IC 443

Key takeaways

IC 443 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect IC 443 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of IC 443 from memory before moving on to harder problems.

Reference excerpt

IC 443 (also known as the Jellyfish Nebula and Sharpless 248 (Sh2-248)) is a galactic supernova remnant (SNR) in the constellation Gemini. On the plane of the sky, it is located near the star Eta Geminorum. Its distance is roughly 5,000 light years from Earth. IC 443 may be the remains of a supernova that occurred 30,000 - 35,000 years ago. The same supernova event likely created the neutron star CXOU J061705.3+222127, the collapsed remnant of the stellar core. IC 443 is one of the best-studied cases of supernova remnants interacting with surrounding molecular clouds.

Global properties

IC 443 is an extended source, having an angular diameter of 50 arcmin (by comparison, the full moon is 30 arcmin across). At the estimated distance of 5,000 ly (1,500 parsec) from Earth, it corresponds to a physical size of roughly 70 light years (20 parsec). The SNR optical and radio morphology is shell-like (e.g. a prototypical shell-like SNR is SN 1006), consisting of two connected sub-shells with different centers and radii. A third, larger sub-shell—initially attributed to IC 443—is now recognized as a different and older (100,000 years) SNR, called G189.6+3.3. Notably, IC 443 X-ray morphology is centrally peaked and a very soft X-ray shell is barely visible. Unlike plerion remnants, e.g. the Crab Nebula, the inner X-ray emission is not dominated by the central pulsar wind nebula. It has indeed a thermal origin. IC 443 shows very similar features to the class of mixed morphology SNRs. Both optical and X-ray emission are heavily absorbed by a giant molecular cloud in the foreground, crossing the whole remnant body from northwest to southeast. The remnant's age is still uncertain. There is some agreement that the progenitor supernova happened between 3,000 and 30,000 years ago. Recent Chandra and XMM-Newton observations identified a plerion nebula, close to the remnant southern rim. The point source near the apex of the nebula is a neutron star, relic of a SN explosion. The location in a star forming region and the presence of a neutron star favor a Type II supernova, the ultimate fate of a massive star, as the progenitor explosion.

The SNR environment

The SNR IC 443 is located in the galactic anticenter direction (l=189.1°), close to the galactic plane (b=+3.0°). Many objects lie in the same region of sky: the HII region S249, several young stars (members of the GEM OB1 association), and an older SNR (G189.6+3.3). The remnant is evolving in a rich and complex environment, which strongly affects its morphology. Multi-wavelength observations show the presence of sharp density gradients and different cloud geometries in the surroundings of IC 443. Massive stars are known to be short lived (roughly 30 million years), ending their life when they are still embedded within the progenitor cloud. The more massive stars (O-type) probably clear the circum-stellar environment by powerful stellar winds or photoionizing radiation. Early B-type stars, with a typical mass between 8 and 12 solar masses, are not capable of this, and they likely interact with the primordial molecular cloud when they explode. Thus, it is not surprising that the SNR IC 443, which is thought to be the aftermath of a stellar explosion, evolved in such a complex environment. For instance, an appreciable fraction of supernova remnants lies close to dense molecular clouds (~50 out of 265 in the Green catalogue), and most of them (~60%) show clear signs of interaction with the adjacent cloud. X-ray and the optical images are characterized by a dark lane, crossing IC 443 from northwest to southeast. Emission from quiescent molecular gas has been observed toward the same direction, and it is likely due to a giant molecular cloud, located between the remnant and the observer. This is the main source of extinction of the low energy SNR emission. In the southeast the blast wave is interacting with a very dense (~10,000 cm−3) and clumpy molecular cloud, such that the emitting shocked gas has a ring-like shape. The blast wave has been strongly decelerated by the cloud and is moving with an estimated velocity of roughly 30–40 km s−1. OH (1720 MHz) maser emission, which is a robust tracer of interaction between SNRs and dense molecular clouds, has been detected in this region. A source of gamma-ray radiation is spatially coincident with IC 443 and most likely originates from interactions of the SNR with the molecular clouds which can be concluded from the detection of the pion bump, a characteristic spectral feature of hadronic gamma-ray production. In the northeast, where the brightest optical filaments are located, the SNR is interacting with a very different environment. The forward shock has encountered a wall of neutral hydrogen (HI), and is propagating into a less dense medium (~10-1,000 cm−3) with a much higher velocity (80–100 km s−1) than in the southern ridge. In the western region, the shock wave breaks out into a more homogeneous and rarefied medium.

See also Gemini (Chinese astronomy) List of stars in Gemini

References

External links

The Case Of The Neutron Star With A Wayward Wake Chandra X-ray Center (ScienceDaily) June 1, 2006 Astronomy Picture of the Day IC 443: Supernova Remnant and Neutron Star 2006 June 2 The Elusive Jellyfish Nebula 2010 May 15 The Elusive Jellyfish Nebula 2013 January 9

Illustrations

Worked examples

Example 1 — a first encounter with IC 443

Start with the simplest possible case. Write down what IC 443 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 IC 443 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 IC 443 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 IC 443

In research: IC 443 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 IC 443 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: IC 443 is common in secondary-school and first-year university syllabi. It links to neighbouring topics 3C objects, Astronomical objects discovered in 1892, Gemini (constellation), so understanding it makes those chapters shorter.
In everyday life: Look for IC 443 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 IC 443 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what IC 443 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 IC 443 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is IC 443 in simple terms?

IC 443 (also known as the Jellyfish Nebula and Sharpless 248 (Sh2-248)) is a galactic supernova remnant (SNR) in the constellation Gemini. On the plane of the sky, it is located near the star Eta Geminorum.

Why does IC 443 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 IC 443?

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 IC 443.