Appendix E. Glossary of Terms.

Appendix E. Glossary of Terms.

Apparent Brightness

Flux (energy) of a star's light arriving at the Earth.

Apparent Magnitude

The measure of the brightness of light from a star or other object as measured from Earth.

Absolute Magnitude

The apparent magnitude that a star would have if it were at a distance of 10 parsecs.


The wavelength range of light over which an observation is made.

Bolometric Magnitude

The magnitude of a star derived either from the total energy that it radiates at all wavelengths or from the total energy of those of its wavelengths that are received on earth.

Deceleration parameter qo

A measure of how fast the cosmological expansion is speeding up or slowing down. qo - a positive value of the deceleration parameter means the universe is slowing down, a value of 0 means the universe is expanding at a constant speed, and a negative value means that the universe is accelerating.

Density parameter W0

The ratio of the actual density of the universe to the critical density. A value greater than one indicates that the universe is denser than the critical value and this corresponds to a closed universe. A value less than one is an open universe.

Elliptical galaxies

Which lack spiral arms, are composed almost entirely of population II stars. Elliptical galaxies are actually of ellipsoidal shape. Normally, elliptical galaxies contain very little or no interstellar matter, and consist of old population II stars only: They appear like luminous bulges of spiral galaxies, without a disk component. These galaxies are nearly all of the same color: they are somewhat redder than the Sun.

NGC 4881


Source: [Date accessed 17 Apr 2000]

Collection of stars (billions), gas and dust . The force of gravity holds a galaxy together.

Interstellar Medium (ISM)

The material between the stars which is composed of gas and dust.

Lookback Times

As light takes a long time to reach us from distant objects, so we seeing them as they were in the past. A time equal to the distance to the time light takes to travel from the objects to us. Examples of look-back times: Moon: 1 second; Sun - 8 minutes; Jupiter 32 to 48 minutes; closest star - 4.2 years; closest galaxy - 150,000 years.

Look Back Times (Ho=75 km/sec/Mpc)



Look-back time

(109 years)

Distance (Mpc)





























v=cz where c= the speed of light 3 x 105 km/sec

Source: "Lecture 34: Cosmology " Author: Terry Herter [Date Accessed 2 June 2000]


Total amount of energy radiated per second. It has units of energy per second (e.g. ergs per second). luminosity of an object is compared to the solar luminosity, the total energy given off per second by the sun. One solar luminosity is 4 1033 ergs per second. Luminosity is denoted as L.


The brightness of a star. Refer Absolute Magnitude and Apparent Magnitude.

Main Sequence Star

A star whose luminosity and surface temperature place it on the main sequence on a Hertzsprung-Russell diagram. A star that derives its energy from core hydrogen burning.

Mass to Luminosity Ratio M/L

A relationship between the masses and luminosities of main-sequence stars. A main sequence star's luminosity is roughly proportional to its mass to the 3.5 power: L ~ M3.5. This relationship was derived from the observations of the masses of various types of main sequence stars, but it has also been demonstrated by the calculation of stellar models of different massed zero age main sequence stars.


The formation of atomic nuclei, started instants after the big bang, as the universe cooled. The chain of thermonuclear fusion processes by which hydrogen is converted to helium, helium to carbon, and so on through all the elements of the periodic table.


One parsec is a unit of distance equal to 3.2616 light years, and a Megaparsec is 3.2616 million light years.


The measurement of light. Specifically refers to the procedure of highly accurate measuring of the apparent magnitudes of astronomical objects. In general, astronomers measure only a portion of the wavelength spectrum when they do photometry. Different types of photometry are defined by the portion of the wavelength that they examine. For example "UBV Photometry" measures the light within three standard regions defined by filters. These are Ultraviolet, Blue and Visual (hence UBV). There are many different photometry systems and standards.

Population I stars

are young, recently formed stars. Population I are second-generation stars formed from the debris of exploded population II stars, these stars have a considerable content of heavy elements that were created by nucleosynthesis in the interiors of the earlier stars. The most luminous population I stars are blue giants. Population I stars, of which the sun is typical, are young stars that still lie mostly on the main sequence of the Hertzsprung-Russell diagram. They are particularly concentrated in the interstellar dust of the spiral arms, where new stars are continually being formed. The very brightest population I stars are not distributed at random, but are grouped in loose associations of several hundred stars that partake in the general galactic rotation and are believed to have a common origin. The categories population I and population II were first introduced by Walter Baade as a result of his studies of the Andromeda Galaxy.

Population II stars

are formed early in the history of the galaxy from pure Hydrogen with a mixture of helium. Because massive blue-white giants burn their nuclear fuel quickly and therefore have lifetimes of only a few million years, no stars of this type are found in population II. The most luminous population II stars are red giants. Population II stars are found both in the spiral arms and in the gas-free and dust-free regions of the spiral galaxies.

Spiral galaxies

Spiral galaxies usually consist of two major components: A flat, large disk which often contains a lot of interstellar matter (visible sometimes as reddish emission nebulae, or as dark dust clouds) and young (open) star clusters and associations which have emerged from them, often arranged in conspicuous and striking spiral patterns and/or bar structures, and an ellipsoidally formed bulge component, consisting of an old stellar population without interstellar matter, and often associated with globular clusters. The young stars in the disk are classified as stellar population I, the old bulge stars as population II. The luminosity and mass relation of these components seem to vary in a wide range, giving rise to a classification scheme. The pattern structures in the disk are most probably transient phenomena only, caused by gravitational interaction with neighboring galaxies. Our sun is one of several 100 billion stars in a spiral galaxy, the Milky Way. Both the arms and the disk of a spiral system are blue in color, whereas its central areas are red like an elliptical galaxy.


Kaufmann, William J. Freedman, Roger A. Universe. (W.H. Freeman and Company. New York 1999. 5th Ed)

"Galaxies" [Date accessed 17 Apr 2000]

"Lifting the Veil on Hubble's Constant" May 25, 1999 Author: John Horack [Date accessed 5 Jun 2000]

"Astronomy 124 Glossary of Terms" [Date accessed 9 June 2000]

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXVIII. Combining the Constraints on the Hubble Constant (Feb 2000). last updated 29 June 1999 [Date accessed 30 May 2000]

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Date Created 1 June 2000
Last Updated 11 June 2000