NGC 4945

Key takeaways

NGC 4945 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 4945 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 4945 from memory before moving on to harder problems.

Reference excerpt

NGC 4945 is a widely-studied barred spiral galaxy in the constellation Centaurus, visible near the optical double star Xi Centauri. It is also known as Caldwell 83. The galaxy was discovered by Scottish astronomer James Dunlop in 1826. It is located at a distance of approximately 11.0 million light-years (3.36 Mpc) from the Milky Way. NGC 4945 hosts one of the closest active galactic nuclei to Earth and is classified as a Seyfert 2 galaxy. NGC 4945 is one of the brightest galaxies of the Centaurus A/M83 Group, a large, nearby group of galaxies. The galaxy is the second brightest galaxy in the subgroup centered on Centaurus A.

Observations The morphological classification of NGC 4945 is SB(s)cd?edge, indicating this is a barred spiral galaxy (SB) with no inner ring structure (s) and possibly loosely-wound spiral arms (cd?). It is inclined at an angle of 90° to the plane of the sky, which means the galactic plane is being viewed edge-on. NGC 4945 is thought to be similar to the Milky Way Galaxy, although the maximum rotation rate of ~ 180 km/s is lower. It has a combined estimated mass of 1.4+1.4−0.7×1011 M☉. The stellar mass of the galaxy is 3.8×1010 M☉, or 38 billion times the mass of the Sun. In 1964, this galaxy was classified as a radio source at the Parkes Observatory. The nucleus was obscured in the optical band but was found to be a quite prominent source of infrared emission. It is the third brightest galaxy in the IRAS point source catalogue, with most of the emission coming from the core. Most of the Galaxy shows a linear rotation curve, although the southwest region showed an infall suggestive of a bar.

In 1979, strong H2O maser emission was detected from the central region. The properties of the nuclear region are suggestive of both a Seyfert type galaxy and an intense starburst region with a complex structure. The active nucleus is heavily obscured by dust. These dust lanes approach the nucleus, forming a tightly-wound structure in the inner 650 ly (200 pc). X-ray emission from the nuclear region indicates a Type 2 Seyfert galaxy. It is a strong emitter of hard X-rays, second only to NGC 4151, and likely harbors a supermassive black hole. The starburst region is thought to be at least 5×106 years old and contributes at least half of the luminosity coming from the core. It is concentrated in the central 330 ly (100 pc) and includes a conical cavity likely produced by supernovae-driven winds. This is taking place in a region of dense molecular clouds, forming a disk of dust and gas, along with many rich star clusters. A hot wind of gas from the nuclear region is carrying away 1.6 M☉ per year. NGC 4945 was the first galaxy outside the Local Group to have stars resolved within its galactic halo. The halo mass is relatively large at ~ 3.5×109 M☉ and it is metal-rich, both of which are typical for a Milky Way-like galaxy. It appears to be counter-rotating compared to the main disk, suggesting the halo has been accreted. The mass of the dominant satellite accreted into the halo is ~ 1.5×109 M☉, which is roughly the same as the Large Magellanic Cloud. In October 2023, researchers using ALMA discovered an unknown object around 200 light years from the center of the galaxy. This object, named Punctum (Latin for "point" or "dot"), is a highly-polarized millimeter continuum source of synchrotron radiation. Punctum has a high non-visible luminosity, with archive data from Chandra reporting a 1×1037 erg s−1 X-ray luminosity in the 3–6 keV range and ATCA data reporting a radio luminosity of 5×1035 erg s−1 at 23 GHz. In a paper published in July 2025 about the object, the researchers said it most resembles a magnetar due to its polarization of 50%±14%, however the millimeter luminosity of said objects is usually much lower.

Supernovae Two supernovae have been observed in NGC 4945:

SN 2005af (Type II-P, mag. 12.8) was discovered by CEAMIG/REA Supernovae Search on 8 February 2005, reaching a peak magnitude of 12.5 on 12 February. SN 2011ja (Type II-P, mag. 14) was discovered by Berto Monard on 18 December 2011, and achieved a maximum magnitude of 11.7 on 9 January 2012. The progenitor star for 2011ja may have been massive at 25 M☉ and located within a massive stellar cluster.

In popular culture "NGC 4945" is the title of a song by Brett Domino on the album Funk.

Image gallery

References

Illustrations

Worked examples

Example 1 — a first encounter with NGC 4945

Start with the simplest possible case. Write down what NGC 4945 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 4945 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 4945 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 4945

In research: NGC 4945 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 4945 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 4945 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1826, Barred spiral galaxies, Caldwell objects, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 4945 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.

How to study NGC 4945 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 4945 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 4945 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 4945 in simple terms?

NGC 4945 is a widely-studied barred spiral galaxy in the constellation Centaurus, visible near the optical double star Xi Centauri. It is also known as Caldwell 83.

Why does NGC 4945 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 4945?

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 4945.