astronomy

KW Sagittarii

KW Sagittarii 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 KW Sagittarii rather than just read about it. In short: KW Sagittarii is a red supergiant or hypergiant star, located approximately 2,420 parsecs (7,900 light-years) away from the Sun in the direction of the constellation Sagittarius. It is one of the largest known stars, with a diameter about 1,000 times larger than the Sun.

Key takeaways

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

Reference excerpt

KW Sagittarii is a red supergiant or hypergiant star, located approximately 2,420 parsecs (7,900 light-years) away from the Sun in the direction of the constellation Sagittarius. It is one of the largest known stars, with a diameter about 1,000 times larger than the Sun. If placed at the center of the Solar System, the star's surface would engulf Mars, coming close to Jupiter's orbit.

Variability

In 1942, Henrietta Hill Swope listed KW Sagittarii as a variable star. It varies erratically in brightness over a range of about two magnitudes. It is classified as a semiregular variable, although the listed period of 670 days is poorly defined. The peculiar cool spectrum has led to comparisons with symbiotic variables, but it is no longer considered to be a cataclysmic binary.

Distance A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association. The parallax derived from the Hipparcos mission is negative so doesn't give much information about the distance except that it is likely to be large. The Gaia Data Release 2 parallax is 0.5281±0.1392 mas and implies a distance of around 1,900 pc. The Gaia result carries a significant statistical margin of error, as well as an indicator that the astrometric excess noise is far beyond acceptable levels so that the parallax should be considered unreliable. A 2021 study published a photogeometric distance of 2,159 pc to KW Sgr, using a parallax published by Gaia DR3 (the successor of Gaia DR2).

Characteristics KW Sagittarii is classed as a dust-enshrouded luminous cool supergiant and varies its spectral type between M0 and M4. Sometimes described as a cool hypergiant, it is losing its mass at 5.6×10−6 M☉ per year and is also found to be a confirmed maser source of SiO masers. H2O and OH masers around the star have been proposed, but neither have been detected. A 2005 study led by Levesque, using a MARCS model, calculated a high luminosity of 363,000 L☉ for KW Sgr and consequently very large radius of 1,460 R☉ based on the assumption of an effective temperature of 3,700 K at a distance of 3.0 kpc. The star was then described as among the four largest and most luminous galactic red supergiants, which includes V354 Cephei, KY Cygni and Mu Cephei. In 2011, its bolometric luminosity was re-measured to be at around 228,000 L☉ based on the same distance. More recently, the Sagittarius OB5 association was calculated to be somewhat closer than previously thought, which would suggest lower estimates for both the radius and luminosity of the star. Based on a distance of 2.4±0.3 kpc, its luminosity was calculated to be 175,000±52,000 L☉ with a radius of around 1,009±142 R☉ based on a measured angular diameter of 3.91±0.25 mas. Its angular diameter was re-measured at 3.00±0.4 mas in 2023.

See also VX Sagittarii — another red supergiant (RSG) in the constellation Sagittarius UY Scuti AH Scorpii

Notes

References

External links Space.Com Universe Today

Worked examples

Example 1 — a first encounter with KW Sagittarii

Start with the simplest possible case. Write down what KW Sagittarii 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 KW Sagittarii 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 KW Sagittarii 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 KW Sagittarii

In research: KW Sagittarii 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 KW Sagittarii 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: KW Sagittarii is common in secondary-school and first-year university syllabi. It links to neighbouring topics Durchmusterung objects, Henry Draper Catalogue objects, Hipparcos objects, so understanding it makes those chapters shorter.
In everyday life: Look for KW Sagittarii 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.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “KW Sagittarii” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study KW Sagittarii in 20 minutes

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

Frequently asked questions

What is KW Sagittarii in simple terms?

Why does KW Sagittarii 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 KW Sagittarii?

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 KW Sagittarii.