Glossary items by letter: o — p

Observable Universe

The portion of the entire universe that can be seen from Earth.


In science, an observation is a fact or occurrence that is noted and recorded. The Hubble Space Telescope is a tool astronomers use to make observations of celestial objects.


A structure designed and equipped for making astronomical observations. Observatories are located on Earth and in space.

Oort Cloud

A vast spherical region in the outer reaches of our solar system where a trillion long-period comets (those with orbital periods greater than 200 years) reside. Comets from the Oort Cloud come from all directions, often from as far away as 50,000 astronomical units.


The degree to which light is prevented from passing through an object or a substance. Opacity is the opposite of transparency. As an object’s opacity increases, the amount of light passing through it decreases. Glass, for example, is transparent and most clouds are opaque.

Open Cluster

Also known as a galactic cluster, an open cluster consists of numerous young stars that formed at the same time within a large cloud of interstellar dust and gas. Open clusters are located in the spiral arms or the disks of galaxies. The Pleiades is an example of an open cluster.

Open Universe

A geometrical model of the universe in which the overall structure of the universe extends infinitely in all directions. The rules of geometry in an open universe are like those that would apply on a saddle-shaped surface.


The point at which a planet appears opposite the Sun in our sky. During the Martian opposition, for example, Mars and the Sun are on opposite sides of the Earth.

Optical Telescope

A telescope that gathers and magnifies visible light. The two basic types of optical telescopes are refracting (using lenses) and reflecting (using mirrors). The Hubble Space Telescope is an example of a reflecting telescope.


A person who grinds lenses and mirrors.


The science that deals with the properties of light; in this case specifically dealing with the way light changes directions when it is either refracted and dispersed by a lens or reflected from a mirror.


The act of traveling around a celestial body; or the path followed by an object moving around a celestial body. For example, the planets travel around, or orbit, the Sun because the Sun’s gravity keeps them in their paths, or orbits.

Ozone Layer

A region in the upper atmosphere that has high concentrations of ozone (triatomic oxygen, 03). The ozone layer protects the Earth by absorbing the Sun’s high-energy ultraviolet radiation.

Parabola vs. sphere

If cross-sections of a spherical surface and a parabolic surface were made by slicing each surface in half, these would be the shapes you would see.


The apparent shift of an object’s position when viewed from different locations. Parallax, also called trigonometric parallax, is used to determine the distance to nearby stars. As the Earth’s position changes during its yearly orbit around the Sun, the apparent locations of nearby stars slightly shift. The stars’ distances can be calculated from those slight shifts with basic trigonometric methods.

Parsec (PC)

A useful unit for measuring the distances between astronomical objects, equal to 3.26 light-years and 3.085678 * 1013 kilometers, or approximately 18 trillion miles. A parsec is also equivalent to 103,132 trips to the Sun and back.

Perfect lens

The perfect lens does not exist. Due to the nature of glass, light is dispersed when passing through glass. In the case of convex lenses, red light bends less than blue light, so the focal points are in different places, making the image blurry. A single lens cannot counter this effect.

Period-Luminosity Law

A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time over which that brightness varies.

Periodic Comet

A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years. Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.

Periodic Table (of the Elements)

A chart of all the known chemical elements arranged according to the number of protons in the nucleus (also known as the atomic number). Elements with similar properties are grouped together in the same column.


Regularly occurring changes in the appearance of the Moon or a planet. Phases of the Moon include new, full, crescent, first quarter, gibbous, and third quarter.

Photoelectric Effect

The release of electrons from a solid material when it is struck by radiant energy, such as visible or ultraviolet light, X-rays, or gamma rays.


An instrument that measures the intensity of light. Astronomers use photometers to measure the brightness of celestial objects.


A technique for measuring the brightness of celestial objects.


A packet of electromagnetic energy, such as light. A photon is regarded as a charge-less, mass-less particle having an indefinitely long lifetime.


The extremely thin, visible surface layer of the Sun or a star. The average temperature of the Sun’s photosphere is about 5800 Kelvin (about 10,000° F). Although the Sun is completely made up of gas, its gas is so dense that we cannot see through it. When we look at the Sun, we are seeing the photosphere.

Pickoff Mirror

One of four flat mirrors inside the Hubble Space Telescope. Each mirror is tilted at a 45-degree angle to the incoming light, diverting a small portion of it to the optical detectors or to one of the fine guidance sensors.


A light-sensitive picture element on a charge-coupled device (CCD) or some other kind of digital camera. A pixel is a tiny cell that, placed together with other pixels, resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each pixel collects light from a celestial object and converts it into a number. The numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image. The greater number of pixels, the sharper the image.

Planck Curve

The graphical representation of the mathematical relationship between the frequency (or wavelength) and intensity of radiation emitted from an object by virtue of its heat energy.


An object that orbits a star. Although smaller than stars, planets are relatively large and shine only by reflected light. Planets are made up mostly of rock or gas, with a small, solid core. In our solar system, the inner planets — Mercury, Venus, Earth, and Mars — are the rocky objects, and most of the outer planets — Jupiter, Saturn, Uranus, and Neptune — are the gaseous ones. Because Pluto is made largely of ice, like a comet, some astronomers do not consider it a true planet.

Planetary Nebula

An expanding shell of glowing gas expelled by a star late in its life. Our Sun will create a planetary nebula at the end of its life.


A small body of rock and/or ice — under 10 kilometers (6 miles) across — formed during the early stages of the solar system. Planetesimals are the building blocks of planets, but many never combined to form large bodies. Asteroids are one example of planetesimals.


A substance composed of charged particles, like ions and electrons, and possibly some neutral particles. Our Sun is made of plasma. Overall, the charge of a plasma is electrically neutral. Plasma is regarded as an additional state of matter because its properties are different from those of solids, liquids, and normal gases.


A column of material that is shaped like a long feather.


A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond Neptune’s orbit where Pluto resides. Pluto was considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. Pluto’s orbit is more elliptical than those of the eight solar system planets.

Potential Energy

The energy of an object owing to its position in a force field or its internal condition, as opposed to kinetic energy, which depends on its motion. Examples of objects with potential energy include a diver on a diving board and a coiled spring.

Primary Mirror

A large mirror in a reflecting telescope that captures light from celestial objects and focuses it toward a smaller secondary mirror. The primary mirror in the Hubble Space Telescope measures 94.5 inches (2.4 meters) in diameter.

Primary lens

A large convex lens in a refracting telescope that captures light from celestial objects and focuses it toward the eyepiece.

Prime Focus

The location where light reflected from the primary mirror of a reflecting telescope comes into focus. Placing a secondary mirror in the light path allows the light to be focused elsewhere, in a more convenient location for the science instruments.

Primordial Nucleosynthesis

Element building that occurred in the early universe when the nuclei of primordial matter collided and fused with one another. Most of the helium in the universe was created by this process.


A prism is usually a triangular-shaped piece of glass used to refract, or bend, light. The shape of the glass causes the light to disperse, or spread out, as it bends, producing a rainbow of colors from the white light.


An eruption of gas from the chromosphere of a star. Solar prominences are visible as part of the corona during a total solar eclipse. These eruptions occur above the Sun’s surface (photosphere), where gases are suspended in a loop, apparently by magnetic forces that arch upward into the solar corona and then return to the surface.

Proper Motion

The apparent motion of a star across the sky (not including a star’s parallax), arising from the star’s velocity through space with respect to the Sun.


Matter that is beginning to come together to form a galaxy. It is the precursor of a present-day galaxy and is sometimes called a “baby galaxy.”


A positively charged elementary particle that resides in the nucleus of every atom.

Proton-Proton Chain

A series of nuclear events occurring in the core of a star whereby hydrogen nuclei (protons) are converted into helium nuclei. This process releases energy.


A small body that attracts gas and dust as it orbits a young star. Eventually, it may form a planetary body.


A collection of interstellar gas and dust whose gravitational pull is causing it to collapse on itself and form a star.


A neutron star that emits rapid and periodic pulses of radiation.

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