astronomy

NGC 7213

NGC 7213 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 7213 rather than just read about it. In short: NGC 7213 is a lenticular galaxy located in the constellation Grus. It is located at a distance of circa 70 million light-years from Earth, which, given its apparent dimensions, means that NGC 7213 is about 75,000 light-years across.

Key takeaways

NGC 7213 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 7213 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 7213 from memory before moving on to harder problems.

Reference excerpt

NGC 7213 is a lenticular galaxy located in the constellation Grus. It is located at a distance of circa 70 million light-years from Earth, which, given its apparent dimensions, means that NGC 7213 is about 75,000 light-years across. It was discovered by John Herschel on September 30, 1834. It is an active galaxy with characteristics between a type I Seyfert galaxy and LINER.

Characteristics The nucleus of NGC 7213 features activity and belongs to the low-luminosity active galactic nuclei (LLAGN). The optical spectrum features broad H-alpha lines and has similar characteristics to a type I Seyfert galaxy, however it is considerably less luminous than other similar galaxies. It also features [Ne V] λ3426, He II λ4686, [O II] λ3726, 3729, 7319, 7330, [O III] λ4363, 4959, and 5007 emission lines. However the intensity ratios of many of these lines is similar to a LINER. Observed in radio waves, NGC 7213 looks like a point-like source that could feature two jets that bend between one 1 arcsec (150 pc) and 10 arcsec (1.5 kpc). It is categorised as a radio-intermediate galaxy, lying between radio-quiet and radio-loud sources. The source of activity in the AGNs is a supermassive black hole (SMBH) lying at the centre of the galaxy. The mass of the SMBH at the centre of NGC 7213 is estimated to be between 8×107 M☉ and 9.6+6.1−4.1×107 M☉ based on the mean velocity dispersion or (3-47) × 106 (106.83+0.84−0.33) M☉ based on the polarization of broad Hα emission. NGC 7213 has been found to be a low-luminosity X-ray source. The 2-10 keV spectrum of NGC 7213 resembles the spectrum of Seyfert I galaxies, but also contains significant emission lines from FeXXV and FeXXVI, which are observed in LINERs and are not present in most classical Seyfert galaxies. Also, the soft X-ray spectrum of NGC 7213 features collisionally ionized thermal plasma, one more characteristic of LINERs. In addition, there is no X-ray emission detected that can be attributed to an accretion disk. Observations by NuSTAR at 3-79 keV showed no evidence for a Compton-reflected continuum. The observations suggest that the accretion disc is truncated in the inner region, with the black hole being fed via radiative-inefficient accretion flows. The X-ray flux of NGC 7213 is fluctuating, presenting a sharp flare, larger than the others, in the 1980s with the flux reducing steadily after for more than 20 years. This flare showed a fast-rise-exponential-decay pattern and has been suggested to be caused by the tidal disruption of a main sequence star by the black hole at the centre of NGC 7213. Smaller scale flares are also observed. The emission variability is also observed in other wavelengths with a time lag. The lag is 24 ± 12 days for 8.4 GHz radio and 40 ± 13 days for the 4.8 GHz radio. Surrounding the nucleus is a ring of starforming regions, lying at a radius of circa 20 arcseconds. Closer to the nucleus can be observed a dusty spiral pattern, with two arms, with the northwest arm being on the near side of the galaxy, and the southeast one to the far side. The ionised gas inflow rate towards the nucleus is estimated to be 0.2 M☉ per year. Although NGC 7213 appears undisturbed in visible light, it shows signs of having undergone a collision or merger when viewed at longer wavelengths, with disturbed patterns of ionized hydrogen including a filament of gas around 64,000 light-years long, itself being part of a larger HI tidal tail southwest of the nucleus.

Nearby galaxies NGC 7213 is the foremost galaxy in a galaxy group known as the NGC 7213 group. Other members of the group include IC 5170, IC 5181, NGC 7232, and NGC 7233. A bit further away lies the NGC 7144 group, which includes NGC 7144, NGC 7145, NGC 7151, and NGC 7155. These galaxies group are part of the Pavo-Indus cloud and the Grus cloud, which lie between the Local Supercluster and Pavo–Indus Supercluster.

See also Other Seyfert galaxies include:

Messier 77 NGC 1106 NGC 5128

References

External links

NGC 7213 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images NGC 7213 on SIMBAD

Illustrations

Worked examples

Example 1 — a first encounter with NGC 7213

Start with the simplest possible case. Write down what NGC 7213 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 7213 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 7213 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 7213

In research: NGC 7213 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 7213 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 7213 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1834, Discoveries by John Herschel, ESO objects, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 7213 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 7213 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 7213 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 7213 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 7213 in simple terms?

Why does NGC 7213 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 7213?

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 7213.