Galaxy spectra show whether a galaxy contains star-forming regions called HII regions. HII is a spectral emission line that corresponds to ionized hydrogen - a hydrogen atom that has lost its electron. HII regions are areas of a galaxy where hydrogen nuclei and electrons are recombining to form neutral hydrogen.
HII spectral lines are at the red end of the visible spectrum and occur when a proton recombines with an electron, giving off a photon. The photon is the equivalence of lost energy in the process.
KILOPARSEC (Kpc)
1 Kpc = 3261.56 light years.
LINER
A low-ionization nuclear emission-line region is a type of galactic nucleus that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms, such as O, O+, N+, and S+. Conversely, the spectral line emission from strongly ionized atoms, such as O++, Ne++, and He+, is relatively weak
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Lenticular galaxies are sometimes called "armless spiral galaxies." They have a central bulge, no spiral arms but may have a ring/cloud of stars around them. Some lenticular galaxies have a bar and are called "barred lenticular galaxies". Their morphological type is labelled SB0. Normal lenticular galaxies are labelled S0.
Planetary nebulae are remnants of stellar explosions where a star throws off it's outer layers, in particular, stars that are about 0.8 solar mass or less. It is thought that the expanding gasses from such explosions become ionized by ultraviolet radiation escaping from the exposed core of the star giving rise to a highly luminous expanding shell. These nebulae exist for only a few thousand years.
Pulsars are 'pulsating radio sources', They are roughly 1.5 times the mass of the sun and thought to be engendered in supernova explosions. They are very small but extremely dense, so that a thimbleful of pulsar matter would weigh a billion tonnes on earth. They travel very fast through space reaching speeds of up to 1200 kms per second. Current theory suggests that a pulsar is a collapsed neutron star that was the result of a supernova. This object rotates about an axis very fast, often milliseconds to complete a rotation. The object also has magnetic poles that may not neccessarily be coincident with the axis of rotation. If not, then charged particles moving along magnetic field lines would get spewed out of the object, forming a circular pattern. If the magnetic poles point in the direction of earth, then we would see this discharge every time the object went through a rotation. Which is exactly what we see every few milliseconds in the case of the fastest rotating pulsars.
Quasar or quasi stellar object. These are objects from the early universe that spew out massive quantities of electromagnetic radiation, including light but unlike pulsars they are not a single object. Instead it is thought that they are of galactic size and powered by super-massive black holes. Some quasars have been shown to have jets of matter flung far out into space. It is also thought that once the super-massive black hole has used up any available matter it will lie 'dormant' until the interaction with another galaxy makes more matter available for the black hole to engender another quasar. They are linked, through theory, to Active galaxies like the Seyfert galaxies and BL Lac objects. The latter are most probably galaxies that emit a strong continuum of electromagnetic radiation from radio frequencies to xray although they spectra do not exhibit any emission lines.
Seyfert galaxies have very bright nuclei, and very bright spectral emission lines from Hydrogen, Helium, Nitrogen and Oxygen.
There are two classification of Seyfert galaxy related to the breadth ot the emission lines. Seyfert I has both broad and narrow while Seyfert II have only narrow lines.
Refers to galaxies that are undergoing an unusually high rate of star formation. So much so that it is thought that this activity will use up all of it's gas reserves during a single rotation. Causes of this effect can be due to interaction with another galaxy. Sometimes gases can be funnelled toward the galactic core along an unstable bar resulting in bursts near the core.
After a star has burned up all of it's fuel i.e. hydrogen and helium, at the end of it's 'red giant' phase, it will undergo a gravitational collapse. This is so extreme that it occurs in minutes. The resulting impact causes a spectacular release of energy so much so that it can outshine a whole galaxy of stars.
There are classifications of supernovae:
Type IA occurs in a binary system where one of the stars is a white dwarf. A white dwarf is an extremely dense star and thus has an intense gravitational field. This field is so strong as to pull matter away from the other star in the binary system. When the white dwarf accretes enough matter to initiate a nuclear chain reaction it explodes and is about 5 billion times brighter than our sun. The white dwarf needs to be about 1.4 times the mass of our sun (the Chandrasekhar limit) for the reaction to start. Since all such systems that explode at this limit ( of mass) we conclude that all explosions of this type give off the same intensity of light. Now we can use the inverse square law against the measured brightness to determine the distance of the star and hence the galaxy containing it.
Type II
UV-excess sources are mostly objects such as white dwarfs, white dwarf binaries, subdwarfs type O and B, emission line stars and QSOs