astronomy

R Hydrae

R Hydrae 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 R Hydrae rather than just read about it. In short: R Hydrae, abbreviated R Hya, is a single star in the equatorial constellation of Hydra, about 2.7° to the east of Gamma Hydrae. It is a Mira-type variable that ranges in apparent visual magnitude from 3.5 down to 10.9 over a period of 389 days.

Key takeaways

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

Reference excerpt

R Hydrae, abbreviated R Hya, is a single star in the equatorial constellation of Hydra, about 2.7° to the east of Gamma Hydrae. It is a Mira-type variable that ranges in apparent visual magnitude from 3.5 down to 10.9 over a period of 389 days. At maximum brightness the star can be seen with the naked eye, while at minimum a telescope of at least 5 cm is needed. This star is located at a distance of approximately 410 light-years from the Sun but is drifting closer with a radial velocity of −10 km/s. This is an aging red giant star with a stellar classification of M6e/M9e:, currently near the thermal-pulsing tip of the asymptotic giant branch. The 1998 detection of weak technicium (Tc) lines in the spectrum suggests the star has recently gone through a third dredge-up. However, a 2010 study failed to detect technicium in the atmosphere, indicating this oxygen-rich AGB star is Tc-poor. O. J. Eggen found the star is a likely member of the Hyades supergroup, which suggests an age of 500 million to one billion years and a mass double that of the Sun.

The first record of observations for this star was in 1662 by Johannes Hevelius. It was found to be variable by Giacomo Filippo Maraldi in 1702; thereafter he tracked its various appearances until 1712. The evidence suggests the period of variability during this period was ~495 days. In between 1770 and 1950, the period declined linearly down to 385 days. It has remained fairly stable since that time. The decline may be due to a prior thermal pulse caused by a helium flash under a hydrogen-fusing shell. The period change would have been accompanied by a decrease in the stellar radius. An alternative explanation is a relaxation of the stellar envelope brought on by a change in the structure within the star.

An extended emission component has been detected in the infrared that indicates a detached, expanding dust shell surrounds the star. The inner boundary of this shell is located at 60–100 times the stellar radius, and it extends outward to at least 1,000 times the radius with an expansion velocity of ~8 km/s. The data indicate a mass loss rate of 2×10−7 M☉·yr−1, which declined by a factor of 20 around the year 1788. Observations using the Spitzer Space Telescope show an extended double shell with a spiral structure, which extends out to 300″. It forms a bow shock where it interacts with the surrounding interstellar medium. The orientation of this feature is consistent with the star's proper motion.

References

Further reading Homan, Ward; et al. (July 2021). "ATOMIUM: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae. I. Morpho-kinematical interpretation of CO and SiO emission". Astronomy & Astrophysics. 651: 19. arXiv:2104.07297. Bibcode:2021A&A...651A..82H. doi:10.1051/0004-6361/202140512. S2CID 233241196. A82.

External links AAVSO Variable Star of the Month. May 2002: R Hydrae finding chart at AAVSO.

Illustrations

Worked examples

Example 1 — a first encounter with R Hydrae

Start with the simplest possible case. Write down what R Hydrae 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 R Hydrae 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 R Hydrae 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 R Hydrae

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

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

Frequently asked questions

What is R Hydrae in simple terms?

Why does R Hydrae 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 R Hydrae?

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 R Hydrae.