The Andromeda Galaxy

Source: Adam Evans CC-BY 2.0

Andromeda galaxy facts

• The only (other) galaxy easily visible to the naked eye.
• 2.5 million light years away +/- 0.1 million light years.
• It is similar in size and mass to the Milky Way.
• Moving towards us (i.e. solar system) at about 300 km/s
• Hubble classification Sb
• Also referred to as M31

Galaxy naming

Galaxies are assigned a number according to several schemes:

• M is the Messier catalogue; M1 to M110; dates to 1771
• NGC is the New General catalogue; 7840 objects; dates to 1888
• IC is the Index Catalogues; 5386 objects; dates to 1895 and 1908

M objects are mostly in NGC, but IC is an extension to NGC. All these catalogues include star clusters, galaxies, nebulas, but galaxies were not distinguished from galaxies until the early 20th century.

Messier objects

Source: NASA Public domain

Hubble classification

Source: NASA/ESA Public Domain

Classification limitations

Classification schemes like Hubble's are useful, but limited:

• They classify only by the appearance of a galaxy.
• This depends partly on our point of view and its orientation.
• Galaxies often do not fall neatly into one classification.

M33 - photograph

Source: Hewolooks CC-BY SA 3.0

M33 - facts

• Magnitude 5.72 - naked eye visible, just
• 2.4 to 3.1 million light years distant
• Moving towards us at 179 +/3 km/s
• Hubble classification Sc
• Smaller than the Milky Way, approx 50,000 ly in diameter, 40 billion stars

M51 Whirlpool galaxy - drawing

Source: Lord Rosse, 1845 Public Domain

M51 - photograph

Source: NASA/ESA Public Domain

M51 - facts

• Magnitude 8.4 - visible in a small telescope
• 23 +/- 4 million light years distant
• 43,000 light years in diameter; 160 billion solar masses
• Moving away at 463 +/3 km/s
• Hubble classification Sa
• It is interacting with NGC 5195

M87 - an elliptical galaxy

Source: NASA/Hubble Public Domain

M87 facts

• Classified as an E0 galaxy.
• Approx. 53 million light years distant.
• Mass uncertain, but many times the mass of the Milky Way.
• The jet visible in the photograph is evidence of a supermassive blackhole at its centre.
• The blackhole's mass is believed to be about 6 billion solar masses.

Irregular galaxies

There are two types of irregular galaxy according to the Hubble classisfaction:

• Irr I where a galaxy contains a hint of structure, e.g. stumps of spiral arm.
• Irr II where there is no discernable structure at all.

M82 - photograph

Source: NASA/ESA/Hubble Public Domain

M82 - facts

• Magnitude 8.4 - visible in a small telescope
• 11.5 +/- 1 million light years distant
• Moving away at 203 +/4 km/s
• An irregular galaxy, with evidence of spiral arms
• 37,000 light years in diameter; 50 billion solar masses
• 5 times more luminous than the Milky Way
• It is interacting with M81, triggering a burst of star formation.
• Often called a "starburst galaxy".

Galaxy statistics

These statistics are broadly correct for all galaxies we can see, but there is noticeable variation throughout space:

• Over 60% are elliptical, mostly dwarfs.
• Under 30% are spiral.
• Under 15% are irregular.

The Local Group

Source: Richard Powell CC BY-SA 2.5

Local Group facts

• Contains at least 54 galaxies, including dwarf galaxies.
• The Milky Way and Andromeda are the two most massive galaxies in the Local Group.
• Its diameter is about 10 million light years.