NGC 6210

Key takeaways

NGC 6210 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NGC 6210 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 6210 from memory before moving on to harder problems.

Reference excerpt

NGC 6210, sometimes also known as the Turtle Nebula or simply the Turtle, is a planetary nebula located approximately 5.4 ± 1.3 kly from the Sun in the constellation of Hercules. It is a bright nebula. It is positioned about 38° above the galactic plane at a vertical distance of about 3.3 kilolight-years (1 kpc), this is unusually high for a nebula. Due to its position, it has little extinction from intervening interstellar dust. The nebula formed as a result of material being shed from a low mass progenitor star less massive than the Sun (~0.9 M☉) the complex structure and appearance of NGC 6210 has led to proposals that the nebula was shaped by mass transfer in a triple star system. This object was first recorded as a star-like feature by Joseph Lalande on March 22, 1799. However, credit for the discovery of a nebula goes to Wilhelm Struve in 1825. John L. E. Dreyer described it as, "a planetary nebula, very bright, very small, round, disc and border".

Characteristics

Structure The structure of NGC 6210 is complex having an unusual morphology being described as very amorphous and irregular in shape (rough ellipsoid) about 0.5 light years in radius. The nebula contains shells, lobes, knots, and haloes. The nebula consists of main two parts; a bright inner region and a faint outer region. The inner region is filled with arches and filaments spanning 13″ × 16". This region has an expansion velocity ranging over 19–24 km/s. The outer region is larger and fainter. It has a pair of "tubular" structures. Emissions from the outer part of the nebula is only about 1% of the total. The nebula contains five distinct axes which material is ejected. The first is the bipolar, inner shell blown from stellar winds. The second is the lop-sided, elliptical, fainter however more massive intermediate shell. Two more axes are bipolar flows that form the point symmetric, high-ionization outer lobes. The final axis traces the collimated outflows of low-ionization knots. Many of these knots that are redshifted have ages greater than 2000 years while those which are blueshifted have ages less than 2000 years. Starting at or before the initial ionization of the nebula around 3500 years ago, the nebula experienced major changes in the direction of its outflows.

Halo Surrounding the nebula is a halo-like structure that is nearly twice as large however with a much fainter surface than the main nebula. There are four arm-like extensions in the halo. Kinematics of the halo show that it is highly disturbed with evidence for at least three circularly symmetric, low-amplitude ripples within the halo.

Abundances The nebula contains a low amount of helium and oxygen. The nitrogen and argon abundances of NGC 6210 is low and nearly that of the Sun. These abundance of carbon is also low, even lower than the solar value. Elements with especially low abundances include silicon, iron, potassium, chlorine, phosphorus, sodium and magnesium. The abundances of oxygen, sulfur and neon is average for nebula located in its galactic position. These low chemical abundances suggest that the star that formed the nebula may not have gone through a second dredge-up phase. The low abundance carbon indicates that the third dredge-up phase may have not occurred. The values seen in NGC 6210 may be inaccurate due to methods of determining these values. For example, the abundance of sulfurs is consistently lower than that of the Sun suggesting errors. An example of possible inaccuracies can be found in the abundance of magnesium. The values for magnesium may be lower due to a small amount of it being present in magnesium sulfide (MgS). There is no indication for magnesium to be associated with dust grains located in the nebula as seen in the infrared spectrum of some other nebulae.

Central star The nebula contains a star located in the central region of NGC 6210. The central star is a hydrogen-rich O(H) type star that has an apparent visual magnitude of 12.66. It is estimated that the temperature of the star is around 65,000 K. The chemical abundances of the nebula, specifically the low values for certain elements such as helium, oxygen, nitrogen and carbon, suggest a low initial mass for the central star, probably ~0.9 M☉. Outflow from this star has been measured with velocities of 2,180 km/s, and the estimated mass loss rate is 2.2×10−9 M☉ yr−1. There appears to be a collaminated jet feature to the northwest, suggesting the central star is ejecting material along two and possibly four such directions. The curvature of these jets suggest a possible rotation of the collimating source.

Gallery

Illustrations

Worked examples

Example 1 — a first encounter with NGC 6210

Start with the simplest possible case. Write down what NGC 6210 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 NGC 6210 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 NGC 6210 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 NGC 6210

In research: NGC 6210 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 NGC 6210 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: NGC 6210 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hercules (constellation), NGC objects, Planetary nebulae, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 6210 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.

How to study NGC 6210 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 6210 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 NGC 6210 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 6210 in simple terms?

NGC 6210, sometimes also known as the Turtle Nebula or simply the Turtle, is a planetary nebula located approximately 5.4 ± 1.3 kly from the Sun in the constellation of Hercules. It is a bright nebula.

Why does NGC 6210 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 NGC 6210?

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 NGC 6210.