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L

Lagrangian points

Points where the gravitational forces of three different massive bodies exactly cancel. For the Sun-Earth-Moon system we have five different Lagrange points, known as L1, L2, L3, L4 and L5.

Lander

A small spacecraft designed to land on another Solar System body.

Latch

Mechanical device that attaches one component, such as a science instrument, to the structure of the telescope and holds it in precisely the right place.

Launch window

The time period within which a spacecraft or rocket must be launched in order to achieve the desired orbit or rendezvous.

Launcher

Powered vehicle used to carry one or more satellites into space.

Lens

Transparent optical element or assembly with either a concave or convex surface, which refracts light to form an image.

Leonids

Meteor shower which occurs around 17 November each year. The meteors appear to radiate from the constellation of Leo. They are caused by dust grains along the orbit of periodic comet Tempel-Tuttle which burn up when they enter the Earth's upper atmosphere. Spectacular displays may occur at approximately 33-year intervals, with the last `storm' taking place in 1966.

Light

All electromagnetic radiation can be called light. However, the term 'light' is commonly used for the electromagnetic radiation that the human eye can detect, that is, the 'visible' or 'optical' light.

Light elements

The chemical elements produced during the first minutes of existence of the Universe. They are helium, deuterium (an isotope of hydrogen), lithium, beryllium and boron. They are called 'light elements' because their nuclei are made of just a few nuclear particles, and thus their mass is very low. The other elements - those produced in the core of stars - are 'heavier'; iron, for instance, has 26 protons in its nucleus. See also Nucleosynthesis and Primordial nucleosynthesis.

Lightning

Sudden high-current discharge caused by a planet atmosphere's electrical breakdown.

Light-year (ly)

The distance travelled by light through space in one year. 1 light-year (ly) equals about 9.5 million million kilometres (=0.3066 parsecs = 63240 AU = a parallax of 3.259 arcsecs).

Line (absorption)

Absorption spectra are formed when continuous spectra from a star shine through a gas that absorbs only certain colours of light. The absorption spectra, therefore, look like continuous spectra with dark bands (absorption lines) at discrete wavelengths. These lines characterise the chemical composition of the gas which surrounds the star.

Luminosity

The amount of radiation emitted by a star or celestial object in a given time.

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