ESO Astronomical Glossary - M


A macrolens is a type of gravitational lens where the lensing object is very massive, such as a . Macrolenses result in image warps, or even multiple images, of faint distant sources, that are strong enough to detect on images. Stuudies of such lensing effects can give information on the distribution of matter, and dark matter in particular, in the Universe.

Main sequence
A term denoting stars that are observed to be in the longest stable section of their lifetime, where hydrogen is fused in the core in a stable chain reaction. Sometimes abbreviated 'MS', the expression 'main sequence' comes from the characteristic pattern or sequence these stars form in a plot of stars' colour and magnitude (this plot is called the Herzsprung-Russell diagram). The typical colour-magnitude relationship observed in main sequence stars is not valid for newly formed stars, and breaks down again once the star enters the red giant stage, which signals the beginning of its death.

Mars is the fourth planet from the Sun in our solar system. It is named after Mars, the god of war in Roman mythology. Its red colour as viewed in the night sky earned it the name "The Red Planet." Mars has two moons, Phobos and Deimos, which are both small and oddly-shaped, possibly being captured asteroids.

Massive Compact Halo Objects (MACHOs)
MACHO is a collective term for objects that reside in the halo of a galaxy and which do not emit enough radiation to be detected from Earth. Examples include brown dwarfs. MACHOs are thought to make up part of the dark matter in the galaxy. In the absence of radiation to detect, MACHOs can be spotted using the technique of microlensing.

Melipal is the name of the third 8.2 m Unit Telescope (UT2) of ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile. In the Mapuche language Melipal means 'The Southern Cross'.

Mercury is the closest planet to the Sun, and the smallest planet in the solar system. The planet remains comparatively little-known: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, and only 40-45% of the planet has been mapped. Mercury has no natural satellites and no atmosphere. A NASA mission, MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging), was launched in 2004 and will perform several flybys in 2008-2009.

The metallicity of an object is the proportion of its material that is made up of metals. In astronomy, the term 'metals' is used for any element heavier than hydrogen or helium. The early Universe contained predominantly hydrogen, helium and lithium, and heavier elements were only formed when stars started to fuse these elemments into heavier byproducts, such as carbon and oxygen. Therefore studying the metallicity of stars in galaxies and clusters can give astronomers information on their stellar history.

A meteor is a flash of light that occurs when a meteoroid burns up in the Earth's atmosphere; also popularly known as a 'shooting star'.

Meteoroids are tiny stones or pieces of metal that travel through space.

A meteorite is a rock from space that survives the passage through Earth's atmosphere and falls to the ground.

Microlensing is a type of gravitational lens, where the foreground lensing object is of low mass. When light from a star is bent around the object, it will cause a temporary brightening, or magnification, of the star. Microlensing is predominantly used in the search for dark matter in the Milky Way galaxy and its nearest neighbours, as it allows us to detect objects that do not emit enough light to be imaged directly, when they act as lenses - for example brown dwarfs. Such objects are collectively known as MACHOs. Microlensing has also become an important method of detecting exoplanets in recent years, as light lensed by a star with an exoplanet appears different to that lensed by a singular star. In January 2006, Danish 1.54-m telescope at ESO's La Silla observatory contributed to the detection of a low-mass exoplanet using the microlensing method.

A micron or micrometre is one-millionth of a metre. Although 'micron' is an outdated term, the correct form being 'micrometre', it is still often used in astronomy.

Microwaves are an energetic form of radio waves. Their wavelength ranges from around 1 mm to 30 cm. Microwave astronomy is very important in the context of the cosmic microwave background radiation.

Milky Way galaxy
The Milky Way galaxy is a spiral galaxy, of which our Sun and solar system are a small part. All of the stars that we can see with the unaided eye are in the Milky Way Galaxy. The plane of the Milky Way looks like a faint band of white in the night sky - hence the name 'milky'. The Milky Way measures about 100,000 light-years in diameter and contains around 200 billion stars. The galaxy is the second largest in the local galactic neighbourhood, called the Local Group.

One milliarcsecond (mas) is one thousandth of an arcsecond (q.v.).