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NASA’s Astronomy Picture of the Day for September 30, 2026, features Arp 78, also known as NGC 772, in the constellation Aries. The galaxy’s prominent outer spiral arm is lined with dust lanes and young blue star clusters; NASA’s explanation says gravitational tides from nearby companion NGC 770 are a likely cause.
NASA’s Astronomy Picture of the Day for September 30, 2026, features Arp 78, a large spiral galaxy in Aries whose prominent outer arm gives it an asymmetric appearance. The image and accompanying explanation identify the galaxy as NGC 772 and say gravitational interactions with its smaller companion, NGC 770, are likely responsible for the arm’s development.
The APOD description places Arp 78 about 100 million light-years beyond the stars and nebulae of the Milky Way. It gives the galaxy a span of roughly 200,000 light-years. The featured telescopic portrait shows a broad spiral structure and an especially conspicuous outer arm, which the explanation describes as overdeveloped compared with the rest of the galaxy.
Along the arm, the image reveals sweeping dust lanes and lines of young blue star clusters. NASA’s account says the arm is “pumped-up by galactic-scale gravitational tides.” It links those tides, likely, to interactions with NGC 770, the more compact galaxy visible directly below Arp 78 in the image.
The APOD description distinguishes the two galaxies by appearance: NGC 770 looks fuzzy and elliptical, while Arp 78 has a clear spiral form. Both are set against faint star streams in the deep exposure. Spiky stars in the foreground belong to the Milky Way, rather than to the distant galaxy pair.
A Companion May Shape the Outer Arm
The image draws attention to how a galaxy’s structure can reflect its surroundings. Arp 78’s pronounced arm is not presented simply as a visual feature: NASA’s explanation connects it to gravitational tides associated with nearby NGC 770. That makes the pair useful to readers as a clear example of the way galaxies can affect one another across large distances.
The account describes this explanation as likely, rather than as a definitive measurement of the forces involved. The image provides a detailed view of the arm’s dust lanes and blue clusters, while the accompanying text offers an interpretation of their broader shape. The distinction matters: the picture shows structure, and the proposed interaction explains how that structure may have formed or been amplified.
APOD’s daily format also gives a specific astronomical subject a concise public explanation. In this entry, the combination of a named galaxy, an approximate distance and scale, and a nearby companion helps place the image in context without suggesting that a single exposure resolves every question about the system.
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Arp 78 and Its Nearby Companion
Arp 78 is also catalogued as NGC 772. NASA’s September 30 entry calls it a peculiar spiral galaxy within the boundaries of Aries, a constellation visible in the night sky. The description’s distance estimate places it well outside the Milky Way, so the stars that appear sharp and spiky in the foreground are much closer objects along the line of sight.
The companion identified in the APOD text is NGC 770, a more compact galaxy seen just below NGC 772 in the portrait. Its fuzzy, elliptical appearance contrasts with the large spiral. The source says faint star streams are visible in the deep telescopic exposure, but it does not provide measurements or further analysis of those streams.
The item is part of NASA’s Astronomy Picture of the Day service, which presents a different astronomical image or photograph each day with an explanation written by a professional astronomer. The September 30 page credits the image to Robert Eder and lists Jerry Bonnell, Cecilia Chirenti, Robert Nemiroff and Keighley Rockcliffe as authors and editors. It also notes that APOD’s email address for image submissions has changed.
How the Tidal Influence Is Inferred
The APOD text says interactions with NGC 770 are likely responsible for Arp 78’s prominent arm; it does not state that this cause has been conclusively established in the entry. The page offers no detailed measurements, research methods or alternative explanations, so readers cannot assess from this description alone how the tidal interpretation was tested.
The explanation also supplies approximate distance and size figures without describing their measurement basis. It does not quantify the age or number of the blue star clusters, the extent of the star streams, or how the galaxies’ interaction has changed over time. Those details are not addressed in the image feature, and the page does not announce a new study or a separate scientific result.
The Next Daily Image
The APOD page labels the next day’s picture, scheduled for October 1, 2026, “a harvest.” The September 30 entry does not provide further details about that image. Readers looking for more on Arp 78’s structure or the proposed interaction with NGC 770 would need material beyond this daily image explanation; the page itself gives no follow-up research timetable.
Key Questions
What is Arp 78?
Arp 78 is a peculiar spiral galaxy, also known as NGC 772, in the constellation Aries. NASA’s APOD description estimates that it is about 200,000 light-years across.
Why does Arp 78 have a prominent outer arm?
The APOD explanation says the arm is likely shaped or amplified by gravitational tides from interactions with the nearby companion galaxy NGC 770. The entry describes this as a likely explanation.
Where is NGC 770 in the image?
NGC 770 appears directly below the larger spiral galaxy. The APOD description calls it more compact and notes its fuzzy, elliptical appearance.
Who made the featured image?
The September 30, 2026, APOD credits the image to Robert Eder.
Source: primary
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