astronomy

Messier 91

Messier 91 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 Messier 91 rather than just read about it. In short: Messier 91 (also known as NGC 4548 or M91) is a barred spiral galaxy that is found in the south of Coma Berenices. It is in the local supercluster and is part of the Virgo Cluster of galaxies.

Key takeaways

Messier 91 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Messier 91 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Messier 91 from memory before moving on to harder problems.

Reference excerpt

Messier 91 (also known as NGC 4548 or M91) is a barred spiral galaxy that is found in the south of Coma Berenices. It is in the local supercluster and is part of the Virgo Cluster of galaxies. It is about 63 million light-years away from the Milky Way. It was the last of a group of eight "nebulae" – the term 'galaxy' only coming into use for these objects once it was realized in the 20th century that they were extragalactic – discovered by Charles Messier in 1781. It is the faintest object in the Messier catalog, with an apparent magnitude of 10.2. As a result of a bookkeeping error by Messier, M91 was for a long time one of the few missing entries in the Messier catalog, not matching any known object in the sky. It was not until 1969 that amateur astronomer William C. Williams realized that M91 was NGC 4548, which was catalogued by William Herschel in 1784. Some sources contend the nearby spiral galaxy NGC 4571 was considered as a candidate for this object by Herschel.

Observation history The object was discovered in 1781 by Messier who described it as nebula without stars, fainter than M90. Messier mistakenly logged its position from Messier 58, where in fact it should have been Messier 89. William Herschel observed the same object in 1784. In 1969 Williams solved this lost Messier object by measuring its right ascension and declination relative to those of the nearby galaxy M89 (notable reference stars angularly nearby are sparse) – rather than M58, a 9th-magnitude galaxy which Messier recorded in 1778. This amended night sky "star-hopping" reference point matches Messier's figures to 0.1 of an arcminute (′) in right ascension and 1′ in declination, a sixtieth of a degree.

Features Inclusion of Messier 91 in the Virgo cluster was confirmed in 1997 from observing Cepheid variables which place it at 52±6 million light years away. Its bar is very conspicuous – it is seen with position angle of 65 to 245 degrees when being measured from the North direction to the East. There is a countering peculiar (local) velocity toward us through the Virgo cluster of about 700 km/s within the cluster's recession velocity of about 1100 km/s, which produces its observed recessional velocity of only about 400 km/s. Messier 91 is also classified as an anemic galaxy, that is: a spiral galaxy with little star formation and gas compared with other galaxies of its type.

Illustrations

Worked examples

Example 1 — a first encounter with Messier 91

Start with the simplest possible case. Write down what Messier 91 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 Messier 91 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 Messier 91 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 Messier 91

In research: Messier 91 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 Messier 91 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: Messier 91 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1781, Barred spiral galaxies, Coma Berenices, so understanding it makes those chapters shorter.
In everyday life: Look for Messier 91 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 Messier 91 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Messier 91 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 Messier 91 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Messier 91 in simple terms?

Messier 91 (also known as NGC 4548 or M91) is a barred spiral galaxy that is found in the south of Coma Berenices. It is in the local supercluster and is part of the Virgo Cluster of galaxies.

Why does Messier 91 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 Messier 91?

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 Messier 91.