The stellar population of spiral galaxies is constantly refurnished by newly formed massive, bright and blue young main sequence stars, so that spirals have blue colours.
Most systematic studies of SFRs in disk and irregular galaxies are
consistent with a common picture, in which the variation
of the UV-visible colors and 
emission properties of
galaxies along the Hubble sequence can be attributed to underlying
variations in their stellar birthrate histories.
In this picture early-type galaxies (type S0-Sb) represent systems
which formed most of their gas into stars on timescales much less than
the Hubble time, while the disks of late-type systems (Sc-Im) have
formed stars at roughly a constant rate since they formed (Fig. 2).
Figure 2: Schematic sketch of the SFR history as a function of Hubble type.
In the following sections we will discuss the physical properties of the gas in star forming galaxies with the tools developed for the analysis of HII regions (Osterbrock, 1989). Clearly, a star forming galaxy is a system much more complex than an HII region. However, the well-spread evidence that star-forming galaxies generally follow relations expected for HII regions (Kennicutt, 1992) suggests that the integrated spectra of a galaxy could be used to roughly derive the characteristic ionization of its gas.
The source of energy that enables the gas of a galaxy to radiate
is ultraviolet radiation from stars. Hot stars, with surface temperature
K, inside or in the vicinity of a
gas-rich region emit ultraviolet photons that tranfer energy to the gas by
photoionization. Hydrogen is by far the most abundant element, and
photoionization of H is thus the main energy input mechanism.
Photons with energy greater than 13.6 eV, the ionization potential of
H, are absorbed in this process, and the excess energy of each
absorbed photon over the ionization potential appears as kinetic
energy of a new liberated photoelectron. Collisions between
electrons, and between electrons and ions, distribute this energy
and maintain a Maxwellian velocity distribution with temperature
T in the range 5000< T< 20000 K.
We summarize now shortly how forbidden and permitted lines
form. For historical reasons, astronomers tend to refer to the chief
emission lines of gaseous nebulae
(
,
etc.) as forbidden lines.
Actually, it is better to think of the bulk of the lines as collisionally
excited lines, which arise from levels within a few volts of the
ground level and which therefore can be excited by collisions with
thermal electrons.
Although downward radiation transitions from these excited levels have very
small transition probabilities,
they are responsible for the emission lines observed.
Indeed, at the low density of typical nebulae
(
) collisional
deexcitation is even less probable.
So, almost every excitation leads to emission of a photon, and the nebula thus emits a forbidden line spectrum that is quite difficult to excite under terrestrial laboratory conditions.
In addition to the collisionally excited lines, the permitted lines of H I, He I, and He II are characteristic features of the spectra of spiral galaxies. They are emitted by atoms undergoing radiative transitions. Indeed, recaptures occur to excited levels, and the excited atoms then decay to lower and lower levels by radiative transitions, eventually ending in the ground level.
To give an overview of the main spectral features visible in the spectra of spiral galaxies we show in Fig. 1 representative spectra for early- to late- and irregular types galaxies.
The spectra of early-type spirals such as NGC 2775 are dominated
by late-type stars and differ only subtly from the spectra of E-S0
galaxies. The main changes are an increase of the flux in the blue,
to which corresponds a decrease in the strength of the
break and the appearance of weak 
and
emission, at the level of a few Å or less in equivalent width
(equation 2).
Except for occasional weak 
emission, no other nebular lines are detected in the integrated spectrum.
Intermediate- to late-type spirals, illustrated by
NGC 4750 and NGC 6181, are characterized by much higher blue flux, more
prominent Balmer absorption lines and nebular emission features
but lines at 
,
,
,
begin to appear.
As one progresses to the bluest spirals and Magellanic irregulars, as illustrated by NGC 4449, the integrated spectrum becomes increasingly dominated by the continua of B-A stars, and by strong nebular emission lines.
The smooth monotonic progression in both the emission and absorption spectra with galaxy type shows clear evidence of the systematic changes in stellar populations and star formation rates along the Hubble sequence.
Finally, we included the spectrum of NGC 7714, a nuclear starburst galaxy.
Nuclear means that most of the starburst activity is concentrated
in an H II region-like nucleus.
Nuclear and global starburst galaxies possess very strong emission
lines, with equivalent width of 
ranging from 70-200 Å,3-10 times higher than in an average Sb or
Sc galaxy. The stellar continua also resemble late-type, active
star-forming galaxies.