astronomy

Phecda

Phecda 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 Phecda rather than just read about it. In short: Phecda , also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.

Key takeaways

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

Reference excerpt

Phecda , also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. Based upon parallax measurements with the Hipparcos astrometry satellite, it is located at a distance of around 83.2 light-years (25.5 parsecs) from the Sun. It is more familiar to most observers in the Northern Hemisphere as the lower-left star forming the bowl of the Big Dipper, together with Alpha Ursae Majoris (Dubhe, upper-right), Beta Ursae Majoris (Merak, lower-right) and Delta Ursae Majoris (Megrez, upper-left). Along with four other stars in this well-known asterism, Phecda forms a loose association of stars known as the Ursa Major moving group. Like the other stars in the group, it is a main sequence star, as the Sun is, although somewhat hotter, brighter and larger. Phecda is located in relatively close physical proximity to the prominent Mizar–Alcor star system. The two are separated by an estimated distance of 8.55 ly (2.62 pc); much closer than the two are from the Sun. The star Merak is separated from Phecda by 11.0 ly (3.4 pc).

Nomenclature γ Ursae Majoris (Latinised to Gamma Ursae Majoris) is the star's Bayer designation. It bore the traditional names Phecda or Phad, derived from the Arabic phrase فخذ الدب fakhth al-dubb ('thigh of the bear'). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Phecda for Gamma Ursae Majoris A. To the Hindus this star was known as Pulastya, one of the seven rishis. In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Gamma Ursae Majoris itself is 北斗三 (Běi Dǒu sān, English: the Third Star of Northern Dipper) and 天璣 (Tiān Jī, English: Star of Celestial Shining Pearl).

Properties

Phecda is an Ae star, which is surrounded by an envelope of gas that is adding emission lines to the spectrum of the star; hence the 'e' suffix in the stellar classification of A0 Ve. It is 2.4 times more massive than the Sun and is 333 million years old. It rotates rapidly with a rotational velocity of 386 km/s at its equator, which causes it to have an oblate shape. The equatorial radius measures 3.39 R☉, while the polar radius measures 2.19 R☉. The effective temperature varies as well, from 6,750 K in the equator to 10,520 K in the poles. Phecda is also an astrometric binary: the companion star regularly perturbs the Ae-type primary star, causing the primary to wobble around the barycenter. From this, an orbital period of 20.5 years has been calculated. The secondary star is a K-type main-sequence star that is 0.79 times as massive as the Sun, and with a surface temperature of 4,780 K.

Notes

References

Illustrations

Worked examples

Example 1 — a first encounter with Phecda

Start with the simplest possible case. Write down what Phecda 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 Phecda 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 Phecda 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 Phecda

In research: Phecda 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 Phecda 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: Phecda is common in secondary-school and first-year university syllabi. It links to neighbouring topics A-type main-sequence stars, Astrometric binaries, Bayer objects, so understanding it makes those chapters shorter.
In everyday life: Look for Phecda 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 Phecda in 20 minutes

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

Frequently asked questions

What is Phecda in simple terms?

Why does Phecda 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 Phecda?

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 Phecda.