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NASA’s Astronomy Picture of the Day for September 30, 2026, features a detailed telescopic image of Arp 78, also known as NGC 772. The galaxy’s prominent outer spiral arm is thought to have been shaped by gravitational interactions with its compact companion, NGC 770.
NASA’s Astronomy Picture of the Day on September 30, 2026, features Arp 78, a large spiral galaxy in the constellation Aries whose pronounced outer arm may have been amplified by gravitational interactions with a nearby companion. The image and accompanying explanation identify the galaxy as NGC 772 and its compact neighbor as NGC 770, offering a view of a system about 100 million light-years from the Milky Way.
According to the NASA Science APOD entry, Arp 78 spans about 200,000 light-years. Its most striking feature is an unusually prominent outer spiral arm. The image description says the arm follows sweeping dust lanes and contains young blue star clusters, features visible in the detailed telescopic portrait.
The explanation says the arm is likely shaped by galactic-scale gravitational tides, with interactions involving the brighter companion galaxy NGC 770 considered the probable cause. NGC 770 appears below Arp 78 in the image and is described as more compact, with a fuzzy, elliptical appearance. The description presents the interaction as a likely explanation, not as a newly reported measurement or a definitively established account of the galaxy’s history.
The deep exposure also reveals faint star streams around the galaxies. Foreground stars in the Milky Way appear sharp and spiky against the more distant, diffuse structures. NASA credits the image to Robert Eder; the APOD entry lists Jerry Bonnell, Cecilia Chirenti, Robert Nemiroff and Keighley Rockcliffe as authors and editors.
A Companion’s Possible Pull
The image makes visible how galaxies can be shaped by their surroundings. Arp 78’s distinctive arm is not simply a decorative feature in the photograph: the NASA explanation links its unusual prominence to tidal forces associated with a neighboring galaxy. That interpretation gives readers a concrete example of how gravity between galaxies may affect the distribution of stars and gas within a spiral system.
The image also shows why deep observations matter. Faint star streams are much less conspicuous than the bright galaxy cores, but their visibility adds detail to the scene and helps convey the scale of the system. The APOD is a public astronomy feature rather than a new research announcement, so its immediate contribution is visual and explanatory: it presents a striking image alongside a concise account of the structures astronomers observe.
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Arp 78 and NGC 770
Arp 78 is also cataloged as NGC 772. The NASA entry locates it within the boundaries of Aries, a constellation whose name means the Ram. Its stated distance—about 100 million light-years—places it far beyond the stars and nebulae of the Milky Way that appear in the foreground of the image.
The object is described as a peculiar spiral galaxy because of its pronounced outer arm. Its smaller apparent companion, NGC 770, sits directly below it in the portrait. NASA’s caption connects the pair’s gravitational interaction to the arm’s development, while noting visual contrasts between the large spiral, the companion’s diffuse elliptical form, and the sharp foreground stars. The entry does not provide a new observing date, measurement method, or detailed account of the galaxies’ orbital history.
What the Image Cannot Resolve
The APOD explanation says interactions with NGC 770 are likely responsible for the enlarged arm, but the entry does not detail how that conclusion was reached or quantify the companion’s contribution. It also does not specify the timing or sequence of any past close encounters. The image alone does not establish those details.
NASA’s page presents the image and a brief explanation, not a new study. It does not report fresh observations, a revised distance estimate, or a change in the accepted interpretation. Further details about the system’s dynamics are not included in the source material.
The Next APOD Entry
NASA’s listing says the next Astronomy Picture of the Day is scheduled for October 1, 2026, with the preview title “a harvest.” The September 30 entry does not announce a follow-up investigation of Arp 78 or specify when additional observations of the galaxy pair might be published.
NASA also notes that its email address for image submissions has changed and directs contributors to the APOD Submissions page. That notice concerns future image submissions; it does not change the information presented about Arp 78.
Key Questions
What is Arp 78?
Arp 78, also known as NGC 772, is a spiral galaxy in the constellation Aries. NASA’s APOD description gives its diameter as about 200,000 light-years.
Why does Arp 78 have a prominent outer arm?
NASA’s explanation says the arm may have been amplified by gravitational tides from interactions with the nearby galaxy NGC 770. The entry describes this as likely, rather than certain.
How far away is Arp 78?
The APOD entry places Arp 78 about 100 million light-years beyond the stars and nebulae of the Milky Way.
What does the image show besides the two galaxies?
The deep telescopic exposure reveals faint star streams. It also shows sharp, spiky foreground stars in the Milky Way, which appear in front of the more distant galaxies.
Source: primary
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