astronomy

I Zwicky 18

I Zwicky 18 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 I Zwicky 18 rather than just read about it. In short: I Zwicky 18 is a blue compact dwarf and Wolf–Rayet galaxy located about 18.44 megaparsecs (60.14 million light-years) away in the constellation Ursa Major. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies.

Key takeaways

I Zwicky 18 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect I Zwicky 18 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of I Zwicky 18 from memory before moving on to harder problems.

Reference excerpt

I Zwicky 18 is a blue compact dwarf and Wolf–Rayet galaxy located about 18.44 megaparsecs (60.14 million light-years) away in the constellation Ursa Major. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. It also possesses an active galactic nucleus.

Age Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted with star formation billions of years after its galactic neighbours. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Early observations with the Hubble Space Telescope suggested an age of 500 million years old for I Zwicky 18. Astronomers have hypothesised the existence of young dwarf galaxies in a cold dark matter-dominated Universe. Dwarfs as young as 500 million years would be, however, much fainter than I Zwicky 18. I Zwicky 18 is thus unlikely to have formed all its stars recently in a cold dark matter Universe. Later observations with the Hubble Space Telescope found faint and old stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.

Contents

Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of hydrogen and helium, the main ingredients created in the Big Bang. The galaxy's primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA's telescopes, including the Spitzer Space Telescope, the Chandra X-ray Observatory, the Far Ultraviolet Spectroscopic Explorer (FUSE), and the James Webb Space Telescope. However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now. In 2015 a study found a very large region of ionised helium in this small galaxy, which tends to be more frequent in very distant galaxies with low presence of metals. The ionization of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms. Scientists therefore hypothesise that I Zwicky 18 must contain Population III stars, which are almost absent in all other galaxies in the local universe.

Distance Astronomers at the Space Telescope Science Institute in Baltimore, Maryland and the European Space Agency discerned the distance by observing Cepheid variable stars within I Zwicky 18. These massive flashing stars pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. The team determined the observed brightness of three Cepheids and compared it with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18's deficiency in heavy elements, indicating the galaxy's stars formed before these elements were abundant in the universe. The Cepheid distance was also validated through the observed brightness of the brightest red stars older than one billion years. The Hubble data suggest that I Zwicky 18 is 59 million light-years from Earth. The galaxy's larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy. The faint, old stars in I Zwicky 18 are almost at the limit of Hubble's resolution and sensitivity.

Classification I Zwicky 18 is classified as a dwarf irregular galaxy and also a blue compact galaxy and a starburst galaxy. I Zwicky 18 is much smaller than our Milky Way and a mere 1.58 kiloparsecs (5,150 light-years) across. I Zwicky 18 may still be creating Population III stars—spectroscopy shows that its stars are composed almost entirely of hydrogen and helium, with heavier elements almost completely absent. The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions are bubbles of gas that have been blown away by stellar winds and supernova explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a new generation of hot, young stars.

Illustrations

Worked examples

Example 1 — a first encounter with I Zwicky 18

Start with the simplest possible case. Write down what I Zwicky 18 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 I Zwicky 18 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 I Zwicky 18 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 I Zwicky 18

In research: I Zwicky 18 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 I Zwicky 18 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: I Zwicky 18 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Dwarf irregular galaxies, Irregular galaxies, Markarian galaxies, so understanding it makes those chapters shorter.
In everyday life: Look for I Zwicky 18 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 I Zwicky 18 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what I Zwicky 18 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 I Zwicky 18 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is I Zwicky 18 in simple terms?

Why does I Zwicky 18 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 I Zwicky 18?

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 I Zwicky 18.