26.2 types of galaxies & 26.3 properties of galaxies & 26.4 the extragalactic distance scale & 26.5 expanding universe

spiral galaxies

• consist of a central bulge, a halo, a disk, and spiral arms

• bright emission nebulae and hot, young stars are present, especially in the spiral arms

  • new star formation occurring

• disks are often dusty

• contain a mixture of young and old stars

• all spiral rotate

• about 2/3 of nearby spiral galaxies have boxy or peanut-shaped bars of stars running through their centers

  • these galaxies are called barred spirals

  • spirals usually begin from the ends of the bar

• luminous parts appear to range in diameter from about 20,000 to more than 100,000 light years

• masses of visible portions, 1 billion-1 trillion Suns

• luminosity; 100 million - 100 billion times the luminosity of our Sun

classification of galaxies

elliptical galaxies

• consist almost entirely of old stars and have shapes that are spheres or ellipsoids (squashed spheres)

• not trace of spiral arms

• light dominated by older reddish stars (population II)

• globular clusters present

• contain small amount of interstellar matter

• show varying degree of flattening

• mass; can be as large as 10^13 Msun

• diameters; extend over several 100,000 light years

• orbits of stars within the galaxies are not in the same direction

  • elliptical galaxies don’t appear to rotate in the same systematic way as spiral galaxies

dwarf elliptical galaxies

• very faint and difficult to see

• luminosity is about equal to the brightest globular clusters

irregular galaxies

• galaxies that do not have the regular shapes and do not fit into the main categories

• lower masses and luminosities than spiral galaxies

• appear disorganised, many are undergoing relatively intense star formation activity

• contain population I and II stars

• e.g. Large Magellanic Cloud and Small Magellanic Cloud

spiral galaxy mass measurements

• use of doppler effect and radial velocity to determine mass

• can measure the rotational speed of the galaxy by obtaining spectra of the gas or stars, and looking for wavelength shifts produced by the Doppler effect

elliptical galaxy mass measurements

• measure the various speeds with which the stars are moving in their orbits around the center of the galaxy, so we can calculate how much mass the galaxy must contain in order to hold the stars within it

• the amount by which each spectra line broadens indicates the range of speeds at which the stars are moving with respect to the galaxy

mass-to-light ratio

• ratio of a galaxy’s mass (in units of the Sun’s mass) to its light output (in units of the Sun’s luminosity)

  • Sun’s mass:light is 1

• tells us roughly what kind of stars make up the most luminous population of the galaxy, and also tells us whether a lot of dark matter is present

• mass:light is greater than 1 for low-mass stars

• galaxies in which star formation is still occuring have many massive stars, and thier mass:light is usually 1-10

• galaxies that have an older stellar population (ellipticals) mass:light ratios of 10-20

Sagittarius A*

• cosmic radio source in the nucleus of the Galaxy

• supermassive black whole

• thousands of old, reddish main-sequence stars within a few hundred light years

• about 100 OB stars that are newly formed

type Ia supernova

• involve the explosion of a white dwarf in a binary system

• all have the same luminosity (about 4.5 × 10^9 Lsun)

expanding universe

• discovered that the more distant the galaxy, the faster it was receding from us

• recession velocities are proportional to distances from us

• Hubble’s law: v = H x d

  • H is Hubble’s constant

• uniformly expanding universe

  • stretching of the universe is what moves things away from us

  • expansion of space

  • as space stretches, the galaxies are carried farther and farther apart