickness of the spiral arm, rather than a random, isolated dust cloud that may not fully be representative of the arm’s location.
While this technique provided major improvements in accuracy according to the researchers, it may be difficult to use it for further measurements because bright gamma-ray bursts that are visible through the plane of the galaxy are rare.
“We’re relying on the universe to provide us with these events, and so far, over 25 years, we’ve only found a handful that we can use,” said co-author Andrea Tiengo of Scuola Universitaria Superiore IUSS Pavia. “That said, we will continue to be on the lookout for more.”
NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
This release features a short video and a series of images, all related to an updated understanding of our home galaxy’s structure. By studying rings of X-ray light echoes, researchers now believe that two of the Milky Way’s spiral arms may be more distant from the center of the galaxy than previously thought.
The updated understanding of the structure of the Milky Way is highlighted in a short video, which compares two artist concept images. In both images, our spiral Milky Way galaxy is shown face-on. It has a bright white core with several arms that spiral out from the center, like long thin clouds corkscrewing counterclockwise. The two longest arms make a full rotation of the spiral galaxy, and curve all the way around to the upper right of the images.
The first image in the video shows the previous understanding of the Milky Way. Here, the two longest arms are curled around the core in a fairly tight spiral. In the second image, which represents the updated understanding, the two longest arms are more loosely spiraled. Visually, this means there is more open space between the curving arms, which are further away from the bright galaxy core. The video fades back and forth between the two artist concept images to illustrate the structural differences between the two understandings.
These findings are further shown by a static image which overlays the new understanding on top of the earlier understanding. In this artist’s concept illustration, dotted lines and different colors are used to differentiate between the two.
A team of astronomers made this discovery by studying gamma-ray bursts that bounce off of dust clouds in the galaxy’s spiral arms. The resulting rings of X-rays, known as light echoes, were detected and mapped by NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. In a supplemental data image, the light echoes resemble concentric arches of neon blue dots trailing across a speckled sky.
Identifying the position of the Milky Way’s spiral arms through X-ray light echoes has allowed astronomers to use geometry, rather than assumptions about galaxy rotation, to better understand the structure of our galaxy.
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