Terminology

[BACK]

What is spectroscopy?

Spectroscopy is the detailed study of the light from an object. Spectrometers are instruments which spread light out into its wavelengths creating a spectrum. Within this spectrum, astronomers can study emission and absorption lines which are the fingerprints of atoms and molecules. An emission line occurs when an electron drops down to a lower orbit around the nucleus of an atom and looses energy. An absorption line occurs when electrons move to a higher orbit by absorbing energy. Each atom has a unique spacing of orbits and can emit or absorb only certain energies or wavelengths. This is why the location and spacing of spectral lines is unique for each atom. Astronomers can learn a great deal about an object in space by studying its spectrum, such as it's composition (what its made of), temperature, density, and it's motion (both it's rotation as well as how fast it is moving towards or away from us).
What is the reddening law?
A "reddening law" is really just a description of how interstellar dust absorbs light that passes through it. When starlight passes through a dust cloud, not only does it get dimmer (because some of the light is being absorbed), it also gets "redder" because the shorter wavelength light is affected more than the longer wavelength light. Interstellar clouds are more efficient at scattering and absorbing blue light than red light, so much less blue light gets through them. That makes stars appear redder when seen through a dust cloud. The "reddening law" allows astronomers to correct this effect, and infer what the star would look like without the dust getting in the way. Different astronomers use different reddening laws, depending on who they think has the better model of exactly how much light is absorbed at different wavelengths. Sometimes this creates arguments and controversy, as no one knows what the right law really is. And getting the right version of the law is important, as it allows astronomers to estimate the actual distances and temperatures of the stars in question.
What is absolute zero?
At a temperature of Absolute Zero there is no motion and no heat. Absolute zero is where all atomic and molecular motion stops and is the lowest temperature possible. Absolute Zero occurs at 0 degrees Kelvin or -273.15 degrees Celsius or at -460 degrees Fahrenheit. All objects emit thermal energy or heat unless they have a temperature of absolute zero.
What are degrees Kelvin and Celsius and how do they relate to degrees Fahrenheit?
In the early years of the eighteenth century, Gabriel Fahrenheit (1686-1736) created the Fahrenheit scale. He set the freezing point of water at 32 degrees and the boiling point at 212 degrees. These two points formed the anchors for his scale.
 
Later in that century, around 1743, Anders Celsius (1701-1744) invented the Celsius scale. Using the same anchor points, he determined the freezing temperature for water to be 0 degree and the boiling temperature 100 degrees. The Celsius scale is known as a Universal System Unit. It is used throughout science and in most countries.
 
There is a limit to how cold something can be. The Kelvin scale is designed to go to zero at this minimum temperature. The relationships between the different temperature scales are:
 
K = 273.15 + C        C = (5/9)*(F-32)        F = (9/5)*C+32
What is the difference between luminosity, absolute magnitude and apparent magnitude?
The luminosity of an object in space is the amount of energy that it radiates each second in all directions. Luminosity is also referred to as the absolute magnitude or absolute brightness of an object. It is the real brightness of a celestial object. The apparent magnitude or apparent brightness of an object is a measure of how bright an object appears to be to an observer. It is the amount of energy from an object in space which reaches a square centimeter of a detector each second. Apparent magnitude is also referred to as flux. It is a measure of how bright a celestial object appears to us. The apparent magnitude of an object depends upon its real brightness and on its distance from us.
What is an Angstrom?
An angstrom is a unit of distance which is commonly used to measure the wavelength of light. 1 Angstrom = 10-10 meters = 0.000,000,000,1 meters. For example, visible light ranges from about 4000 Angstroms (blue) to about 7000 Angstroms (red).
What is an Astronomical Unit?
An astronomical unit (A.U.) is the average distance between the Earth and the Sun, which is about 93 million miles or 150 million kilometers. Astronomical units are usually used to measure distances within our solar system. For example, the planet Mercury is about 1/3 of an A.U. from the Sun, while the farthest planet, Pluto, is about 40 A.U. from the Sun (that's 40 times as far away from the Sun as the Earth is).
What is a light-year?
Most objects in space are so far away, that using a relatively small unit of distance, such as an astronomical unit, is not practical. Instead, astronomers measure distances to objects which are outside our solar system in light-years. A light-year (ly) is the distance that light can travel in one year in a vacuum (empty space). The speed of light is about 186,000 miles or 300,000 kilometers per second. So, in one year light travels a distance of about 5,880,000,000,000 miles or 9,460,000,000,000 kilometers or 63,240 A.U.. This distance is 1 light-year. For example, the nearest star to us is about 4.3 light-years away. Our galaxy, the Milky Way, is about 150,000 light-years across, and the nearest large galaxy, Andromeda, is 2.3 million light-years away.
What are arcminutes and arcseconds?
In astronomy, we often use angular measurements to describe the apparent size of an object in space and the apparent distances between objects. Often these angles are very small. Angles are also used to describe an object's location in space. The angular measure of an object is usually expressed in degrees, arcminutes or arcseconds. Just as an hour is divided into 60 minutes and a minute into 60 seconds, a degree is divided into 60 arcminutes and an arcminute is divided into 60 arcseconds. To give you an idea of how small an arcsecond is, imagine the width of a dime as seen from 2 kilometers or 1 1/4 miles away.
 
1 degree = 1 = 1/360 of a circle
1 arcminute = 1' = 1/60 of a degree
1 arcsecond = 1" = 1/60 of an arcminute = 1/3600 of a degree

 
To get a rough estimate of the angular size of objects in space, you can go out on clear night when the moon is up. Extend your arm towards the sky. Your fist, at arms length, covers about 10 degrees of the sky, your thumb covers about 2 degrees, and your little finger covers about 1 degree. If you look at the Moon, it should take up about 1/2 a degree in the sky. The Big Dipper should be about 20 degrees (two fists at arms length) from one end to the other.
What is spatial resolution?
The spatial resolution of a telescope depends on the size of its lenses or mirrors and the size of the pixels in its detectors. The resolution is also limited by air turbulence (for ground based observatories) and by the smoothness of a telescope's mirrors or lenses. The spatial resolution of a telescope is proportional to the wavelength of light being detected divided by the diameter of the telescope. Larger telescopes have better spatial resolution. However, it is the size of the telescope relative to the wavelength that really counts. The longer the wavelength, the larger the telescope needs to be to get good resolution.
What is the sensitivity of a telescope?
The sensitivity of a telescope is the smallest signal that it can clearly measure from a source in space. It is the minimum brightness that a telescope can detect. A telescope with high sensitivity can detect very dim objects, whereas a low sensitivity telescope can only detect the brighter objects in space. Many objects in space are very dim as seen from the Earth. Some are naturally dim objects because they do not emit much light. Others only appear to be dim because they are a great distance from us. It is important for a telescope to have the greatest sensitivity possible, so that it can observe the many different types of objects in space.

[BACK]