astronomy

Messier 81

Messier 81 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 Messier 81 rather than just read about it. In short: Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a grand design spiral galaxy about 12 million light-years away in the constellation Ursa Major. It has a D25 isophotal diameter of 29.44 kiloparsecs (96,000 light-years).

Key takeaways

Messier 81 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Messier 81 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Messier 81 from memory before moving on to harder problems.

Reference excerpt

Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a grand design spiral galaxy about 12 million light-years away in the constellation Ursa Major. It has a D25 isophotal diameter of 29.44 kiloparsecs (96,000 light-years). Because of its relative proximity to the Milky Way galaxy, large size, and active galactic nucleus (which harbors a 70 million M☉ supermassive black hole), Messier 81 has been studied extensively by professional astronomers. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers. In late February 2022, astronomers reported that M81 may be the source of FRB 20200120E, a repeating fast radio burst.

Discovery Messier 81 was first discovered by Johann Elert Bode on 31 December 1774. Thus, it is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, hence listed it in the Messier Catalogue.

Visibility The galaxy is to be found approximately 10° northwest of Alpha Ursae Majoris (Dubhe) along with several other galaxies in the Messier 81 Group. Its apparent magnitude due to its distance means it requires a good night sky and only rises very briefly and extremely low at its southernmost limit from Earth's surface, about the 20th parallel south. Messier 81 and Messier 82 are considered ideal for viewing using binoculars and small telescopes. The two objects are generally not observable to the unaided eye, although highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a very dark sky. Telescopes with apertures of 8 inches (20 cm) or larger are needed to distinguish structures in the galaxy.

The galaxy is best observed during April.

Interstellar dust Most of the emission at infrared wavelengths originates from interstellar dust. This interstellar dust is found primarily within the galaxy's spiral arms, and it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and thus enhancing the infrared dust emission from these regions.

Globular clusters It is estimated M81 has 210 ± 30 globular clusters. In late February 2022, astronomers reported that M81 may be the source of FRB 20200120E, a repeating fast radio burst.

Supernova

Only one supernova has been detected in Messier 81. The supernova, named SN 1993J, was discovered on 28 March 1993 by F. García in Spain. At the time, it was the second brightest Type II supernova observed in the 20th century, peaking at an apparent magnitude of 10.7. The spectral characteristics of the supernova changed over time. Initially, it looked more like a Type II supernova (a supernova formed by the explosion of a supergiant star) with strong hydrogen spectral line emission, but later the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a Type Ib. Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other Type II supernovae, but did resemble the variations observed in Type Ib supernovae. Hence, the supernova has been classified as a Type IIb, a transitory class between Type II and Type Ib. The scientific results from this supernova suggested that Type Ib and Ic supernovae were formed through the explosions of giant stars through processes similar to those taking place in Type II supernovae. Despite the uncertainties in modeling the unusual supernova, it was also used to estimate a very approximate distance of 8.5 ± 1.3 Mly (2.6 ± 0.4 Mpc) to Messier 81. As a local galaxy, the Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M81 along with M31 and M33.

SMBH In the center of M81 there exists a supermassive black hole (SMBH) with a mass of about 7×107 M☉. The SMBH is active, having an accretion disk and one-sided relativistic jet. The observation also demonstrate that there may exist a second SMBH that orbits the primary SMBH with a period of around 30 years. The mass of the secondary SMBH is estimated at 0.1 of the primary.

Environment Messier 81 is the largest galaxy in the M81 Group, a group of 34 in the constellation Ursa Major. At approximately 11.7 Mly (3.6 Mpc) from the Earth, it makes this group and the Local Group, containing the Milky Way, relative neighbors in the Virgo Supercluster. Gravitational interactions of M81 with M82 and NGC 3077 have stripped hydrogen gas away from all three galaxies, forming gaseous filamentary structures in the group. Moreover, these interactions have allowed interstellar gas to fall into the centers of M82 and NGC 3077, leading to vigorous star formation or starburst activity there.

Distance The distance to Messier 81 has been measured by Freedman et al to be 3.63 ± 0.34 Megaparsecs (11.8 ± 1.1 million light years) by using the Hubble Space Telescope to identify classical Cepheid variables and measure their periods using the period-luminosity relation discovered by Henrietta Swan Leavitt.

See also List of galaxies List of Messier objects Messier object New General Catalogue

References

External links

StarDate: M81 Fact Sheet M81, SEDS Messier pages SST: Messier 81 NASA Astronomy Picture of the Day: Bright Galaxy M81 (20 June 2002) NightSkyInfo.com – M81, Bode's Galaxy Messier 81 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images ESA/Hubble images of M81 Galaxy Messier 81 (Bode's Galaxy) Spitzer Image Gallery Helkit Observatory Deep image of the M81 Area Bode's Galaxy at Constellation Guide

Illustrations

Worked examples

Example 1 — a first encounter with Messier 81

Start with the simplest possible case. Write down what Messier 81 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 Messier 81 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 Messier 81 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 Messier 81

In research: Messier 81 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 Messier 81 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: Messier 81 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1774, Discoveries by Johann Elert Bode, IRAS catalogue objects, so understanding it makes those chapters shorter.
In everyday life: Look for Messier 81 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 Messier 81 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Messier 81 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 Messier 81 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Messier 81 in simple terms?

Why does Messier 81 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 Messier 81?

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 Messier 81.