Share

News Release Archive:

News Release 204 of 955

April 6, 2010 09:00 AM (EDT)

News Release Number: STScI-2010-03

Small Companion to Brown Dwarf Challenges Simple Definition

Back

Fast Facts Help/Glossary

Below are explanations of the Fast Facts that accompany our images. Not all Fast Facts will appear in every news release.

About the Object

This section describes the object that is the subject of an image or science discovery. These facts include:

Object Name

A name or number that astronomers use to identify an astronomical object. This can be a descriptive name (Crab Nebula, Whirlpool Galaxy) or an abbreviation and number (M1, NGC 1952) that reflects its designation in any one of many catalogs of astronomical objects. In those examples, for instance, the M would show that an object is listed in the Messier Catalog, while the NGC would indicate the New General Catalogue.

Object Description

The type of astronomical object being presented. Astronomers classify objects according to their properties with varying levels of detail, such as star, planet, nebula, cluster, galaxy, etc. Within these broad classifications are finer divisions: spiral galaxy, elliptical galaxy, irregular galaxy, etc.

Position

The position of the object in the sky. A celestial coordinate system maps positions in the sky similar to the geographic coordinate system used on the surface of the Earth. It provides a consistent way to locate celestial objects with any telescope. For objects showing substantial motion on the sky over a relatively short time, such as planets, the position — when stated — is given at the time of the observations. Positions are often not reported for moving objects.

J2000

The "epoch" of the celestial coordinates. The coordinates must refer to a particular date because they are relative to the orientation of the Earth in space. Earth's rotation axis changes over thousands of years. The measurements of an object's coordinates on the sky 100 years ago will not be identical today. J2000 refers to the coordinates based in the year 2000. Astronomers use the J2000 coordinates along with the date of observation to calculate the correct coordinates.

R.A.

Right ascension is one of two coordinates (with declination) used by astronomers to locate stars and other celestial objects in the sky. Right ascension is comparable to longitude, but is measured and displayed in units of time: hours (h), minutes (m), and seconds (s) because the entire sky appears to pass overhead over a period of 24 hours. The zero hour corresponds to the apparent location of the Sun with respect to the stars at the time of the vernal (spring) equinox (approximately March 21).

Dec.

Declination is one of two celestial coordinates (with right ascension) required to locate an astronomical object, such as a star, on the celestial sphere. Declination is the measure of angular distance of a celestial object north or south of the celestial equator and is comparable to latitude. Declination is measured and displayed in angular units: degrees (°), minutes (') and seconds (") of arc.

Constellation

Traditionally, a constellation is a pattern of stars in the sky, with names often based on mythological themes. Astronomers now divide the sky into 88 distinct constellations defined by exact boundaries. Every location on the celestial sphere falls in one of these formal constellations. Leo (the Lion) is one example of the 88 constellations formally identified by astronomers.

Distance

The physical distance from Earth to the astronomical object. Distances within our solar system are usually measured in Astronomical Units (AU), the average distance between the Sun and Earth, about 93 million miles or 150 million kilometers. Distances between stars are usually measured in light years, the distance that a particle of light will travel in a year — about 6 trillion miles or 10 trillion km. Interstellar distances can also be measured in parsecs, with one parsec equal to 3.26 light-years. For objects whose distance changes over a relatively short time, such as planets, the distance is given at the time of the observation.

Dimensions

This value refers to the physical size of an astronomical object and/or to the apparent angle it subtends on the sky. Because the sky appears "flat" from Earth, the apparent size of the object must be described in terms of how much of the sky it covers. To do this, astronomers split the sky into angles. The full sky is 360 degrees, or a circle. Both the Moon and Sun appear to take up the same amount of space on the sky — about 0.5 degrees, or 30 arcminutes — and thus have similar apparent sizes. When we know the distance to an object, however, we can also give a true, physical size. For instance, the physical diameter of the Moon is 2,159 miles (3,475 km), and the physical diameter of the Sun is about 870,000 miles (1.4 million km).

Back to top

About the Data

These items describe the data used to produce the images being presented and to analyze the object.

HST Proposal

A detailed description of the observations, their scientific justification, and links to the data available in the Hubble archive.

Science Team

The astronomers who planned the observations and analyzed the data.

Instrument

The Hubble Space Telescope instrument used to produce the data. Find more information about Hubble's instruments at Nuts and Bolts.

Filters

The individual filters that were used in the science observations and to reconstruct the image. Filters in Hubble's cameras isolate a portion of the light from the object. Comparing observations in different filters allows a more detailed analysis of the object under study.

Exposure Dates

The dates that Hubble made its observations. Each science program typically requires many separate exposures and observations, sometimes over a long period of time, even many years. Individual observations can vary from a few seconds to many minutes. Exposures can be combined into a single observation with a cumulative time of many hours or even days.

Back to top

About the Release

These terms relate to the news release describing a science discovery or photo release presenting a Hubble image.

Release Credit

The formal credit listing institutions and individuals responsible for the content. The individual cited is usually the investigator primarily responsible for the data or analysis or the formal Principal Investigator (PI) of the Hubble observing proposal.

Acknowledgement

Others responsible for the science results or images being presented.

Release Date

The date the release content became public.

Color

A brief description of the methods used to convert Hubble data into the color image being presented.

Orientation

The rotation of the image on the sky with respect to the north pole of the celestial sphere.

Diameter of Object

See Dimensions.

Visual Magnitude

A value representing the apparent brightness of a celestial object as seen from Earth. For historical reasons, fainter stars have numerically larger magnitude.

Back to top