astronomy

HD 861

HD 861 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 861 rather than just read about it. In short: HD 861 is a spectroscopic binary star system in the deep northern constellation of Cassiopeia. With an apparent magnitude of 6.622, the star is faintly visible to the naked eye under very dark skies and readily visible using binoculars.

Key takeaways

HD 861 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 861 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HD 861 from memory before moving on to harder problems.

Reference excerpt

HD 861 is a spectroscopic binary star system in the deep northern constellation of Cassiopeia. With an apparent magnitude of 6.622, the star is faintly visible to the naked eye under very dark skies and readily visible using binoculars. It is located approximately 403 light-years (124 parsecs) distant according to Gaia DR3 parallax measurements, and is moving further away at a heliocentric radial velocity of 8.80 km/s.

Stellar properties The primary star is a typical Am star, enriched in iron and especially so in barium but depleted in carbon, oxygen and calcium. At an age of 724 million (108.86) years, it is currently a main-sequence star fusing hydrogen into helium at its core. It will continue to do so for the next 320 million years until it runs out of core hydrogen at 1.05 billion (109.02) years old, at which point it will leave the main sequence and enter the subgiant phase. The secondary star is a G-type main-sequence star slightly less massive than the Sun and less than half as luminous.

Orbit The orbital properties of the companion were first determined in 1971 by Acker, with an orbital period of 11.2153 days and an eccentricity of 0.22. In 2002, however, Debernardi found an entirely different set of orbital parameters in his PhD thesis and also discovered the stellar spectra of the secondary star. This new orbit has a longer period of 15.9696 days and a lower eccentricity of 0.124. This was backed up by Budaj et al., who also independently found the secondary spectra and obtained a mass ratio between the two stars that agreed with Debernardi's research.

References

Illustrations

Worked examples

Example 1 — a first encounter with HD 861

Start with the simplest possible case. Write down what HD 861 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 861 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 861 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 861

In research: HD 861 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 861 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 861 is common in secondary-school and first-year university syllabi. It links to neighbouring topics A-type main-sequence stars, Am stars, Cassiopeia (constellation), so understanding it makes those chapters shorter.
In everyday life: Look for HD 861 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 “HD 861” →

Affiliate

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

How to study HD 861 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HD 861 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 861 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HD 861 in simple terms?

Why does HD 861 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 861?

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 861.