astronomy

S Persei

S 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 S Persei rather than just read about it. In short: S Persei is a red supergiant or hypergiant located near the Double Cluster in Perseus, north of the open cluster NGC 869. It is a member of the Perseus OB1 association and is one of the largest known stars.

Key takeaways

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

Reference excerpt

S Persei is a red supergiant or hypergiant located near the Double Cluster in Perseus, north of the open cluster NGC 869. It is a member of the Perseus OB1 association and is one of the largest known stars. If placed in the Solar System, its photosphere would engulf the orbit of Jupiter. It is also a semiregular variable, a star whose variations are less regular than those of Mira variables.

Discovery S Persei was named by German astronomer Adalbert Krueger in 1874 after observing that it varied in brightness. It was subsequently listed in major stellar catalogues of that era as HD 14528 and BD+57°552.

Variability

S Persei varies slowly by several magnitudes, a factor of over 40 in brightness. It has a main period of somewhat over two years, but shows significant unpredictability. There is a strong variation in the amplitude from around one magnitude to about four magnitudes, and these have been interpreted as beats due to a second period of about 940 days. Other analyses find only the primary period of 813 ± 60 days. S Persei is classified as a semiregular variable star of type SRc, indicating that it is a supergiant, and it has one of the largest visual amplitudes of any variable of this type. While the General Catalogue of Variable Stars lists it as varying between magnitudes 7.9 and 12.0, it has since been seen fainter. The spectral type of S Persei also varies. Typically it is a red supergiant of spectral class M3 or M4, but particularly at deep visual minima it may show a much cooler spectral type of M7 or M8, highly unusual for a supergiant.

Properties Many of the visually bright variable stars belong to this class of semiregulars, as these stars are extremely large and luminous, and hence visible across long distances. S Persei has been described as a hypergiant and has a radius over 700 times the sun's radius (R☉). Its angular diameter has been measured directly and found to be somewhat elliptical. Modelled as a uniform disk, the radius corresponds to 1,212 ± 124 R☉. The temperature has been calculated from the spectrum using a DUSTY model, giving an effective photospheric temperature of 3,500 K and a temperature of 1,000 K for the surrounding dust torus. This is consistent with previous studies, but the derived luminosity from different authors varies from 86,000 L☉ to 186,000 L☉. Older studies frequently calculated higher luminosities, lower temperatures, and consequently larger values for the radius. The mass of S Persei is also uncertain, but expected to be around 20 M☉. Mass is being lost at 2.4 to 2.6×10−5 M☉ per year, leading to an extensive and complex circumstellar environment of gas and dust.

Location S Persei is surrounded by clouds containing water molecules which produce maser emission. This allows the distance to be measured very accurately using very long baseline interferometry, giving an annual parallax of 0.413 ± 0.017 milliarcseconds. For comparison, the Gaia Data Release 2 parallax is 0.2217±0.1214 mas. It lies somewhat further away than the centres of the Double Cluster open clusters, but definitely within the Per OB1 association and the Perseus Arm of the galaxy. S Persei is a double star. The red supergiant has an A0 11th magnitude companion at 69". There are also several other 8th to 10th magnitude stars within half a degree of S Persei.

See also VX Sagittarii – A comparable late-type red supergiant or hypergiant with both strong spectral variability and one of the largest visual amplitudes Similar red supergiants or hypergiants with strong spectral variability

BC Cygni HV 11423 KW Sagittarii Westerlund 1 W26 Similar red supergiants or hypergiants with a large visual amplitude

AH Scorpii VY Canis Majoris Other comparable red supergiants or hypergiants

PZ Cassiopeiae UY Scuti

References

Illustrations

Worked examples

Example 1 — a first encounter with S Persei

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

In research: S 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 S 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: S Persei is common in secondary-school and first-year university syllabi. It links to neighbouring topics Double stars, Durchmusterung objects, Emission-line stars, so understanding it makes those chapters shorter.
In everyday life: Look for S 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 S Persei in 20 minutes

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

Frequently asked questions

What is S Persei in simple terms?

Why does S 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 S 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 S Persei.