This document discusses the properties and life cycles of stars. It begins by explaining that stars are separated by vast distances, which astronomers measure in light years. The closest star to our sun is Proxima Centauri. Astronomers use parallax to determine the distances to nearby stars. Stars are classified based on their color, size, brightness, chemical composition and mass. Color indicates a star's temperature, with the hottest stars appearing blue. Stars are formed from dense clouds of gas and dust, and their lifespan depends on their mass. Over time stars evolve through different stages until their fuel is exhausted, ending as white dwarfs, neutron stars or black holes. Stars often exist in groups like clusters and galaxies. The universe has been expanding

1. Stars

2. Distances To The Stars
 Stars are separated by
vast distances.
 Astronomers use units
called light years to
measure the distance
between the stars
 A light-year is the
distance that light travels
in the vacuum during a
year
 Proxima Centauri, is the
closest star to the sun.

3. Parallax
 Astronomers have developed various methods of
determining the distance between the stars.
 The change in the position of an object with respect to
a distant background is called parallax.
 As the Earth moves on its orbit, astronomers are able
to observe stars from two different positions.
 Astronomers measure the parallax of nearby stars to
determine their distance from the Earth

4. Properties of the Stars
 Astronomers classify stars by
their colour, size, and
brightness. Other properties
of the stars are their
chemical composition and
mass.
 Colour and Temperature –
a star’s colour indicates the
temperature of its surface.
 The hottest stars appear blue
 The cooler stars appear red
 The spectrum of colour in a
star is from blue to green, to
yellow and red.

5. Brightness
 The brightness of a star as
viewed from the Earth is
dependent on many factors
such as colour intensity and
distance.
 Apparent Brightness – is
the brightness of a star as it
appears from the Earth.
 The apparent brightness
decreases as its distance
from you increases
 Absolute Brightness – is
how bright a star really is.
The absolute brightness is a
characteristic of the star and
is not dependent on its
distance from the Earth.

6. Composition
 Each star has its
own spectrum.
 Most stars have a
chemical makeup
that is similar to
the sun, with
hydrogen and
helium together
making up from 96
to 99.9 % of a
star’s mass.

7. How Stars Are Formed
 A nebula is a large cloud
of gas and dust spread
out over a large volume
of space.
 Some nebulas are
glowing clouds lit from
within by bright stars.
 A star is formed when a
contacting cloud of gas
and dust becomes so
dense and hot that
nuclear fusion begins.

8. Life Cycles of the Stars
 Adult Stars – A star’s
mass determines the
star’s place on the main
sequence and how long it
will stay.
 The amount of gas and
dust available when a
star is formed deter-
mines the mass of each
young star.
 The larger the star the
more energy it produces.
 Since blue stars burn
brightly, they use up
their fuel quickly and are
short lived.

10. The Death of a Star
 The dwindling
supply of fuel in a
star’s core leads to
the star’s death as
a white dwarf,
neutron star, or
black hole.

11. Groups of Stars
 Astronomers have
determined that more
than half of all stars are
members of the star
systems.
 There are three basic
kinds of star clusters:
open clusters,
associations, and
globular clusters.
 Astronomers classify
galaxies into four main
types: spiral, barred-
spiral, elliptical, and
irregular.

13. The Expanding Universe
 The observed red shift in the spectra of the
galaxies shows that the universe is expanding.
 Astronomers theorize that the universe came
into being in an event called the Big Bang.
 Dark matter can only be detected by observing
its gravitational effects on visible matter.

14. Pavel Elena-Maria, Sav Ioana, Ispas Darius
XC-matematica-informatica
Colegiul National “Horea, Closca si Crisan” Alba Iulia