astronomy

NGC 4156

NGC 4156 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 4156 rather than just read about it. In short: NGC 4156 is a barred spiral galaxy in the constellation Canes Venatici. The galaxy lies about 220 million light years away from Earth based on redshift-independent methods, which means, given its apparent dimensions, that NGC 4156 is approximately 90,000 light years across.

Key takeaways

NGC 4156 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 4156 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 4156 from memory before moving on to harder problems.

Reference excerpt

NGC 4156 is a barred spiral galaxy in the constellation Canes Venatici. The galaxy lies about 220 million light years away from Earth based on redshift-independent methods, which means, given its apparent dimensions, that NGC 4156 is approximately 90,000 light years across. Based on redshift the galaxy lies 325 million light years away. It was discovered by German-British astronomer William Herschel on March 17, 1787.

Characteristics The galaxy has a narrow bar and two high-surface brightness spiral arms. The bar appears slightly asymmetric, with the eastern part slightly longer than the western. A dust lane is visible in the bulge. The arms form a pseudoring about 0.34 arcminutes across. A chain of HII regions in the ring runs along the edge of the lens in the southern part of the pseudoring but not at the north. The north chain of HII regions is prominent along the middle part of the ring rather than near the edge of the bar, which is the norm. The eastern arms appear to bifurcate towards its end. Several faint spiral arms are also visible, two of which are connected with the main arms towards their brighter ends. Ultraviolet emission is detected along the optical arms. A northwards filament emerges from the galaxy and seems to encompass another galaxy towards its end.

Nucleus NGC 4156 has a small, bright nucleus. The nucleus emits far and mid ultraviolet light and X-ray emission was detected by the Einstein Observatory. The observed flux was observed to decrease by a factor of two in a few months. Weak emission lines were also observed. The X-ray emission means that the nucleus of NGC 4156 is active and the narrow emission lines observed in optical light lead to its categorisation as a LINER, as the optical narrow-line flux ratios made it unfeasible to unambiguously denote it as a Seyfert galaxy. The most accepted theory for the energy source of active galactic nuclei is the presence of an accretion disk around a supermassive black hole. The mass of the black hole in the centre of NGC 4156 is estimated to be (1.4±0.1)×108 M☉. Optical data taken in 2019 have revealed the appearance of a broad-line region (BLR) component in the Hα and Hβ line profiles, together with a rising blue continuum. The BLR components have been confirmed by 2022 follow-up observations, whereas the rising continuum has disappeared. In both 2019 and 2022, the broad Hα is bright and well detected, whilst the broad Hβ emission line is barely detected and harder to identify, especially in 2022. This suggests that in 2019 and 2022 NGC 4156 was in two intermediate states of its 2-to-1 type round-trip evolution, being a type 1.5 in 2019 and type 1.8 in 2022 and thus classifying NGC 4156 as a change-looking Active Galactic Nucleus. The spectral evolution and type transitions observed in the optical over the last twenty years might be the result of variable dust absorption, shielding the central AGN along the line of sight, or changes in the accretion rate. XMM-Newton data show no clear signs of absorption. Therefore, if absorption is the main responsible for the observed change in the optical, such obscuring material is not attenuating the flux at higher energies.

Supernovae Two supernovae have been observed in NGC 4156:

SN 1974A was discovered by Halton Arp on 28 January 1974 at an apparent magnitude of 20. Its type is unknown. SN 2025bvm was discovered on 17 February 2025 by Asteroid Terrestrial-impact Last Alert System at an apparent magnitude of 18.8. Based on its spectrum it was categorised as a Type Ia supernova several days before maximum.

Nearby galaxies NGC 4156 lies 5.2 arcminutes from NGC 4151 and 7 arcminutes from Holm 345c. The north outer arm of NGC 4151 nearly overlaps with NGC 4156. However the pairing is optical as NGC 4156 lies at a much larger redshift than NGC 4151. NGC 4151 forms a triplet with UGC 7071 and KUG 1208+386, which lie at a similar redshift.

Gallery

References

External links NGC 4156 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images NGC 4156 on SIMBAD

Illustrations

Worked examples

Example 1 — a first encounter with NGC 4156

Start with the simplest possible case. Write down what NGC 4156 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 4156 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 4156 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 4156

In research: NGC 4156 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 4156 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 4156 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1787, Barred spiral galaxies, Canes Venatici, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 4156 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 4156 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 4156 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 4156 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 4156 in simple terms?

Why does NGC 4156 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 4156?

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 4156.