astronomy

WR 138a

WR 138a 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 WR 138a rather than just read about it. In short: WR 138a is a Wolf-Rayet star in the constellation Cygnus. It is of a very late spectral type of WN9h.

Key takeaways

WR 138a belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect WR 138a to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of WR 138a from memory before moving on to harder problems.

Reference excerpt

WR 138a is a Wolf-Rayet star in the constellation Cygnus. It is of a very late spectral type of WN9h. The WR is also at the centre of a ring nebula (typical of WRs) and is a runaway.

Position and Discovery WR 138a was first identified as a star with H-Alpha emission in 1997. Its Wolf-Rayet nature was discovered in 2009, along with its physical parameters. Although WR 138a is located in the Cygnus X complex from our viewpoint, in actually it is further away (4,200 pc compared to ~1,800 pc) and unrelated to the complex. The star is also very reddenned, and in the visible wavelength, it is reddenned by 7.4 magnitudes. It is also a runaway star, with a peculiar velocity of 50 km/sec−1, and is located about 230 pc above the galactic plane.

Nebula WR 138a has a ring nebula around it, which measures 2.3 arcminutes across, and in reality measures about 1.4 pc across. WR 138a is offset from the centre of the nebula by around 0.2 arcminutes.

Properties

WR 138a is a relatively dim WNL star. Modelling WR 138a's spectrum with PoWR gives a luminosity of around 200,000 L☉ and a temperature of approximately 40,000 K. Using the Stefan-Boltzmann Law the star's radius can be calculated, which turns out at approximately 9.4 R☉. WR 138a has a very strong stellar wind, typical of Wolf-Rayet stars, and it loses 10−4.7 M☉ (about 2×10−5 M☉) per year because of this stellar wind, which has a terminal velocity of about 700 kilometres per second. Therefore, the star loses around 1 solar mass every 50,000 years.

Evolution WR 138a has a current mass of about 13 M☉, and probably evolved from a star with an initial mass of about 30 M☉, and is about 6.7 million years old. The small size and nearly circular shape of the nebula around WR 138a imply that the stellar wind interacts with the dense ambient medium comoving with the star, which is what shapes the nebula. This consideration suggests that the immediate precursor of WR 138a was a red supergiant (i.e. the star's initial mass is below 40 M☉) and that the WR wind still propagates and ploughs through the high-density region around the star, which is occupied by material shed by the star during its RSG phase. According to evolution models, stars with initial masses of 25 to 40 M☉ have lifecycles of O → RSG → WN, and the evolution of WR 138a is consistent with the models. Considering average cumulative RSG mass loss values, and velocities, a dynamical age of WR 138a's nebula can be obtained, which turns out at 14,000 years old, which suggests that WR 138a only recently entered the WR phase.

References

Worked examples

Example 1 — a first encounter with WR 138a

Start with the simplest possible case. Write down what WR 138a 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 WR 138a 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 WR 138a 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 WR 138a

In research: WR 138a 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 WR 138a 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: WR 138a is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cygnus (constellation), Runaway stars, Wolf–Rayet stars, so understanding it makes those chapters shorter.
In everyday life: Look for WR 138a 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.

Affiliate

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

How to study WR 138a in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what WR 138a 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 WR 138a out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is WR 138a in simple terms?

WR 138a is a Wolf-Rayet star in the constellation Cygnus. It is of a very late spectral type of WN9h.

Why does WR 138a 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 WR 138a?

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 WR 138a.