astronomy

NGC 4993

NGC 4993 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 4993 rather than just read about it. In short: NGC 4993 (also catalogued as NGC 4994 in the New General Catalogue) is a lenticular galaxy located about 140 million light-years away in the constellation Hydra. It was discovered on 26 March 1789 by William Herschel and is a member of the NGC 4993 Group.

Key takeaways

NGC 4993 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 4993 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 4993 from memory before moving on to harder problems.

Reference excerpt

NGC 4993 (also catalogued as NGC 4994 in the New General Catalogue) is a lenticular galaxy located about 140 million light-years away in the constellation Hydra. It was discovered on 26 March 1789 by William Herschel and is a member of the NGC 4993 Group. NGC 4993 was the site of GW170817, a collision of two neutron stars, the first astronomical event detected in both electromagnetic and gravitational radiation, a discovery given the Breakthrough of the Year award for 2017 by the journal Science. The detection of gamma-ray burst GRB 170817A simultaneous with the gravitational wave event, for example, provided the first direct confirmation that binary neutron star collisions generate short gamma-ray bursts.

Physical characteristics NGC 4993 has several concentric shells of stars and a large dust lane—with a diameter of approximately a few kiloparsecs—which surrounds the nucleus and is stretched out into an "s" shape. The dust lane appears to be connected to a small dust ring with a diameter of ~330 ly (0.1 kpc). These features in NGC 4993 may be the result of a recent merger with a gaseous late-type galaxy that occurred about 400 million years ago. However, Palmese et al. suggest that the galaxy involved in the merger was a gas-poor galaxy.

Dark matter content NGC 4993 has a dark matter halo with an estimated mass of 193.9×1010 M☉.

Globular clusters NGC 4993 has an estimated population of 250 globular clusters. The luminosity of NGC 4993 indicates that the globular cluster system surrounding the galaxy may be dominated by metal-poor globular clusters.

Supermassive black hole NGC 4993 has a supermassive black hole with an estimated mass of roughly 80 to 100 million solar masses (8×107 M☉).

Galactic nucleus activity The presence of weak O III, NII and SII emission lines in the nucleus of NGC 4993 and the relatively high ratio of [NII]λ6583/Hα suggest that NGC 4993 is a low-luminosity AGN (LLAGN). The activity may have been triggered by gas from the late-type galaxy as it merged with NGC 4993.

Neutron star merger observations

On 17 August 2017, rumors circulated that NASA's Fermi and ESA's INTEGRAL had detected GRB 170817A, a short gamma-ray burst (sGRB) of the type conjectured to be emitted in the collision of two neutron stars. On 16 October 2017, the LIGO and Virgo collaborations announced that, 1.7 seconds before the GRB signal, they had detected GW170817, a gravitational wave event which matched predictions for such mergers. The gravitational wave signal had a duration of about 100 seconds, and was indeed later confirmed as the first gravitational wave detection of a neutron star merger. The simultaneous detection of the GW and sGRB signals suggested that they were emitted by the same astrophysical event, so researchers scrambled to combine their data and narrow down its location in the sky. An intense search effort ensued, with hundreds of astronomers across the world pre-empting telescope schedules to scan the region of the sky constrained by the GW / sGRB data. Eleven hours after the gravitational wave and gamma-ray signals, an optical transient, kilonova AT 2017gfo (initially designated SSS17a), was detected in NGC 4993, allowing the location of the merger to be precisely known. Together with GW170817, GRB 170817A and AT 2017gfo marked the first observation and localization of electromagnetic counterparts to a gravitational wave observation, an important milestone in the field of multi-messenger astronomy. For example, the initial GW and sGRB signals had traveled for 140 million years at the speed of light before arriving at earth only 1.7 seconds apart, empirically tightening the previous constraint on the speed of gravity by around 14 orders of magnitude.

Gallery

See also Gravitational-wave astronomy List of gamma-ray bursts List of gravitational wave observations Neil Gehrels Swift Observatory Ultra-Fast Flash Observatory Pathfinder

References

External links

"NGC 4993". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 28 August 2017. "NGC 4993". DSO Browser. Archived from the original on 15 February 2015. Retrieved 28 August 2017. Courtney Seligman. "New General Catalog Objects: NGC 4950 – 4999". Retrieved 28 August 2017. Hartmut Frommert. "Revised NGC Data for NGC 4993". SEDS. Retrieved 28 August 2017. GRB 170817A – NASA/IPAC Extragalactic Database (NED) GRB 170817A – Max Planck Institute for Extraterrestrial Physics (MPE) GRB 170817A - INTEGRAL Science Data Center (ISDC) The galaxy NGC 4993 in the constellation of Hydra Starmap NGC 4993 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Illustrations

Worked examples

Example 1 — a first encounter with NGC 4993

Start with the simplest possible case. Write down what NGC 4993 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 4993 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 4993 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 4993

In research: NGC 4993 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 4993 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 4993 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Active galaxies, Astronomical objects discovered in 1789, Elliptical galaxies, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 4993 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 4993 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 4993 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 4993 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 4993 in simple terms?

Why does NGC 4993 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 4993?

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 4993.