astronomy

HD 81817

HD 81817 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 81817 rather than just read about it. In short: HD 81817 is a possible binary star system with two brown dwarf companions in the northern circumpolar constellation of Draco. It has an orange hue and is visible to the naked eye with an apparent visual magnitude of 4.28.

Key takeaways

HD 81817 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 81817 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HD 81817 from memory before moving on to harder problems.

Reference excerpt

HD 81817 is a possible binary star system with two brown dwarf companions in the northern circumpolar constellation of Draco. It has an orange hue and is visible to the naked eye with an apparent visual magnitude of 4.28. The system is located at a distance of approximately 990 light years from the Sun based on parallax, and is drifting closer with a radial velocity of −7 km/s. It is a member of the IC 2391 moving group. The primary component of this system is an aging giant star with a stellar classification of K3III. The star's chromosphere is of the type called "hybrid", displaying a cool stellar wind in combination with hot emission lines. It appears to be the source for the X-ray emission coming from this system. The star is 150 million years old with 4.3 times the mass of the Sun. With the supply of hydrogen at its core exhausted, the star has expanded to 83.8 times the Sun's radius. It is radiating 1,823 times the luminosity of the Sun from its photosphere at an effective temperature of 4,140 K. A possible companion star was discovered in 1984 based upon its ultraviolet spectrum. The distribution of the far ultraviolet flux matches that of a white dwarf star of class DA. A 2020 study finds it unlikely that there is a white dwarf companion; instead claiming the radial velocity variations are caused by a substellar object HD 81817 b, probably a brown dwarf, and possibly another substellar object. HD 81817 b would have a minimum mass of 27.1 MJ and orbit at 3.3 AU with a period of 1047.1 days and an eccentricity of 0.17. Gaia DR2 astrometry also suggested a companion with a mass of about 124 MJ (with a high margin of error) orbiting at 2.67 AU, consistent with the radial velocity detection. If the latter mass estimate is correct, this object would be a low-mass star, probably a red dwarf. Further observations through 2022 have confirmed that the companion is indeed a brown dwarf, in addition to the detection of a second brown dwarf on a closer orbit.

See also List of exoplanets discovered in 2020 - HD 81817 b

References

Illustrations

Worked examples

Example 1 — a first encounter with HD 81817

Start with the simplest possible case. Write down what HD 81817 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 81817 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 81817 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 81817

In research: HD 81817 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 81817 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 81817 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bright Star Catalogue objects, Brown dwarfs, Draco (constellation), so understanding it makes those chapters shorter.
In everyday life: Look for HD 81817 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 81817 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HD 81817 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 81817 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HD 81817 in simple terms?

Why does HD 81817 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 81817?

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 81817.