astronomy

Kepler-21

Kepler-21 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 Kepler-21 rather than just read about it. In short: Kepler-21, also known as HD 179070, is a star with a closely orbiting exoplanet in the northern constellation of Lyra. At an apparent visual magnitude of 8.25 this was the brightest star observed by the Kepler spacecraft to host a validated planet until the discovery of an exoplanet orbiting HD 212657 in 2018.

Key takeaways

Kepler-21 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kepler-21 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kepler-21 from memory before moving on to harder problems.

Reference excerpt

Kepler-21, also known as HD 179070, is a star with a closely orbiting exoplanet in the northern constellation of Lyra. At an apparent visual magnitude of 8.25 this was the brightest star observed by the Kepler spacecraft to host a validated planet until the discovery of an exoplanet orbiting HD 212657 in 2018. This system is located at a distance of 354 light-years (109 parsecs) from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −18.2 km/s.

The spectrum of HD 179070 presents as an evolving F-type subgiant star with a stellar classification of F6 IV. This suggests the star has exhausted the supply of hydrogen at its core and is evolving into a giant star. It is an estimated 2.6 billion years old and is spinning with a rotation period of 12.6 days. With 1.4 times the mass of the Sun it currently has 1.9 times the Sun's radius. The star is radiating five times the luminosity of the Sun from its photosphere at an effective temperature of 6,305 K. A faint nearby source was detected in 2011 and determined to be a co-moving stellar companion in 2016. Designated HD 179070 B, it lies at an angular separation of 0.75″ along a position angle of 129° relative to the primary. At the distance of this star, this corresponds to a projected separation of 87 AU. It is possible that this companion star had a significant influence on the exoplanet formation and subsequent orbital evolution.

Planetary system

A candidate transiting exoplanet was discovered based on data from the first four months of photometry from the Kepler spacecraft. Confirmation was obtained in 2012 after extensive follow-up observations and analysis of the Kepler light curves. The calculated density of the planet is approximately 6.4 g·cm−3, similar to Earth's 5.5 g·cm−3, which suggests a rocky composition. With an equilibrium temperature of 2,025 Kelvin, the top few-hundred kilometers of the planet is probably molten. Calculations of the rate of orbital decay from tidal effects results in a decrease in the orbital period of 3.88 milliseconds per year, since this would be a change of only 4 seconds every thousand years it would be undetectable in any reasonable length of time. Kepler-21 might have another exoplanet, a gas giant with at least 3.7 times the mass of Jupiter, named Kepler-21c.

References

Illustrations

Worked examples

Example 1 — a first encounter with Kepler-21

Start with the simplest possible case. Write down what Kepler-21 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 Kepler-21 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 Kepler-21 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 Kepler-21

In research: Kepler-21 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 Kepler-21 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: Kepler-21 is common in secondary-school and first-year university syllabi. It links to neighbouring topics F-type subgiants, Henry Draper Catalogue objects, Hipparcos objects, so understanding it makes those chapters shorter.
In everyday life: Look for Kepler-21 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 Kepler-21 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kepler-21 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 Kepler-21 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kepler-21 in simple terms?

Why does Kepler-21 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 Kepler-21?

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 Kepler-21.