Astronomical Glossary

All-sky Survey

An all-sky image constructed from many small images of the night sky. Astronomers have these in visible light, which is the light we see with our eyes, but also have images in infrared, radio, X-Ray, and Gamma Ray areas of the electromagnetic spectrum. Also, the cosmic microwave background images are an example of a processed all-sky image. These images are two-dimensional in nature, so we wrap them onto a sphere that surrounds our vantage point.

Astronomical Unit

An Astronomical Unit is the average distance between the Sun and Earth. The adopted value is 149,597,870,700 meters, or 149.6 million kilometers, or 93 million miles.

Astrophysical dust

Astronomers refer to dust as the microscopic rocks that float around the universe. These rocky particles range from a few molecules to maybe a millimeter in diameter. From our point of view, dist is a nuisance and obscures our view of objects behind it. However, infrared telescopes allow us to see through the dust. Dust scatters light and can dim light and make objects appear more red than they are. Just as the daytime sky is blue because the atoms in our atmosphere scatter blue light, dust scatters red light and reddens objects, which astronomers call extinction of light.

Blackbody

A theoretical body that absorbs and re-emits all light incident on it. The light emitted from such a body only depends on the temperature of the body and that temperature remains constant given it emits all light (energy) incident on it. We use the term as it relates to the cosmic microwave background (CMB) light, which has been referred to as the most perfect blackbody in nature. The CMB is a nearly constant 2.7 Kelvin, and , departing from an ideal blackbody by only one part in 100,000.

Completeness

Data completeness is an attribute of a data set that relates to its overall quality. One might be tempted to think of this as gaps in observations, just as the Sloan DSS Galaxies did not look at the entire sky, so there are gaps in the data. But, this is not the gist of completeness. Completeness tells you the quality of a data set. For example, for the Hipparcos catalog of stars, we know the catalog has a completeness down to a magnitude of +7.5 magnitude. That means for stars brighter than 7.5, the catalog is complete and we can treat as such when examining it. This complete subset of stars numbers about 25,000. But, Hipparcos has over 100,000 stars. This means that for stars dimmer than 7.5 magnitude, Hipparcos didn’t capture them all in its data. The data set is incomplete for these dimmer stars.

Ecliptic

The ecliptic is the imaginary line in the sky on which the Sun travels throughout the year. Because it is defined by the annual path of Sun, it traverses the zodiacal constellations. Extrapolated to three dimensions, the ecliptic, then, defines the plane shared by the Sun and Earth.

Finger of god

A “finger of god” is a line of galaxies that point radially back to the Milky Way. Early in the process of mapping the local universe, astronomers noticed that galaxies, when plotted, appeared on lines that pointed radially back to Earth. They saw these “lines” of galaxies pointing at Earth from all directions in the sky. It turns out we see fingers of god mostly in clusters of galaxies.

Within a cluster of galaxies, there are random motions as the galaxies all orbit the center of the cluster. We derive the distance to galaxies using their relative velocity, and, in a sense, the velocities inside a cluster are contaminated by these random, local motions (as opposed to the more global recessional velocity of the expanding universe). This results in a radial spread in distance for the galaxies in a cluster. And, all these radial “lines” all point back to the observer here on Earth, puzzling astronomers who first observed this in the earliest maps of the universe.

Habitable zone

The habitable zone for a planetary system is a region where liquid water can exist. It depends on the type of host star and the nature of the planet. Here, we tend to assume a rocky, earth-like planet and a star akin to the Sun. For the Solar System, Earth is comfortably in the habitable zone, while Venus flirts with the inner boundary, and Mars lies within but closer to the outer boundary. We also show exoplanet habitable zones in the module.

Light year

A light year is the distance over which light travels in one year. Light speed is 300,000 km/s (186,000 miles/sec). So, 1 light second is just 300,000 km (186,000 miles), 1 light day is 26 billion km (16 billion miles) and 1 light year is 9.5 trillion km (6 trillion miles).

Lookback time

Lookback time is the difference between when we detect the light from an object here on Earth versus when the light was emitted by the object. Because the universe is expanding, and spacetime can expand faster than the speed of light (matter cannot, but the intervening fabric of spacetime can), it produces a discrepancy between how far an object was when its light left versus how far it actually is at this moment. The lookback time tells us when the light left the object.

Observable universe

That part of the universe visible to us. Our sightline is limited because the early universe was opaque—light was constantly scattering off free electrons. About 380,000 years after the Big Bang, the universe expanded and cooled to the point where electrons combined with protons to make neutral hydrogen. At that time, light was able to travel across the universe, and this is the oldest light we can see.

Each point of observation in the universe has its own spherical observable universe. For our observation point on Earth, we have a 45-billion-light0year sphere surrounding us. And, for any other point, they will have their own similar sphere centered on them, revealing areas of the universe that are invisible to us while we can see areas invisible to them.

Parsec

A parsec is a unit of distance in astronomy. The word comes from “one parallax second.” Parallax is the technique by which we measure the distance to stars. It is a geometrical method where we measure a star’s position relative to the background stars on one side of Earth’s orbit, then six months later measure the star’s position again. The difference in positions is called parallax. (More on parallax.) When this Earth-Sun distance of 1 Astronomical Unit subtends an angle of 1 arcsecond on the sky due to the star’s apparent motion as we orbit the Sun, then it is said to be 1 parsec away. One parsec equals 206,265 Astronomical Units exactly, and 3.26 light years.

Redshift/blueshift

Redshift refers to the shifting of light as the source moves away from the observer. It is the light equivalent of the Doppler shift in sound when, for example, a train’s sound changes as it approaches you and recedes from you. We use redshift in astronomy to calculate the distance to objects outside the Milky Way. Almost all objects outside the Galaxy are moving away from us as the universe expands, and we can measure the shift of light in the spectral lines of a galaxy, and once we know that shift in wavelength, we can compute its distance. If the object is moving away from us, the light is shifted toward the red in the electromagnetic spectrum, and if the object is moving toward us, the light is shifted toward the blue end of the spectrum.

Spacetime

Spacetime is a mathematical model that incorporates both space and time into one, unified continuum. It is not needed to describe the non-relativistic world, where Euclidean space and classical mechanics describes the universe accurately. But, in a relativistic universe the three-dimensional space cannot be separated from time. Gravity, and therefore matter, when moving close to the speed of light, slows time down for an outside observer. A massive object’s gravity warps spacetime, creating valleys for the light to traverse, which takes slightly longer than it would in the absence of that mass.

Uncertainty

In science, uncertainty refers to doubt in a measurement value. These doubts arise from several things, but often derive from the limitations of the instrument doing the measurement. For example, a ruler has lines for each millimeter, but if the object being measured is somewhere between 2 mm and 3 mm, you have to estimate what the final value. We know 2 mm is too low, and 3 mm is too high, so the value will be some guess, with a “give or take” range, for example, maybe we estimate it to be 2.5 mm plus or minus 0.5 mm. In astrophysics, these limitations derive from limitations of telescopes, observational conditions, and the nature of the object we’re measuring.

Zone of avoidance

This is a term invented by astronomers and describes the areas of the sky obscured by the Milky Way—that band of light across our sky. It is named as such because early galaxy maps showed areas where galaxies appeared to avoid. We prefer to use “zone of obscuration” because it has a more physical meaning and is inherently more clear.