Page 19 - Curriculum Visions Dynamic Book

Classifying a star
Grouping, or classifying, stars helps identify patterns both in the stars and in the galaxies that contain them.
There are many ways in which stars can be grouped. For example, astronomers label young stars (those less than 100 million years old and which still contain heavy elements such as iron, nickel, and carbon) as Population I stars. The Sun
is a Population I star. All of these kinds of stars occur in the arms of spiral galaxies. By contrast, stars that are older contain little by way of heavy elements because, in the earliest years of the Universe, such elements had not yet been created. They are called Population II stars. They occur in the globular clusters at the heart of spiral galaxies and throughout elliptical galaxies.
Each new generation of stars contains a higher proportion of heavy elements than its predecessor. That is why, over time, the Universe gradually contains less hydrogen and more of every other
element. The gas clouds emitted by Population I stars will eventually be used again to form a new generation of stars.
Another way of classifying stars is through their size and colour.
The size of a star is often related both to how bright it is and to its colour. Small stars like the Sun are a few thousand degrees Celsius at their surfaces. They are yellow. Large stars are several tens of thousands of degrees at the surface because they are burning more fiercely. As a result, they shine with a blue-white light.
Astronomers classify the hottest stars as type
O and the coldest as M. A complete sequence of categories is O B A F G K M. The Sun ranks as a G star.
Stars can also be thought of as having a life cycle, as we shall see from page 20. As a result, it is useful to describe where stars are in their life. The numbers I (for the largest and youngest stars) to V (for the old dWarf stars) are used for this.
Star clusters
Stars are not isolated in their galaxies. It is common to find groups of stars bound together by their
own gravity. They are called star clUsters. There are two types of star clusters: open clusters and globular clusters.
Open clusters have a hundred to a thousand stars. There does not appear to be any regular organization to an open cluster.
Globular clusters are simply vast. They can contain hundreds of thousands, or millions, of stars, all concentrated together.
In both types of cluster the stars were formed at the same time from the same cloud of gas and dust. As they developed, some grew faster than others, gathering more matter to themselves and depriving others of the chance to grow.
Because big stars evolve so much more rapidly than small stars (see page 23), some stars in a cluster have gone farther through their life cycles than others.
center of gravity The point at which all of the mass of an object can be balanced.
dWarf star A star that shines with a brightness that is average or below.
fUsion The joining of atomic nuclei to form heavier nuclei.
gravity The force of attraction between bodies.
main seqUence The 90% of stars in the Universe that represent the mature phase of stars with small or medium mass.
milky Way The spiral galaxy in which our star and Solar System are situated.
optical Relating to the use of light.
protostar A cloud of gas and dust that begins to swirl around; the resulting gravity gives birth to a star.
star clUster A group of gravitationally connected stars.
stellar Wind The flow of tiny charged particles (called plasma) outward from a star.
19