astronomy

NGC 720

NGC 720 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 720 rather than just read about it. In short: NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of about 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across.

Key takeaways

NGC 720 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 720 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 720 from memory before moving on to harder problems.

Reference excerpt

NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of about 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across. It was discovered by William Herschel on October 3, 1785. The galaxy is included in the Herschel 400 Catalogue. It lies about three and a half degrees south and slightly east from zeta Ceti.

Characteristics NGC 720 is an elliptical galaxy with elongated shape in the northwest to southeast axis as seen from Earth. Observations by the Hubble Space Telescope of the core of NGC 720 did not reveal the presence of dust, disk, or inner spiral. As observed in X-rays by the Chandra X-ray Observatory in 2000, the galaxy features a slightly flattened, or ellipsoidal triaxial halo of hot gas that has an orientation different from that of the optical image of the galaxy. The observations by Chandra X-ray Observatory fit predictions of a cold dark matter model. Although the data also fit the expectations from modified Newtonian dynamics (MOND). The galaxy lacks emission in radio waves, meaning it does not host an active galactic nucleus. The total mass of the galaxy with its dark matter halo is estimated to be (3.1±0.4)×1012 M☉, with the total gas mass exceeding the stellar one. The observations of hot gas fit models that are nearly hydrostatic. 42 X-ray point sources were detected in the galaxy, including a possible central source. Twelve of them are located within 2 arcsec of globular cluster candidates. NGC 720 features nine ultraluminous X-ray sources, the most found in an early type galaxy as of 2003. Observations made in 1996 suggested the galaxy had 660 ±190 globular clusters in the central 30kpc, a number considered small for such a galaxy. The allocation of the clusters resembled the ellipticity, position angle and surface brightness of the galaxy. However, in 2012 it was observed that the blue globular clusters subpopulation had a similar slope with the X-ray surface brightness profile. Further observations by the SLUGGS Survey (2016), with wider field data, raised the number of globular clusters in the galaxy to 1489 ± 96 and their distribution was less elliptical than the surface profile of the galaxy. The clusters have bimodal distribution as far as color is concerned, with the clusters characterised as red or blue, with the blue clusters having a stronger connection with the galactic halo. Optical long slit spectrography of the galaxy showed a strong age gradient along the semimajor axis of NGC 720, which has been explained on the grounds of two distinct population components. At the centre of the galaxy lie stars whose age is estimated to be 13 billion years (Gyrs) old up to 0.73 kpc, where stars with solar metallicity age (5 Gyrs) dominate. These older stars form a small bulge-like spheroid. At distances over 1 kpc dominate stars with age at 2,5 Gyrs. Based on the Mg2 gradient and its mass, it is proposed that NGC 720 underwent an unequal mass galaxy merger about 4 Gyrs ago.

Nearby galaxies NGC 720 is the foremost galaxy in a small galaxy group, the NGC 720 group, which also includes the galaxy Arp 4. NGC 720 lies at the centre of the group and the rest of the galaxies of the group are dwarf galaxies, which are at least 2 mag fainter than NGC 720. There is extended intragroup X-ray emission. The high fraction of early-type galaxies suggest that NGC 720 may be a fossil group, despite its low mass. Further away lie the edge-on spiral galaxies NGC 681, NGC 701, and NGC 755.

Gallery

References

External links

NGC 720 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images NGC 720 on SIMBAD

Illustrations

Worked examples

Example 1 — a first encounter with NGC 720

Start with the simplest possible case. Write down what NGC 720 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 720 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 720 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 720

In research: NGC 720 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 720 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 720 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1785, Cetus (constellation), Discoveries by William Herschel, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 720 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 720 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 720 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 720 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 720 in simple terms?

Why does NGC 720 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 720?

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 720.