• ABSOLUTE MAGNITUDE. A very distant, very bright star might appear dimmer than a nearby star
that is actually dim because some of the brighter star's light is diffused by interstellar dust and gas.
Absolute magnitude is the measure of how bright a star truly is, when observed from a standard
distance of exactly 10  parsecs (32.6 light years). Each increment of 5 corresponds to a factor of
100 in brightness, so a star with an absolute magnitude of -10 is 100 times brighter than a star
with an absolute magnitude of -5. A star with an absolute magnitude of -5 is 100 million times
brighter than a star with an absolute magnitude of +15.

• LUMOSITY. The total amount of light energy emitted by a celestial object when compared to the
sun. The sun emits 384,600,000,000,000,000,000,000,000 watts (an ordinary light bulb might
emit 100 watts). Thus, an object with a lumosity of 1 emits the same amount of wattage as the sun.
An object with a lumosity of 10,000 is 10,000 times brighter than the sun, and an object with a
lumosity of 0.0001 emits just a tiny fraction of the light (so it's really dim).

• SURFACE TEMPERATURE. The temperature of a star's surface determines its color. Very hot stars
are blue and purple, while cooler stars are orange and red. Though it's not pictured on the above
image because it would be too confusing, every spectral class is further divided into one of ten
subclasses based solely upon temperature, with 0 being the hottest and 9 being the coolest. So a
star classed as type G1 would be a smokin' hot G-type star, while a G9 would at the cooler end of
the G-type spectrum.

• SPECTRAL CLASS. A letter assigned to a star to denote its type. There are nine class types (O, B, A,
F, G, K, M, L, and T), each of which is associated with a different color; for example, any red star is
considered Class M, and any yellow star (like the sun) is Class G.  However, since there can be vast
differences in size and brightness within a class, stars are further divided into eight more specific
groups based upon lumosity.


The first five groups are fairly self-explanitory, with each type representing different varieties of
very large stars. These stars are generally quite rare, and most stars fall into the sixth category, the
main sequence.

A star in the main sequence is generally a stable, middle-aged star whose dominant energy
production process is the fusion of hydrogen in its core. Over the course of its life, a star will spend
most of its time within the main sequence, remaining near its initial position on sequence until it
has expended a significant amount of hydrogen in its core. At that point, the star will begin to
evolve out of the main sequence--typically moving up and to the right on the HR diagram (a class G
star like the sun would evolve into a sub-giant or a bona fide giant, while a class O star would
transition into a hyper giant).

Class O and B stars that evolve out of the main sequence will eventually die violent deaths in the
form of a supernova and become either a black hole or a neutron star (and completely removing it
from the HR diagram in the process). Smaller stars like the sun have far less violence in their
futures; after a period of vast expansion, the star will just fizzle away, becoming a white dwarf and
eventually a lifeless brown dwarf.

So, using the HR diagram, the sun would be classified as type G2V... where G indicates a yellow
star, the 2 indicates that it's a pretty hot when compared to other yellow stars, and the V places it
on the main sequence, just the way we like it.

A type O8Ia star would indicate a purple star, relatively cool compared to other purple stars, that
has evolved out of the main sequence and transitioned into a hyper giant. This guy's on it's last
legs! Watch out for a super nova!
The Hertzsprung-Russell
Diagram (HRD) is a plot of
a star's brightness and
temperature, and can be
used to determine its
spectral classification.