Hoag's Object

Key takeaways

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

Reference excerpt

Hoag's Object is an unusual ring galaxy in the constellation of Serpens Caput. It is named after Arthur Hoag, who discovered it in 1950 and identified it as either a planetary nebula or a peculiar galaxy. The galaxy has a D25 isophotal diameter of 45.41 kiloparsecs (148,000 light-years).

Characteristics A nearly perfect ring of young hot blue stars circles the older yellow nucleus of this ring galaxy c. 600 million light-years away in the constellation Serpens. The ring structure is so perfect and circular that it has been referred to as "The most perfect ring galaxy". The diameter of the 6 arcsecond inner core of the galaxy is about 17±0.7 kly (5.3±0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75±3 kly (24.8±1.1 kpc) and an outer 45″ diameter of 121±4 kly (39.9±1.7 kpc). The galaxy is estimated to have a mass of 700 billion suns. By comparison, the Milky Way galaxy has an estimated diameter of around 100 kly and consists of between 100 and 400 billion stars and a mass between 800 billion and 1.54 trillion suns. The gap separating the two stellar populations may contain some star clusters that are almost too faint to see. Though ring galaxies are rare, another more distant ring galaxy can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at roughly the one o'clock position in the image shown here. Noah Brosch and colleagues showed that the luminous ring lies at the inner edge of a much larger neutral hydrogen ring. A few other galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none matches Hoag's Object in symmetry, these galaxies are sometimes called Hoag-type galaxies.

History and formation Even though Hoag's Object was clearly shown on the Palomar Star Survey, it was not included in either the Morphological Catalogue of Galaxies, the Catalogue of Galaxies and Clusters of Galaxies, or the catalogue of galactic planetary nebulae. In the initial announcement of his discovery, Hoag proposed the hypothesis that the visible ring was a product of gravitational lensing. This idea was later discarded because the nucleus and the ring have the same redshift, and because more advanced telescopes revealed the ring's knotty structure, which would not be visible if the galaxy were a product of gravitational lensing. Many of the galaxy's details remain mysterious, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy, producing a density wave in the disk that leads to a characteristic ring-like appearance. Such an event would have happened at least 2–3 billion years ago, and may have resembled the processes that form polar-ring galaxies. However, there is no sign of any second galaxy that would have acted as the "bullet", and the likely older core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis implausible. Observations with one of the most sensitive telescopes have also failed to uncover any faint galaxy fragments that should be observable in a collision scenario. However, a team of scientists that analyzes the galaxy admits that "if the carnage happened more than 3 billion years ago, there might not be any detritus left to see." Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" that occurred a few billion years ago in a barred spiral galaxy. Schweizer et al. claim this is an unlikely hypothesis because the nucleus of the object is spheroidal, whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily.

See also LEDA 1000714 UGC 4599 NGC 6028 NGC 7742

Notes

References

Further reading Adelman-Mccarthy, Jennifer K.; et al. (2008). "The Sixth Data Release of the Sloan Digital Sky Survey". The Astrophysical Journal Supplement Series. 175 (2): 297–313. arXiv:0707.3413. Bibcode:2008ApJS..175..297A. doi:10.1086/524984.

External links Media related to Hoag's Object at Wikimedia Commons Hubble Space Telescope image of Hoag's Object. Accretion Rings Galactic Rings – Fund. Cosmic Physics, 1996. Vol. 17, pp. 95–281 A Wheel within a Wheel (09/05/2002) News Release Number: STScI-2002-21 – HubbleSite ESA/Hubble image of Hoag's Object Hoag's Object on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Illustrations

Worked examples

Example 1 — a first encounter with Hoag's Object

Start with the simplest possible case. Write down what Hoag's Object 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 Hoag's Object 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 Hoag's Object 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 Hoag's Object

In research: Hoag's Object 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 Hoag's Object 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: Hoag's Object is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1950, Peculiar galaxies, Principal Galaxies Catalogue objects, so understanding it makes those chapters shorter.
In everyday life: Look for Hoag's Object 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 Hoag's Object in 20 minutes

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

Frequently asked questions

What is Hoag's Object in simple terms?

Hoag's Object is an unusual ring galaxy in the constellation of Serpens Caput. It is named after Arthur Hoag, who discovered it in 1950 and identified it as either a planetary nebula or a peculiar galaxy.

Why does Hoag's Object 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 Hoag's Object?

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 Hoag's Object.