astronomy

GJ 3991

GJ 3991 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 GJ 3991 rather than just read about it. In short: GJ 3991 (also known as Gliese 3991 and G 203-47) is a binary star system located 24.78 light-years (7.60 parsecs) away in the constellation Hercules. Due to their small separation, the two stars were initially detected from the radial velocity changes of GJ 3991, making the system a spectroscopic binary.

Key takeaways

GJ 3991 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect GJ 3991 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of GJ 3991 from memory before moving on to harder problems.

Reference excerpt

GJ 3991 (also known as Gliese 3991 and G 203-47) is a binary star system located 24.78 light-years (7.60 parsecs) away in the constellation Hercules. Due to their small separation, the two stars were initially detected from the radial velocity changes of GJ 3991, making the system a spectroscopic binary. It consists of a red dwarf star with 20–30% the mass of the Sun, and a white dwarf star roughly 50% the mass of the sun. The two components orbit each other in a tight orbit only 0.11 astronomical units from each other, with an orbital period of only 14.91 days. This system is the nearest example of a post-common-envelope binary.

White dwarf GJ 3991 B was first identified in 1997 by astronomers I. N. Reid and J. E. Gizis through significant radial velocity variations of GJ 3991 A, but they were unable to identify the nature of the secondary object. In 1998, another group of astronomers was able to determine the secondary as a cold white dwarf star, the compact remnant that remains after a low-mass star such as the Sun is no longer able to fuse elements for energy. It was directly detected for the first time, and spectroscopically confirmed as a white dwarf, in 2026. GJ 3991 B is the 9th nearest white dwarf, after Sirius B, Procyon B, Van Maanen 2, Gliese 440, 40 Eridani B, Stein 2051 B, G 240-72, and LP 658-2. Despite the close orbit of the white dwarf, it is not tidally locked, with a slow rotation period likely exceeding 100 days. Some sources call the component stars A & B, while others call them Aa & Ab.

See also List of star systems within 20–25 light-years

References

Worked examples

Example 1 — a first encounter with GJ 3991

Start with the simplest possible case. Write down what GJ 3991 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 GJ 3991 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 GJ 3991 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 GJ 3991

In research: GJ 3991 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 GJ 3991 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: GJ 3991 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Binary stars, Gliese and GJ objects, Hercules (constellation), so understanding it makes those chapters shorter.
In everyday life: Look for GJ 3991 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.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “GJ 3991” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study GJ 3991 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what GJ 3991 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 GJ 3991 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is GJ 3991 in simple terms?

Why does GJ 3991 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 GJ 3991?

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 GJ 3991.