OK, so now youíre fairly good at reading the diagram. Letís look
at why we called it a type of "family portrait" for stars. In previous
activities with the star life cycle, the focus was on equilibrium
and the battle between gravity and gas pressure to maintain a stable
star. Remember that a protostar (forming star) is similar to human
pregnancy Ė the star is forming, gaining mass, and itís not truly
"born" yet. Once nuclear fusion begins in a star, the star is born.
Stars live the majority of their lives in the main sequence phase,
and then as they destabilize and burn fuels other than hydrogen,
they expand, reduce in core temperature, and eventually die. The
HR diagram has stars on it at every phase in the life cycle. It
is like taking a family picture. There will be new babies, teenagers,
parents, and grandparents. Think of the HR diagram as a representation
of stars throughout their lives. Today, a star could be residing
on the main sequence (adult), but when the star ages by hundreds
of millions of years, it destabilizes. The next "family portrait"
may have that exact same star off the main sequence as a red giant
We have animated the HR diagram to help you understand how a star
changes throughout its life. In this animation, a star will move
throughout the diagram. DO NOT BE DECEIVED! The star does not
physically zip around the universe as it ages. The star physically
stays in the same location, but the movement is to show how luminosity
(brightness) and surface temperature change with star age. Remember,
itís a diagram, not a star map.
This animated version shows how a star's brightness and surface
temperature are measured throughout its life cycle.
Click the picture to view the activity.
More HR diagram Fun
This "family portrait" is an HR diagram with five stars on it and
a background of the color each star would look through the filter
of a spectrophotometer. There are some questions to answer about
Click the picture for interactive version with questions.