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Properties of Stars
Brightness of Stars
• The brightness of a star observed by us depends upon
– The distance of a star
– The actual amount of light given off by the star (its energy)
• How bright a star appears to us is called its apparent
magnitude.
• The actual total radiant energy given off by a star is
called its luminosity and is sometimes referred to as
the absolute magnitude.
Brightness of Stars
• The brightness of stars as viewed from Earth is often
measured using the apparent magnitude scale.
• The smaller the value, the brighter the star.
• The Sun has a value of -27. A full moon has value -13.
Star colours and Temperature
• Stars come in all colours, from dull
red, through orange, yellow and white
to blue.
• There is a direct relationship between
a star’s surface temperature and its
colour.
Star colours and Temperature
• The largest stars are the red giants – which
can be hundreds of times the diameter of the
sun. Betelgeuse is a red giant.
• The smallest stars are white dwarfs and are
about the size of Earth.
The Hertzsprung-Russell (H-R) Diagram
• Between 1911 and 1913, two astronomers – one
Danish (Ejnar Hertzsprung) and one American
(Henry Russell) – independently developed a
diagram to show the relationship between the
luminosity or absolute magnitude of a star and
its surface temperature as deduced from its colour.
• The diagram is called the Hertzsprung-Russell or
H-R diagram and is used extensively by
astronomers.
The Hertzsprung-Russell (H-R) Diagram
The Hertzsprung-Russell (H-R) Diagram
The Hertzsprung-Russell (H-R) Diagram
• Stars can be classified into characteristic types
depending on their position on the H-R diagram.
• Most stars line up along a slightly curved diagonal
line called the main sequence. Our Sun is located
on the main sequence.
• On the main sequence, low mass stars tend to be
cooler and less bright whereas high mass stars are
hotter, brighter and located at the top of the
sequence.
The Hertzsprung-Russell (H-R) Diagram
The Hertzsprung-Russell (H-R) Diagram
• Red giants are located in the upper right corner of the
H-R diagram. They are both cool and very bright. Their
brightness is mainly due to their massive size.
The Hertzsprung-Russell (H-R) Diagram
• White dwarfs lie below the main sequence. These stars
are white hot and yet have low brightness. With white
dwarfs it is their small size which accounts for why they
are not as bright as expected.
The Hertzsprung-Russell (H-R) Diagram
• Astronomers suggest that all stars begin their life in the main
sequence and spend the largest part of their life there.
• This explains why most of the stars observed at a particular
time are found in the main sequence.