Chapter 15 Flashcards
(26 cards)
Hubble classification scheme
Method of classifying galaxies according to their appearance, developed by Edwin Hubble
Spiral galaxy
Galaxy composed of a flattened, star-forming disk component that may have spiral arms and a large central galactic bulge.
Barred-spiral galaxy
Spiral galaxy in which a bar of material passes through the center of the galaxy, with the spiral arms beginning near the ends of the bar.
Elliptical galaxy
Category of galaxy in which the stars are distributed in an elliptical shape on the sky, ranging from highly elongated to nearly circular in appearance.
S0 galaxies
Galaxy that shows evidence of a thin disk and a bulge, but that has no spiral arms and contains little or no gas
SB0 galaxies (Lenticular galaxies)
An S0-type Galaxy whose disk shows evidence of a bar.
Irregular galaxies
A galaxy that does not fit into any of the other major categories in the Hubble classification scheme.
Magellanic clouds
Two small irregular galaxies that are gravitationally bound to the Milky Way Galaxy.
Hubble sequence
Hubble’s arrangement of elliptical, S0, and spiral galaxies, often used to classify galaxies but not thought to represent a true evolutionary sequence in any sense.
Standard candle
Any object with an easily recognized appearance and known luminosity, which can be used in estimating distance. Supernova, which all have the same peak luminosity (depending on type), are good examples of standard candles and are used to determine distances to other galaxies.
Tully-Fisher relation
A relation used to determine the absolute luminosity of a spiral galaxy. The rotational velocity, measured from the broadening of spectral lines, is related to the total mass, and hence the total luminosity.
Local group
The small galaxy cluster that includes the Milky Way Galaxy.
Galaxy cluster
A collection of galaxies held together by their mutual gravitational attraction.
Hubble’s law
Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is directly proportional to its distance away.
Cosmological redshift
The component of the redshift of an object that is due
Cosmological redshift
The component of the redshift of an object that is due only to the Hubble flow of the universe.
Hubble constant
The constant of proportionality that gives the relation between recessional velocity and distance in Hubble law.
Active galaxies
The most energetic galaxies, which can emit hundreds or thousands of times more energy per second than the Milky Way, mostly in the form of long-wavelength nonthermal radiation.
Starburst galaxies
Galaxy in which a violent event, such as near collision, has caused an intense episode of star formation in the recent past.
Active galactic nucleus
Region of intense emission at the center of an active galaxy responsible for virtually all of the galaxy’s nonstellar luminosity.
Seyfert galaxies
Type of active galaxy whose emission comes from a very small region within the nucleus of an otherwise normal-looking spiral system.
Radio galaxy
Type of active galaxy that emits most of its energy in the form of long-wavelength radiation.
Radio lobe
Roundish extended region of radio-emitting gas, lying well beyond the center of a radio galaxy.
Blazar
Particularly intense active galactic nucleus in which the observer’s line of sight happens to lie directly along the axis of a high-speed jet of particles emitted from the active region.
