Thecosmic distance ladder (also known as theextragalactic distance scale) is the wayastronomers measure thedistance of objects inspace. No one method works for all objects and distances, so astronomers use a number of methods.
A realdirect distancemeasurement of anastronomical object is possible only for those objects that are close enough to Earth (within about a thousandparsecs). It is the larger distances which are the problem. Several methods rely on astandard candle, which is an astronomical object that has a known standardluminosity.
The ladder analogy arises because no one technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.
Theastronomical unit is the mean (average) distance of the Earth from the Sun. This we know quite accurately.Kepler's Laws tell the ratios of the distances of planets, andradar tells the absolute distance to innerplanets andartificial satellites in orbit around them.
Parallax is the use oftrigonometry to discover the distances of objects near to thesolar system.
As the Earth orbits around the Sun, the position of nearby stars will appear to shift slightly against the more distant background. These shifts are angles in a righttriangle, with 2AU making the short leg of the triangle and the distance to the star being the long leg. The amount of shift is quite small, measuring 1arcsecond for an object at a distance of 1 parsec (3.26 light-years)
This method works for distances up to a few hundred parsecs.
Objects of known brightness are calledstandard candles. Most physical distance indicators are standard candles. These are objects which belong to a class that has a known brightness. By comparing the knownluminosity of the latter to its observed brightness, the distance to the object can be computed using theinverse-square law.
In astronomy, the brightness of an object is given in terms of itsabsolute magnitude. This quantity is derived from thelogarithm of its luminosity as seen from a distance of 10parsecs. Theapparent magnitude is the magnitude as seen by the observer. It can be used to determine the distanceD to the object in kiloparsecs (kiloparsec = 1,000 parsecs) as follows:
wherem the apparent magnitude andM the absolute magnitude. For this to be accurate, both magnitudes must be in the same frequency band and there can be no relative motion in the radial direction.
Some means of accounting forinterstellar extinction, which also makes objects appear fainter and more red, is also needed. The difference between absolute and apparent magnitudes is called thedistance modulus, and astronomical distances, especially intergalactic ones, are sometimes tabulated in this way.
Two problems exist for any class of standard candle. The principal one iscalibration, finding out exactly what the absolute magnitude of the candle is.
The second lies in recognizing members of the class. The standard candle calibration does not work unless the object belongs to the class. At extreme distances, which is where one most wishes to use a distance indicator, this recognition problem can be quite serious.
A significant issue with standard candles is the question of how standard they are. For example, all observations seem to indicate that Type Iasupernovae that are of known distance have the same brightness, but it's possible thatdistant Type Ia supernovae have different properties thannearby Type Ia supernovae.[1]
With few exceptions, distances based on direct measurements are available only up to about a thousand parsecs, which is a modest portion of our own galaxy. For distances beyond that, measures depend upon physical assumptions, that is, the assertion that one recognizes the object in question, and the class of objects is homogeneous enough that its members can be used for meaningful estimation of distance.
Physical distance indicators, used on progressively larger distance scales, include:
In aHertzsprung-Russell diagram the absolute magnitude for a group of stars is plotted against thespectral classification of the stars. Evolutionary patterns are found that relate to the mass, age and composition of the star. In particular, during theirhydrogen burning period, stars lie along a curve in the diagram called themain sequence.
By measuring the properties from a star's spectrum, the position of a main sequence star on the H-R diagram can be found. From this the star'sabsolute magnitude is estimated. A comparison of this value with theapparent magnitude allows the approximate distance to be determined, after correcting forinterstellar extinction of the luminosity because of gas and dust.
In a gravitationally-boundstar cluster such as theHyades, the stars formed at approximately the same age and lie at the same distance. This allows relatively accurate main sequence fitting, providing both age and distance determination.
This is not a complete list of methods, but it does show the ways astronomers go about estimating the distance of astronomical objects.