astronomy

Messier 75

Messier 75 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 75 rather than just read about it. In short: Messier 75 or M75, also known as NGC 6864, is a globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same year.

Key takeaways

Messier 75 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 75 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Messier 75 from memory before moving on to harder problems.

Reference excerpt

Messier 75 or M75, also known as NGC 6864, is a globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same year. M75 is about 67,500 light years away from Earth and is 14,700 light years away from, and on the opposite side of, the Galactic Center. Its apparent size on the sky translates to a true radius of 67 light years. M75 is classified as class I, meaning it is one of the more densely concentrated globular clusters known. It shows a slow rotation around an axis that is inclined along a position angle of −15°±30°. The absolute magnitude of M75 is about −8.5, equating to 180,000 times more luminous than the Sun (L☉). The cluster has a half-light radius of 9.1 ly (2.80 pc) with a core radius of about 1.6 ly (0.5 pc) and appears not to have undergone core collapse yet. The mass density at the core is 7.9×104 M☉·pc−3. There are 38 RR Lyrae variable stars and the cluster appears to be Oosterhoff-intermediate in terms of metallicity. 62 candidate blue stragglers have been identified in the cluster field, with 60% being in the core region. Messier 75 is part of the Gaia Sausage, the hypothesized remains of a dwarf galaxy that merged with the Milky Way. It is a halo object with an orbital period of 0.4 billion years to travel around the galaxy on a very pronounced ellipse, specifically eccentricity of 0.87. The apocenter (maximal distance from Earth) is about 57,000 ly (17,500 pc).

Gallery

Illustrations

Worked examples

Example 1 — a first encounter with Messier 75

Start with the simplest possible case. Write down what Messier 75 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 75 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 75 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 75

In research: Messier 75 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 75 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 75 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1780, Discoveries by Pierre Méchain, Gaia-Enceladus, so understanding it makes those chapters shorter.
In everyday life: Look for Messier 75 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 75 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Messier 75 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 75 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Messier 75 in simple terms?

Why does Messier 75 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 75?

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 75.