astronomy

US 708

US 708 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 US 708 rather than just read about it. In short: US 708 is a hyper-velocity class O subdwarf in Ursa Major, in the halo of the Milky Way Galaxy. One of the fastest-moving stars in the galaxy, the star was first surveyed in 1982.

Key takeaways

US 708 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect US 708 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of US 708 from memory before moving on to harder problems.

Reference excerpt

US 708 is a hyper-velocity class O subdwarf in Ursa Major, in the halo of the Milky Way Galaxy. One of the fastest-moving stars in the galaxy, the star was first surveyed in 1982.

Discovery US 708 was first discovered in 1982 by Peter Usher and colleagues of Pennsylvania State University as a faint blue object in the Milky Way halo. Sloan Digital Sky Survey measured the star again in 2005.

Research In 2015, Stephan Geier of the European Southern Observatory led a team that reported in Science that the velocity of the star was 1,200 km/s (4,300,000 km/h; 2,700,000 mph), the highest ever recorded in the galaxy at that time. The star's high velocity was originally suspected to be caused by the massive black hole at the center of the galaxy. But now it is found out that the star must have crossed the galactic disk about 14 million years ago and thus it did not come from the center of the galaxy; hence the speed now possessed by the star may not be attributed to the black hole. However, closer study suggested it had been one element of a pair of close binary stars. Its companion had already entered its white dwarf stage when US 708 entered its red giant phase. Their respective orbits changed as its companion took gas from the outer layers of US 708. Then its companion acquired enough mass to go supernova, which triggered US 708 being flung away at its high velocity, not by the black hole at the center of our galaxy. The team behind the new observations suggests that it was orbiting a white dwarf roughly the mass of the Sun with an orbital period of less than 10 minutes.

Structure The star is a high speed rotating, dense helium star which is supposed to be formed by the interaction of a companion star nearby. These stars are composed of helium, which is the remnant of a massive star which had lost its envelope of hydrogen. Geier's team describe the star as the "fastest unbound star in the galaxy" and employed the Echellette Spectrograph and Imager attached to the 10 meter Keck II telescope in Hawaii. Its velocity exceeds the escape velocity of our galaxy.

See also List of star extremes S5-HVS1 – fastest moving star as of November 2019

References

Illustrations

Worked examples

Example 1 — a first encounter with US 708

Start with the simplest possible case. Write down what US 708 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 US 708 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 US 708 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 US 708

In research: US 708 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 US 708 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: US 708 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1982, Hypervelocity stars, O-type subdwarfs, so understanding it makes those chapters shorter.
In everyday life: Look for US 708 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 US 708 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what US 708 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 US 708 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is US 708 in simple terms?

US 708 is a hyper-velocity class O subdwarf in Ursa Major, in the halo of the Milky Way Galaxy. One of the fastest-moving stars in the galaxy, the star was first surveyed in 1982.

Why does US 708 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 US 708?

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 US 708.