Universal Expansion
1845 -- Earl of Rosse made first drawing of the Whirlpool Galaxy (M51).1912 -- Henrietta Levitt studied Cepheid variables (Type I) in the Small Magellanic Cloud. Type I are metal-rich and Type II metal-poor. Cepheids dim and brighten in a regular manner. The time between maximum brightness is called the period of the variable. Levitt found that the period was related to the absolute luminosity of the Cepheid variable.
1915 -- Harlow Shapley measured the position of 93 globular clusters using RR Lyrae stars. These stars, like Cepheids, are variables stars and have absolute luminosities that depend upon the period of light variation. Shapley found that most of these clusters were located in one part of the sky, near Sagittarius.
1917 -- Shapley conjectured that the globular clusters orbit the galactic center and that the latter was in the direction of Sagittarius. That's why the globular clusters appeared to be mostly in one part of the sky.
1920 -- In April, Harlow Shapley debated Heber D. Curtis. Shapley argued that the nebulae (which later proved to be galaxies) lie within the Milky Way. Curtis thought they resided far from our galaxy. The issue was not resolved by that debate.
1923 -- Hubble photographed the Andromeda Galaxy (M31) and showed (in 1924) that M31 is at about 2.2 million light years from us. This was conclusive proof that the galaxy lay beyond the Milky Way. The universe was much vaster than previously thought.
DiskNear the sun the average density of stars is about 1 star per 330 cubic light years.
Bulge
In the galactic core there are about 10 million stars per cubic light year.
HaloThe globular clusters form a halo about the galaxy.
DiameterAbout 120,000 light years. The sun is about 28,000 light years from the center and takes about 240 million years to make a complete orbit, at about 220 km/s.
AgeFrom the oldest stars we know that the galaxy is about 15 billion years old.
Luminous MassBetween 100 to 500 billion solar masses.
Star Birth RateAbout 3 to 5 stars per year.
Population IPopulation I stars are young compared with the age of the galaxy. These stars lie in the disk and the spiral arms--like the sun. Their ages are in the range 1 million to a few billion years. They are relatively rich in heavier elements--about 3% heavier elements, because they are of the second or third generation.
Population IIPopulation II stars are old with ages around 10 billion years. They populate the galactic bulge and halo. These stars contain only trace amounts of heavier elements.
Open Clusters
These are loose groupings of stars. They contain a few hundred stars in a volume with a radius between 7 and 20 light years. The stars are mainly (young) population I stars. A beautiful example is the Pleiades.
Globular ClustersThese are spherical groupings of stars that can contain thousands to millions of stars. These clusters have radii in the range 40 to 160 light years. A good example is the Great Globular Cluster in Hercules, M13. The large number of stars causes a strong gravitational pull towards the center, which causes the stars to form tight spherical clusters. The stars are always (old) population II stars.
Interstellar Dust causes light from background stars to be reddened and dimmed. Sometimes the dust is so thick that light cannot get through. We call such a dust cloud a dark nebula. (A beautiful example is the Horsehead nebula in Orion). Sometimes, however, the dust acts like a kind of mirror and reflects light that falls upon it creating reflection nebulas.Interstellar Gas can glow because of stars embedded in them. A spectacular example of one of these emission (and reflection) nebulas is the Great Nebula in Orion.
The further an object is from a central mass the slower it moves in its orbit about the central mass. This is a consequence of Kepler's Laws and is certainly true of the planets that orbit the Sun. Pluto moves a lot slower than does the Earth.However, the stars in the Milky Way (and other galaxies) do not obey this rule: as we move away from the center of the Milky Way the orbital speeds of stars do indeed fall, as expected, however, beyond about 8000 light years or so from the center the orbital speeds of stars remain roughly constant. For example, the Sun and stars further out than the Sun move along their orbits, about the galactic center, at about 220 km/s.
The orbital speed as a function of the radial distance from the galactic center is called the rotation curve.
Some scientists have suggested that this observation implies a breakdown of Newton's law of gravity on large distance scales. Another explanation is that galaxies are shrouded in a large amount of unseen matter, called dark matter, that causes additional gravitational effects. This, currently, is the favored hypothesis. The nature of the dark matter, however, is unknown. But perhaps in a few years we shall be better informed as the results of the many experiments being conducted worldwide become available.
On very large scales galaxies are found to form clusters. Our galaxy, the Milky Way, inhabits a cluster we call the Local Group which contains about 30 galaxies of which the Milky Way, M31 and M33 are the three largest galaxies.
On even greater scales the galaxy clusters themselves form clusters called superclusters that can be tens to hundreds of millions of light years across. These superclusters resemble huge ribbons or sheets in space. Between these superclusters are great voids containing relatively few galaxies. Why the galaxies are distributed like this is still a mystery.
There is considerable evidence that galaxies, like everything else in the universe, evolve; that is, they change with time. One cause of galactic evolution is galactic collisions. Indeed, many astronomers believe that such collisions play a dominant role in the evolution of galaxies. (See animation.)
The galaxies in clusters are bound together by gravity. But calculations show that the visible matter in galaxies does not provide enough mass, and therefore enough gravitational binding, to keep the galaxies from drifting away from the cluster.
Moreover, as noted above, many galaxies have flat rotation curves, just like the Milky Way. The best explanation for these observations is that there is a lot more matter in galaxies than appears; that is, galaxies and galaxy clusters are immersed in huge halos of dark matter.
Radio Galaxies
They emit energy in the radio part of the electromagnetic spectrum--sometimes up to millions of times that emitted by a normal galaxy. The emission comes usually from the galactic core and from regions well outside the core.
Seyfert GalaxiesThese galaxies, named after the American astronomer Carl Seyfert, are spiral galaxies whose cores emit as much energy (at all wavelengths) as the entire radiation output of the Milky Way. A striking feature of these galaxies is that their luminosity can change rapidly, sometimes in a matter of minutes!
QuasarsThese galaxies are the most powerful energy emitters known.
The energy output of active galaxies is so enormous that ordinary mechanisms of energy generation are unable to explain their energy output. The favored explanation, at the moment, is to suppose that one or more supermassive black holes exist, at the core of these galaxies, into which matter is falling. As the matter falls into the black hole the gravitational energy of the material is converted into kinetic energy (heat energy), thereby causing the temperature of the in-falling gas to rise to millions of Kelvin and radiate energy.
The Sombrero Galaxy (M104) in the constellation of Virgo is thought to have a billion solar mass black hole lurking at its center. The Hubble Space Telescope recently found evidence that the galaxy M87 (which is about 50 million light years from us) contains a supermassive black hole at its core that may be the size of our solar system!
In the 1920s Hubble and others established that the light from very distant galaxies was red-shifted. In 1929, Hubble, proposed that the red-shift is a Doppler effect, caused by the motion of galaxies away from us. He discovered that galaxies were moving away from us according to a simple law, now called Hubble's Law.
The law can be stated in a simple formula:
v = Hdwhere v is the recession velocity, d is the distance between us and the galaxy and H is a constant called the Hubble constant. The Hubble constant is measured to be between 50 km/s/Mpc (read "50 kilometers per second per megaparsec") and 90 km/s/Mpc.that is, 30,000 km/s. This is one tenth the speed of light! Galaxies that are relatively close to us do not necessarily recede from us; some are actually approaching us because of the gravitational attraction between them and the Milky Way In fact, about 5 billion years from now, just as the Sun is about to become a red giant the Andromeda Galaxy will collide with the Milky Way!Example: A galaxy that is 600 Mpc from us would be receding at a speed of
v = (50 km/s/Mpc) x (600 Mpc)
