astronomy

Janus (star)

Janus (star) 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 Janus (star) rather than just read about it. In short: Janus, also known by its name ZTF J203349.8+322901.1, is a transitioning white dwarf located more than 1,300 light-years (400 pc) away in the constellation Cygnus, discovered in 2019 by the Zwicky Transient Facility (ZTF), located at the Palomar Observatory, while looking for periodically variable white dwarfs. Subsequent observations using the Low-Resolution Imaging Spectrometer (LRIS) on the W.

Key takeaways

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

Reference excerpt

Janus, also known by its name ZTF J203349.8+322901.1, is a transitioning white dwarf located more than 1,300 light-years (400 pc) away in the constellation Cygnus, discovered in 2019 by the Zwicky Transient Facility (ZTF), located at the Palomar Observatory, while looking for periodically variable white dwarfs. Subsequent observations using the Low-Resolution Imaging Spectrometer (LRIS) on the W. M. Keck Observatory discovered its two-faced nature, with one hemisphere dominated by hydrogen, and the other dominated by helium.

Properties ZTF J203349.8+322901.1 is located more than 1,300 light-years (400 pc) away in the constellation Cygnus, with a mass between 1.2 M☉ and 1.27 M☉ (1.21 for an oxygen–neon core and 1.27 for a carbon–oxygen core), a radius of 3400+700−600 km and a surface temperature of approximately 35,000 Kelvin. It is rare in that it has two hemispheres of different gases, one dominated by hydrogen, and the other dominated by helium. Another star, GD 323, shares this feature, albeit much more subtle. Janus' rotation period was observed by using CHIMERA, a high-speed imaging photometer, and HiPERCAM, a quintuple-beam imager (data collected on the nights of 6 and 9 September 2021 for a total of 2.1 hours), both located on the Gran Telescopio Canarias, which revealed a period of 14.97 minutes, which is much faster than what is usually observed in white dwarfs (hours to days). As it rotates, its spectrum transitions from only hydrogen lines to only helium lines at phases ≈ 0 and 0.5, respectively. There was no Zeeman splitting observed. The two hemispheres were measured to be at different temperatures, with the hydrogen side at 34900+1300−1500 K and the helium side at 36700+1300−1600 K.

Theories There are some theories as to why ZTF J203349.8+322901.1's hemispheres are so starkly defined and of different compositions. The first theory revolves around the belief that white dwarfs undergo an evolutionary phase, where helium sinks towards the bottom and hydrogen rises towards the top due to their masses. It is theorised that Janus was observed in an intermediate stage of this phase, and is exiting the DB gap on its way to becoming a DB white dwarf. The second theory is based on asymmetric magnetic fields: If one hemisphere has a stronger magnetic field than the other, then the magnetic pressure at the pole will be higher, causing the hydrogen to diffuse towards the pole due to the ion pressure gradient, requiring a magnetic field of at least tens of kGs, and less than a few MG. The third theory revolves around ZTF J203349.8+322901.1 being the result of a merger of two white dwarfs, due to its large mass and short rotation period.

References

Worked examples

Example 1 — a first encounter with Janus (star)

Start with the simplest possible case. Write down what Janus (star) 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 Janus (star) 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 Janus (star) 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 Janus (star)

In research: Janus (star) 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 Janus (star) 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: Janus (star) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cygnus (constellation), Discoveries by the Zwicky Transient Facility, White dwarfs, so understanding it makes those chapters shorter.
In everyday life: Look for Janus (star) 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 Janus (star) in 20 minutes

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

Frequently asked questions

What is Janus (star) in simple terms?

Why does Janus (star) 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 Janus (star)?

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 Janus (star).