astronomy

Xi Ursae Majoris

Xi Ursae Majoris 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 Xi Ursae Majoris rather than just read about it. In short: Xi Ursae Majoris is a quintuple star system 28.5 light-years (8.7 parsecs) away in the constellation of Ursa Major. It has the traditional name Alula Australis; Xi Ursae Majoris is the Bayer designation, which is Latinised from ξ Ursae Majoris and abbreviated Xi UMa or ξ UMa.

Key takeaways

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

Reference excerpt

Xi Ursae Majoris is a quintuple star system 28.5 light-years (8.7 parsecs) away in the constellation of Ursa Major. It has the traditional name Alula Australis; Xi Ursae Majoris is the Bayer designation, which is Latinised from ξ Ursae Majoris and abbreviated Xi UMa or ξ UMa. It is a variable star with a small amplitude. Xi Ursae Majoris is found in the left hind paw of the Great Bear. Xi Ursae Majoris is the second-nearest quintuple system, after V1054 Ophiuchi.

History of observations Xi Ursae Majoris was found to be a visual double star by William Herschel in 1780, and in 1804 he presented it as a likely physical binary star based on observed orbital motion. It then became the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828.

Stellar system

The two main components are yellow main-sequence stars. The brighter component (designated Xi Ursae Majoris A), has a mean apparent magnitude of +4.41. The companion star (Xi Ursae Majoris B) has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years. They are currently (2022) separated by 2.3 arcseconds, and will widen to a maximum 3.0 arcseconds in 2035.

Each component of this double star is itself a single-lined spectroscopic binary. The orbit of the A pair has been determined from spectroscopy and speckle interferometry, giving a period of 669 days and an eccentricity of 0.53. B's binary companion (Xi Ursae Majoris Bb) has not been detected visually, but the radial velocity variations of the spectral lines show a circular orbit with a period of 3.98 days. The masses of both A and B's companions (Ab and Bb) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) indicate that they are probably red dwarfs, Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf. However, component Ba has been found to be enriched in barium relative to component Aa, suggesting that its companion Bb may be a white dwarf. In 2012 Wright et al. discovered the fifth component and the first brown dwarf of the system using Wide-field Infrared Survey Explorer (WISE) data—a T8.5 brown dwarf WISE J111838.70+312537.9 with angular separation 8.5 arcmin, and the projected physical separation about 4,000 AU.

Variable star

ξ Ursae Majoris is classified as an RS Canum Venaticorum variable and its brightness varies by 0.01 magnitude. Component B is believed to be the variable star, showing characteristic emission lines in its spectrum that are not present for component A.

Nomenclature ξ Ursae Majoris (Latinised to Xi Ursae Majoris) is the star's Bayer designation. It also bore the traditional names Alula Australis (and erroneously Alula Australe). Alula (shared with Nu Ursae Majoris) comes from the Arabic phrase Al Ḳafzah al Ūla 'the First Spring' and Australis is Latin for 'southern'. 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 Alula Australis for Xi Ursae Majoris Aa. In Chinese, 三台 (Sān Tái), meaning Three Steps, refers to an asterism consisting of Xi Ursae Majoris, Iota Ursae Majoris, Kappa Ursae Majoris, Lambda Ursae Majoris, Mu Ursae Majoris and Nu Ursae Majoris. Consequently, the Chinese name for Xi Ursae Majoris itself is 下台二 (Xià Tái èr, English: Star of Second Lower Step).

See also List of star systems within 25–30 light-years

References

Note

External links Animation of the orbits of the stars in the Alula Australis System at SolStation.com Main Article on Alula Australis at SolStation.com Alula Australis by Dr. Jim Kaler.

Illustrations

Worked examples

Example 1 — a first encounter with Xi Ursae Majoris

Start with the simplest possible case. Write down what Xi Ursae Majoris 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 Xi Ursae Majoris 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 Xi Ursae Majoris 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 Xi Ursae Majoris

In research: Xi Ursae Majoris 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 Xi Ursae Majoris 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: Xi Ursae Majoris is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bayer objects, Bright Star Catalogue objects, Durchmusterung objects, so understanding it makes those chapters shorter.
In everyday life: Look for Xi Ursae Majoris 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 Xi Ursae Majoris in 20 minutes

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

Frequently asked questions

What is Xi Ursae Majoris in simple terms?

Why does Xi Ursae Majoris 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 Xi Ursae Majoris?

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 Xi Ursae Majoris.