astronomy

NGC 765

NGC 765 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 765 rather than just read about it. In short: NGC 765 is an intermediate spiral galaxy located in the constellation Aries. It is located at a distance of circa 220 million light years from Earth, which, given its apparent dimensions, means that NGC 765 is about 195,000 light years across.

Key takeaways

NGC 765 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 765 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 765 from memory before moving on to harder problems.

Reference excerpt

NGC 765 is an intermediate spiral galaxy located in the constellation Aries. It is located at a distance of circa 220 million light years from Earth, which, given its apparent dimensions, means that NGC 765 is about 195,000 light years across. It was discovered by Albert Marth on October 8, 1864. The galaxy has an extensive hydrogen (HI) disk with low surface brightness, whose diameter is estimated to be 240 kpc (780,000 light years).

Characteristics The morphological classification of NGC 765 in the De Vaucouleurs system is SAB(rs)bc, where the 'SAB' denotes a weak-barred spiral, '(rs)' indicates a spiral structure which lies between that of a ring and a non-ring galaxy, and 'bc' means the spiral arms are intermediately wound. NGC 765 has at least three spiral arms, that can be traced up to a radius of 50 kpc. It has been identified to be a low surface brightness galaxy with a prominent bulge. Emission by low ionisation elements, with strong [N II] and [S II] lines, as well as [O I], has been detected at the nuclear region of the galaxy indicating the presence of an active galactic nucleus. The nucleus also emits X-ray, which were detected by the Chandra X-ray Observatory. The X-ray flux in 0.2 - 10 KeV is estimated to be 1.1×1040 erg/s, which is comparable to some low luminosity active galactic nuclei, like LINERs. Diffuse X-ray emission from the galactic bulge has been attributed to the presence of gas. NGC 765 holds a large amount of hydrogen gas [HI], which has been estimated to be 4.67×1010 M☉ based on the observed HI emission by the Very Large Array and the Westerbork Synthesis Radio Telescope wide-field survey. This mass is about an order of magnitude over the hydrogen mass observed in other low surface brightness galaxies, and also three times more than the mean mass of hydrogen in spiral galaxies. The HI mass-to-luminosity ratio for NGC 765 is MH I/LB~ 1.6. The mass of HI in the galaxy means it is categorised as a massive low surface brightness galaxy. The HI is distributed on a large disk, which measures 240 kpc across. This is the largest observed disk in the nearby universe and it is a similar size to the disk of Malin 1. This disk appears asymmetric, with the east side showing a rapid decline in column density between 100 and 120 kpc, while the decline in the west side is smoother. Inside the disk are observed regions with higher brightness. They are compact, measuring less than 10kpc across and have a mass similar to that of dwarf irregular galaxies, and they hold about 10% of the galactic HI. It has been suggested that these clumps of HI are the remnants of accreted galaxies.

Nearby galaxies

Two satellite galaxies have been detected; UGC 1453, an irregular galaxy which lies to the south, and NGC 765A, which was first detected in HI towards the north-east and has a faint visual counterpart. UGC 1453 is located 14.8 arcminutes from NGC 765, which corresponds to a projected distance of 310 kpc, and NGC 765A lies 16.5 arcminutes away, at a projected distance of 338 kpc. NGC 765 is a member of the galaxy group known as LGG 41. Other members of the group include NGC 776, IC 187, IC 188, UGC 1451, and UGC 1479. The group is part of the Perseus–Pisces Supercluster.

See also Malin 1 - prototype giant low surface galaxy NGC 262 - a galaxy with a similar giant hydrogen disk

References

External links

NGC 765 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images NGC 765 on SIMBAD

Illustrations

Worked examples

Example 1 — a first encounter with NGC 765

Start with the simplest possible case. Write down what NGC 765 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 765 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 765 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 765

In research: NGC 765 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 765 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 765 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aries (constellation), Astronomical objects discovered in 1864, Discoveries by Albert Marth, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 765 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 765 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 765 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 765 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 765 in simple terms?

Why does NGC 765 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 765?

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 765.