astronomy

Smith's Cloud

Smith's Cloud 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 Smith's Cloud rather than just read about it. In short: Smith's Cloud is a high-velocity cloud of hydrogen gas located in the constellation Aquila at Galactic coordinates l = 39°, b = −13°. The cloud was discovered in 1963 by Gail Bieger, née Smith, who was an astronomy student at Leiden University in the Netherlands.

Key takeaways

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

Reference excerpt

Smith's Cloud is a high-velocity cloud of hydrogen gas located in the constellation Aquila at Galactic coordinates l = 39°, b = −13°. The cloud was discovered in 1963 by Gail Bieger, née Smith, who was an astronomy student at Leiden University in the Netherlands.

Properties

Using the National Science Foundation's Robert C. Byrd Green Bank Telescope, radio astronomers have found that Smith's cloud has a mass of at least one million solar masses and measures 3,000 parsecs (9,800 ly) long by 1,000 pc (3,300 ly) wide in projection. The cloud is between 11,100 pc (36,000 ly) and 13,700 pc (45,000 ly) from Earth and has an angular diameter of 10 to 12 degrees, approximately as wide as the Orion constellation, or about 20 times the diameter of the full moon, although the cloud is not visible to the naked eye. The cloud is apparently moving towards the disk of the Milky Way at 73 ± 26 kilometers per second. Smith's Cloud is expected to merge with the Milky Way in 27 million years at a point in the Perseus Arm. Astronomers believe it will strike the Milky Way disk at a 45° angle, and its impact may produce a burst of star formation or a supershell of neutral hydrogen. Projecting the cloud's trajectory backwards through time, it is estimated that it had passed through the disk of the Milky Way some 70 million years ago. To have survived this previous encounter, astronomers have suggested that it is embedded inside a massive dark matter halo. The fact that it survived this previous encounter means that it is likely to be much more massive than previously thought, and may be a candidate for being a dark galaxy. In this scenario it would be a failed dwarf galaxy, with the ingredients to form a stellar galaxy, but few if any detectable stars. However, chemical abundance measurements from the Hubble Space Telescope argue against this hypothesis; these measurements show that the Smith Cloud has an average metallicity of one half of the solar value, indicating that its gas originates in the Galaxy, not from an extragalactic source. The cloud's orbit and metallicity are both consistent with an origin in the outer disk of the Milky Way. The mechanism by which this gas was released is not known.

References

External links

Finley, Dave (Jan 11, 2008). "Massive Gas Cloud Speeding Toward Collision With Milky Way". National Radio Astronomy Observatory (NRAO). The leading edge of this cloud is already interacting with gas from our Galaxy— Felix J. Lockman Rincon, Paul (12 Jan 2008). "Huge gas cloud will hit Milky Way". BBC.

Illustrations

Worked examples

Example 1 — a first encounter with Smith's Cloud

Start with the simplest possible case. Write down what Smith's Cloud 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 Smith's Cloud 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 Smith's Cloud 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 Smith's Cloud

In research: Smith's Cloud 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 Smith's Cloud 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: Smith's Cloud is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aquila (constellation), Astronomical objects discovered in 1963, Dark galaxies, so understanding it makes those chapters shorter.
In everyday life: Look for Smith's Cloud 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.

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How to study Smith's Cloud in 20 minutes

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

Frequently asked questions

What is Smith's Cloud in simple terms?

Why does Smith's Cloud 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 Smith's Cloud?

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 Smith's Cloud.