astronomy

Holmberg 15A

Holmberg 15A 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 Holmberg 15A rather than just read about it. In short: Holmberg 15A (abbreviated to Holm 15A) is a supergiant elliptical galaxy and the brightest galaxy of the Abell 85 galaxy cluster in the constellation Cetus, about 241.4 megaparsecs (787.3 million light-years) from Earth. It was discovered c. 1937 by Erik Holmberg.

Key takeaways

Holmberg 15A belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Holmberg 15A to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Holmberg 15A from memory before moving on to harder problems.

Reference excerpt

Holmberg 15A (abbreviated to Holm 15A) is a supergiant elliptical galaxy and the brightest galaxy of the Abell 85 galaxy cluster in the constellation Cetus, about 241.4 megaparsecs (787.3 million light-years) from Earth. It was discovered c. 1937 by Erik Holmberg. It became well known when it was reported to have the largest core ever observed in a galaxy, spanning some 15,000 light years, however this was subsequently refuted.

Supermassive black hole It has been postulated that the primary component of the galactic core is a supermassive black hole with a mass of 40 ± 8 billion solar masses (M☉), although no direct measurement has yet been made. Previous estimates by Lauer et al. have jointed a mass value as high as 310 billion M☉ using the gamma ray point break radius method. Kormendy and Bender gave a value of 260 billion M☉ in a 2009 paper. Lower estimates were given by Kormendy and Ho et al. in 2013 at 2.1 and 9.2 billion M☉. The paper by Lopez-Cruz et al. stated: "Therefore, we conservatively suggest that Holm 15A hosts an SMBH with a mass of 1010 M☉." Kormendy and Ho et al derived these equations using the M–sigma relation and the size of the outer bulge of the galaxy, which are indirect estimates. Rusli et al derived a value of 170 billion M☉ using break radius methodology. In addition, Abell 85 has its velocity dispersion of dark matter halo at ~750 km/s, which could be explained only by a black hole with a mass greater than 150 billion M☉, although Kormendy and Ho et al stated that "dark matter halos are scale-free, and the SMBH-dark matter coevolution is independent from the effects of baryons". This makes it one of the most massive black holes ever discovered, with it being classified as an ultramassive black hole.

Illustrations

Worked examples

Example 1 — a first encounter with Holmberg 15A

Start with the simplest possible case. Write down what Holmberg 15A 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 Holmberg 15A 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 Holmberg 15A 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 Holmberg 15A

In research: Holmberg 15A 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 Holmberg 15A 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: Holmberg 15A is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1937, Cetus (constellation), Compact object stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Holmberg 15A 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 Holmberg 15A in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Holmberg 15A 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 Holmberg 15A out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Holmberg 15A in simple terms?

Why does Holmberg 15A 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 Holmberg 15A?

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 Holmberg 15A.