astronomy

HD 117939

HD 117939 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 HD 117939 rather than just read about it. In short: HD 117939 is a Sun-like star in the southern constellation of Centaurus. With an apparent visual magnitude of 7.29 it is too faint to be viewed with the naked eye, but is within the range of binoculars or a small telescope.

Key takeaways

HD 117939 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect HD 117939 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HD 117939 from memory before moving on to harder problems.

Reference excerpt

HD 117939 is a Sun-like star in the southern constellation of Centaurus. With an apparent visual magnitude of 7.29 it is too faint to be viewed with the naked eye, but is within the range of binoculars or a small telescope. It is located at a distance of 98.5 light years from the Sun based on parallax measurements, and is drifting further away with a radial velocity of +82 km/s. This is an intermediate disk star with a high proper motion, traversing the celestial sphere at an angular rate of 0.68 arcsec yr−1. An ordinary G-type main-sequence star with a stellar classification of G4V, this star is an "excellent photometric match for the Sun"; the atmospheric properties of the star make it a near solar twin. It is older than the sun at 6.1 billion years, but is more chromospherically active. To date no exact solar twin (precisely matching all important properties of the Sun) has been found. However, there are some stars that come very close to being identical to the Sun, and as such are dubbed solar twins by astronomers. An exact solar twin would be a 4.6 billion years old G2V star with a 5,772K temperature, the correct metallicity, and a 0.1% solar luminosity variation. G2V stars with an age of 4.6 billion years or more have typically reached their most stable state. Proper metallicity and size are also important to low luminosity variation.

Sun comparison Chart compares the sun to HD 117939.

Worked examples

Example 1 — a first encounter with HD 117939

Start with the simplest possible case. Write down what HD 117939 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 HD 117939 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 HD 117939 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 HD 117939

In research: HD 117939 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 HD 117939 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: HD 117939 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Centaurus, Durchmusterung objects, G-type main-sequence stars, so understanding it makes those chapters shorter.
In everyday life: Look for HD 117939 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 HD 117939 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HD 117939 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 HD 117939 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HD 117939 in simple terms?

Why does HD 117939 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 HD 117939?

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 HD 117939.