astronomy

NGC 891

NGC 891 is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand NGC 891 rather than just read about it. In short: NGC 891 is an edge-on unbarred spiral galaxy about 30 million light-years away in the constellation Andromeda. Alternatively, it known as Caldwell 23, the Silver Sliver Galaxy, or the Outer Limits Galaxy.

Key takeaways

NGC 891 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NGC 891 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 891 from memory before moving on to harder problems.

Reference excerpt

NGC 891 is an edge-on unbarred spiral galaxy about 30 million light-years away in the constellation Andromeda. Alternatively, it known as Caldwell 23, the Silver Sliver Galaxy, or the Outer Limits Galaxy. It was discovered by William Herschel on October 6, 1784. The galaxy is a member of the NGC 1023 group of galaxies in the Local Supercluster. It has an H II nucleus. The object is visible in small to moderate size telescopes as a faint elongated smear of light with a dust lane visible in larger apertures. In 1999, the Hubble Space Telescope imaged NGC 891 in infrared. In 2005, due to its attractiveness and scientific interest, NGC 891 was selected to be the first light image of the Large Binocular Telescope. In 2012, it was again used as a first light image of the Lowell Discovery Telescope with the Large Monolithic Imager. Supernova SN 1986J was discovered on August 21, 1986 at apparent magnitude 14.

Properties NGC 891 looks as the Milky Way would look like when viewed edge-on (some astronomers have even noted how similar to NGC 891 our galaxy looks as seen from the Southern Hemisphere) and, in fact, both galaxies are considered very similar in terms of luminosity and size; studies of the dynamics of its molecular hydrogen have also proven the likely presence of a central bar. Despite this, recent high-resolution images of its dusty disk show unusual filamentary patterns. These patterns are extending into the halo of the galaxy, away from its galactic disk. Scientists presume that supernova explosions caused this interstellar dust to be thrown out of the galactic disk toward the halo. It may also be possible that the light pressure from surrounding stars causes this phenomenon.

The galaxy is a member of a small group of galaxies, sometimes called the NGC 1023 Group. Other galaxies in this group are the NGCs 925, 949, 959, 1003, 1023, and 1058, and the UGCs 1807, 1865 (DDO 19), 2014 (DDO 22), 2023 (DDO 25), 2034 (DDO 24), and 2259. Its outskirts are populated by multiple low-surface brightness, coherent, and vast substructures, like giant streams that loop around the parent galaxy up to distances of approximately 50 kpc. The bulge and the disk are surrounded by a flat and thick cocoon-like stellar structure. These have vertical and radial distances of up to 15 kpc and 40 kpc, respectively, and are interpreted as the remnant of a satellite galaxy disrupted and in the process of being absorbed by NGC 891.

In popular culture NGC 891 appears alongside M67, the Sombrero Galaxy(M104), the Pinwheel Galaxy(M101), NGC 5128, NGC 1300, M81, and the Andromeda Galaxy in the end credits of the Outer Limits TV series, which is why it is occasionally called the Outer Limits Galaxy. The soundtrack of the 1974 film Dark Star by John Carpenter features a muzak-style instrumental piece called "When Twilight Falls on NGC 891". The first solo album by Edgar Froese, Aqua, also released in 1974, contained a track called "NGC 891". Side 2 of the album, which included this track, was unusual in having been a rare example of a commercially issued piece of music recorded using the artificial head system.

Illustrations

Worked examples

Example 1 — a first encounter with NGC 891

Start with the simplest possible case. Write down what NGC 891 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NGC 891 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NGC 891 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NGC 891

In research: NGC 891 appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NGC 891 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NGC 891 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Andromeda (constellation), Caldwell objects, NGC 1023 Group, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 891 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study NGC 891 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 891 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NGC 891 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 891 in simple terms?

Why does NGC 891 matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NGC 891?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NGC 891.