astronomy

Phi Persei

Phi Persei 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 Phi Persei rather than just read about it. In short: Phi Persei, Latinized from (Phi Per, φ Persei, φ Per) formally named Dajiangjunbei, is a class B2Vep fourth-magnitude star in the constellation Perseus, located about 720 light-years from Earth. Nomenclature Phi Persei (Phi Per, φ Persei, φ Per) is the star's Bayer designation.

Key takeaways

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

Reference excerpt

Phi Persei, Latinized from (Phi Per, φ Persei, φ Per) formally named Dajiangjunbei, is a class B2Vep fourth-magnitude star in the constellation Perseus, located about 720 light-years from Earth.

Nomenclature Phi Persei (Phi Per, φ Persei, φ Per) is the star's Bayer designation. Flamsteed followed Ptolemy in treating φ Persei as being in Andromeda and gave it the designation 54 Andromedae. It is isolated from the main stars of Perseus, but lies within its formal borders. In Chinese, 天大將軍 (Tiān Dà Jiāng Jūn), meaning Heaven's Great General, refers to an asterism consisting of φ Persei, γ Andromedae, 51 Andromedae, 49 Andromedae, χ Andromedae, υ Andromedae, τ Andromedae, 56 Andromedae, β Trianguli, γ Trianguli and δ Trianguli. Consequently, the Chinese name for φ Persei itself is 天大將軍二 (Tiān Dà Jiāng Jūn èr, English: the Second Star of Heaven's Great General.). The IAU Working Group on Star Names approved the name Dajiangjunbei for Phi Persei A on 25 August 2025 and it is now so entered in the IAU Catalog of Star Names. This means the northern star of Dajiangjun, since the determinative star was already named (Almach).

System

Phi Persei is spectroscopic binary consisting of a blue main sequence primary of class B2 and a hot subdwarf secondary. The two stars have an orbit of 217 days and are separated by about 200 R☉. Phi Persei is a runaway star and extrapolating its space velocity backwards by the modelled age of the system (57 million years) places it within the Alpha Persei Cluster. The primary star rotates rapidly with a projected equatorial velocity of 440 km/s. Due to its rapid rotation, the primary star has a polar radius about 5.5 R☉ and an equatorial radius of about 8.0 R☉. With an effective temperature of nearly 30,000 K, it has a bolometric luminosity nearly 15,000 times higher than the Sun. The rapidly-spinning star is surrounded by a circumstellar disk. The binary orbit, the spin of the primary star, and the disk are all seen nearly edge-on. There are no eclipses, but models of the system show that the disk significantly obscures the primary star. The primary formed as a 3.8 M☉ star and has accreted material from its companion. The secondary star is also a class O subdwarf, hotter than the primary but smaller and less massive. It is proposed that it is the core of a 7 M☉ star, with the outer layers stripped as it expanded away from the main sequence. Its luminosity is higher than expected for a normal 1.2 M☉ helium star, which suggests it may have evolved to helium shell burning. Although the subdwarf has a bolometric luminosity about half that of the primary, it emits most of its radiation as ultraviolet, being only about 3% as bright as the primary at visible wavelengths.

Variability

Phi Persei is a variable star with both rapid and longterm variations in its brightness and spectrum. Variations occur on a daily timescale which may be related to the rotation of the primary star. Any variations originating with the secondary are difficult to detect due to its comparative faintness. Slower variations, including deep fades, are also seen. Some of the variations may correspond to the orbital period, possibly eclipses of gas streams or hot spots, but the occasional deep fades do not match any particular orbital phase. The variations are classified as γ Cassiopeiae-type, shell stars with eruptions and irregular fading.

References

External links HST Press release on the φ Per binary system

Illustrations

Worked examples

Example 1 — a first encounter with Phi Persei

Start with the simplest possible case. Write down what Phi Persei 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 Phi Persei 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 Phi Persei 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 Phi Persei

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

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

Frequently asked questions

What is Phi Persei in simple terms?

Phi Persei, Latinized from (Phi Per, φ Persei, φ Per) formally named Dajiangjunbei, is a class B2Vep fourth-magnitude star in the constellation Perseus, located about 720 light-years from Earth. Nomenclature Phi Persei (Phi Per, φ Persei, φ Per) is the star's Bayer designation.

Why does Phi Persei 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 Phi Persei?

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 Phi Persei.