Astronomy 104 - Chapter 23 - Key Terms

Particle undetectable at any electromagnetic wavelength, but can be inferred from its gravitational influence.

Thick distribution of warm gas and stars around the galactic center.

The center of the Milky Way, or any other, galaxy. The point about which the disk of a spiral galaxy rotates.

Flattened region of gas and dust that bisects the galactic halo in a spiral galaxy. This is the region of active star formation.

Region of a galaxy extending far above and below the galactic disk, where globular clusters and other old stars reside.

Small central high-density region of a galaxy. Nearly all of the radiation from an active galaxy is emitted from the nucleus.

Plot of rotation speed versus distance from the center of a galaxy.

Time taken for objects at the distance of the Sun (about 8 kpc) to orbit the center of the Galaxy, roughly 225 million years.

A huge assembly of stars containing from millions to hundreds of billions of stars plus dust and gas. The Sun is a star in the Milky Way Galaxy.

The effect induced on the image of a distant object by a massive foreground object. Light from the distant object is bent into two or more separate images.

The spiral galaxy in which the Sun resides. The disk of our Galaxy is visible in the night sky as the faint band of light known as the Milky Way.

A relation between the pulsation period of a Cepheid variable and its absolute brightness. Measurement of the pulsation period allows the distance of the star to be determined.

A star whose luminosity varies in a predictable, periodic way. Also known as an intrinsic variable star (in contrast to an eclipsing variable star).

Plot of the orbital speed of disk material in a galaxy against its distance from the galactic center. Analysis of rotation curves of spiral galaxies indicates the existence of dark matter.

Variable star whose luminosity changes in a characteristic way. All RR Lyrae stars have more or less the same period.

Strong radio source corresponding to the supermassive black hole at the center of the Milky Way. The most intense portion of the Sgr A region is called Sgr A* (read as Sagittarius A-star).

Mode of star formation in which shock waves produced by the formation and evolution of one generation of stars triggers the formation of the next.

Distribution of material in a galaxy in a pinwheel-shaped design apparently emanating from near the galactic center.

(i) A wave of matter formed in the plane of planetary rings, similar to ripples on the surface of a pond, which wrap around the rings forming spiral patterns similar to grooves in a record disk. Spiral density waves can lead to the appearance of ringlets. (ii) A proposed explanation for the existence of galactic spiral arms, in which coiled waves of gas compression move through the galactic disk, triggering star formation.

Galaxy composed of a flattened, star-forming disk component which may have spiral arms and a large central galactic bulge.

Historical name for spiral galaxies, describing their appearance.

A star whose luminosity changes with time. The star can be an intrinsic or pulsating variable star or an eclipsing binary star system.